Principles of Cancer Biotherapy

Principles of Cancer Biotherapy 5th Edition

Principles of Cancer Biotherapy 5th Edition

Edited by Robert K. Oldham Hematology-Oncology Associates of Southeast Missouri Hospital Southeast Medical Plaza Cape Girardeau, MO USA

Robert O. Dillman Hoag Cancer Center Newport Beach, CA USA University of California Irvine, CA USA

Robert K. Oldham Hematology-Oncology Associates of Southeast Missouri Hospital Southeast Medical Plaza Cape Girardeau, MO USA

Robert O. Dillman Hoag Cancer Center Newport Beach, CA USA University of California Irvine, CA USA

e-ISBN 978-90-481-2289-9 ISBN 978-90-481-2277-6 DOI 10.1007/978-90-481-229-9 Springer Dordrecht Heidelberg London New York Library of Congress Control Number: 2009929387 First edition published 1987 by Raven Press Second edition published 1993 by Williams & Wilkins Third and Fourth edition published 1998 Kluwer Academic Publishers © Springer Science+Business Media B.V. 2009 No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission from the Publisher, with the exception of any material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com)

Preface

The idea for the first edition of Principles of Cancer Biotherapy was formulated in the early 1980s. As the founding director of the Biological Response Modifiers Program for the National Cancer Institute from 19801984, one of us (rko) envisioned a textbook that would embody the principles of the then fledgling fourth modality of cancer treatment - biotherapy. Contributing authors were solicited in 1985, and the first edition came off the presses in 1987. Principles represented the first comprehensive textbook on the use of cancer biotherapy and summarized the work done in this field through 1986. The second edition of Principles was published in 1991 about the time biotherapy was more broadly recognized as the fourth major cancer treatment modality. Subsequent textbooks by DeVita, Hellman and Rosenberg [1] in 1991, Mitchell [2] in 1993 and Rosenberg [3] in 2000 validated the importance of this modality in cancer care. This third edition was published in 1998 and confirmed the tremendous progress that had been made in the previous five years using biologicals in cancer treatment. It was generally agreed that biopharmaceuticals were producing major opportunities for new cancer therapies. Cancer biotherapy was emerging as a more specific and targeted form of systemic cancer treatment. Cancer growth control was also becoming an effective method of treatment complementing cancer destruction as mechanisms of cancer treatment and “cure”. The fourth edition of Principles was published in 2003. It was apparent that biotherapy and the use of biopharmaceuticals had not only become recognized as the fourth modality of cancer treatment, it was clear that biopharmaceuticals had become the dominant form of new cancer therapeutics which in the future will replace less selective, more toxic forms of therapy. For years, the chemical manipulation of small molecules has been pursued in drug development. We now have all the tools for the biological manipulation of natural substances for therapeutic use. In fact, as we better understand the interaction between biological molecules and their receptors, it is clear that biological manipulation and chemical manipulation are coming together to bring molecular medicine to the bedside. Many biological molecules are large and have functions other than

those mediated by their active sites. There is increasing evidence that drug development will focus on the interaction between the smaller active regions of these large biological molecules and their receptors. This opens up a broad field of molecular design for extending and improving the therapeutic activities of natural biological molecules. This has recently been extended to small molecules interacting with DNA/RNA (anti-sense), with enzymes such as tyrosine kinase and with growth and vascular factor receptors. The last decade has been extraordinarily productive in the development of new anticancer drugs through chemical and biological manipulation of these natural molecules. Thus, the body itself has become the “medicine cabinet” of the future. In the 1990s and beyond into this millennium, medicine will face extraordinary demands. While technology brings us tremendous opportunity, it also highlights problems in our medical care system. Most new technology is expensive and, as it comes from the laboratory to the clinic, is by its very nature untried and unproven. Our medical care system involves a private and government insurance reimbursement system that favors paying for marginally effective medical care of the past rather than innovative medical treatments of the future. Such a system is inhibitory to the development of effective new anti-cancer medicines. To more rapidly and efficiently exploit the opportunities in cancer biotherapy in the future, patients, employers, insurers, universities, and government must come together and redefine the system of reimbursement to maximize the patient’s opportunity for access to new and potentially effective cancer therapies. To simply reimburse for old ineffective or marginally effective treatment is not the answer. Provisions must be made to fund clinical research and afford these new approaches broader use at the bedside. We must develop methods to allow our patients access to the opportunities of the future, while maintaining solid support for effective therapies of the past. No longer is it acceptable to pay only for medical care that utilizes old technology, such as chemotherapy, that is approved but only marginally effective. Across the broad spectrum of human malignancies, most chemotherapeutic drugs are toxic and of limited medical value. We must support clinical research

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vi in its efforts to bring newer methods of cancer treatment to the clinic, methods that are less toxic and more effective. We believe cancer biotherapy will ultimately replace much of what we utilize today in cancer treatment. In light of this view, we want to thank all the authors for their dedication to purpose in writing this fifth edition of Principles. This book summarizes an evolving science and a rapidly changing medical practice. As we progress into the millennium, it now becomes possible to envision a much more diversified system of cancer research and treatment that will afford greater opportunities for our patients. As indicated in some of the chapters in Principles, there is increasing evidence that our historical “kill and cure” outlook in cancer treatment is in need of modification. Some forms of cancer biotherapy use the strategy of tumor growth stabilization and control through continued biological therapy over a longer period of time, akin to the use of insulin in the treatment of diabetes.

Preface These chapters illustrate some of these new methods of thinking and illustrate new strategies for the treatment and control of cancer. It is always difficult to move from past dogmas to future opportunities, but this fifth edition of Principles of Cancer Biotherapy illustrates why it is so important for researchers, regulators and clinicians to explore and apply these new opportunities in cancer biotherapy to the benefit of our patients. Robert K. Oldham, M.D.

Robert O. Dillman MD

References 1. DeVita VT Jr, Hellman S, Rosenberg SA. Biologic Therapy of Cancer. J.B. Lippincott, 1991. 2. Mitchell MS. Biological Approaches to Cancer Treatment. McGraw-Hill, 1993. 3. Rosenberg SA. Principles and Practices of the Biologic Therapy of Cancer. J.B. Lippincott, 2000.

Contents

Preface Robert K. Oldham

v

Contributors

ix

1. Cancer biotherapy: general principles Robert K. Oldham

1

2. The pathogenesis of cancer metastasis: relevance to therapy Sun-Jin Kim, Cheryl Hunt Baker, Yasuhiko Kitadai, Toru Nakamura, Toshio Kuwai, Takamitsu Sasaki, Robert Langley, and Isaiah J. Fidler

17

3. Developmental therapeutics and the design of clinical trials Robert K. Oldham

41

4. Recombinant proteins and genomics in cancer therapy Kapil Mehta, Bulent Ozpolat, Kishorchandra Gohil, and Bharat B. Aggarwal

53

5. Current concepts in immunology Robert K. Oldham

85

6. Therapeutic approaches to cancer-associated immune suppression Robert K. Oldham

101

7. Cancer vaccines Kenneth A. Foon and Malek M. Safa

147

8. Cytokines Walter M. Lewko and Robert K. Oldham

155

9. Interferons: therapy for cancer David Goldstein, Robert Jones, Richard V. Smalley, and Ernest C. Borden

277

10. Monoclonal antibody therapy Robert O. Dillman

303

11. Immunotoxins Arthur E. Frankel, Jung-Hee Woo, and David M. Neville

407

12. Drug Immunoconjugates Malek Safa, Kenneth A. Foon, and Robert K. Oldham

451

13. Targeted radionuclide therapy of cancer John M. Pagel, Otto C. Boerman, Hazel B. Breitz, and Ruby Meredith

463

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viii

Contents

14. Stem cell/bone marrow transplantation as biotherapy Robert K. Oldham

497

15. Cellular immunotherapy (CI), where have we been and where are we going? John R. Yannelli

505

16. Growth and differentiation factors as cancer therapeutics Kapil Mehta, Robert K. Oldham, and Bulent Ozpolat

527

17. Granulocyte colony-stimulating factor: biology and clinical potential MaryAnn Foote and George Morstyn

569

18. Granulocyte-macrophage colony-stimulating factor Maryann Foote and George Morstyn

581

19. Cancer gene therapy Donald J. Buchsbaum, C. Ryan Miller, Lacey R. Mcnally, and Sergey A. Kaliberov

589

20. Regulatory process for approval of biologicals for cancer therapy Antonio J. Grillo-López

613

21.1. Cancer biotherapy: 2009 disease-related activity Robert K. Oldham and Robert O. Dillman

631

21.2. Biological therapy of melanoma Robert K. Oldham

633

21.3. Biological therapy of genitourinary cancer Robert K. Oldham

645

21.4. Biological therapy of colon cancer Robert O. Dillman

659

21.5. Biological therapy of breast cancer Robert O. Dillman

669

21.6. Biological therapy of lung cancer Robert O. Dillman

679

21.7. Biological therapy of B and T cell lymphoproliferative disorders Robert O. Dillman

693

21.8. Biological therapy of multiple myeloma Robert K. Oldham

711

21.9. Biological therapy of squamous cell cancers of the head and neck Robert O. Dillman

713

21.10. Biological therapy of glioblastoma Robert O. Dillman

723

22. Speculations for 2009 and beyond Robert K. Oldham

733

Index

739

Contributors

Robert K. Oldham Hematology-Oncology Associates of Southeast Missouri Hospital Southeast Medical Plaza Cape Girardeau, MO USA

Toshio Kuwai Department of Cancer Biology M. D. Anderson Cancer Center The University of Texas Houston, TX USA

Robert O. Dillman Hoag Cancer Center Newport Beach, CA USA University of California Irvine, CA USA

Takamitsu Sasaki Department of Cancer Biology M. D. Anderson Cancer Center The University of Texas Houston, TX USA

Sun-Jin Kim Department of Cancer Biology M. D. Anderson Cancer Center The University of Texas Houston, TX USA Cheryl Hunt Baker Department of Cancer Biology M. D. Anderson Cancer Center The University of Texas Houston, TX USA Yasuhiko Kitadai Department of Cancer Biology M. D. Anderson Cancer Center The University of Texas Houston, TX USA Toru Nakamura Department of Cancer Biology M. D. Anderson Cancer Center The University of Texas Houston, TX USA

Robert Langley Department of Cancer Biology M. D. Anderson Cancer Center The University of Texas Houston, TX USA Isaiah J. Fidler Department of Cancer Biology M. D. Anderson Cancer Center The University of Texas Houston, TX USA Kapil Mehta Department of Experimental Therapeutics M. D. Anderson Cancer Center The University of Texas Houston, TX USA Bulent Ozpolat Department of Experimental Therapeutics M. D. Anderson Cancer Center The University of Texas Houston, TX USA

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x Kishorchandra Gohil Department of Internal Medicine The University of California Davis, CA USA Bharat B. Aggarwal Department of Experimental Therapeutics M. D. Anderson Cancer Center The University of Texas Houston, TX USA Kenneth A. Foon Department of Hematological Malignancies Nevada Cancer Institute Las Vegas, NV USA Malek M. Safa Division of Hematology / Oncology University of Cincinnati Cincinnati, OH USA Walter M. Lewko Cancer Cellular Therapeutics, Inc. Cape Girardeau, MO USA David Goldstein Department of Medical Oncology Prince of Wales Hospital Randwick, Sydney Australia Robert Jones Centre for Oncology and Applied Pharmacology CRUK Beatson Laboratories Glasgow, Scotland UK Richard V. Smalley (deceased) Synertron Inc. Madison, WI USA Arthur E. Frankel Cancer Research Institute of Scott &White Temple, TX USA

Contributors Jung-Hee Woo Cancer Research Institute of Scott &White Temple, TX USA David M. Neville Angimmune LLC, Bethesda, MD USA John M. Pagel The Fred Hutchinson Cancer Research Center Seattle, WA USA Otto C. Boerman University Medical Center Nijmegen The Netherlands Hazel B. Breitz Poniard Pharmaceuticals, Inc. Seattle, WA USA Ruby F. Meredith Department of Radiation Oncology University of Alabama Birmingham Birmingham, Alabama USA John R. Yannelli Department of Microbiology, Immunology and Molecular Genetics Markey Cancer Center University of Kentucky, School of Medicine Lexington, Kentucky USA MaryAnn Foote Thousand Oaks, CA USA Dr George Morstyn Department of Microbiology Monash University Victoria Australia

Contributors

xi

Donald J. Buchsbaum Department of Radiation Oncology University of Alabama at Birmingham Birmingham, AL USA

C. Ryan Miller Department of Pathology University of North Carolina Chapel Hill, NC USA

Lacey R. McNally Department of Radiology University of Alabama at Birmingham Birmingham, AL USA

Antonio J. Grillo-López Rancho Santa Fé, CA USA

Sergey A. Kaliberov Department of Radiation Oncology University of Alabama at Birmingham Birmingham, AL USA

1

Cancer biotherapy: general principles ROBERT K. OLDHAM

The term biotherapy encompasses the therapeutic use of any biological substance, but more specifically, it connotes the use of products of the mammalian genome. With modern techniques of genetic engineering, the mammalian genome represents the new “medicine cabinet.” Biological response modifiers (BRM) are agents and approaches whose mechanisms of action involve the individual’s own biological responses. Biologicals and BRM work through diverse mechanisms in the biotherapy of cancer. They may (a) augment the host’s defenses through the administration of cells, natural biologicals, or the synthetic derivatives thereof as effectors or mediators (direct or indirect) of an antitumor response; (b) increase the individual’s antitumor responses through augmentation or restoration of effector mechanisms, or decrease a component of the host’s reaction that is deleterious; (c) augment the individual’s responses using modified tumor cells or vaccines to stimulate a greater response, or increase tumor cell sensitivity to an existing biological response; (d) decrease transformation and/or increase differentiation or maturation of tumor cells; (e) interfere with growth-promoting factors and angiogenesis inducing factors produced by tumor cells; (f) decrease or arrest the tendency of tumor cells to metastasize to other sites; (g) increase the ability of the patient to tolerate damage by cytotoxic modalities of cancer treatment; and/or (h) use biological molecules to target and bind to cancer cells and induce more effective cytostatic or cytocidial antitumor activity. While several of these approaches involve the augmentation or use of biological responses, an understanding of the biological properties of immune response molecules, growth and maturation factors, and other biological substances will assist in the development of specific molecular entities that can act on biological responses and/or act directly on tumor cells. Thus, one can visualize the development of biological approaches with response-modifying as well as direct cytolytic, cytostatic, growth-inhibiting (antiproliferative), or maturational effects on tumor cells. Biotherapy is the fourth modality of cancer therapy and can be effective alone or in association with surgery, radiotherapy, and chemotherapy. To put biotherapy into perspective, it is important to dispel a historical miscon-

R.K. Oldham and R.O. Dillman (eds.), Principles of Cancer Biotherapy, © Springer Science + Business Media B.V. 2009

ception associated with immunotherapy: biotherapy can be effective against clinically apparent, even bulky, cancer, and treatment should not be restricted to situations where the tumor cell mass is imperceptible [64, 71]. Thus, the clinical trial designs for biotherapy can be similar to those used previously for other modalities of cancer treatment, as long as one measures both pharmacokinetics and the biological responses affected by these approaches [65]. Testing is continuing for biotherapy using the interferons, lymphokines, chemokines and cytokines, growth and maturation factors, angiostatic factors, monoclonal antibodies and their immunoconjugates, vaccines, and cellular therapy [68].

Historical Perspectives The use of chemical and biological compounds to modulate biological responses has been under active investigation for over 30 years. Although various chemicals, bacterial extracts, and viruses have been found to modulate immune responses in experimental animals, and, to a more limited extent, in humans, these “nonspecific” immunomodulators have not been highly effective in clinical trials [71]. Molecular biologists have recently developed techniques for the isolation of genes and their subsequent translation into appropriate production systems. These methods make available virtually unlimited quantities of highly purified biological compounds for experimental and therapeutic use. As a result, several classes of biologicals are being evaluated in preclinical models and clinical trials (Table 1). The continued investigation of nonspecific immunomodulators and adjuvants, as well as the recent advent of genetically engineered biologicals, makes the need for predictive preclinical assays of biological activity and efficacy apparent [33]. In vitro assays of biological activity (bioassays) are generally used to define and quantitate the activity of a given biological substance. Subsequently, flow cytometry, immunoperoxidase staining, enzyme-linked immunosorbent assays (ELISA), tetramer assays, radioimmunoassays (RIA), and variations of these methods allow the precise determination of levels of these molecules and activities in appropriate fluids and tissues.

1

2 Table 1. Biologicals and BRM Immunomodulators (chemicals, bacterial extracts, viruses, etc.) Lymphokines/cytokines α, β, γ-interferon; IL-1–32, tumor necrosis factor (TNF); etc. Growth/maturation factors (CSF, IL-2, EPO, etc.) Effector cells (cytotoxic and helper T cells, NK, and LAK cells, gene-engineered cells, etc.) Tumor-associated antigens and gene-engineered cellular vaccines Monoclonal antibodies Immunoconjugates

Finally, there is the need to assess the in vivo activity of these materials in preclinical models to develop predictive assays for clinical efficacy and provide information useful in the rational selection of agents and the design of clinical trials [33, 54]. Given the variability in the biological behavior of cancer and its interface with the human outbred host, it is not surprising that trials of nonspecific and specific immunotherapy, as translated from artificially constructed animal models, have not been uniformly successful in cancer treatment [4, 33]. Naturally occurring cancers arise in a particular organ from one cell or a few cells under some carcinogenic stimulus. In humans, these initial foci of cancer cells may grow – and sometimes lie dormant – over very long periods of time (from 1% to 30% of the human life span) before there is clinical evidence of cancer. Dissemination of cells from the initial focus may occur at any time during the development of the primary tumor. Subsequently, growth and metastasis occur over periods of months to years from primary and secondary foci, causing complex biological interactions to occur. In contrast, experimentally induced cancer is an artificial (even artifactual) situation. A short-duration, highdose carcinogen may be used to induce cancer quickly so that experiments can proceed rapidly. In transplantable models, the tumor cells are injected into young, normal, syngeneic animals, thereby circumventing the influences of environmental or genetic factors that may be operative in the natural host during tumor development. Many of the experimental models are transplantable tumors that have been maintained for decades and have only the most remote relevance to cancer in humans. The injection represents a single, instantaneous point source for a defined tumor load that has been manipulated in vitro. Regardless of whether that tumor load is 10 or 106 cells, it is being placed artificially in a single site and allowed to grow and metastasize from that selected and artificial, single site. Thus, these transplantable cancers are simply not analogous to clinical

Cancer biotherapy: general principles cancer and the conclusions drawn from them are unlikely to be broadly applicable to human cancers [33]. The modern era of cancer treatment began in the 1950s with the recognition that most cancers were systemic problems. It became obvious that lymphatic and blood-borne metastases often occurred simultaneously with local growth and regional spread. The early success of chemotherapy in leukemia and lymphoma prompted a massive and enthusiastic search for chemicals that might have cytolytic or cytostatic effects on cancer cells. Millions of compounds have been “screened” for antitumor activity [11], and this effort has given clinicians less than 75 “approved and active” anticancer chemotherapy drugs [84]. In addition, there are about 15 approved hormonal agents, ten targeted small molecules, and 25 biological drugs. There is now widespread recognition that only a few drugs in cancer treatment can effectively palliate and sometimes cure [19]. The development of three modalities (surgery, radiotherapy and chemotherapy) and their subsequent integration into what is now multimodal cancer treatment has been summarized [18, 20]. Between 1975 and 1990 a plateau was reached in cancer treatment. New surgical techniques (e.g., debulking, intra-operative methods for radiotherapy, and catheter isolation/infusion for chemotherapy) and new methods of radiotherapy (e.g., neutrons, protons, interstitial therapy, isotopes) continue to be developed, but these two modalities are primarily useful in local and regional cancer treatment. There has been continued, but slow, progress in the treatment of highly replicative, drug-sensitive malignancies over the past 15 years. It is now apparent that further progress with chemotherapy will depend on a greater understanding of the metabolic and enzymatic processes of cancer cells and the differences between these and normal cells. In addition, there are the problems of drug resistance, selectivity of action, and drug delivery. Cancer cells are more like than unlike normal cells with respect to chemotherapy sensitivity but they are more likely to become chemotherapy resistant with drug exposure. Many chemotherapeutic agents are highly cytolytic, but the problems of normal tissue toxicity, drug delivery, and tumor-cell resistance remain [19, 46]. Thus, cancer remains a systemic problem, and further systemic but more selective approaches are required for more effective treatment [20]. In addition to standard chemotherapy drugs that are approved and in clinical use, there are approximately 15 hormonal drugs available. These are primarily used in breast cancer and prostate cancer, and many are variations on the theme of blocking hormonal receptors. The evolution in this area has been primarily testing new

