- Email: [email protected]
Role of angiogenesis in tumor growth and metastasis
Role of Angiogenesis in Tumor Growth and Metastasis Judah Folkman Angiogenesis is required for invasive tumor growth and metastasis and constitutes an important point in the control of cancer progression. Its inhibition may be a valuable new approach to cancer therapy. Avascular tumors are severely restricted in their growth potential because of the lack of a blood supply. For tumors to develop in size and metastatic potential they must make an “angiogenic switch” through perturbing the local balance of proangiogenic and antiangiogenic factors. Frequently, tumors overexpress proangiogenic factors, such as vascular endothelial growth factor, allowing them to make this angiogenic switch. Two strategies used in the development of antiangiogenic agents involve the inhibition of proangiogenic factors (eg, antivascular endothelial growth factor monoclonal antibodies) as well as therapy with endogenous inhibitors of angiogenesis, such as endostatin and angiostatin. Therapy with endogenous angiogenic inhibitors such as endostatin and angiostatin may reverse the angiogenic switch preventing growth of tumor vasculature. Preclinical studies have shown that endostatin effectively inhibits tumor growth and shrinks existing tumor blood vessels. Phase 1 clinical trials of endostatin and angiostatin are ongoing, and preliminary results show minimal toxicities. Semin Oncol 29 (suppl 16):15-18. Copyright 2002, Elsevier Science (USA). All rights reserved.
T
HE GROWTH of new capillary blood vessels, or angiogenesis, is required for tumor growth and metastasis, and thus constitutes an important control point in the progression of cancer. For this reason, tumor angiogenesis has become the focus of extensive investigation,1,2 and its inhibition is emerging as a rational and potentially valuable new approach to cancer therapy. Antiangiogenic therapies in development target various factors implicated in tumor angiogenesis, and all of these therapies inhibit endothelial cell proliferation (or induce endothelial cell apoptosis) and migration into the tumor bed, which is the initial step in capillary formation. Antiangiogenic agents may be particularly effective in the context of combination therapy because they enhance the delivery and therapeutic efficacy of treatment modalities that directly target cancer cells (eg, radiation, chemotherapy, monoclonal antibody therapy). THE ROLE OF VASCULATURE IN CANCER GROWTH
Autopsy studies in accident victims show that the incidence of in situ tumors, particularly those Seminars in Oncology, Vol 29, No 6, Suppl 16 (December), 2002: pp 15-18
of the breast, prostate, and thyroid gland, is substantially higher than the documented prevalence of cancer,3 indicating that tumor metastasis is a relatively infrequent phenomenon. It has been proposed that the low metastatic activity of in situ tumors may be related to the fact that they have not acquired an angiogenic phenotype (ability to recruit vasculature), because tumor growth and metastasis have been shown to require increased expression of angiogenic factors and increased vascularization.4-6 In other words, although in situ tumors may replicate rapidly, their growth and metastatic properties are severely restricted by the absence of adequate blood supply. THE ANGIOGENIC SWITCH
The change from a quiescent to an invasive