Robert K. Oldham hormonal agents for superiority in therapy and for lesser degrees of toxicity when used in large-scale trials against hormonally sensitive tumors. Then newest area of drug development is in targeted therapy. Here, small molecules are tested for the ability to block a particular receptor, enzyme, or target which may be involved in cell replication or cell metabolism. Eight targeted agents are currently available in the clinic, and a very large number of these small molecules are currently in clinical trials, most of which are variations on the theme of blocking a particular receptor or enzyme system with many of these drugs being multifunctional in that they can block more than one receptor or system. The scientific basis for biotherapy as the fourth modality of cancer treatment is now firm [21–23, 68, 70, 81, 82, 94, 97]. Historically, there was an attempt to establish immunotherapy in this role. Whereas immunotherapy was reproducible under specific experimental protocols, it was not strikingly effective in animals bearing palpable tumors and did not translate well to patients. Given the observation that immunotherapy was more effective with small tumor burdens, investigators began to study both “specific” and “nonspecific” immunotherapy as treatment for minimal residual disease. Although it became widely accepted that the treatment of animals with minimal residual disease was analogous to the postsurgical treatment of cancer in humans, this analogy was often stretched beyond reason. Immunotherapy in young, normal, syngeneic animals was often begun on the day of the tumor transplant (or within 1 or 2 days), using a transplant of a very small number of tumor cells (1–1,000) to a single site. In many of these studies, and in studies in which the tumor was surgically resected and no evident disease remained, the effects of immunotherapy were reasonably reproducible; the therapy was most beneficial when the tumor mass was less than 106 cells. These experimental results produced a dogma that immunological manipulation or immunotherapy could work only when the tumor cell mass was imperceptible [4, 71]. This posed real problems for clinical immunotherapy, since the tumor cell mass at clinical diagnosis or after surgery is usually three orders of magnitude (or more) greater than 106 cells. Despite the obvious difficulties with the experimental models and the translation to humans, clinicians began larger scale immunotherapy trials in the 1970s. The results of initial, small, uncontrolled trials were often reported as positive. Larger, randomized, controlled studies were done to confirm the efficacy of a particular immunotherapeutic regimen in a particular type of cancer. Although some of the controlled studies were positive, most yielded marginal or

3 negative results. Thus, immunotherapy had a poor image by the end of the 1970s [71, 109]. Immunotherapy failed to establish itself as a major mode of cancer treatment for several reasons. One important factor was the lack of definition and purity of immunotherapeutic agents. Many of the nonspecific approaches involved the use of complex chemicals, bacteria, viruses, and poorly defined extracts in an attempt to “stimulate” the immune response. Thus, molecular definition of the actual stimulating entity was not available. Given the lack of analogy between model systems and humans, the poorly characterized reagents, and the problems of variability in experimental procedures, the lack of demonstrable clinical efficacy was predictable [71]. Immunotherapy is not an appropriate term for the modern use of biologicals and BRM in medicine. Biological control mechanisms should be envisioned on a much broader basis than the immune system. While immunotherapy remains a subcategory of biotherapy, growth and differentiation factors, chemokines and angiostatic factors, the use of synthetically derived molecular analogs, and the pharmacological exploitation of biological molecules is much broader than immunotherapy (Table 1) [68]. Certain specific developments led to biotherapy becoming the fourth modality of cancer therapy. Advances in molecular biology have given scientists the capability to clone individual genes and produce significant quantities of highly purified genomic products as medicines. Unlike extracted and purified biological molecules, available in small quantities as semi-purified mixtures, the products of cloned genes have a level of purity on a par with drugs. They can be analyzed alone or in combination as to their effects in cancer biology. In addition, progress in nucleic acid sequencing and translation, protein sequencing and synthesis, the isolation and purification of biological products, and mass cell culture has given the scientific community the opportunity to alter nucleic acids and proteins at the nucleotide or amino acid level to manipulate then optimize their biological activity [42]. The elucidation of the human genome and the encoded products broadened the opportunities for advancements in biotherapy. As a result of gene cloning, a major approach in cancer treatment has evolved. Interleukin-2 can be used to stimulate the growth of a broad range of lymphocytes (T, NK, and LAK cells). This has given clinicians the ability to have large quantities of specific subclasses of effector cells for cancer treatment. Emerging evidence suggests that these effector cells can be helpful in the regional and systemic treatment of advanced, bulky

4 cancer. It was the availability of Interleukin-2 that allowed this technology to prosper [83]. In addition, IL-2 is now being used as the T cell growth factor for gene engineered lymphocytes containing new genes to enhance their cancer treatment capacity [80, 103]. Another major technical advance was the discovery of hybridomas. A major limitation on the use of antibodies had been the inability to make reproducible hightiter, specific antisera and to define these preparations on a molecular basis. Immunoglobulin reagents can now be produced with the same level of molecular purity as cloned gene products and drugs. Processes to easily “humanize” antibodies have produced therapeutic antibodies with excellent specificity, low immunogenicity and optimal pharmacokinetics. These antibodies are also powerful tools in the isolation and purification of tumor-associated antigens, lymphokines/cytokines, and other biologicals, which can then be used in biotherapy. The advances in molecular biology and hybridoma technology have eclipsed previous techniques for the isolation and purification of biological molecules [63, 77]. Technical advances in instrumentation, computers, and computer software have been critically important in the isolation and purification of biological molecules. The construction of nucleotide or amino acid sequences to fit any biological message is now possible. While this synthetic capability is currently limited to smaller gene products, techniques by which analysis and construction of nucleotide sequences will occur in an automated way, making enormously complex molecules possible to synthesize and manufacture, are rapidly becoming available.

Cancer biotherapy: general principles neoplasms [33]. There may exist in animals and in humans specific or nonspecific defects important in the development of their autochthonous tumors. Corrections of such defects may require a form of biotherapy totally different from that required to assist the normal host in controlling a transplanted cancer.

Model Screening Criteria Theoretically, an ideal procedure for screening new biologicals should employ a system of sequential and progressively more demanding protocols designed to select a maximum number of effective agents. The term screening denotes a series of sequential assays through which promising agents are tested for therapeutic potential. For some biotherapies, a general screening procedure may be inappropriate. For example, the activity of a monoclonal antibody with antitumor specificity would not be detected by use of the general screen of biological activity. Design considerations for general screening in biotherapy have been extensively reviewed [33, 54, 56, 61]. A step-by-step approach to the screening of potential BRM was developed to define their effects on T-cell, B-cell, NK-cell, and macrophage functions. A progressive in vitro and then in vivo sequence allows the variables of dose, schedule, route, duration and maintenance of activity, adjuvanticity, and synergistic potential to be explored in an orderly fashion [33]. Unfortunately, the screening program was used only briefly by the NCI and no replacement program is now in use.

Evaluation of Screening Programs

Preclinical Models Biological Activity in Preclinical Models Central to the identification of biotherapy that might be useful in cancer patients is the recognition that, in the main, the challenge in humans is the eradication of metastases. Metastases can result from the dissemination of different subpopulations of cells within the primary neoplasm [32]. This may explain the observation that cells within a metastasis can be antigenically distinct from those that predominate in the parental tumor [32]. Metastases may also emanate from other metastases. The implications of cellular heterogeneity as it relates to the outcome of the specific immunotherapy are obvious. In addition, normal animals are not comparable to animals or humans bearing autochthonous

Screening programs for chemotherapeutic agents were initiated in the mid-1950s, and attempts have been made to randomly examine thousands of compounds for antitumor activity [11]. Such large screening programs are empirically rather than rationally based, and are no longer appropriate [4, 5, 20, 33, 71]. Whether induced or transplantable animal tumor systems are valid models for testing therapeutic approaches for human cancer has been a controversial issue [33]. In patients, therapy successful against one type of cancer may not be successful against another type, or even for another patient with the same histological type of cancer. Unlike the model systems, in which treatment can be given with precise timing relative to the metastatic phase of a resected tumor or injected tumor cells, cancer diagnosis in humans is generally late, and micrometastases (and often macro-metastases) have become

Robert K. Oldham well established before treatment can be initiated. Thus, screening programs can only provide tentative indications on agents and approaches of interest.

Biotherapy: Specific Agents and Approaches Nonspecific Immunomodulators Since the early 1900s, immunotherapy with bacterial or viral products has been utilized with the hope of “nonspecifically” stimulating the host’s immune response [71]. These agents had been useful as adjuvants and as nonspecific stimulants in animal tumor models, but human trials have been disappointing. Perhaps purified viruses or specific chemicals will lead to the development of more effective adjuvants or stimu-

5 lants of the immune response. Bacillus Calmette-Guerin (BCG) and other whole organisms were used early in immunotherapy. The use of a purified derivative of bacterial components, such as muramyl di- or tripeptide, “packaged” in liposomes as a method to stimulate macrophages to greater anticancer activity has also been tried. Such adjuvants may prove useful with genetically engineered or synthetic tumor-associated antigens, active specific immunotherapy, or immunoprophylaxis. Multiple agents that appear capable of augmenting one or more immune functions already exist. Several of these have been associated with prolongation of survival in prospectively randomized trials involving patients with a wide variety of malignancies (Table 2). Although some of these agents have produced modest clinical benefits, and do represent a potential method of immune augmentation, it is doubtful that they will have a major role in future cancer therapy. Great problems

Table 2. Biologicals and biological response modifiers Immunomodulating agents Alkyl lysophospholipids (ALP) Azimexon BCG Bestatin Brucella abortus Cornyebacterium parvum Cimetidine Sodium diethyldithiocarbamate (DTC) Endotoxin Glucan ‘Immune’ RNAs Krestin Lentinan T-cell growth factors (‘TCGF’ – interleukin 2 [IL-2]) Levamisole Muramyldipeptide (MDP), tripeptide (MTP) Maleic anhydride-divinyl ether (MVE-2) Mixed bacterial vaccines (MBV) Nocardia rubra cell wall skeletons (CWS) Picibanil (OK432) Prostaglandin inhibitors (aspirin, indomethacin) Thiobendazole Tuftsin Interferons and interferon inducers Interferons (α, β, γ) Poly IC-LC Poly A-U GE-132 Brucella abortus Tilorone Viruses Pyrimidinones Thymosins Thymosin alpha-1 Thymosin fraction 5 Other thymic factors

Lymphokines, cytokines, growth/maturation factors Antigrowth factors Chalones Colony-stimulating factors (CSF) Growth factors (transforming growth factor, TGF) Lymphocyte activation factor (LAF-interleukin 1 [IL-1]) Lymphotoxins (TNF, α, β, LT) Macrophage activation factors (MAF) Macrophage chemotactic factor Macrophage cytotoxic factor (MCF) Macrophage growth factor (MGF) Migration inhibitory factor (MIF) Maturation factors Interleukin 3–18, etc. Thymocyte mitogenic factor (TMF) Transfer factor Transforming growth factor (TFR, α, β) Antigens Tumor-associated antigens Molecular vaccines Cell-engineered cellular vaccines Effector cells Macrophages NK cells Cytotoxic T cells LAK cells T helper cells Miscellaneous approaches Allogeneic immunization Liposome-encapsulated biologicals Bone-marrow transplantation and reconstitution Plasmapheresis and ex vivo treatments (activation columns, immunoabsorbents, and ultrafiltration) Virus infection of cells (oncolysates)

6 exist for most of the agents, including lack of chemical definition, low purity, and poor reproducibility from one lot to another. An additional problem has been the inability to define clearly a mechanism of action for these agents in humans. The preclinical screening established by the Biological Response Modifiers Program (BRMP) of the National Cancer Institute was one mechanism to do this [31]. Data from this type of screening with subsequent phase I and phase II clinical data could have provided interesting insights and correlations [98, 104, 105]. Unfortunately, this approach has been abandoned. The ability to specifically control and manipulate immune responses with highly purified, defined molecules obtained by genetic engineering is the future. Thus, it seems probable that nonspecific immunotherapy as a sole modality has become obsolete, although as adjuvants some may find a role in active specific immunotherapy (Chapter 6).

Active Specific Immunotherapy There has been a substantial effort to produce active immunization of autochthonous or syngeneic hosts with irradiated or chemically modified tumor cells in an attempt to use active specific immunotherapy (AST) [46]. Inherent is the assumption that tumor cells express immunogenic tumor-associated antigens (TAA). Treatment of tumor cells with a variety of unrelated agents, such as irradiation, mitomycin, lipophilic agents, neurominidase, viruses, or admixtures of cells with bacterial or chemical adjuvants, has produced non-tumoroigenic tumor cell preparations that are immunogenic upon injection into syngeneic hosts (Table 3). AST using BCG-tumor cell (“antigens”) vaccines has been reevaluated using a syngeneic guinea pig hepatocarcinoma. The definition of several variables of vaccine preparation, such as a ratio of bacterial organisms to viable, metabolically active tumor cells, the procedures of tumor cell dissociation and cryobiologic preservation, and the irradiation attenuation of cells, has resulted in the development of an effective non-tumorigenic vaccine. It has proven effective in both micro- and macro-metastatic disease [47]. The nature of the anatomic alteration in metastatic nodules after AST was explored using a specific monoclonal antibody to assess vascular permeability within these tumor nodules [50, 51]. Immunohistologic analysis demonstrated significantly more antibody in tumors from vaccinated animals than in comparable tumors from unvaccinated guinea pigs. These data support the hypothesis that the anatomic characteristics of tumor foci restrict drug and antibody access, thus pro-

Cancer biotherapy: general principles Table 3. Studies of immunotherapy with random designs Specific cancer and type of immunotherapy Leukemia BCG/AML BCG/Cells/AML MER/AML CP/Cells/AML All/Hodgkin’s NHL/MM Lev/ALL BDG/NHL Lev/MM Lung cancer IT BCG IP BCG IP BCG & Lev IP CP Lev Thy Fr V BCG/Cells TAA/Freund’s Adjuvant Breast cancer Poly A/Poly U BCG Lev Colon cancer BCG MER CP Lev Melanoma IL/BCG BCG/BCG + Cells BCG CP Lev

Positive studies

+ + + +

+

+ + + + +

+ + +

+ + +

+ +

+ + + +

+

+ + + + + + +

Genitourinary cancer IC BCG/bladder BCG/prostate

+ +

Gynecological cancer CP/cervix CP/ovary BCG/ovary

+ +

Other cancers Lev/H & N CP/H & N BCG/Cells/Sarcoma MER/Neuroblastoma

Equivocal Negative studies studies

+ + + +

+

+ + + +

tecting tumors not only from immunotherapy but from other forms of treatment as well [45].

Robert K. Oldham The regulation of the blood supply to neoplastic tissue may be unique in comparison to normal tissue. Tumor metastatic nodules may have a vascular “barrier,” which contributes to a limitation in delivery for chemotherapeutic agents, monoclonal antibodies, and immune effector cells. Such vascular barriers may provide an environment in which some tumor cells survive blood-borne chemotherapeutic and biologic agents. Thus, solid tumor nodules may serve as “pharmacologic sanctuaries,” allowing even drug-sensitive tumor cells to continue to grow [45–47]. Hanna and co-workers [47] demonstrated that strategically timed chemotherapy subsequent to immunotherapy can effectively double the number of survivors attainable with immunotherapy alone. This effect was not drug specific. These results suggest a new basis for AST in the treatment of solid tumors. Inflammatory disruption of anatomic barriers of metastatic nodules combined with strategically administered chemotherapy or biotherapy may prove useful in the design of future biotherapy trials in humans. Another approach used more recently involves the delivery of lymphokines and cytokines, such as interleukins, colony stimulating factors, tumor necrosis factor, lymphotoxins, macrophage cytotoxic factors, and activated complexes (such as those generated by plasma perfusion over protein A columns) to the tumor and its vascular bed. These substances are known to have powerful effects on tumor vasculature and cellular infiltrations, something leading to tumor necrosis. This approach may, in addition, increase the access of antibody, immunoconjugates, drugs, and activated cells to the cancer nodule [45]. A more recent method to influence the blood supply of tumor nodules has been the use of a monoclonal antibody, Bevacizumab, which targets the vascular endothelial growth factor (VEGF) receptor. Bevacizumab is primarily used with chemotherapy, and it may be there are similar mechanisms at play with regard to increasing chemotherapy drug access to tumor nodules by the use of a monoclonal antibody targeting the VEGF receptor [38, 49, 52]. A major limitation of AST has been the availability of purified TAA. Whereas whole cell vaccine preparations contain viable, non-tumorigenic cells prepared from individual tumors, purified TAA is now prompting largescale immunization. TAA purification and characterization followed by genetic engineering of the antigen for vaccine production is underway and several purified antigens are in clinical trials, especially in melanoma. Alternatively, synthetic peptide sequences of the active portion of TAA may prove useful in the near future. Even the combining site of antibody to TAA has recently been suggested as a potential vaccine. These technologies

7 are now undergoing extensive preclinical and clinical evaluation (see Chapter 7).

Thymic Factors It has been known for years that thymic extracts have immunological activity [40, 41]. Thymosin fraction 5 and thymosin alpha-1 have received the most attention in the laboratory and the clinic. Thymosin fraction 5 is an extract containing a variety of thymic polypeptides, and alpha-1 is a synthetic polypeptide component present in many thymic extracts. Thymic preparations have been shown to enhance and suppress immune responses in both intact and thymectomized animals. Many investigators have reported that the thymosins can correct selected immunodeficiency states, both natural and experimentally induced. There have also been reports that thymic factors can augment suppressed or depressed T-cell responses in patients with cancer. Studies in preclinical screening have demonstrated stimulation of T-cell activity [99], but clinical studies have not shown striking effects [25, 98, 99], and none have been approved for clinical use.

Recombinant DNA Technology Recombinant DNA technology, commonly referred to as genetic engineering, has provided us with the tool for the biosynthesis and subsequent mass production of a significant number of biologicals [8, 29, 39]. This is highly relevant to lymphokines/cytokines as well as growth and maturation factors, and should revolutionize the treatment of cancer over the next 10 years. The process involves the incorporation (recombination) of a segment of a DNA molecule containing a desired gene into a vector, usually a plasmid, which in turn is inserted into a host organism, usually an Escherichia coli, although other bacteria, yeasts, insects, and mammalian cells have been utilized. The cells are cloned and the cells producing the desired protein or polypeptide are selected. This clone is mass produced using fermentation techniques, and the protein molecule is harvested and purified. The resultant product is a highly purified (95–99%) protein solution, and has a high specific activity (i.e., biological activity per weight of protein). The relevant DNA can be obtained by a variety of methods. Once messenger RNA (mRNA) is isolated, complementary DNA (cDNA) can be produced through the use of reverse transcriptase. Alternatively, an artificial DNA molecule can be constructed once the nucleotide or amino acid sequence is known. This can be used to isolate the complementary sequence, which is then isolated and cloned. RNA molecules can also be used in cell-free systems

8 Table 4. Clinical lymphokines/cytokines Colony-stimulating factors Erythropoietin Interferons: α, β, γ Interleukins 1–32, etc. Lymphotoxincs: α, β (TNF) Macrophage-activating factors Thymosins Transfer factor(s)

to produce these biologicals. There are over 200 restriction enzymes that can cut desired fragments of DNA and lead to their isolation. These enzymes can uniquely cleave a DNA molecule at specific, predictable sites relative to the nucleotide sequence. These fragments are then incorporated into the plasmid, and combine with plasmid DNA. Plasmids have a symbiotic relationship with selected bacteria inducing resistance to a variety of antibiotics, which allows for selection of engineered clones. A number of alpha interferons, as well as beta and gamma interferon, have been genetically engineered [24, 39, 106, 111]; multiple interleukins, colony-stimulating factors, and tumor necrosis factor have been cloned [107]. The number of cloned biological products increases yearly (see Chapter 4). These biological products and their receptors (Table 4) are rapidly being translated into highquality pharmaceuticals for clinical testing and approval for general use.

Lymphokines/Cytokines Lymphokines and cytokines are molecules secreted by a variety of cells (Table 4). They provide one means through which the cells involved in the immune process communicate with one another and direct the overall process [40]. Lymphokines/cytokines with specific effects on cell proliferation have been identified and may prove useful as anticancer agents. Interleukins (IL) 1–32 are among the multiple lymphokines that appear to be involved in a cascade phenomenon leading to the induction of a variety of immune responses [86]. Other examples include multiple subclasses of colony-stimulating factors (CSF) chemokines and ligands. The list of lymphokines/cytokines is long, and the potential for therapeutic manipulation is great [73, 74]. The identification of these biological activities is the start of a process that should ultimately provide us with the knowledge and the tools to identify more accurately and control a number of immune responses. Physicians may then be able to manipulate the immune system (Fig. 1) intelligently, in favor of the host. Further, by selective activation

Cancer biotherapy: general principles and subsequent cloning in vitro, T-cell lines with specific cytotoxic and helper capabilities can be obtained and utilized in autologous and allogeneic adoptive immunotherapy [16, 27, 93, 103, 112]. Many investigators have held the rather simplistic view that the immune system (Fig. 1) might be manipulated in vivo and corrected to better deal with the cancer problem. Evidence to date suggests that the pharmacologic use of biologicals, in rather high doses, is a more effective method for cancer treatment, with immunoactivation and immunomodulation, playing the dominant roles [69, 70]. Another specific use of lymphokines may be in the pharmacological regulation of tumors of the lymphoid system. Although many of these tumors are considered to be generally unresponsive to normal growth-controlling mechanisms mediated by lymphokines, it is possible that large quantities of pure lymphokines administered as medicinals, or the use of certain molecular analogs of these naturally occurring lymphokines, may be useful in the treatment of lymphoid malignancies. The use of certain lymphokines/cytokines has been extended to other cancers, in that in vitro observations suggest an antiproliferative activity in some solid tumors. These antiproliferative effects might be maximized by testing the tumor cells of each patient to “custom tailor” the treatment rather than giving these biologicals as general treatment, as has been done with anticancer drugs [70]. IL-1, originally known as lymphocyte activating factor, is a macrophage-derived cytokine that has an enhancing effect on murine thymocyte proliferation. Both IL-1 and viable macrophages are necessary for the initial step in activation of IL-2 (Fig. 2). Cloning of IL-1 and IL-2 has made large quantities of highly purified materials available for clinical studies. Preclinical studies with IL-2 have been oriented around in vitro cell production protocols and induction or maintenance of antitumor T-cell effects in vivo [15, 16, 67, 70]. IL-2 has been used to activate peripheral blood cells and initially stimulated much interest [93, 104]. These activated cells are generally more cytotoxic against cancer cells than normal cells; however, their lineage can be T cell or NK cell (LAK cells) depending on the technique employed. This approach, though expensive and technically demanding, illustrates that cellular therapy is feasible and can be effective [67, 70, 85, 103]. A lymphotoxic product of antigen/mitogen-stimulated leukocytes was called lymphotoxin [92]. Lymphotoxin may be a principle effector of delayed hypersensitivity and, although conflicting data have been reported, may also be involved in the cytoxic reactions of T-cell-mediated

Robert K. Oldham

9

Figure 1. Immune response

lysis and NK- or K-cell lysis. Depending upon the type of tumor cell involved, the in vitro effect of lymphotoxin may be either cytolytic or cytostatic. Mouse tumor cells are frequently killed by homologous and heterologous lymphotoxins, whereas in other species reversible inhibition of tumor cell proliferation is more common [48]. Human lymphotoxin is of at least two species, alpha and beta [30, 37, 60, 88, 108]. Alpha lymphotoxin is tumor necrosis factor (TNF). Both have been cloned, and TNF has undergone rather extensive clinical trials [31, 91, 101]. While some antitumor responses have been seen, the lymphotoxins have not been highly efficacious as single agents in the treatment of advanced human tumors and the toxicity of systemic administration has been unacceptable. Continued trials are underway combining lymphotoxins such as TNF with other lymphokine/cytokines and with chemotherapy. Targeted delivery of these molecules may prove more efficacious since they have high toxicity administered intravenously with what is probably minimal delivery to the tumor cell

site [60, 28]. Thus, intratumoral and regional perfusion studies with TNF have yielded positive results in patients with melanoma and sarcoma [17, 53, 59, 60, 90]. There is now evidence that the combined use of various lymphokines may produce enhanced antiproliferative effects. Selective assays for lymphokine antiproliferative cocktails may prove useful in “tailoring” such preparations for individual patients [6, 95, 97, 102, 100, 110]. More than 100 biological molecules have already been described and named as lymphokines/cytokines (Table 5). Biologicals such as the interferons, lymphotoxins, TNF, CSF, IL-1–22 are now under evaluation (see Chapter 8). The studies require quantities of material obtained through genetic engineering, using sensitive and rapid assay procedures to monitor production, purification, and bioavailability.

Interferons Interferons are small, biologically active proteins with antiviral, antiproliferative, and immunomodulatory

10

Cancer biotherapy: general principles Monocytic Macrophage

Ag Stimulated T Cell

? CSF

Activated Macrophage

IL-1

Activated T cells

Ag Stimulated T cell

IL -2

Effector T cell Precursor

Mature Effector T Cells

Figure 2. Model for interleukin stimulation of T-cell immune responses

activities (see Chapter 9). Each interferon has distinctive capabilities in altering a variety of immunological and biological responses. As a class, the interferons appear to have some growth-regulating capacity in that antiproliferative effects are measurable with in vitro assays and in animal model systems. The relative efficacy of the mixtures of natural interferons that occur after virus stimulation as compared to the cloned interferons remains to be precisely determined. There are more than 20 interferon molecules (and theoretically hundreds of recombinant hybrids there from), and efforts are underway altering individual interferon molecules in specific ways, so the range of biological activities of the interferons as antiviral agents, as immunomodulating agents, and as antiproliferative agents may be very broad [66, 69]. In addition to antiviral and antiproliferative activity, the interferons have profound effects on the immune system. Low doses enhance antibody formation and lymphocyte blastogenesis, while higher doses inhibit

these functions. Low to moderate doses may inhibit delayed hypersensitivity while enhancing macrophage phagocytosis and cytotoxicity, natural killer activity, and surface antigen expression. Interferons prolong and inhibit cell division (both transformed and normal cells). In addition, interferons stimulate the induction of several intracellular enzyme systems with resultant profound effect on macromolecular activities and protein synthesis. We can draw some preliminary conclusions about interferon therapy for human cancer [9, 36, 43, 44, 57, 69, 96] (Tables 6 and 7). One is that the Cantell, lymphoblastoid, and recombinant alpha-interferons are surprisingly similar, both quantitatively and qualitatively, in their toxicity, antitumor efficacy, and other biologic effects. Second, objectively defined antitumor responses in phase I alpha-interferon trials (mostly involving lymphoma, myeloma, Kaposi’s sarcoma, melanoma, and renal cancer) were observed in approximately 10% of all patients treated. That level of activity may not seem impressive, but it does exceed the average response rate of 1–2% in phase I trials of chemotherapeutic agents. We should also note that very few responses in patients with tumors of the breast, colon, lung, or lower genitourinary system have been seen with alpha-interferon as a single agent. Overall, even though interferons have toxicities, they were more tolerable and less permanent than those observed in early-phase chemotherapy testing. A third impression, suggested by increased response rates with higher alpha-interferon doses, is that interferons may produce their acute antitumor effect by a direct cytostatic action, rather than an indirect immunomodulatory mechanism [12, 23, 57, 66, 69, 72, 98]. Finally, clinical experience with beta- and gammainterferons indicated that both produce response rates and response patterns similar to those obtained with alpha-interferon, although beta-interferon is only approved for multiple sclerosis and gamma only for chronic granulomatous disease. What is clear from the current preclinical and clinical studies is that the interferons have antitumor activity even in bulky, drug-resistant cancers [102]. Clinical activity has been seen most reproducibly in several lymphomas and leukemias (Table 6), but responses in many other tumor types with approval in melanoma and renal cancer [36].

Growth and Maturation Factors Using technology similar to that employed for lymphokine/cytokines, scientists have cloned and produced a variety of growth and maturation factors. The

Robert K. Oldham

11

Table 5. Antigen-nonspecific mediators,a unrestricted by MHC Helper factors LAF (lymphocyte-activating factor, IL-1) NMF (normal macrophage factor) BAF (B-cell-activating factor) TRF (T-cell-replacing factor) MP (mitogenic protein) TDF (thymus differentiation factor) Transferrin MF (mitogenic [blastogenic] factor) NSF (nonspecific factor) TDEF (T-cell-derived enhancing factor) TEF (thymus extracted factor) Complement components DSRF (deficient serum-restoring factor) Suppressor factors Inhibitor(s) of DNA synthesis AIM (antibody-inhibitory material) IDA (inhibitor of DNA synthesis) LTF (lymphoblastogenesis inhibition factor) FIF (feedback inhibition factor) MIFIF (Mif inhibition factor) SIRS (soluble immune response suppressor) IRF (immunoregulatory gamma-globulin) Chalones IF (interferons) AFP (alpha-fetoprotein) LDL (low-density lipoproteins) CRP (C-reactive protein) Fibrinogen degradation products NIP (normal immunosuppressive protein) LMWS (low-molecular-weight suppressor) HSF (histamine-induced suppressor factor) TCSF (T-cell suppressive factor) Factors acting on inflammatory cells MIF (migration inhibitory factor) MCF (macrophage chemotactic factor) MSF (macrophage slowing factor) MEF (migration enhancement factor) MAF (macrophage activating factor) MFF (macrophage fusion factor) PRS (pyrogen-releasing substance) LIF (leukocyte inhibition factor) NCF (neutrophil chemotactic factor) PAR (products of antigenic recognition) BCF (basophil chemotactic factor) ECF (eosinophil chemotactic factor) a

ESP (eosinophil stimulation factor) LCF (lymphocyte chemotactic factor) LTF (lymphocyte trapping factor) Factors acting on vascular endothelium SRF (skin reactive factor) TPF (thymic permeability factor) LNPF (lymph node permeability factor) LNAF (lymph node activating factor) AIPF (anaphylactoid inflammation promoting factor) IVPF (increased vascular permeability factor) Factors acting on other cells Interferons TMIF (tumor cell migration inhibition factor) OAF (osteoclast activating factor) Fibroblast chemotactic factor Pyrogens FAF (fibroblast activating factor) Growth-stimulating factors BCGF (B-cell growth factor) BCDF (B-cell differentiation factor) MGF (macrophage growth factor) MF (mitogeni [blastogenic] factor) LIAF (lymphocyte-induced angiogenesis factor) CSF (colony-stimulating factor) TDF (thymus differentiation factor) Thymopoietin, thymosin TCGF (T-cell growth factor, IL-2) IL-3 (interleukin 3) EGF (epidermal growth factor) Direct-acting factors Lysosomal enzymes CTF (cytotoxic factors) MTF or MCF (macrophage toxic [cytotoxic] factor) SMC (specific macrophage cytotoxin) MCF (macrophage cytolytic factor) ACT (adherent cell toxin) Chromosomal breakage factors Microbicidal factors LT (lymphotoxin) PIF (proliferation inhibitory factor) CIF (cloning inhibition factor) IDS (inhibitor of DNA synthesis) Transforming factors TNF (tumor necrosis factor)

These names/factors are based on biological activity. Several may represent the activity of a single molecule.

clinical trials have focused mainly on erythropoietin and the colony-stimulating factors. The former is a drug now approved for use in refractory anemia, and GM-CSF and GCSF are approved for the treatment of bone marrow dysplasia and chemotherapy-induced marrow suppression. Oprelvekim to stimulate platelet production is now approved. These factors are reviewed in later chapters, but it should be noted that they represent only the early beginnings of the very broad field of growth and matu-

ration factors (See Chapter 16). It is now clear that a variety of biological substances up- and down-regulate growth of both normal and neoplastic cells. These substances may stimulate or inhibit growth and may change the maturation cycle of various normal and neoplastic cells. Contained within this broad category of factors are the tumor growth factors, colony-stimulating factors, and a variety of still to be defined factors important in the regulation of cell growth and maturation. Future

12

Cancer biotherapy: general principles

Table 6. Apha-interferon activitya Active Hairy-cell leukemia Chronic myelogenous leukemia Myeloproliferative disorders Non-Hodgkin’s lymphoma Cutaneous T-cell lymphoma Kaposi’s sarcoma Multiple myeloma Melanoma Renal cell carcinoma Bladder cancer (intravesical) Inactive Breast cancer Colon cancer Lung cancer Prostate cancer Acute myelogenous leukemia a

As a single agent.

Table 7. Malignancies: Summary of responses to alphainterferon with date of FDA approval Tumor type

Response rate (%)

Multiple myeloma Hairy cell leukemia Low-grade lymphoma High-grade lymphoma Kaposi’s sarcoma Chronic myelogenous leukemia Melanoma Renal cancer

18–27 80–90 38–73 0–10 25–40 80–90 20–30 10–20

Approved 1986 1997 1988 1995 1995

therapeutic use of these factors may regulate growth and spread of cancer. Such a chronic growth restraining strategy may not “cure” cancer; rather treatment may be more analogous to using insulin in diabetes. This is a field that is undergoing explosive growth and should be watched carefully over the coming decade for molecules with therapeutic activity.

Monoclonal Antibodies The advent of hybridoma technology in the late 1970s made available an important tool for the production of monoclonal antibodies for therapeutic trials [10] (see Chapter 10). These antibodies are now being produced in huge quantities and in highly purified form for cancer treatment. The humanization of murine antibody combining sites has yielded a whole new class of therapeutic antibodies [84]. They will undoubtedly define a new range of

antigens on the cell surfaces, which will improve our understanding of cell differentiation and of cancer biology. Major problems in understanding the biology of the cancer cell have been the difficulties of isolating, purifying, and characterizing tumor-associated antigens (TAA). The use of monoclonal antibody technology will improve the definition of the neoplastic cell surface and identify its differences from the normal counterpart. This will be of great value in cancer diagnosis and histopathologic classification, and will be useful in the imaging of tumor cell masses and in the therapy of cancer [1, 7, 13, 14, 32, 33, 34, 35, 50, 51, 55, 63, 75, 84, 89]. Finally, antibodies may be a useful reagent in treating certain immune deficiencies and in altering immune responses. The removal of T cells from bone marrow to improve bone marrow transplantation techniques is an example of using antibody as a BRM [63]. A more recent example is the use of anti-CTLA-4 antibodies to regulate the immune response. Two monoclonal antibodies are in development that have influence on T-regulatory cells, which influence the immune response to cancer. It is already clear that this class of monoclonal antibodies can influence T-regulatory cells in a yet undefined manner, such that regression of bulky melanoma can occur. At the same time, they give a range of interesting toxicities with regard to uveitis, colitis, and depigmentation syndromes relative to their use in melanoma [26]. In spite of encouraging data from the use of antibodies and, especially, immunoconjugates to target toxic substances to cancer, the heterogeneity of cancer is an important consideration [2, 71–78, 87]. If a single antibody or a fixed combination of a few antibodies covering only a portion of the tumor cells is used, and if that preparation does not eliminate the true replicating cell population (stem cell) from the patient’s tumor population, eventual outgrowth of viable, perhaps resistant cells is inevitable. Therefore, it seems logical to proceed with attempts to type human tumors and to deliver toxic substances to them utilizing “cocktails” of antibodies sufficient to cover all the tumor cells suspected of replication in each patient. This type of approach may require a considerable amount of testing for each patient and a “typing” of one or more tumors from each patient [3, 58, 62, 78, 79, 87]. Such approaches may be more individually designed than is easily approachable through the product development paradigm that has been used in the development of new anticancer drugs. If however, the spectrum of human tumor heterogeneity is great, the goal of the ideal antibody conjugated to the ideal toxic agent may not be achievable.

Robert K. Oldham

Future Perspectives How rapidly will biotherapy develop and what role will it have in cancer treatment in the next decade? It is certain that we now have much more powerful tools for improving cancer therapy in the future. We now have the techniques to decipher the major problems in cancer biology down to the genetic level. These techniques, along with the recognition that biotherapy can provide increased selectivity in cancer treatment, support the belief that new and highly effective approaches are near. Biotherapy provides an additional technique, which may work effectively in combination with surgery or radiotherapy (to decrease the local and regional tumor) or with chemotherapy (to reduce the systemic tumor burden). It may work very effectively via antibodies in directing radioisotopes selectively to the tumor site, and with chemotherapy, toxins, and other cytostatic or cytotoxic molecules in directing the agent to the tumor bed, enhancing activity and selectivity. The use of biotherapy is at an early stage. We have already seen that highly purified biologicals can be effective in patients with clinically apparent, even advanced, bulky cancer. Clinical studies with alpha-interferon have demonstrated the responsiveness of radiation- and drugresistant lymphoma, melanoma, and renal carcinoma. IL-2 with effector cells or alone (in high doses) produces partial and complete remissions in melanoma and kidney cancer. These results, along with the clinical results using monoclonal antibody alone and conjugated to toxic substances in selected cancers (lymphoma, melanoma, gastrointestinal, leukemia and breast cancer), confirm the concept that we need not think of biotherapy as a tool that can be used only in patients with undetectable and minimal residual tumor burdens [84]. While this modality may work best with minimal tumor burdens (a situation that is also true for chemotherapy), biotherapy can be used as a single modality in clinically apparent disease. It may be even more effective in multimodality treatment regimens. Biotherapy offers the hope for selective treatment to enhance the therapeutic/toxic ratio and lessen the problem of nonspecific toxicity, a major impediment to the development of more effective cancer treatment. This decade will provide many opportunities to pursue new approaches in cancer treatment. These approaches will employ new techniques in the laboratory and clinic, requiring special training and expertise. The medical oncologist trained in the administration of chemotherapy drugs, is not necessarily well prepared to give biologicals for cancer treatment. Biotherapy uses biological substances that are often active on, or work in

13 association with, the immune system. The tremendous diversity of this system is best understood by clinical immunologists and cell biologists who are well suited to assist in the translation of biotherapeutic approaches to the clinic. Given these new techniques and new approaches, we must redesign many of the mechanisms for developmental therapeutics [59]. It may well be that the specificity of biologicals will require that biotherapy be developed in an individualistic fashion and applied to each patient in a specific, personalized way. This concept was conceptualized in 1977 by the Nobel laureate Sir Peter Medawar: The cure of cancer is never going to be found. It is far more likely that each tumor in each patient is going to present a unique research problem for which laboratory workers and clinicians between them will have to work out a unique solution.

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2

The pathogenesis of cancer metastasis: relevance to therapy SUN-JIN KIM, CHERYL HUNT BAKER, YASUHIKO KITADAI, TORU NAKAMURA, TOSHIO KUWAI, TAKAMITSU SASAKI, ROBERT LANGLEY, AND ISAIAH J. FIDLER

Introduction Metastasis, the spread of malignant tumor cells from a primary neoplasm to distant parts of the body where they multiply to form new growths, is a major cause of death from cancer. The treatment of cancer poses a major problem to clinical oncologists, because by the time many cancers are diagnosed, metastasis may already have occurred, and the presence of multiple metastases makes complete eradication by surgery, radiation, drugs, or biotherapy nearly impossible. Metastases can be located in different organs and in different locations within the same organ. These aspects significantly influence the response of tumor cells to therapy and the efficiency of anticancer drugs, which must be delivered to tumor lesions to destroy cells without leading to undesirable side effects. Similarly, immune effector cells of current biotherapeutic regimens may not reach or localize in metastases with different organs. Exacerbating the problems of treating metastatic disease is the fact that tumor cells in different metastases and in some instances even in different regions within a single metastasis may respond differently to treatment. Tumor cell resistance to current therapeutic modalities is the single most important reason for the lack success in treating many types of solid neoplasms. In part, the emergence of treatment-resistant tumor cells is due to the heterogeneous nature of malignant neoplasms. This diversity, which permits selected variants to develop from the parent tumor, implies not only that the primary tumor and metastases can differ in their response to treatment, but also that individual metastases can differ markedly from one another. Insight into the molecular mechanisms that regulate the pathogenesis of cancer metastasis as well as a better understanding of the interaction between metastatic cells and the host microenvironment should provide a foundation for the design of new therapeutic approaches. Moreover, the development of adequate models that allow for the isolation and characterization of cells possessing metastatic potential will be invaluable in the

R.K. Oldham and R.O. Dillman (eds.), Principles of Cancer Biotherapy, © Springer Science + Business Media B.V. 2009

design of more effective therapeutic modalities. In this chapter, we discuss the biology of cancer metastasis with special emphasis on recent data demonstrating that the microenvironment of different organs can influence the biological behavior of tumor cells at different steps of the metastatic process and the development of biologic diversity in malignant neoplasms. These findings have obvious implications for the therapy of neoplasms in general and metastases in particular.

The Pathogenesis of Cancer Metastasis The process of cancer metastasis is dynamic, complex and consists of multiple, sequential, and interrelated steps shown schematically in Fig. 1. To produce a clinically relevant lesion, metastatic cells must survive all the steps of the process. If the disseminating tumor cell fails to complete any one of these steps, it will fail to produce a metastasis. Thus, the successful metastatic cell has been likened to a decathlon champion who must be proficient in all ten events to be successful, not just a few [58]. The outcome of this process depends on both the intrinsic properties of the tumor cells and their interactions with host factors [48, 56, 57, 99]. The essential steps in the formation of a metastasis are: (1) After the initial transforming event, either unicellular or multicellular, growth of neoplastic cells must be progressive, with nutrients for the expanding tumor mass initially supplied by simple diffusion. (2) Extensive vascularization must occur if a tumor mass is to exceed 1–2 mm in diameter. The synthesis and secretion of proangiogenic angiogenesis factor probably plays a key role in establishing a neocapillary network from the surrounding host tissue. (3) Local invasion of the host stroma by some tumor cells could occur by several nonmutually exclusive mechanisms. (4) Thin-walled venules, like lymphatic channels, offer very little resistance to penetration by tumor cells and provide the most

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The pathogenesis of cancer metastasis: relevance to therapy

THE PATHOGENESIS OF A METASTASIS TRANSFORMATION

ANGIOGENESIS

MOTILITY & INVASION

Capillaries, Venules, lymphatic vessels

ARREST IN CAPILLARY BEDS

EMBOLISM & CIRCULATION

TRANSPORT

ADHERENCE

EXTRAVASATION INTO ORGAN PARENCHYMA

MULTICELL AGGREGATES (lymphocytes,platelets)

RESPONSE TO MICROENVIRONMENT

METASTASIS OF METASTASES TUMOR CELL PROLIFERATION & ANGIOGENESIS

METASTASES

Figure 1. The pathogenesis of cancer metastasis

common pathways for tumor cell entry into the circulation. Although clinical observations have suggested that carcinomas frequently metastasize and grow via the lymphatic system, whereas malignant tumors of mesenchymal origin more often spread by the hematogenous route, the presence of numerous venolymphatic anastomoses invalidates this concept. (5) Detachment and embolization of small tumor cell aggregates occurs next, the vast majority of circulating tumor cells being rapidly destroyed. (6) Once the tumor cells have survived the circulation, they must (7) arrest in the capillary beds of organs, either by adhering to capillary endothelial cells or by adhering to subendothelial basement membrane, which may be exposed. (8) Extravasation occurs next, probably by the same mechanisms that influence initial invasion. (9) Proliferation within the organ parenchyma completes the metastatic process (10). To continue

growing, the micrometastases must develop a vascular network and continue to evade the host immune system. Moreover, the cells can invade, penetrate blood vessels, and enter the circulation to produce additional metastases (Fig. 1).

NeovascularizationAngiogenesis Oxygen can diffuse from capillaries for only 150–200 μm. When distances of cells from a blood supply exceed this, cell death follows [80]. Thus, the expansion of tumor masses beyond 1 mm in diameter depends on neovascularization, i.e., angiogenesis [78, 79]. The formation of new vasculature consists of multiple, interdependent steps. It begins with local degradation of the basement

Sun-Jin Kim et al. membrane surrounding capillaries, followed by invasion of the surrounding stroma and migration of endothelial cells in the direction of the angiogenic stimulus. Proliferation of endothelial cells occurs at the leading edge of the migrating column, and the endothelial cells begin to organize into three-dimensional structures to form new capillary tubes. Differences in cellular composition, vascular permeability, blood vessel stability, and growth regulation distinguish vessels in neoplasms from those in normal tissue [1, 3, 79, 156]. The onset of angiogenesis involves a change in the local equilibrium between proangiogenic and antiangiogenic molecules [55]. Some of the common proangiogenic factors include bFGF, which induces the proliferation of a variety of cells and has also been shown to stimulate endothelial cells to migrate, to increase production of proteases, and to undergo morphogenesis [80]. Likewise, VEGF/VPF has been shown to induce the proliferation of endothelial cells, to increase vascular permeability, and to induce production of urokinase plasminogen activator by endothelial cells [46]. Additional proangiogenic factors include IL-8 [268], platelet-derived endothelial cell growth factor, which has been shown to stimulate endothelial cell DNA synthesis and to induce production of FGF, hepatocyte growth factor (HGF), or scatter factor, that increases endothelial cell migration, invasion, and the production of proteases, and platelet-derived growth factor (PDGF). The production of bFGF, VEGF, and IL-8 by tumor or host cells or the release of proangiogenic molecules from the extracellular matrix is known to induce the growth of endothelial cells and formation of blood vessels. Further, the organ microenvironment can directly contribute to the induction and maintenance of the proangiogenic factors bFGF [226, 227] and IL-8 [228]. The production of angiogenic molecules, e.g., VEGF, bFGF, and IL-8 by melanoma cells is regulated by complex interactions with keratinocytes in the skin [106]. Reports from our laboratory showed that IL-8 is an important molecule in melanoma growth and progression. Constitutive expression of IL-8 directly correlated with the metastatic potential of human melanoma cells. Further, IL-8 induced proliferation, migration, and invasion of endothelial cells and, hence, neovascularization [97]. Several organ-derived cytokines (produced by inflammatory cells) are known to induce expression of IL-8 in normal and transformed cells [106]. Since IL-8 expression in melanocytes and melanoma cells can be induced by inflammatory signals, the question of whether specific organ microenvironments could influence the expression of IL-8 was analyzed. Melanoma cells were implanted into the subcutis, the spleen (to produce liver metastasis),

19 and intravenously (to produce lung metastasis) of athymic nude mice. Subcutaneous tumors, lung lesions, and liver lesions expressed high, intermediate, and no IL-8, respectively, at both the mRNA and protein levels. Melanoma cells established from the tumors growing in vivo exhibited similar levels of IL-8 mRNA transcripts as continuously cultured cells, thus demonstrating that the differential expression of IL-8 was not due to the selection of a subpopulation of cells [97]. IL-8 expression can be upregulated by coculturing melanoma cells with keratinocytes (skin) and inhibited by coculturing melanoma cells with hepatocytes (liver). We also investigated the effects of two cytokines produced by keratinocytes (IL-1, IFN-β) and two cytokines produced by hepatocytes (TGF-α, TGF-β) on the regulation of IL-8 in human melanoma cells. IL-1 upregulated the expression of IL-8 in human melanoma cells at both the mRNA and protein levels in a dose- and timedependent manner in the presence of de novo protein synthesis. IFN-β did not affect constitutive IL-8 mRNA and protein production in human melanoma cells, but it did block the induction of IL-8 by IL-1. TGF-β inhibited the expression of IL-8, while TGF-α had no effect on IL-8 expression [226]. Patients with renal cell carcinoma exhibit high levels of bFGF in the serum or the urine that inversely correlates with survival [181, 184]. Human renal cancer cells implanted into the kidney of nude mice were highly metastatic to the lung, whereas renal cancer cells implanted subcutaneously remain local [227]. The subcutaneous or intramuscular tumors expressed a lower level of mRNA transcripts for bFGF than did tumor cells growing in culture, whereas the renal tumors had 20 times higher levels of bFGF mRNA and protein levels as compared with the cultured cells. A histopathologic examination of the tumors demonstrated that the subcutaneous tumors had few blood vessels, whereas the renal tumors had many [227, 228]. Direct correlation between the level of bFGF and advanced disease was also reported for patients diagnosed with colon carcinoma. Patients with Duke’s C or D tumors had markedly higher levels of bFGF in the blood than patients with Duke’s B. In situ hybridization analysis revealed that bFGF was overexpressed at the tumor periphery associated with cell division. Northern blot analysis detected no mRNA transcripts for bFGF [129]. In a follow-up study of patients with colon cancer, bFGF expression was found to be highest in the primary tumors of patients who presented with metastatic disease and therefore identify a cohort of patients who appeared to be free of metastatic disease at the time of surgery (low bFGF expression) as compared to another cohort of patients who did develop metastatic disease (high bFGF) [130].

20

Tumor Cell Invasion To reach blood vessels or lymphatics, tumor cells must penetrate host stroma that includes basement membrane. The interaction with the basement membrane consists of attachment, matrix dissolution, motility and penetration [154]. At least three nonmutually excluding mechanisms can be involved in tumor cell invasion of tissues. First, mechanical pressure produced by rapidly proliferating neoplasms may force cords of tumor cells along tissue planes of least resistance [154, 155]. Second, increased cell motility can contribute to tumor cell invasion. Most tumor cells possess the necessary cytoplasmic machinery for active locomotion and increased tumor cell motility is preceded by a loss of cell-to-cell cohesive forces. In epithelial cells, the loss of cell-to-cell contact is associated with downregulation of the expression of E-cadherin, a cell surface glycoprotein involved in calcium-dependent homotypic cell-to-cell cohesion. Reduced levels of E-cadherin are associated with a decrease in cellular/ tissue differentiation and increased grade in carcinomas [120]. Many differentiated carcinomas express higher levels of E-cadherin mRNA, as do adjacent normal epithelial cells, whereas poorly differentiated carcinomas do not. Mutations in the E-cadherin gene and abnormalities of α-catenin, which is an E-cadherin-associated protein, have been associated with the transition of cells from the noninvasive to the invasive phenotype [261]. Third, invasive tumor cells secrete enzymes capable of degrading basement membranes, which constitute a barrier between epithelial cells and the stroma. Epithelial cells and stromal cells produce a complex mixture of collagens, proteoglycans, and other molecules, which contains ligands for adhesion receptors and is permeable to molecules but not to cells [230]. To invade the basement membrane, a tumor cell must first attach to extracellular matrix (ECM) components by a receptor-ligand interaction. One group of such cell surface receptors are the integrins which specifically bind cells to laminin, collagen, or fibronectin [218]. Many integrins that bind to different components of the ECM are expressed on the surface of human carcinoma cells. Tumor progression has been associated with a gradual decrease of integrin expression suggesting that the loss of integrins, coupled with the loss of E-cadherin, may facilitate detachment from a primary neoplasm. Subsequent to binding, tumor cells can degrade connective-tissue ECM and basement membrane components [177]. The production of enzymes such as type IV collagenase (gelatinase, matrix metalloproteinase) and heparinase in metastatic tumor cells correlates with invasive capacity of human carcinoma cells. Type IV collagenolytic

The pathogenesis of cancer metastasis: relevance to therapy metalloproteinases with apparent molecular masses of 98, 92, 80, 68, and 64-kDa have been detected in highly metastatic cells. Poorly metastatic cells, on the other hand, appear to secrete very low amounts of only the 92-kDa metalloproteinase [171, 172].

Lymphatic Metastasis Early clinical observations led to the impression that carcinomas spread mainly by the lymphatic route and mesenchymal tumors spread mainly by means of the bloodstream. We now know, however, that the lymphatic and vascular systems have numerous connections and that disseminating tumor cells may pass from one system to the other [27]. For these reasons, the division of metastasis into lymphatic spread and hematogenous spread is arbitrary. During invasion, tumor cells can easily penetrate small lymphatic vessels and be passively transported in the lymph. Tumor emboli may be trapped in the first lymph node encountered on their route, or they may bypass regional draining lymph nodes to form distant nodal metastases (“skip metastasis”). Although this phenomenon was recognized in the late 1800s [193], its implications for treatment were frequently ignored in the development of surgical approaches to treat cancers [263, 264]. Regional lymph nodes (RLN) in the area of a primary neoplasm may become enlarged as a result of hyperplasia or growth of tumor cells in the node. Although the use of morphologic criteria for assessing prognoses based on lymph node appearance is debatable, lymphocytedepleted lymph nodes are believed to indicate a less favorable prognosis than those demonstrating reactive morphologic characteristics [16]. Hyperplastic responses could indicate reactivity to autochthonous tumors, and this could benefit the host. Whether the RLN can retain tumor cells and serve as a temporary barrier for cell dissemination is controversial. In most experimental animal systems used to investigate this question, normal lymph nodes were subjected to a sudden challenge with a large number of tumor cells, a situation that may not be analogous to RLN at the early stages of cancer spread in humans, when small numbers of cancer cells continuously enter the lymphatics. This issue is important because of practical considerations for surgical management of such neoplasms as cutaneous melanoma. It raises the question, is elective prophylactic lymph node dissection appropriate for the treatment of micrometastases? The biologic justification for elective lymph node dissection in patients with melanoma presumes that metastasis of some cutaneous melanomas

Sun-Jin Kim et al. occurs first in the RLN, and that only at a later time do tumor cells gain access to the circulation to reach distant organs. If this is the case, and RLN can act as a temporary barrier to the spread of cancer, removing the RLN with micrometastases could clearly increase the cure rate in subgroups of patients with melanoma. Some evidence exists that patients with melanomas of intermediate thickness (1–4 mm) do in fact have an improved survival rate subsequent to elective lymph node dissection. Similarly, some data suggest that an improved survival rate can be achieved for selected patients with head and neck cancers by elective lymph node dissection or local treatment with x-irradiation [25]. Recent advances in mapping of the lymphatics draining cutaneous melanoma (by the use of dyes or radioactive tracers) have allowed surgeons to identify the lymph node draining the tumor site (i.e., the sentinel lymph node) [36]. The presence of melanoma micrometastases in sentinel lymph nodes is correlated with poor prognosis and hence indicates wide field dissection. A series of more than 500 melanoma cases with longer than 4 years’ median clinical follow-up concluded that absence of disease in sentinel lymph node correlates with increased disease-free status (in other nodes) and few or no skip metastases [119, 175]. These data suggest that elective lymph node dissection when metastatic cells are present in sentinel lymph nodes produces beneficial results in patients with melanoma.

Hematogenous Metastasis During blood-borne metastasis, tumor cells must survive transport in the circulation, adhere to small blood vessels or capillaries, and invade the vessel wall. The mere presence of tumor cells in the circulation does not in itself constitute metastasis, since most cells released into the bloodstream are eliminated rapidly [60, 263, 264]. Using radiolabeled tumor cells, we found that by 24 h after entry into the circulation, less than 1% of the cells are still viable, and less than 0.1% of tumor cells placed into the circulation eventually survive to produce metastases [60]. Although most tumor cells are destroyed in the bloodstream, it seems that the greater the number of cells released by a primary tumor, the greater the probability that some cells will survive to form metastases. The number of tumor emboli in the circulation appears to correlate well with the size and clinical duration of the primary tumor, and the development of necrotic and hemorrhagic areas in large tumors facilitates this process by providing tumor cells easy access to the circulation [263].

21 To a large degree, the rapid death of most circulating tumor cells is probably due to such simple mechanical factors as blood turbulence. Tumor cell survival can be increased by aggregation. Tumor cells can aggregate with each other or with host cells, such as platelets and lymphocytes [67]. Once metastatic cells reach the microcirculation, they interact with cells of the vascular endothelium. These interactions include nonspecific mechanical lodgment of tumor cell emboli as well as formation of stable adhesions between tumor cells and small-vessel endothelial cells. The organ distribution of metastatic foci is believed to depend, in part, on the ability of blood-borne malignant cells to adhere to specific endothelium and produce endothelial cell retraction [185, 186]. The formation of fibrin clots at sites of tumor cell arrest in the microcirculation can result in blood vessel damage [45, 46]. In some tumor systems, fibrin formation is not essential for tumor cell implantation or metastasis formation. The increased coagulability often observed in patients with cancer may be related to the high levels of thromboplastin found in certain tumors or to production of high levels of procoagulant-A activity, which can directly activate factor X in the clotting process. Since reduced blood flow could lead to increased trapping of circulating tumor cells and perhaps to their increased survival, the use of anticoagulants in the treatment or control of metastasis has been tried, albeit to a limited success. The adhesion of tumor cells to the vascular endothelium is regulated by mechanisms similar to those used by leukocytes. The initial attachment of leukocytes to vascular endothelial cells is regulated by the selectin family of adhesion molecules which consists of three closely related cell surface molecules. E-selectin, which is expressed by endothelial cells, mediates initial attachment of lymphocytes (and tumor cells) by interaction with specific carbohydrate ligands that contain sialylated fucosylated lactosamines. The expression of mucin-type carbohydrates on the surface of human colon carcinoma has been correlated with their metastatic potential [161], perhaps through differential interaction with E-selectins expressed on specific endothelial cells. The development of firm adhesion requires the interaction of other adhesion molecules, another selective process in metastasis. Several classes of cell-to-cell adhesion molecules regulate this adhesion. These include the hyaluronate receptor CD44 and its splice-variants [183], the integrins α5β1, α6β1, and α6β4, and the galactoside-binding galectin-3 [218]. The arrest of tumor cells in capillary beds leads to the retraction of endothelial cells and the exposure of the tumor cells to the ECM. The adhesion of metastatic cells

22 to components of the ECM, such as fibronectin, laminin, and thrombospondin, facilitates metastasis to specific tissues, and peptides containing sequences of these components of the ECM can reduce formation of hematogenous metastases [246]. Extravasation of arrested tumor cells is believed to operate by mechanisms similar to those responsible for local invasion. Tumor cells can grow and destroy the surrounding vessel, invade by penetrating the endothelial basement membrane, or they can follow migrating white blood cells [50]. The ability of malignant cells to extravasate into surrounding tissues of particular organs seems to arise, in part, from their selective adherence to and invasion of certain tissues [187]. Malignant cells frequently penetrate thin-walled capillaries but rarely invade arteries or arteriole walls, which are rich in elastin fibers. This resistance to invasion is not necessarily mediated by mechanical strength alone. Connective tissues have been shown to produce protease inhibitors, and these may block enzyme-dependent processes of invasion. The invasion, survival, and growth of malignant cells at particular secondary sites also involve their responses to tissue or organ factors. Tumor cells can recognize tissue-specific motility factors that direct their movement and invasion [196]. After tumor cells invade organ parenchyma, they must also respond to organ-specific factors that influence their growth [187].

Metastasis of Metastases The tumor cells proliferating within metastases can invade host stroma, penetrate blood vessels, and enter the circulation to produce secondary metastases, the so-called “metastasis of metastases” [73, 235, 236]. Hart and Fidler used the preferential growth of B16 melanoma metastases in specific organs. Following the intravenous injection of B16 melanoma cells into syngeneic mice, tumor growths developed in the lungs and in fragments of lung or ovarian tissue implanted intramuscularly into the quadriceps femoris but not in renal tissue implanted as a control [101]. Tumor growth in the specific transplanted organ could have been caused by the arrest and growth of tumor cells immediately following intravenous injection, i.e., “initial metastases.” Alternatively, tumor cells injected intravenously could have been arrested in the lungs, where they developed; once metastases were established, tumor cells could enter the circulation to be arrested at other organs and produce “secondary metastases” [235]. To distinguish between these possibilities, Nicolson and Fidler performed

The pathogenesis of cancer metastasis: relevance to therapy several experiments [73]: Two weeks after normal, tumor-free mice were joined parabiotically to metastasisbearing animals, there was no evidence of any tumor growth in the “guest” animals. However, when the parabiont animals were allowed to survive for 4 weeks after separation from the metastasis-bearing animals, 40% developed lung metastases. Since the host mice did not have primary tumors at the time of parabiosis, the metastases in the guest mice could have only arisen as metastasis from metastases [101].

The Biologic and Metastatic Heterogeneity of Neoplasms Only a few tumor cells that enter the circulation can produce metastases. In fact, since less than 0.01% of circulating cells are likely to produce a secondary growth, the development of metastases could represent the fortuitous survival of a few tumor cells or the selection from the parent tumor of a subpopulation of metastatic cells endowed with properties that enhance their survival [65, 66, 71]. Data generated by our research group and many others prove that neoplasms are biologically heterogeneous and that metastasis is indeed a selective process. The first experimental proof of metastatic heterogeneity of neoplasms was provided by Fidler and Kripke in 1977 working with the murine B16 melanoma [71]. Using the modified fluctuation assay of Luria and Delbruck [157], they showed that different tumor cell clones, each derived from an individual cell isolated from the parent tumor, varied dramatically in their ability to produce pulmonary nodules after intravenous inoculation into syngeneic recipient mice. Control subcloning procedures demonstrated that the observed diversity was not a consequence of the cloning procedure [71]. The finding that preexisting tumor cell subpopulations proliferating in the same tumor exhibit heterogeneous metastatic potential has since been confirmed in many laboratories with a wide range of experimental animal tumors of different histories and histologic origins [reviewed in 58, 63, 64, 208, 209]. In addition, studies using young nude mice as models for metastasis of human neoplasms have shown that several human tumor lines and freshly isolated tumors such as colon carcinoma, renal carcinoma, and prostate cancer also contain subpopulations of cells with widely differing metastatic properties [64]. We studied the biologic and metastatic heterogeneity in a mouse melanoma induced in C3H mice by chronic exposure to ultraviolet B radiation and painting with

Sun-Jin Kim et al. croton oil [134, 135]. One mouse thus treated developed a melanoma designated by Kripke as K-1735 [134]. The original K-1735 melanoma was established in culture and immediately cloned [69]. In an experiment similar in design to the one described for the B16 melanoma, the clones differed greatly from each other and from the parent tumor in their ability to produce lung metastases. In addition to differences in number of metastases, we also found significant variability in the size and pigmentation of the metastases. Metastases to the brain, heart, liver and skin were found as well; those growing in the brain were uniformly pigmented, whereas those growing in the lymph nodes, heart, liver, or skin generally had no pigment. To determine whether the absence of metastasis production by some (but not all) clones of the K-1735 was a consequence of their immunologic rejection by the normal host [134–136], we examined their metastatic behavior in young nude mice. In addition to the lack of a functional T-cell system, unstressed 3-week-old nude mice are also deficient in natural killer cell activity. In such recipients, the immunologic barrier to metastatic cells that also may be highly immunogenic is removed, and they may thus successfully complete the process. This was true for cells of two clones that did not produce metastases in normal syngeneic mice but produced tumor foci in the young nude recipients. Most of the nonmetastatic clones were nonmetastatic in both normal syngeneic and in the nude recipients. Therefore, the clones’ failure to metastasize in syngeneic mice probably was not caused by their immunologic rejection by the host but by their inability to complete one or another step in the complex metastatic process.

Enhanced Metastatic Potential of Tumor Cells Harvested from Metastases Our studies and most data reported by others have led us to conclude that metastasis is a highly selective process regulated by a number of as yet imperfectly understood mechanisms. This belief is contrary to the once widely accepted notion that neoplastic dissemination is the ultimate expression of cellular anarchy. In fact, suggesting that cancer metastasis is a selective process is a more optimistic view in terms of cancer therapy than one that postulates that tumor dissemination is an entirely random event. Belief that certain rules govern the spread of neoplastic disease implies that elucidation and understanding of these rules will lead to better therapeutic interventions.

23 We addressed the question of whether the cells that survive to form metastases possess a greater metastatic capacity than most cells in the parent neoplasm [243]. Some support for this possibility comes from the initial in vivo selection experiments of the highly metastatic B16-F10 cell line derived from the parent B16 melanoma [66]. Comparable results have been obtained with the K-1735 tumor. When cells derived from the parent tumor were injected intramuscularly into the hind foot pads of syngeneic mice, the resulting skin tumors produced spontaneous pulmonary metastases. Four cell lines were established from four individual lung nodules harvested from four different mice. The finding that all the lines derived from metastatic deposits produced significantly more metastases than cells of the parent line was good evidence for the hypothesis that metastasis is a selective process, that is, cells populating metastases have an increased metastatic capacity [243]. Our studies demonstrate that the K-1735 melanoma is heterogeneous and contains both nonmetastatic and metastatic cells. In contrast, individual metastases (spontaneous pulmonary metastases) are more uniform. This suggests that metastatic foci could develop by a clonal expansion of a few surviving metastatic cells. Moreover, it explains the observations showing that tumor cells in primary and metastatic lesions differ in their antigenic properties, biochemical characteristics, and response to cytotoxic drugs [reviewed in 61–63].

Clonal Origin of Cancer Metastases Multiple metastases proliferating in a host, even in the same organ, often exhibit diverse biological characteristics of, for example, hormone receptors, antigenicity or immunogenicity, and response to various chemotherapeutic agents. This diversity may result from the nature of the pathogenesis of metastasis, the process of tumor evolution and progression, or both. Pathologists have long been aware that neoplasms frequently exhibit different morphological appearances in different areas. For this reason, the malignant or benign nature of a tumor cannot be determined with confidence unless multiple sections obtained from all parts of the tumor are examined. The zonal differences in tumors are not restricted to morphology alone but include biological characteristics such as growth rates, sensitivity to cytotoxic drugs, antigenicity, and pigmentation [74]. Since primary tumors are not uniform, it is possible that tumor cell aggregates entering the circulation from one

24 zone of the tumor may be different from those entering from another zone. If an embolic aggregate originates from a primary tumor’s homogeneous zone, regardless of whether only one cell or several cells survived to proliferate in distant organs, the resulting metastasis would be like a primary tumor of unicellular origin. If a mixed embolus derived from an area of zonal junctions enters the circulation, the unicellular or multicellular origin of the metastasis would depend on whether a single cell or multiple cells survived to proliferate. To determine whether individual metastases are clonal and whether different metastases can be produced by different progenitor cells, Talmadge et al. [244] performed a series of experiments using the fact that x-irradiation of tumor cells induces random chromosome breaks and rearrangements. Analyzing the karyotype composition of 21 individual melanoma lung metastases after cultivating cells from individual lesions, this research group found unique karyotypic patterns of abnormal, marker chromosomes in most of the lines established from metastases, which suggested that each metastasis originated from a single progenitor cell. Similar results have been obtained in other rodent tumor systems. These studies revealed that the majority of metastases are of clonal origin. Moreover, variant clones with diverse phenotypes are formed, rapidly resulting in the generation of significant cellular diversity within individual metastases [244]. Cancer metastases of a clonal origin can be produced by two different mechanisms, proliferation of a single cell or of many cells. In the case of the second possibility, the cell aggregate at the metastatic site must have a homogeneous composition. To determine which of these possibilities is responsible for the generation of clonal K-1735 melanoma metastases, we injected C3H mice intravenously with aggregates of K-1735 cells consisting of two distinct subpopulations [75]. Cells of line K-1735-M2 are highly metastatic and exhibit a stable normal karyotype. Cells of the X-met-21 line are also highly metastatic but exhibit a stable, submetacentric chromosomal marker. After mixed aggregates (>20 cells) of these two cell types were injected, individual lung metastases were recovered and cultured, and each metastatic line was subjected to chromosome analysis. We reasoned that if an experimental metastasis originated from a single proliferating cell, all tumor cells within the metastatic focus should express either the K-1735-M2 or the X-met-21 chromosomal profile. This indeed was the case. Analysis of the distribution and fate of circulating tumor emboli has demonstrated that multicellular aggregates are more likely to give rise to a metastasis than a single-tumor-cell embolus. This is probably so because tumor cells not on the periphery of circulating emboli

The pathogenesis of cancer metastasis: relevance to therapy can be protected from destruction in the circulation, and a large aggregate of cells can more readily arrest in the capillary bed of an organ. Since the aggregates we injected were large, each containing more than 20 cells, the results suggest that the melanoma lung metastases resulted from the proliferation of a single viable cell within the embolus. Thus, regardless of whether an embolus is initially homogeneous or heterogeneous, metastases can be unicellular in origin. Collectively, these observations indicate that different metastases arise from different progenitor cells and account for the well-documented differences in behavior of different metastases. Among individual metastases of proved clonal origins, however, heterogeneity can develop rapidly to create significant intralesional heterogeneity.

Development of Biological Diversity within and Among Metastases Clinical and histologic observations of neoplasms have suggested that tumors undergo a series of changes during the course of the disease. A tumor initially diagnosed as benign, for example, can evolve over a period of many months into a malignant tumor. This can best be demonstrated in the case of human cutaneous melanoma where the transformation of normal melanocytes and their conversion into metastatic cells has been studied in detail by Clark and coworkers [32, 33]. This progression is gradual and consists of a series of discrete irreversible steps [107]. To explain the process of tumor evolution and progression as originally defined by Foulds in 1954 [81], Nowell [190] suggested that acquired genetic variability within developing clones of tumors, coupled with host selection pressures, can result in the emergence of new tumor cell variants that exhibit increasing growth autonomy or malignancy. Nowell’s hypothesis [190] predicted that accelerating tumor progression toward malignancy can be accompanied by increasing genetic instability of the evolving cells. To test this hypothesis, we have examined the metastatic stability and rates of mutation of paired metastatic and nonmetastatic cloned lines isolated from four different mouse neoplasms. We found that highly metastatic cells were phenotypically less stable than their nonmetastatic counterparts. Moreover, in highly metastatic clones, the rate of spontaneous mutation was found to be severalfold higher than in low-metastatic clones. These results are in accord with the hypothesis that tumor progression occurs as a result of acquired genetic alterations.

Sun-Jin Kim et al. Similar data have been reported for other neoplasms [reviewed in 62, 65]. Evidence that genetic mechanisms can be responsible for tumor progression comes from mutagenesis experiments using nitrosoguanidine. The finding that metastatic cells exhibit higher mutation rates than nonmetastatic cells [31], and that heterogeneity develops more rapidly in tumors containing few subpopulations of cells [186, 208, 210] suggest that accelerated tumor evolution and progression will result in the rapid development of biologic diversity in metastases, especially when such lesions are of clonal origin.

Intratumoral Heterogeneity for Expression of Tyrosine Kinase Growth Factor Receptors It is well recognized that tumor progression, angiogenesis, and metastasis are regulated by the interaction of tumor cells with the host organ microenvironment [70]. The expression of growth factors and their receptors varies within different zones of any given organ and may also differ among different cells within a tumor [62, 63]. Overexpression of transforming growth factor-alpha (TGF-α), epidermal growth factor (EGF), and the EGF receptor (EGFR) has been reported to be associated with poor prognosis in different neoplasms [2, 111, 170, 253, 271]. Expression of vascular endothelial growth factor (VEGF) is associated with increased vascular permeability, cell proliferation, and survival of endothelial cells [85, 86, 148, 272]. The expression of VEGF has also been correlated with microvessel density of neoplasms [52, 242] metastasis, and, hence, poor prognosis [53, 116, 140, 234, 262]. The expression of PDGF and its receptor (PDGFR) by tumor cells, tumor-associated endothelial cells [126, 251], and pericytes and myofibroblasts [127, 131, 151, 269, 270] is common to many neoplasms. In clinical specimens of human colon carcinomas, while PDGF is produced by tumor cells, the expression of PDGFR is restricted to stromal cells, including tumorassociated endothelial cells [131], i.e., tumor-associated stromal cells generate a microenvironment favorable to the survival and progressive growth of tumor cells [213]. Specifically, PDGFR signaling is related to recruitment of pericytes and control of interstitial fluid pressure [14, 192, 202, 213], which are favorable to the survival and progressive growth of neoplasms [155]. The expression of TGF-α, EGF, VEGF, and PDGF-β and their respective receptors was examined in 12 human colon cancer surgical specimens and in orthotopic tumors in nude mice produced by two distinct human colon

25 carcinoma cell lines. Inter- and intratumoral heterogeneity in expression of these growth factors and their receptors was reported. Different proteins produced by tumor cells can regulate the interaction between tumor cells and the organ microenvironment [57]. Many protein tyrosine kinase receptors on tumor cells and tumor-associated endothelial cells can be induced or upregulated by ligands produced by tumor cells via autocrine and paracrine pathways [55]. In many clinical trials, the presence of targets in patients was not confirmed, and the response rate was unpredictable [247]. For inhibition of phosphorylated EGFR, the presence of mutated receptors was reported to be a predictable variable [158], but significant therapeutic responses of cancer cells with wild-type receptors has also been reported [23, 269, 270]. These confusing criteria for selection of patients for treatment strongly indicate the need for better methodologies. The data demonstrating inter- and intratumoral heterogeneity for expression of EGFR, VEGFR2, and PDGFR-β in different human colon cancer specimens of different stages suggest that targeting a single tyrosine kinase receptor is not likely to provide significant therapeutic results. In other words, targeted therapy is effective only against its target and eliminating tumor cells that are dependent on one pathway, e.g., EGFR, is not likely to prevent the proliferation of tumor cells that are independent of this pathway. These results agree with a recent report demonstrating biological heterogeneity of tyrosine kinase receptors in other cancers [30, 173]. Indeed, simultaneous blocking of two protein tyrosine kinase pathways produce more efficient therapeutic effects in prostate [128] and pancreatic [269] carcinoma than did blocking of a single such pathway. Therapeutic efficacy was further increased when three protein tyrosine kinase pathways were inhibited [270]. Cancers are biologically heterogeneous for multiple properties, including antigenicity, sensitivity to chemotherapeutic agents, invasion, and metastasis [76, 77]. The progressive growth, metastasis, and survival of tumor cells depend on their interaction with the organ microenvironment. The expression of multiple tyrosine kinase receptors by different tumor cells within a single neoplasm indicates that targeting a single tyrosine kinase may not produce eradication of the disease.

Zonal Heterogeneity for Gene Expression A better understanding of the cross-talk between cancer cells and the organ microenvironment involving multiple genes first requires the identification of gene expression

26 profiles within the tumor. Among current methods for establishing these profiles, microarray analysis is the most powerful, and several such studies have been undertaken to predict disease outcome for individual patients [9, 10, 29, 115, 207, 216, 258, 259] and to identify cancer patients who should receive chemotherapy [17]. For example, pretherapeutic gene expression profiling has been undertaken to identify breast cancer patients who should receive specific chemotherapy [17], to predict response of rectal cancer patients to preoperative chemoradiotherapy [89], and to predict response of breast cancer patients to docetaxel [28], and a specific gene expression pattern was correlated with recurrence in Dukes’ B colon carcinomas [265]. None of these studies, however, accounted for the biologic heterogeneity of gene expression within a single tumor. Tumor cells depend on multiple and redundant pathways for growth, survival, and adaptation to the host microenvironment [70, 76, 77, 155]. In malignant neoplasms, genetic instability of cancer cells frequently yields extensive intratumoral heterogeneity in the pattern of structural chromosome aberrations [90, 92, 93, 95]. Three dimensional tumor growth and uneven fractional division of tumor cells within a single tumor mass can give rise to biologically different central and peripheral zones. The microenvironment of the central zone differs significantly from that of the peripheral zone, and zonal heterogeneity for several molecules in different tumor systems has been reported [102, 105, 129, 130, 138, 139, 191, 225]. Among these, bFGF, matrix metalloproteinase-2 (MMP-2), and MMP-9 have been shown to be highly expressed at the invasive edge of tumors [105, 130, 131, 138, 139], whereas the cell-to-cell cohesion molecule, E-cadherin, was downregulated at the periphery of the tumors [105, 130, 131, 138, 139, 191]. In fact, the ratio of expression of MMP-2 and MMP-9 to E-cadherin (MMP/E-cadherin ratio) at the periphery of the tumors correlated with metastatic potential and recurrent disease [138, 139]. Affymetrix HG-U133-Plus 2.0 array and laser capture microdissection techniques, demonstrated that different zones of the same pancreatic tumor exhibit differential expression of genes [179, 180]. The study of 1,222 genes demonstrated statistically significant differences. Bioinformatic functional analysis revealed that 346 upregulated genes in the peripheral zone were related to cytoskeleton organization and biogenesis, cell cycle, cell adhesion, cell motility, DNA replication, localization, integrinmediated signaling pathway, development, morphogenesis, and IκB kinase/NF-κB cascade. In the central zone, 876 upregulated genes were related to the regulation of cell proliferation, transcription, transmembrane

The pathogenesis of cancer metastasis: relevance to therapy receptor protein tyrosine kinase signaling pathways, response to stress, small GTPase-mediated signal transduction, hexose metabolism, cell death, response to external stimulus, and carbohydrate metabolism. These data clearly demonstrate zonal heterogeneity for gene expression profiles within single tumors and suggest that characterization of zonal gene expression profiles is essential if microarray analyses of genetic profiles are to produce reproducible data, predict disease prognosis, and allow design of specific therapeutics [179, 180].

Tumor-Organ Interaction: The “Seed and Soil” Hypothesis Clinical observations of cancer patients and studies with experimental rodent tumors have led cancer biologists to conclude that the metastatic pattern of certain tumors is organ-specific and independent of vascular anatomy, rate of blood flow, and number of tumor cells delivered to each organ [reviewed in 62, 65, 263, 264]. Indeed, the distribution and fate of hematogenously disseminated, radiolabeled melanoma cells in experimental animals conclusively demonstrated that tumor cells can reach the microvasculature of many organs, but growth in the organ parenchyma occurs in only specific organs [60]. These findings, however, were not new. More than 100 years earlier, Paget reached a similar conclusion. In 1889, he asked, “What is it that decides what organs shall suffer in a case of disseminated cancer?” Paget’s study was motivated by the discrepancy between considerations of blood flow and the frequency of metastases in different organs. He examined the autopsy records of 735 women who died of breast cancer and many other patients with different neoplasms and noticed the high frequency of breast cancer metastasis to the ovaries and the variations in incidence of skeletal metastases produced by different primary tumors. These findings were not compatible with the view that metastatic spread was due to “a matter of chance” or that tissues “played a passive role” in the process. Paget [193] concluded that metastasis occurred only when certain favored tumor cells (the “seed”) had a special affinity for the growth milieu provided by certain specific organs (the “soil”). The formation of metastasis required the interaction of the right cells with the compatible organ environment. In 1928, Ewing [50] challenged Paget’s seed and soil theory and hypothesized that metastatic dissemination occurs purely by mechanical factors that are a result of the anatomical structure of the vascular system. Both of these explanations have been evoked separately or

Sun-Jin Kim et al. together in order to explain the metastatic site preference of certain types of neoplasms. In a review of clinical studies on site preferences of metastases produced by different human neoplasms, Sugarbaker [235] concluded that common regional metastatic involvements could be attributed to anatomical or mechanical considerations, such as efferent venous circulation or lymphatic drainage to regional lymph nodes, but that metastasis in distant organs from numerous types of cancers were indeed sitespecific. Experimental data supporting Paget’s 1889 seed and soil hypothesis were provided a century later by Hart and Fidler [101], who studied the preferential growth of B16 melanoma metastases in specific organs. Following the intravenous (i.v.) injection of B16 melanoma cells into syngeneic C57BL/6 mice, tumor growths developed in the natural lungs and in grafts of pulmonary or ovarian tissue implanted either subcutaneously (s.c.) or intramuscularly (i.m.). In contrast, neoplastic lesions failed to develop in control grafts of similarly implanted renal tissue or at the site of surgical trauma. Parabiosis experiments suggested that the growth of the B16 melanoma in ectopic lung or ovarian tissue was due to the immediate arrest of circulating neoplastic cells and not to shedding of malignant cells from foci growing in the natural lungs. Quantitative analysis of tumor cell arrest and distribution using cells labeled with [125I]-5-iodo-2´-deoxyuridine indicated that the growth of tumors in the implanted organs was not due to an enhanced initial arrest of B16 cells. No significant differences in immediate tumor cell arrest were detected between implanted fragments of lungs (tumorpositive) and kidney (tumor-negative) or between organbearing and contralateral control limbs. These data demonstrated that the outcome of metastasis is dependent on both tumor cell properties and host factors and supported the seed and soil hypothesis as an explanation of the nonrandom pattern of cancer metastasis. The introduction of peritoneovenous shunts for palliation of malignant ascites provided an opportunity to study some of the factors affecting metastatic spread in humans. Tarin et al. [245] described the outcome in patients with malignant ascites draining into the venous circulation, with the resulting entry of viable tumor cells into the jugular veins. Good palliation with minimal complications was reported for 29 patients with various neoplasms. The autopsy findings in 15 patients substantiated the clinical observations that the shunts did not significantly increase the risk of metastasis. In fact, despite continuous entry of millions of tumor cells into the circulation, metastases in the lung (the first capillary bed encountered) were rare. These results provide compelling verification of the seed and soil hypothesis.

27 A clear demonstration of organ-site specific metastasis comes from studies of experimental brain metastasis. Two murine melanomas were injected into the carotid artery to simulate the hematogenous spread of tumor emboli to the brain. The K-1735 melanoma generated lesions only in the brain parenchyma, whereas the B16 melanoma produced only meningeal growths [219]. Similarly, different human melanomas [220] injected into the internal carotid artery of nude mice also produced unique patterns of brain metastasis. Distribution analysis of radiolabeled melanoma cells injected into the internal carotid artery ruled out the possibility that the patterns of initial cell arrest in the microvasculature of the brain predicted the eventual sites of growth. Rather, the different sites of tumor growth in the brain involved interactions between the metastatic cells and brain endothelial cells, and the response of tumor cells to local growth factors. In other words, site-specific metastases were produced by tumor cells that are receptive to their new environment.

Regulation of Tumor Cell Gene Expression by the Organ Microenvironment Tumor cells with different metastatic capabilities have been shown to differ in expression of proteins, such as collagenases, E-cadherin, IL-8, bFGF, and many others [65]. In most cases, however, these differential expressions were most evident in tumor cells growing at anatomically correct sites. Many biologic investigations of solid tumor cells often use cell lines growing in vitro as monolayer cultures. While easy to accomplish, monolayer cultures are not subjected to any cross talk, e.g., paracrine signaling pathways associated with growth in vivo [37]. Clinical observations of cancer patients and studies in rodent models of cancers have concluded that certain tumors tend to metastasize to certain organs [264]. The concept that metastasis results only when certain tumor cells interact with a specific organ microenvironment was originally proposed in Paget’s venerable “seed and soil” hypothesis [193]. Indeed, spontaneous metastasis is produced by tumors growing at orthotopic sites, whereas the same tumor cells implanted into ectopic sites fail to produce metastasis [124]. To determine the influence of the microenvironment on changes in gene expression, microarray analyses were done on three variant lines of a human pancreatic cancer with different metastatic potentials [141, 179].

28 The variant lines were grown in tissue culture in the subcutis (ectopic) or pancreas (orthotopic) of nude mice. Compared with tissue culture, the number of genes whose expression was affected by the microenvironment was upregulated in tumors growing in the subcutis and pancreas. In addition, highly metastatic pancreatic carcinoma cells growing in the pancreas expressed significantly higher levels of 226 genes than did the low metastatic variant cells. Growth of the tumor cells in the subcutis did not yield similar results, indicating that the orthotopic microenvironment significantly influences gene expression in pancreatic cancer cells [141, 179].

Regulation of the Invasive Phenotype by the Microenvironment The metastatic capacity of human colon cancer cells growing in orthotopic tissues of nude mice directly correlates with the level of collagenase type IV activity [233]. Histological examination of the human colon carcinomas growing in the subcutis, wall of the colon, or kidney of nude mice revealed a thick pseudocapsule around the subcutaneous but not cecal or kidney tumors [61, 70, 76]. These differences suggested that the organ environment could influence the ability of metastatic cells to invade host stroma. Significant differences were found in the levels of secreted type IV collagenases between human colon cancer cells growing subcutaneously or in the cecum of nude mice. In the medium conditioned by human colon cancer cells derived from subcutaneous implants, we detected only a latent form of the 92-kDa type IV collagenase. In contrast, both latent and active forms of the 92-kDa type IV collagenase were found in culture medium conditioned by tumor cells harvested from cecal tumors. Moreover, cancer cells grown in the cecum secreted more than twice as much enzymes as the subcutaneous tumors [178]. The invasive ability of human colon cancer cells is directly influenced by organ-specific fibroblasts. Primary cultures of nude mouse fibroblasts from skin, lung, and colon were established, and invasive and metastatic human colon cancer cells were cultured alone or with the fibroblasts. The cancer cells grew on monolayers of all three fibroblast cultures but did not invade through skin fibroblasts [51]. Cancer cells growing on plastic and on colon or lung fibroblasts produced significant levels of latent and active forms of type IV collagenase, whereas colon cancer cells cocultivated with nude mouse skin fibroblasts did not. One possible explanation is that fibroblasts from the skin produce IFN-β, whereas those

The pathogenesis of cancer metastasis: relevance to therapy from the kidney or colon do not. The incubation of human colon cancer cells [51] and human renal cancer cells [91] with IFN-β significantly reduced the expression and activity of collagenase type independently of antiproliferative activity.

Regulation of Angiogenesis by the Organ Microenvironment The intensity of the angiogenic response varies considerably among different types of tumors and within different zones of a single tumor [48]. The rate of tumor cell division is still several orders of magnitude greater than the rate of neovascularization. As tumors expand, their microenvironment is often hypoxic [260]. Hypoxia is often associated with the activation of the transcription factor hypoxia-inducible factor-1 alpha (HIF-1α) to initiate the transcription of genes, e.g., VEGF/VPF [54]. VEGF/VPF increases the permeability of blood vessels by stimulating the functional activity of vesicular-vacuolar organelles, clusters of cytoplasmic vesicles and vacuoles located in microvascular endothelial cells [45–47]. VEGF also induces migration, protease production, and endothelial cell proliferation [54]. In addition, VEGF/ VPF regulates endothelial cell survival by activating the phosphatidylinositol-3 kinase/Akt signal transduction pathway and stimulating expression of the antiapoptotic proteins Bcl-2 and A1 [110]. HIF-1α signaling also encodes the polypeptide chains of PDGF [100]. The functional activity of PDGF is, to a large extent, determined by the anatomical location of a specific tumor. For example, in pancreatic tumors, PDGF has been shown to stabilize developing vascular networks by recruiting pericytes to support the immature blood vessel walls [13, 14]. In tumors of the central nervous system, PDGF stimulates the release of VEGF from the tumor-associated endothelium [96]. In contrast, tumors in the skin rely on PDGF signaling to regulate the level of interstitial fluid pressure in the tumor [199]. In prostate cancer bone metastasis (see next section), PDGF functions as a survival factor for tumor endothelial cells by activating the intracellular effectors MAPK and Akt [142, 143]. The vascular endothelium is regarded as structurally and functionally heterogeneous [88]. To examine this diversity, we generated a broad panel of microvascular endothelial cells from various organs of H-2Kb-tsA58 transgenic mice [142]. cDNA expression profiles generated on the endothelial cells predicted significant organspecific differences in expression levels of tyrosine kinase

Sun-Jin Kim et al. receptors, chemokine receptors, and proteins that regulate the efflux of toxic substrates; these were confirmed at the protein level. Endothelial cells derived from the mouse brain expressed measurable levels of PDGF-Rβ, the chemokine receptor CXCR-2, and P-glycoprotein, whereas endothelial cells from the pulmonary circulation did not express detectable levels of these proteins. The organ-derived endothelial cells also exhibited vast differences in response to stimulation with endothelial cell mitogens. Endothelial cells originating from the brain and liver showed the greatest increase in cell division in response to basic fibroblast growth factor, while EGF was the most potent mitogen for endothelial cells derived from the lung and uterus.

Influence of Lymphoid Cells on Angiogenesis The regulation of angiogenesis by T lymphocytes, macrophages, and mast cells, is well established [55, 59, 68, 83, 160, 164, 169, 204, 223, 240]. For example, invasive cutaneous melanoma is often associated with a local inflammatory reaction involving T lymphocytes and macrophages, a condition often associated with an increased risk of metastasis [19, 217]. The relatively slow growth of tumors in aging mice has been closely linked with decreased vascularization [133] associated with a diminished immunological response [38, 92]. The role of tumor vascularization and its effect on tumor growth in immunosuppressed mice was investigated and it was concluded that the growth of the immunogenic B16 melanoma was delayed in myelosuppressed mice as compared to control mice [98]. Similarly, tumor growth in mice pretreated with doxorubicin (DXR) and then injected with normal splenocytes 1 day before tumor challenge was comparable to that in the control mice, implicating myelosuppression as a cause of retardation of tumor growth and vascularization. Similar results were obtained with athymic nude mice [98]. Many other studies have recognized the importance of macrophages in tumor angiogenesis [5, 204–206, 239]. Macrophages can produce more than 20 molecules that influence endothelial cell proliferation, migration, and differentiation [204, 205]. Macrophages may modify the extracellular matrix, thereby modulating angiogenesis either through direct production of extracellular matrix components or proteases that alter the structure and composition of the extracellular matrix [240]. Macrophages have also been shown to produce antiangiogenic molecules, such as thrombospondin-1 [40, 152, 153, 205].

29 Macrophage-derived metalloelastase has been shown to be responsible for the generation of angiostatin in Lewis lung carcinoma [42], and the addition of plasminogen to 3LL Lewis lung carcinoma cells cultured in vitro did not result in the generation of angiostatin. However, its addition to co-cultured macrophages and carcinoma cells did [42], suggesting that elastase activity in macrophages was significantly enhanced by the cytokine GM-CSF which was secreted by the tumor cells [137], leading to the generation of plasminogen.

Regulation of Response to Chemotherapy by the Organ Microenvironment Clinical observations suggest that the organ environment can influence the response of tumors to chemotherapy. For example, in women with breast cancer, lymph node and skin metastases are more sensitive to chemotherapeutic agents than are metastases residing in either the lung or bone [41]. Several intrinsic properties of tumor cells can render them resistant to chemotherapeutic drugs, including increased expression of the mdr genes, leading to overproduction of the transmembrane transport protein P-glycoprotein (P-gp) [18, 249]. Expression of P-gp often parallels increases in dose or duration of chemotherapy. Indeed, increased levels of P-gp can be induced by selecting tumor cells for resistance to natural product amphiphilic anticancer drugs. Nevertheless, elevated expression of P-gp accompanied by development of the multidrug resistance (MDR) phenotype has also been found in many solid tumors of the colon, kidney, and liver that had not been previously exposed to chemotherapy [249]. One of the most striking examples of site-specific variations in therapeutic response was observed following implantation of colon carcinoma cells into different anatomic locations of nude mice (using the highly metastatic KM12L4 human colon carcinoma cell line) or syngeneic BALB/c mice (using the CT-26 murine colon carcinoma). Mice received injections of the KM12L4a cells into either the subcutis (ectopic site), spleen (leading to experimental liver metastasis), or cecum (growth at the orthotopic site). Tumor-bearing mice were given doxorubicin and subsequently evaluated for response to treatment. Up to 80% growth of subcutaneous tumors was inhibited by two i.v. injections of doxorubicin as compared to about 40% inhibition of the intracecal tumors and less than 10% inhibition of lesions in the liver [267]. Similar results were obtained with murine

30 colon cancer cells. Subcutaneous tumors were sensitive to treatment with doxorubicin; lung metastases were insensitive [42]. However, the tumor cells at both of these sites were equally sensitive to 5-FU, a drug whose activity is not influenced by expression of the MDR phenotype. Northern blot analysis showed that the relative expression of the mdr1 and mdr3 genes was greatest in the cecum, liver, and lungs of mice, and this expression correlated with the relative resistance of tumor cells. Indeed, this expression of mdr was transient and the subsequent culture of cells from a liver metastasis for 7–10 days resulted in a decrease of mdr expression to the level in tumor cells maintained in culture [43]. These events resulted from organ-specific modulation of tumor cell properties. These findings are not restricted to experimental systems. In patients with colon carcinoma, elevated P-gp expression is found on the invasive edge of the primary tumor (growing in the colon) and in metastases located in lymph node, lung, and liver [129]. Whether this finding is due to selection or adaptation is unclear.

The pathogenesis of cancer metastasis: relevance to therapy to up-regulate the growth factor receptors and the activation of these receptors on tumor cells (autocrine effect) and on tumor-associated endothelial cells (paracrine effect). These effects can lead to downstream signaling that activate mitogen-activated protein kinases and phosphatidylinositol-3 kinase pathways and lead to downregulation of the apoptotic proteins caspases 3 and 8 [8, 142, 143, 229]. Activation of growth factor receptors on tumor cells and on the tumor-associated endothelial cells is likely to increase the expression of antiapoptotic proteins, causing endothelial cell resistance to chemotherapy, even though these cells divide every 30–40 days. Inhibition of this growth factor receptor activation by receptor antagonists in tumor-associated endothelial cells (and tumor cells) can lead to BAX induction, activation of caspase-8, and down-regulation of BCL-2 and NF-κB [1, 24], thereby increasing the susceptibility of tumors to chemotherapy. If the tumorassociated endothelial cells continue to cycle, the use of receptor antagonists [6, 7, 125–128] combined with anticycling drugs can cause the destruction of the vasculature within neoplasms, leading to the apoptosis of adjacent tumor cells [20, 21, 193, 232].

Targeting the Vasculature Because systemic antitumor therapy fails primarily because of the genetic instability and biologic heterogeneity of neoplasms, therapeutic agents that target a tumor’s vasculature, a genetically stable and essential component of tumors, have been explored as an alternative to conventional therapy. The structure and architecture of tumor vasculature can differ dramatically from those of normal organs [48, 182, 189]. Modern techniques, such as phage display targeting, have defined “vascular addresses” that may be distinct for different organs and for tumors in those organs [194]. In normal human tissues, for example, endothelial cells are long lived and recycle every 3–5 years, whereas tumor-associated endothelial cells can recycle every 30–40 days. The rate of endothelial cell turnover has been reported to be <0.1% in normal human tissues, whereas in malignant human tumors, it is 2–9.9% [48, 49]. Endothelial cells that line blood vessels in different organs are also phenotypically distinct. The expression of various cytokines and growth factors and their relevant receptors on both tumor cells and organ-specific endothelial cells differs in different organ microenvironments, and the interaction of these growth factor ligands (i.e., EGF, TGF-α, VEGF, and PDGF) with their relevant receptors is essential for endothelial cell survival and growth [6, 7, 128, 142, 143]. For example, tumor cells that produce growth factor ligands are likely

Targeting the Expression of Platelet-Derived Growth Factor Receptor by Reactive Stroma Colon carcinoma cells produce various growth factors and cytokines that contribute to progressive growth and metastasis [117]. One example is the family of PDGFs, members of a family of dimeric disulfide-bonded growth factors exerting their biological effects through activation of two structurally related tyrosine kinase receptors, the PDGF-α- and -β receptors [104]. PDGF consists of dimeric forms, including PDGF-AA, PDGF-BB, PDGF-AB, PDGF-CC, and PDGF-DD [15, 144, 149]. The α-receptor binds all possible forms of PDGF except PDGF-DD, whereas the PDGF-β receptor preferentially binds PDGF-BB. PDGF-Rβ is expressed on many tumor types, and PDGF-BB is an important autocrine growth factor for many cell types, including gliomas, sarcomas, pancreatic carcinoma, and prostate cancer [103]. PDGF-R signaling has also been reported to stimulate angiogenesis [213], recruit pericytes [14, 192], and control the interstitial fluid pressure in stroma, thereby affecting transvascular transport of chemotherapeutic agents [200–203]. A recent study of human colon cancer clinical specimens revealed that PDGF-Rβ and phosphorylated PDGF-Rβ are not expressed on tumor cells but, rather, are predominantly

Sun-Jin Kim et al. expressed in stromal cells and in pericytes surrounding the tumor microvessels [131, 132, 238, 241]. Imatinib, a derivative of 2-phenylaminopyrimidine, was originally developed as a competitor for an ATPbinding site of the Abl protein tyrosine kinase [44]; however, it is also a potent tyrosine kinase inhibitor of c-Kit and PDGF-R [22]. We have reported that imatinib can slow both the progressive growth of human pancreatic carcinoma in nude mice [114, 270] and the growth of experimental bone metastasis of human prostate cancer [127, 251]. To examine the role of PDGFR on colon cancer stromal cells, we examined the therapeutic effect of imatinib administered as a single agent or in combination with the chemotherapeutic irinotecan against human colon carcinoma cells growing in orthotopic (cecum and liver) and ectopic (subcutis) organs of nude mice. Blockade of PDGF-Rβ signaling by oral administration of imatinib or imatinib combined with irinotecan significantly inhibited the growth of orthotopic tumors and the incidence of lymph node metastasis in nude mice. Histopathological analysis of human colon carcinoma growing in the cecum and liver of nude mice treated with imatinib alone or with imatinib combined with irinotecan demonstrated decreased stromal reaction and inhibition of phosphorylated PDGF-Rβ in the tumor-associated stromal cells. These effects were associated with the inhibition of tumor cell proliferation, an increase of apoptosis in stromal cells, and a decrease in the number of pericytes surrounding the tumor-associated microvessels. In contrast, treatment of mice with imatinib alone or imatinib combined with irinotecan did not affect the growth of colon carcinoma cells in the subcutaneous space, demonstrating once again that the biology of tumors differs with the organ microenvironment [58, 76]. In general, tumor cells in a neoplasm are biologically heterogeneous and their phenotype can be modified by the organ microenvironment [63, 74]. Histologically, human carcinoma tissues are composed of both parenchyma and stroma. Tumor stroma consists of fibroblasts, smooth muscle cells, inflammatory cells, microvessels, and abundant extracellular matrix (ECM) [99]. Tumorassociated stroma at both primary sites and metastatic sites generate a favorable microenvironment for the survival of cancer cells [155]. Fibroblasts are activated by various growth factors and cytokines that are released by cancer cells, for example TGF-β, PDGF, and bFGF. Activated fibroblasts express αSMA, leading to the term ‘myofibroblasts’ [45]. Stromal reaction (desmoplasia), such as myodifferentiation of fibroblasts [109, 145, 215] clearly alters the stromal phenotype. In the liver, hepatic stellate cells are the only mesenchymal cells present in

31 the parenchyma. These cells are activated by various stimuli from tumor cells and undergo transformation into myofibroblasts, which are characterized by expression of αSMA [211]. In general, tumor-associated stroma is an abundant source of tumor-promoting growth factors and cytokines, and stromal cells activated by cancer cells can in turn regulate the growth and progression of carcinoma cells [112, 165, 222, 250]. The progressive growth of colorectal tumors in experimental animals has been correlated with proliferation of myofibroblasts, whereas regression of colon tumors has been linked to a fibrous capsule, suggesting that the presence of reactive myofibroblasts may inhibit growth of tumor cells [150]. Wounding has been associated with tumorpromoting effects [224] and, functionally, tumor-associated stromal cells are similar to stromal cells in healing wounds insofar as expression of myodifferentiation markers, production of ECM, expression of growth factors and cytokines, and neovascularization [121–123]. The combination of imatinib and irinotecan completely inhibited tumor cell growth at the abdominal wound healing site induced at the time of tumor cell injection into the cecum [131]. Imatinib may inhibit stromal reaction at the orthotopic primary site and the surgical wound. PDGF-R signaling pathway also plays an important role in increasing tumor interstitial hypertension, which may block the accumulation of antitumor drugs [201–203]. The microvasculature in both tumor tissue and normal colon mucosa consists of endothelial cells, pericytes (mural cells or smooth muscle cells), and basement membranes. All of these components are thought to be abnormal in tumor vessels. Pericytes are key cells in vascular development, stabilization, maturation, and remodeling [12, 87]. Functional-blocking antibodies that target PDGF-Rβ block pericyte recruitment during vascular development [252]. In our study, desmin-positive pericytes were found on microvessels in both normal organs and colon cancers, albeit with different morphological characteristics. Specifically, pericytes in tumor-associated vessels, but not those in vessels of normal colon mucosa, were enlarged and overexpressed PDGF-Rβ and p-PDGF-Rβ [131]. In agreement with earlier reports [14, 269, 270], we found that treatment with imatinib decreased pericyte coverage on tumor-associated endothelial cells. The inhibition of PDGF-R signaling by a protein tyrosine kinase inhibitor decreased pericyte recruitment and attachment to endothelial cells and destabilized tumor vasculature by killing pericytes. In the absence of decreased vessel density in tumors treated with imatinib combined with irinotecan, functional rather than quantitative changes of the tumor vasculature by imatinib may diminish tumor growth [84].

32 Recently, targeting the VEGF pathway to inhibit tumor angiogenesis is attracting attention as a novel cancer therapy [113]; however, it has been suggested that VEGFtargeting therapies are mostly active against immature vessels [13], i.e., normalization of the tumor vasculature [118]. Because imatinib inhibits pericyte coverage on tumor vasculature, it will be very interesting to determine if the combined inhibition of PDGF-R and VEGF-R signaling may produce synergistic antivascular effects [45, 118, 200]. Multi-target tyrosine kinase inhibitors are under current investigation in clinical trials. Sorafenib and sunitinib target not only multiple VEGF-Rs but also PDGF-Rβ, and phase II studies show promising activity of these molecules in renal cell carcinoma [180], likely due to inhibition of angiogenesis [266].

Anti-Vascular Therapy of Prostate Cancer Bone Metastasis In clinical specimens of human prostate cancer bone metastasis and in experimental models of orthotopic human prostate cancers in the long bones of nude mice, the expression of PDGF and PDGF-R correlates with the growth of metastatic tumor cells in the bone parenchyma. Specifically, prostate cancer cells growing adjacent to bone tissue express high levels of the PDGF protein and the PDGF-R, which is phosphorylated, and tumor-associated endothelial cells express high levels of phosphorylated PDGF-R [127, 128]. Oral administration of the PDGF-R tyrosine kinase inhibitor imatinib (STI571) to nude mice blocks the PDGF signaling pathway by inhibiting phosphorylation of its receptors [251]. Oral administration of STI571 or STI571 plus injectable taxol to male nude mice reduced the incidence and size of primary tumors and bone lesions and prevented bone lysis as measured by digital radiography. Immunohistochemical analysis of control, untreated bone lesions, demonstrated that human prostate cancer cells growing adjacent to the bone expressed high levels of PDGF and activated (phosphorylated) PDGF-R, whereas tumor cells growing in the adjacent musculature following lysis of the bone did not. Treatment with STI571 and more so with STI571 plus taxol significantly inhibited phosphorylation of PDGF-R on tumor cells and endothelial cells, decreased tumor cell proliferation, and induced significant apoptosis in tumor cells and tumor-associated endothelial cells. These data indicate that targeting PDGF-R phosphorylation can produce significant therapeutic effects against prostate cancer bone metastasis [125–128].

The pathogenesis of cancer metastasis: relevance to therapy To determine whether the PDGF-R expressed on tumor-associated endothelial cells could be the primary target of imatinib, we selected a multidrug-resistant variant of the human PC-3MM2 prostate cancer. These cells were implanted into the tibia of nude mice, and when the lesions were growing progressively, the mice were randomized to different treatment regimens. The bone tumors were resistant to taxol, whereas treatment with imatinib and taxol produced a significant decrease in tumor incidence, bone lysis, and incidence of lymph node metastasis. Initially, this treatment produced apoptosis of tumor-associated endothelial cells (but not tumor cells), followed 1 week later by apoptosis of the tumor cells. Collectively, these studies demonstrated that the primary targets for imatinib treatment are the tumor-associated endothelial cells, i.e., an anti-vascular therapy [126].

Targeting Platelet-Derived Growth Factor Receptor on Endothelial Cells of Multidrug Resistant Prostate Cancer The major cause of death from prostate cancer is due to metastases that are resistant to conventional therapies. Genetic instability of tumor cells leading to biological heterogeneity is largely responsible for the development of hormone-refractory prostate cancer cells [10, 11, 56, 197, 198, 254–257], and selection pressures by chemotherapeutic agents results in the emergence of cells that are resistant to chemotherapy [122, 147, 159, 198, 221, 231]. These resistant cells often express the MDR1 gene and its product, P-glycoprotein (P-gp) [94, 248, 257]. Methods to overcome multidrug resistant activity by blocking transmembrane drug efflux pump action [82, 166–168, 176], liposomal encapsulation of cytotoxic drugs [18], immunotherapeutic monoclonal antibody targeting P-glycoprotein [214] or protein toxins selectively killing cells expressing P-glycoprotein on their surfaces [168] are under clinical investigation. Multidrug resistance-associated protein (MRP) was found in multidrug resistant prostate cancer cell lines that do not express P-glycoprotein [34]. MRP1 [237, 256] and MRP2 [255] have been reported to be expressed in prostate cancer cell lines, but there are very few compounds with proven MRP1-associated multidrug resistant-modulating capacity at present. Glutathione and glutathione-S-transferase (GST) detoxification systems are another drug-resistance mechanism reported to

Sun-Jin Kim et al. protect cancer cells against the lethal effects of chemotherapy [174]. GST-π inactivation is related to early steps of prostate carcinogenesis [146, 237], whereas the hormone-independent disseminated prostate cancers clearly expressed GST-π [256]. However, reversal of multidrug resistance in prostate cancer by challenging the glutathione pathway has been observed in vitro [212]. Many changes in the regulatory processes of apoptosis apparently contributed to the malignant phenotype of prostate cancer cells [35, 39, 188, 256], and methods to stimulate apoptosis are currently under development. However, despite progress in our understanding of the biology of MDR, therapeutic approaches that directly target tumor cells have not been successful. To overcome multidrug resistance of prostate cancer, one may wish to affect the host factors in the organ microenvironment, such as the vasculature [26, 63, 72, 78–80, 122]. Because all cells in the body depend on an adequate supply of oxygen and nutrients and the ability to remove toxic molecules, therapy directed against tumor-associated endothelial cells can destroy tumor cells, regardless of their biological heterogeneity [77]. Recent reports have demonstrated statistically significant therapeutic results in experimental human prostate cancer bone metastasis in nude mice treated with a combination of paclitaxel and various tyrosine kinase inhibitors directed toward PDGFR or EGFR [125–128, 251]. These experimental data have been successfully translated to clinical trials [45, 46]. Since both tumor cells and tumorassociated endothelial cells can express these activated growth factor receptors, it has not been clear whether the combination treatment targeted the tumor cells or the endothelial cells or both. We reported that multidrug resistant PC-3MM2-MDR human prostate cancer cells growing in the prostate of nude mice are resistant to systemic administration of paclitaxel. Targeting the phosphorylated PDGF-R on tumor-associated endothelial cells leads to regression of the multidrug resistant prostate cancer and inhibition of lymph node metastasis. As stated previously, regardless of sensitivity or resistance to hormones or chemotherapeutic drugs, all tumor cells are dependent on a viable vasculature for growth and survival [56, 62, 123]. Tumor cells interact with host factors in the microenvironment to induce growth and expansion of vasculature [70, 72, 121]. We have reported [126–128] that PDGF produced by prostate cancer cells growing adjacent to bone can increases and activate the expression of PDGF-R on tumor-associated endothelial cells by a paracrine mechanism. The systemic administration of imatinib inhibited the phosphorylation of PDGF-R (but not EGF-R) [128], and the combination of imatinib and paclitaxel induced apoptosis of

33 tumor-associated endothelial cells as well as PDGF-Rexpressing tumor cells. The expression of phosphorylated PDGF-R is known to activate anti-apoptotic pathways involving Akt, PI3K, MAPK, and Bcl-2 [142, 143], and treatment with imatinib can inhibit this effect. These data suggest that a major target for imatinib and paclitaxel therapy could well be the tumor-associated endothelial cell. We tested this possibility by using MDR human prostate cancer cells. The acquisition of multidrug resistance by leukemia cells [162], hepatocellular carcinoma [273], and human lung carcinoma [163] has been shown to decrease tumorigenicity and prolong tumor cell doubling time. In agreement with these reports, the in vivo growth of the human prostate cancer PC-3MM2 selected for multidrug resistance (PC-3MM2-MDR) was slower than the parental tumor cells. The ‘seed and soil’ theory [63, 193] held true regardless of the MDR phenotype. Similar to parental cells, MDR tumor cells growing in the bone (but not in the muscle) expressed IL-8, bFGF, EGF, EGF-R, PDGF, and PDGF-R. Endothelial cells of tumor-associated vessels in bone lesions also expressed PDGF-R on their surface, and treatment with imatinib and paclitaxel inhibited the phosphorylation of the PDGF-R on both tumor cells and endothelial cells. The PC-3MM2-MDR bone lesions responded to systemic administration of imatinib and paclitaxel (but not to paclitaxel administered alone), raising the possibility that imatinib could have sensitized the tumor cells to paclitaxel. Immunohistochemical analyses revealed that early apoptosis (after 2–3 weeks of chemotherapy) was mostly limited to tumor-associated endothelial cells. The multidrug resistant tumor cells were still treatment-resistant. In contrast, parental cells underwent extensive apoptosis in mice treated with imatinib and paclitaxel. The PDGF-R has been shown to regulate the interstitial hypertension and transcapillary transport of molecules [108], and the administration of imatinib to mice bearing tumors can increase the concentration of a systemically administered chemotherapeutic drug within a tumor by twofold to threefold [201–203]. Since the PC-3MM2-MDR cells were >50-fold more resistant to paclitaxel even in the presence of imatinib, the in vivo therapeutic response of the PC-3MM2-MDR bone tumors to imatinib and more so to imatinib plus paclitaxel could not have been due to changes in interstitial fluid pressure. Endothelial cells in normal tissues rarely divide, whereas 2–3% of endothelial cells in prostate cancer divide daily [4, 48]. These dividing endothelial cells should be sensitive to anticycling drugs such as paclitaxel. As stated above, the first wave of apoptosis in bone tumors from mice treated with imatinib and

34

The pathogenesis of cancer metastasis: relevance to therapy

paclitaxel for only 2 weeks occurred in tumor-associated endothelial cells, followed by apoptosis of tumor cells and ultimately necrosis. By the fourth week of treatment with imatinib and paclitaxel or imatinib alone, concurrent apoptosis of tumor cells and tumor-associated endothelial cells was observed. Without paclitaxel, imatinib may produce therapeutic effects by the blockade of PDGF-R, which serves as a survival factor [142, 143]. Therefore, targeting the PDGF-R on tumor-associated endothelial cells by imatinib and paclitaxel can produce therapeutic results in multidrug resistant human prostate cancer experimental bone metastasis.

using homogeneous therapy. In other words, it is unlikely that therapy of cancer in general and metastasis in particular can be accomplished using a single modality. Second, cancer is a chronic disease. A chronic disease must be treated chronically, i.e., by management. Third, cancer is the disease of the “seed” and the “soil”. Since the outcome of metastasis depends on multiple interactions of metastatic cells with homeostatic mechanisms in the organ microenvironment, therapy for metastasis should be targeted not only against tumor cells, but also against the homeostatic factors that helps metastatic cells grow and survive.

Conclusions

References

A current definition of the seed and soil hypothesis consists of three principles. First, neoplasms are biologically heterogeneous and contain subpopulations of cells with different properties. Second, the process of metastasis is selective for cells that must succeed in all the steps of this complex process. Although some of the steps in this process contain stochastic elements, as a whole, metastasis favors the survival and growth of a few subpopulations of cells that preexist within the parent neoplasm. Thus, metastases can have a clonal origin, and different metastases can originate from the proliferation of different single cells. Third, the outcome of metastasis depends on multiple interactions (“cross-talk”) of metastatic cells with homeostatic mechanisms, which the tumor cells can exploit. A convincing example of this principle is the establishment of tumor-associated vasculature. Tumor growth and metastasis depend on an adequate blood supply. The extent of angiogenesis within and around tumors is regulated by the balance between proangiogenic and antiangiogenic molecules. The crosstalk between tumor cells, normal cells, leukocytes, and stromal cells occurs through growth factors, growth factor receptors, and cytokine signaling, all of which are released and act on cells within the tumor microenvironment. These interactions enhance the establishment of angiogenesis that in turn regulates the proliferation and survival of metastatic cells. Interruption of one or more of these interactions can lead to the inhibition or regression of metastasis. Because the growth of primary tumors is often controlled with surgery and/or irradiation, antiangiogenic agents may be most beneficial to prevent widespread and metastatic disease. To succeed, several principles must be considered. First, it is essential to remember that all neoplasms are biologically heterogeneous. One cannot treat a heterogeneous disease

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3

Developmental therapeutics and the design of clinical trials ROBERT K. OLDHAM

New techniques in biotechnology and the use of biologicals in cancer treatment have made it apparent that developmental therapeutics for biotherapy are different from standard drug development. Millions of chemicals have been screened as anticancer agents, but less than 75 have reached the clinic as commercial pharmaceuticals. Perhaps 20 of these drugs can be classed as moderately effective; the rest are only marginally useful and all can be highly toxic. With the discovery of monoclonal antibodies and conjugates thereof, the exploitation of bioengineering to produce purified, characterized lymphokines/ cytokines and other biologicals, and further information on the mechanism of action of these natural molecules, the rate of development for biological therapeutics has risen dramatically. Because of their selectivity and with the implicit biological diversity of cancer, new approaches are needed to efficiently bring biotherapy to the clinic. Previously, there have always been fewer promising agents (drugs) to test than patients who needed new approaches. In fact, the drug development paradigm has been a slow and laborious, with each drug taking some 8–12 years to gain commercial approval, at a cost of more than US$500 million per drug. Taxol, a drug approved for general use, entered clinical trials in the late 1980s and only became generally available in 1998; taking more than a decade to pass through our current system of drug development. On average, fewer than four new chemotherapy drugs per year have been approved for general use by oncologists. This expensive and slow paradigm reflects both the toxicity and marginal effectiveness of chemotherapeutic drugs as well as a bureaucratic regulatory system more fearful of criticism over toxicity than a willingness to pursue opportunities for seriously ill patients [33, 70, 71, 83]. More recently, a large number of biological substances and targeted therapies have been making their way to the clinic, increasing the difficulty of decisions as to the order and amount of preclinical and clinical testing. Hundreds of biologic agents are in clinical testing with more agents to test than there are easily available patients for testing. The current system for drug development is not sufficiently flexible, and needs major changes in direction and technique to optimize clinical testing and

R.K. Oldham and R.O. Dillman (eds.), Principles of Cancer Biotherapy, © Springer Science + Business Media B.V. 2009

speed the translation of new approaches to patients [6, 8, 17, 21, 22, 24, 30, 32, 33, 41, 55, 57, 61, 76–82, 90, 91, 94, 98, 99, 103]. Although the concept of biotherapy is not new, the use of recombinant genetics to produce highly purified biologicals as medicinals dates from about 1980 [31]. A member of the alpha-interferon family was the first biological produced by recombinant methods to be used as an anticancer medicine in humans [103, 105]. In the 25 years since the first alpha-interferon molecule was prepared by recombinant methods, a large number of recombinant molecules (lymphokines, cytokines, monoclonal antibodies, growth and differentiation factors, angiogenesis factors and cell receptors) have become available or are being prepared for testing in the clinic (see Chapter 8). Historical aspects in the development of immunotherapy have been reviewed [12, 75]. Before the 1980s, the term immunotherapy was considered to be synonymous with biological therapy by most investigators. However, it is now clear that there are many biological approaches that may affect cancer growth and metastases, yet are not within the immune system. Thus, biotherapy now refers to agents derived from biological sources and/or the use of agents that affect biological responses. The term “biologicals” describes agents extracted from or produced from biological materials. With biotechno-logy, this involves the use of recombinant genetics to isolate the gene, transfect it into an appropriate producer organism, and then the isolation and purification of the protein product. Genomics and proteonomics are processes to yield the “code” to eventually prepare biological therapy by chemical synthesis. The types of materials that alter biological responses for the benefit of the patient have been called biological response modifiers (BRM). The use of biologicals and BRM in the treatment of cancer and other diseases is biotherapy (a term I initially used in 1984 to describe this fourth modality of cancer treatment) [84, 85]. As is always the case, nomenclature can be confusing and terms may be variously defined by different individuals. In the broadest sense, biotherapy includes blood products, transplanted organs, bone

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42 marrow/stem cell transplants, antibiotics (often derived by extraction from biological organisms and later synthesized), and a variety of other agents and approaches. However, in this chapter, the focus will primarily be on BRM, recombinant biologicals and gene engineered or activated cells being developed as medicinals. Immunotherapy has had a checkered past. Kari Cantell said it well when describing some of the early interferon research: “Much second class research was carried out with third class preparations slightly contaminated with interferon” [5]. In addition, there have been questionable approaches (“alternative medicine”) used by certain practitioners, and even frank quackery by others who purported to deliver “immunotherapy” for cancer patients. Even for the very dedicated scientists who have explored immunotherapy over the past several decades, there have been many pitfalls relating to the purity of their preparations, the source of the materials, and the assays and techniques by which their measurements were made. Huge expenditures by the National Institute of Health (NIH) in the area of molecular, biological and viral oncology over the last 40 years led, in large part, to the current technology of “genetic engineering.” In contrast to previous attempts to develop biological therapy, we now have techniques available that can isolate a single gene and use it in a bio-manufacturing process to produce absolutely pure proteins identical to those found in the body. It is with these techniques that thousands of biological compounds and their synthetic analogs and components are being developed. Many of these biologicals will be candidates for use as medicinals. A major value of the research done thus far with interferon is that it may be viewed as a model for the development of other biological substances as medicinals [61, 76]. Interferon research in particular and biotherapy in general must be seen as a new challenge in developmental therapeutics, with agents to be tested appearing at a still accelerating rate. No longer can the historical drug development paradigm be used [55]. The methods used by the Food and Drug Administration [11], National Institutes of Health, and the pharmaceutical industry in the development of drugs must be drastically changed to a new paradigm that will accommodate the realities relevant to biologicals and their use in medicine [33, 55, 71, 82, 86, 87, 90].

Drug Development The current process of drug development involves a very long and costly set of procedures [33, 55, 80]. This includes the initial concept, extraction or synthesis,

Development therapeutics and the design of clinical trials formulation, documentation of biological activity and purity, early studies in the laboratory and in experimental animals to determine the mechanism of action and toxicity, and compilation of all the preclinical data into an investigational new drug application (INDA). It costs millions of dollars for a pharmaceutical company to file a single INDA. During preclinical development, companies make projections about the potential market size and profitability as justification for the investment. These projections are difficult, and are most accurate when related to an existing drug in a known market. It follows that predicting the market size for a totally new agent or new approach is much more difficult. Subsequent to the INDA, further preclinical work on the mechanism of action and preclinical toxicology of a new drug is done. In addition, early-phase clinical studies are begun to determine biological activity in humans. Although the process is not uniform for all classes of pharmaceuticals, studies are termed “phase I” when the dose of the drug is escalated to determine its biological activity and its toxicity. Based on the preclinical toxicology information in small animals and sometimes in primates, projections are made for the starting dose in humans. A low starting dose is selected to avoid severe toxicity in the initial patients in a phase I clinical trial. The rate of dose escalation in these trials and the acceptability of toxic side effects vary with the population at risk and are related to the seriousness of the disorder and the treatment alternatives for the patient. Because the starting doses are very low and because of phase I trial design, using different patients at each dose level (to avoid cumulative toxicity), the chance of a therapeutic effect for the initial patients receiving a new agent is nearly zero. It is axiomatic that the phase I trials are designed to accumulate the maximum amount of clinical information with the least toxicity. Generally, the end point for phase I trials is the achievement of a maximum tolerated dose (MTD), the dose at which side effects are unacceptable to the physician and the patient within the design of that clinical trial. These trials may require 100–500 patients to reach agreement on the MTD in two to four schedules and routes of administration. In addition, pharmacokinetic and metabolism studies are done as part of Phase I. Once the MTD is established, subsequent investigators can be reasonably assured that the upper limit for the dose to be administered is well defined and that therapeutic activity will not be missed in later phase trials because of sub-therapeutic doses. Phase II studies are then conducted to determine the therapeutic activity of the new drug in various types of cancer. Patients with specific cancers are selected in order that these trials can be conducted in reasonably

Robert K. Oldham uniform patient groups. Based on the preclinical information and the clinical toxicology studies, as well as pharmacokinetic and metabolic considerations, therapeutic doses are selected that represent the investigator’s “best guess” as to the therapeutically active dose range. In addition, schedule and route of administration must be considered. In phase II trials, it is often necessary to administer an agent at different doses by different routes (e.g., intramuscular, intravenous, subcutaneous, oral) and on different schedules (e.g., once a day, three times a day, 24-h infusion, etc.). Phase II trials are considered complete when a substantial body of data exists concerning the therapeutic activity of a new drug with reference to best dose, route of administration, and schedule for therapeutic efficacy. If one accepts classical criteria for cancer subtypes, over 100 histologic types of cancer exist. If one assumes a standard statistical criterion of at least 14 patients treated to look for one response so as not to miss a 20% response rate, then a new drug would need to be tested in a minimum of 1,400 patients by each schedule and route to assure clinicians of its inactivity in each tumor type; and if one assumes three routes and five schedules must be tested, over 21,000 patients would need to participate just to prove a specific drug ineffective. Such studies demonstrating inactivity sacrifice patients “for the good of the system.” Such FDA-mandated testing stretches ethical standards to the limit. Following the completion of adequate phase I and phase II trials, phase III trials to compare a new agent with standard treatment are conducted. The extent of phase III trials depends on the treatment alternatives available. In the case of diseases for which other therapies are effective, such trials may need to be extensive, controlled, and employ random designs, sometimes with blinding of the study both to the physician and to the patient. Such phase III trials have produced an enormous literature on the subject of ethics, trial design, end points, and the assessment of efficacy [9, 23, 28, 38, 71, 82, 91, 92, 95, 106, 107, 110]. If, in such phase III trials, a new agent proves therapeutically superior without unacceptable side effects, in comparison with or in addition to “standard” therapy, the new agent is very likely to receive FDA approval. Since “standard therapy” is often only marginally effective, large randomized trials proving a new treatment is significantly (usually marginally) better means most patients derive little or no real therapeutic benefit in the trials. Where efficacious therapy with an approved drug is not available, fewer phase III data may be needed to gain approval, and patients with advanced cancer are often randomized against best supportive care or a placebo. This latter practice also stretches ethical practice to the limit [47, 71, 95].

43 Most drugs fail at the phase I/II level, being either too toxic or inactive. Drugs that complete phase II trials with acceptable toxicity and activity in at least one cancer have a greater than 50% chance of successfully passing phase III with eventual approval by the FDA. For this reason, it has been suggested that drugs should be approved for general use at the end of phase II to speed up the process of bringing new drugs to the clinic [47, 83]. The final step in the commercial development of a new drug requires the filing of a new drug application (NDA). All the information available under the INDA and all of the data from the phase I, II, and III testing are made available to the FDA for review and consideration. Only the FDA is authorized to approve a new drug for sale in the United States; this usually involves defining the drug dose, route of administration, schedule, toxicity, and therapeutic activity in specific diseases (indications). After FDA approval, the company may begin to advertise the agent for the approved use only. Uses outside the specific FDA approval indication(s) are termed “off label” uses. Perhaps 50% of all cancer drug therapy is “off label” use by physicians but the pharmaceutical industry is specifically prohibited by the FDA from providing clinical trial information to clinicians on “off label” drug activity. The long time and enormous expenditure in the current drug development system have been justified as being in the public interest inasmuch as the FDA requires extensive testing so active agents with reasonable toxicities (safe drugs) can be made available as medicinals. The definitions of reasonable toxicity and of therapeutic efficacy have been the subject of considerable debate, both in the general sense and relative to specific drugs. Often, the long-term effects and/or toxicities of approved drugs have prompted secondary revisions in the regulatory process, further lengthening and expanding the steps necessary for new drugs to be approved. This process would seem to be totally in the public interest. However, when viewed in another context, the process clearly has some exclusionary aspects [47, 66, 71, 83]. The FDA defines a single standard for drug development in the United States. Marked differences exist between countries as to a regulatory body’s role in drug development, and there are marked differences in the number of drugs available to patients in different countries. The regulatory agency of each country has its own view of what is ethical and in the public interest (different standards, different ethics). Thus, there can be reasonable debate on which rules are the best for the development of new drugs. Finally, it is obvious that the long development times and the huge expenditures required effectively

44 restrict competition in the area of drug development. Such restriction of competition gives major pharmaceutical firms a virtual monopoly on drug development. Although most pharmaceutical firms would undoubtedly say that they would prefer a lower cost for drug development, one may view the process as being in their best interests (barrier to entry), since it restricts competition from smaller firms that cannot marshal the resources to carry out these extensive and expensive studies [47, 55, 71, 79, 80, 82, 83]. Thus, our drug development paradigm is highly restrictive. It has worked reasonably well over the past 50 years only because of public acceptance of a conservative regulatory structure and because of the small number of relatively toxic drugs that actually were available for clinical testing. The advent of biologicals has placed great pressure on this paradigm. Indeed, the effective development of biologicals will require changes in the regulations and policies for the development of new pharmaceuticals [47, 55, 71, 78, 82, 90, 91].

Biologicals and BRM Development A process of developmental therapeutics similar to that described for chemicals (drugs) is now being applied to biopharmaceuticals. Historically, for new biologicals, the information on composition, formulation, and purity was often imprecise. In contrast to drugs, which are generally small, synthesized molecules with a chemical definition that is quite straightforward, biologicals have had a more variable developmental process. Preparing a biological for testing has involved extraction from a microorganism, from a fraction of a cell culture, or from a tissue (complex chemical mixture). Such extractions yielded complex mixtures with defined biological activity, where the precise chemical composition was incomplete or unknown [75]. Biotechnology is now making available a range of biologicals (lymphokines/cytokines, monoclonal antibodies, antigens, growth and maturation factors) that are pure and as well defined as small molecule drugs [55, 57]. Market size, the potential for profit, the long development period of preclinical and clinical testing, and the large investment necessary to develop new therapeutic agents have defined the scope of drug development. The development and testing of new biologicals will require extensive procedural and regulatory changes if we are to be successful in bringing these agents to the clinic quickly and efficiently [47, 55, 83, 92].

Development therapeutics and the design of clinical trials Perhaps the most cogent example relates to the development of monoclonal antibodies. For years there have been sufficient data to indicate that monoclonal antibodies are eventually going to be very useful both diagnostically and therapeutically [10, 26, 36, 40, 51, 52, 54, 62, 73, 94, 102, 103]. Now there are therapeutic antibodies approved for colon and breast cancer, lymphoma and leukemia with hundreds more in testing [8, 19, 21, 41, 94, 103]. The problems of developing new monoclonal antibodies for therapy is quite different from those previously encountered with drugs. The most striking of these problems relate to market size. Generally speaking, one looks at the population afflicted with a particular disease and makes the presumption that a new drug will be active in a certain percentage of patients with that disease. The percentage is often reasonably high and allows the market forecast to be applicable to a substantial number of patients. For example, the number of patients at risk per year with lung cancer is reasonably well defined, and once some evidence of clinical activity of a new drug in lung cancer is available, the market size for that drug can easily be calculated. For monoclonal antibodies, the situation is very different. At the extreme is anti-idiotypic monoclonal antibody therapy. It has been demonstrated in preclinical models and, to a more limited extent, in humans that anti-idiotypic antibody can be made to the specific tumor antigen (idiotype) of the neoplastic cell [11, 29, 39, 51, 65]. Such antiidiotypic antibodies or idiotype vaccines derived therefrom have proven useful in controlling the growth of the B-cell lymphomas. However, a critical problem for the development of these reagents is market size; here, it is the individual patient with that particular neoplasm. Thus, the market size might be as small as one. Even with some cross-reactivity, many of these antibodies or vaccines are expected to apply only to very small populations, much smaller than even the “orphan drugs” envisioned by FDA policy. Obviously, most pharmaceutical companies will never develop these kinds of “individualized or personalized medicines” under existing FDA guidelines [37, 71, 93, 94]. A less extreme example relates to the development of monoclonal antibodies for tumor-associated antigens that may be restricted to subpopulations of cancer cells [19, 20]. Considerable heterogeneity exists within any one histologic type of cancer (between patients) and even within a single patient’s cancer [54, 64]. It may be that for any one cancer only a small portion of the patient population will have a particular antigen or array of antigens on the cancer cell surface. Therefore, a monoclonal antibody might be applicable only to 1%, 5%, or 10% of the patients with a particular type of neoplasm.

Robert K. Oldham Because of clonal heterogeneity, both between patients and in different clones within individual patients, there may be the need to use multiple antibodies (“cocktails”) in treatment [1, 46, 69, 90]. Given the over 100 histologic types of cancer and the heterogeneity within each cancer type, market calculations may define very narrow applications [55, 63]. These considerations restrict market size to a level unapproachable given drug development costs under the current drug development paradigm. Perhaps less obvious, but equally problematic, will be the development of other biologicals for treatment. For lymphokines and cytokines, there is already evidence of both antigen-specific and non-antigen-specific signals that may have growth regulatory effects (See Chapter 8). One can visualize, with an antigen-specific lymphokine, how the signal may relate to the specific antigen and be applicable in a single patient, or a very few patients (e.g., transfer factor, IgE suppressor factors). Less restricted, but still highly restricted compared to standard drug development, is the development of non-antigen-specific lymphokines (e.g., interferons, lymphotoxins, interleukins), which are active against certain classes of cells, thus making them clinically applicable only in selected populations [55, 57].

Interferons: the Early Model Interferons represented early models for new biological approaches in cancer treatment [57, 61, 76]. “Natural” extracted interferons from stimulated white blood cells were used in initial clinical trials. The low purity, lotto-lot variation, and expense of stimulating leukocytes to produce interferon, along with the difficulties in the extraction and purification methods, limited the clinical use of these materials. These preparations were typical of early forms of nonspecific immunotherapy [5, 75]. With the advent of increased interferon availability through recombinant genetics, and with the very high purity (greater than 99%) of these preparations, extensive trials were completed for alpha-interferon preparations which led to the approval of alpha-interferon as the first genetically engineered anticancer biopharmaceutical [42, 76]. The design of phase I biotherapy trials should differ markedly from those for drugs. The dose-response curve for these agents may be very broad (and sometimes multiphasic), with peak effects at different doses for each system responding to the biopharmaceutical. For example, the immunomodulatory activity of alpha-interferon can be seen at very low doses, whereas the antiproliferative activity appears to be more reproducible at higher doses. In biotherapy, there is a need to measure biological

45 responses in the context of the clinical trials [37, 48, 50, 62, 105]. Since one may be administering the biological to stimulate a particular biological response, for which the dose-response curve may not be known a priori, one must perform studies with pharmacokinetics to assay serum availability, and also measure the desired biological effects to determine the optimal dose at which it might alter a particular biological response (optimal biological response modification, OBRM). Finally, schedule and route of administration have already proven important [42, 76]. The pharmacokinetics after intravenous and intramuscular administration differ for the alpha interferons [43], and there has been a variable lack of absorption of intramuscularly administered beta- and gamma-interferons [34, 74]. Thus, the proper design of phase I biotherapy trials must take into account appropriate measurements of bioavailability, pharmacokinetics, biological response modification, and toxicity, all in the context of escalating doses, to determine the dose– response curves for each of these properties. Responses to biologicals vary substantially between patients, and escalating doses in individual patients can yield valuable data without subjecting patients to “tests” with no therapeutic opportunity. Like most biologicals, the interferons may act through a diverse set of biological mechanisms. Cell surface receptors for their activity exist, and responses to the administration of biologicals are somewhat predetermined by the condition and biological receptor repertoire of the patient. This situation is in direct contradistinction to that of drugs, in which a totally new chemical is often administered to a patient in whom no standard biological response mechanism existed a priori. Thus, biologicals may be viewed in their physiologic role of correcting immunodeficiency states, as well as in the pharmacologic role of augmenting host responses and perhaps having direct antitumor effects. Clearly, it was rational to test chemical drugs to MTD and to treat just below this dose in a “kill or cure” approach to cancer therapy. Current evidence suggests the OBRM dose and the MTD dose should be determined in cancer biotherapy to properly design effective therapeutic trials. Once determined, the design of therapeutic trials for biotherapy may differ greatly from classical chemotherapy studies.

Biotherapy Trial Strategies Many strategies exist or can be envisioned for conducting clinical trials with new biologicals. Some of these strategies have already been utilized in the early-phase

46 testing of the interferons [4, 27, 59, 60, 103]. Two underlying principles are apparent wherein biologicals and drugs differ: when biologicals are administered, patients already have physiologic mechanisms and receptors that respond to them; and biologicals are derivatives of natural products of the mammalian or human genome, and may be expected to have less acute and chronic toxicity than drugs at similar biologically effective doses. However, when high doses are used to exploit a certain action of a biological, acute and chronic toxicities may appear. These two considerations will not necessarily apply to those BRM that are well-defined chemical entities and behave more in the manner of drugs with regard to toxicity.

Empirical Clinical Testing In many early interferon trials a set dose of a “natural” interferon was given to a variety of patients with neoplastic or viral disorders [57, 62, 76]. The chosen dose was the one expected, based on preclinical information or other clinical data, to be relatively nontoxic and yet to have sufficient biological effects to be therapeutically active. In this context, most early clinical trials with leukocyte interferon preparations were conducted at doses under five million units per day, although it became apparent later that doses up to 60 million units per day could be tolerated by the patients for a certain number of days. These trials were conducted as preliminary feasibility trials or pilot phase II trials to gain some information on the biological effects of the interferon preparations. Much of the information derived from them was anecdotal, but they yielded preliminary information about the biological effects and toxicities of alpha-interferons. Now that the development of biotherapy is proceeding more rapidly, other strategies are to be preferred.

Escalating dose Trials Using Different Groups of Patients Phase I trials for biologicals have been modeled after the standard phase I trial design used for drugs. Groups of patients (usually three to five) are treated with a particular dose of a new biological with a single route of administration [103]. These trials are begun with very low (sub-therapeutic) doses based on preclinical information, and the schedule is designed to increase the dose gradually to levels where toxicity occurs [55, 74]. Generally, new patients are entered at each higher dose level after all the patients have been entered on the previous dose and have received at least several treatments. In this way, each patient group provides toxicity

Development therapeutics and the design of clinical trials information before the higher dose is initiated in new patients. By utilizing different groups of patients at each dose level, cumulative toxicity for any one patient is avoided. Patients receive a predetermined number of doses at each dose level and then are followed. During this type of study, biological response modification and clinical toxicity are assessed [4, 48–50, 53]. Pharmacokinetics are also done in selected patients, so that the bioavailability can be determined [103]. The dose-escalation scheme can be a modified Fibonacci series, or some variation of this classical doseescalation method. The considerations involved in doseescalation methods dictated by drug toxicities do not necessarily apply to biologicals. Thus, some inves-tigators have used much faster dose escalations for biologicals. Often, the dose escalation schedule is rather empirical, the one selected being based on the best guess of the investigators involved. This phase I drug development strategy is based on historical data using new drugs and has several potential disadvantages. Since each patient receives only a particular dose for a defined period of time, it is highly likely that a large percentage of the patients will receive sub-therapeutic doses if antitumor effects are observed only at higher dose levels. Early clinical trials with biologicals have not produced severe cumulative toxicity. This clinical trial strategy, which was designed to avoid the cumulative toxic effects seen with drugs, is inappropriate for biotherapy. Since tolerance (tachyphylaxis) may develop for some effects (fever), entering different patients on progressively higher doses may expose patients to avoidable acute toxicities. Finally, the patients who are exposed to only one dose level are not individually assessed for biological response or for antitumor response over a wide dosage range. This could give them a greater opportunity for optimization of biological and therapeutic response [59]. This may be particularly true in the very-early-phase trials in which biological effects and antitumor effects are totally unknown.

Escalating-dose Trials within Individual Patients An alternative clinical trial strategy is the use of an escalating-dose trial within individual patients [58, 59, 74, 94]. There are several variations of this theme, but each involves starting patients at a low dose and escalating the dose in each patient to determine the biological response modifying effects and toxicities over a broad dose range. This clinical trial strategy is very conservative of patients, in that studies in small numbers of patients can give a large amount of information over a

Robert K. Oldham broad dose range in each biotherapy trial. In the context of this dose escalation, pharmacokinetics and biological response modification can be measured in each patient. It is important in such a clinical trial strategy that bioavailability studies be done and the information be available concomitantly with the study. Appropriate “wash-out” periods between doses can be utilized so that the administered biological does not circulate in increasing quantities as the trial continues. For example, with the interferon trials, one can determine the serum level after each dose and administer the next dose when interferon is no longer detectable. This allows the patient to avoid the possibility of severe acute toxic effects based on cumulative serum levels. Obviously, biological and therapeutic effects can still be cumulative and must be monitored by appropriate clinical and biological measurements throughout this type of trial. This strategy offers the possibility of low-, medium-, and high-dose therapy for the individual patient in the context of a single clinical trial. It maximizes the opportunity for the investigator to learn the optimal biological response modifying dose and the toxic dose, and perhaps to gain therapeutic information, all in the context of a single trial. Theoretically, rational maintenance regimens could be designed based on the observations made during the escalating dose trial for each patient. For monoclonal antibody, this strategy may be particularly important, in that the delivery of the monoclonal antibody to the tumor site may be the most important consideration in developmental therapeutics [25, 36, 54, 63, 94]. Giving a low dose and then progressively higher doses to the same patient, with subsequent determination of antibody localization, is a useful way of determining the correct dose for delivery of the antibody and/or its conjugates to the appropriate target organ in the context of a toxicity study. This trial design rationally ties targe-ting (delivery), toxicity, and therapeutic effects together in combined phase I/II studies in a manner perfectly appropriate for antibody and conjugates, in direct contradistinction to drug development [2, 3, 17, 18, 59, 96, 98]. In studies that employ escalating doses, a useful variation is to enter individual patients for a limited number of doses. With this trial design, the initial three or four patients may enter at the lowest dose level and progress through dose level 4 or 5, at which time the second group of patients may be entered at dose 3 and escalated upward in a manner that allows the second group to follow the first group in dosage escalation toward the MTD. A third group may be entered at dose level 6 and so on. This strategy allows one to avoid administering the full dose range to all patients, avoids most cumulative toxicity, and helps avoid sub-therapeutic doses.

47 It increases the patient’s therapeutic opportunity without undue risk of toxicity. This strategy is ethically preferable, most acceptable to patients and quite easy to describe in the informed consent [71].

Schedule In the initial clinical trials, the schedule of administration is generally empirical or is based on preclinical observations. Once there is information on bioavailability, different schedules of administration can be designed rationally. Both the bioavailability of the molecule and its biological effects are relevant. There are some biologicals that have a very short serum half-life (measured in a few seconds to minutes) but may produce much longer lasting biological effects. It may be useful to compare the biological effects and toxicities of biologicals when administered under conditions of rather constant exposure as against intermittent exposure in order to gain a preliminary sense of which schedule is most relevant [35, 74, 108]. Once these data are available, schedules of administration for phase II studies can be more rationally designed.

Route The route of administration and delivery of biotherapy to selected target organs is critical. There are data indicating that certain biologicals are inactivated, or poorly absorbed, when given intramuscularly or subcutaneously. For such biologicals intravenous administration is quite important. When data on bioavailability are not known from preclinical models, it is probably important to conduct early clinical trials with the intravenous route, since such trials can provide early data on serum pharmacokinetics and provide the best opportunity for broad biodistribution of the administered biological. It is important in phase I biotherapy trials to determine if serum levels are measurable and if biological effects are seen in the presence or absence of serum levels for agents given by any route of administration. There may be instances in which second mediators are involved, with useful biological effects occurring without apparent serum bioavailability. Effusions may contain both tumor cells and reactive immunological elements, in which case the administration of a biological into a restricted space such as the thoracic or peritoneal cavity might be appropriate. There are indications that certain immunomodulators may be effective in this setting [56]. Patients with tumors that remain confined to a single compartment for a prolonged period of time may offer unique opportunities for biotherapy [72, 88]. Ovarian cancer, with its propensity for

48 remaining localized in the abdominal peritoneum, may be ideal for evaluating the antitumor activity of cells [72], biologicals, and BRM in a relatively closed space. Interleukin-2 with activated cells is active when infused into selected anatomical sites and visceral cavities where tumor is present [44, 88]. It seems clear that targeting of monoclonal antibody and its conjugates may be improved by selective organ or region perfusion or infusion. Considerations for the design of early-phase trials in these spaces differ markedly from those using systemic administration.

Patient Selection Historically, patient selection for phase I drug trials has been broad, and patients bearing many tumor types have been entered into the trials. While this may be appropriate for drugs, and for selected lymphokines that act broadly on immunological responses or may act in a general antiproliferative manner across a broad spectrum of tumors, it is not appropriate for those biologicals that act more specifically. Thus, a monoclonal antibody that has been specifically designed to recognize a particular type of cancer can be appropriately tested only in patients with that type of cancer [45, 59]. In vitro determinations of specificity and activity may play a greater role in the selection of the patient populations for phase I biotherapy trials. The escalating-dose phase I trials, with the dose being escalated within individual patients, preselected on the basis of in vitro specificity and/or activity, appear to be most relevant and efficient for determining the distribution, biological effects, and toxicity of monoclonal antibodies. The same considerations may apply to certain lymphokines/cytokines, in which case selection as to activity may be on an individual patient basis. These trials may be more appropriately termed phase I/II trials. In fact, given the heterogeneity of cancer, both with respect to the specificity of recognition by antibody and with respect to activity by immunoconjugates and lymphokines/cytokines, in vitro determinations may play a major role in clinical trial design. Biological systems are diverse, and single patients may require specifically designed treatments when specificity and activity restrict the applicability of biotherapy. The design of early-phase [biotherapy] clinical trials needs to be radically changed [55, 58, 71]. There has been much speculation concerning the types of patients most appropriate for trials with biologicals. Many believe that these agents should be tested primarily in the adjuvant setting, where the tumor burden is quite small and the biological responses to be modified are still healthy [75]. Although this sort of strategy may be optimal, it is also prohibitively expensive

Development therapeutics and the design of clinical trials and very difficult to design. It would be virtually impossible to investigate a significant number of biologicals in phase II or III trials utilizing this type of clinical trial design. The number of patients required for trials in the adjuvant setting is enormous, in that the recurrence rate cannot be precisely predicted without a concurrent random control group. The lack of certainty for recurrence prompts ethical questions in designing phase II or III trials for these patients, since dose, route, schedule, OBRM and therapeutic efficacy have not been determined. It has now been well demonstrated that certain biologicals have activity in patients with bulky and resistant disease [4, 7, 13–16, 27, 57, 58, 67–68, 76, 89, 94, 108]. Even though biotherapy may be more effective, as chemotherapy and radiotherapy are, when the tumor burden is small, that does not mean that it is totally ineffective in patients with bulky disease. Given the large number of biologicals available to the clinic, there is a need to develop methods of rapid clinical testing in early-phase trials, and this will necessitate testing in patients with apparent disease [55, 57, 75, 108]. Indeed, the initial clinical trials with interferons, interleukins, monoclonal antibodies, and cellular therapies generally selected such patients with a good performance status and a reasonably functional immune system. These patients have shown evidence of biological response modification, and they have shown antitumor responses. Thus, such clinical trials can be used as indicators for biological activity of new agents [57, 67, 94].

Future Prospects Developmental therapeutics for biotherapies is well underway. From the inception of this field in 1980 to the present, a great number of new approaches have become available in biotherapy [42, 57, 71, 94, 108]. Unlike the field of drugs, where very large numbers of compounds are screened in a preclinical testing program and very few reach the clinic, a much higher percentage of the biologicals selected rationally and tested actually go on to clinical trials. This is due to selection based on the known physiologic activities of biologicals as opposed to random testing of chemicals in drug development. The strategies for these clinical trials need to be cost efficient, advantageous to the patient, ethical [71, 97], and scientifically interpretable to the clinician/scientist [64, 75]. Phase I strategies are most important in that they give the initial leads on which the design of early phase II and III trials are based. With proper selection clinical trial strategies, new biologicals can be more effectively screened and evaluated as potential anticancer agents [59, 71].

Robert K. Oldham

49

Biotherapy is the fourth modality of cancer treatment [57, 58, 84, 85, 86, 94]. These agents and this technology have far broader applications in medicine than cancer therapeutics. Given the large number of cloned biologicals and the virtually unlimited number of recombinant molecules that can be produced therefrom, along with unlimited variety of monoclonal antibodies that can now be produced, it is apparent that the coming years will produce new challenges for those involved in developmental therapeutics [33]. This tremendous expansion of agents and approaches available for cancer therapeutics will increase our opportunities and amplify the complexity in the preclinical evaluation of these agents and their translation to the clinic. The numbers of biologicals to be evaluated and their inherent selectivity call for new methods in the selection of the “most likely to succeed.” Additionally, the methods used historically by pharmaceutical manufacturers and the regulations established by governmental agencies for approving anticancer drugs are inhibitory to the rapid development of biotherapy [47, 55, 83]. Novel approaches will be necessary if these biologicals are to be effectively and rapidly translated to clinical trials [47, 55, 64, 95]. Alpha-interferon, IL-2 and monoclonal antibodies and their conjugates have anti-tumor effects, even in patients with bulky disease. This finding is likely to

emerge for other forms of biotherapy [42, 57, 59, 67, 71, 104, 108, 109]. This should give pause to those who believe the immunological dogma that biotherapy can only be active in minimal residual disease. Like chemotherapy and radiotherapy, biotherapy may be more active with lesser tumor burdens, but current data indicate that activity and hence selection of compounds for further study can be assessed in patients with advanced disease. The potential for the use of biotherapy is great. We are in a new era in cancer therapeutics. To develop biotherapy, we must begin to approach this new era in pharmaceutical research and to contemplate novel methods for the efficient and timely development of biotherapy rather than simply continuing in old paradigms (Fig. 1) [33, 55, 71, 82, 83]. Some would still say no new treatment can be judged without randomized, clinical trials. Such trials are valid in searching for small differences between a new treatment and an ethically acceptable control (standard treatment) [101]. Whether placebo controls are ever acceptable in cancer treatment is debatable; however, pilot studies without controls can be very useful even to the point of defining efficacy if the treatment effect is large and obvious. A recent and innovative “n of 1” trial also bears consideration in trials of biotherapy [35].

Preclinical Testing (in vitro/animal model) Phase I Clinical Trial Phase II Clinical Trial Phase III Clinical Trials

Unresponsive or poorly responsive neoplasm; Less than partial responses Partial responses to newer active single agents Increased partial responses and rare complete responses To some single and combined agents

Figure 1. Although radically different, chemotherapy and biotherapy share a systemic approach to cancer treatment. For those cancers that chemotherapy has been able to cure, reaching that goal required a sequence of steps in which new and increasingly more effective agents and combinations of agents had to be identified. Biotherapy can be expected to travel a similar path

Increased complete responses to more effective Combination therapy Increased survival in complete responders with or Without continued treatment during remission A portion of long-term survivors are cured

50

Development therapeutics and the design of clinical trials

We must not just continue to simply do what has been done historically [71]. Development therapeutics for biologicals certainly represents the kind of science Lewis Thomas described: It is hard to predict how science is going to turn out, and if it is really good science it is impossible to predict. This is in the nature of the enterprise. If the things to be found are actually new, they are by definition unknown in advance, and there is no way of telling in advance where a really new line of inquiry will lead. You cannot make choices in this matter, selecting things you think you’re going to like and shutting off the lines that make for discomfort. You either have science or you don’t, and if you have it you are obliged to accept the surprising and disturbing pieces of information, even the overwhelming and up heaving ones, along with the neat and promptly useful bits. It is like that.

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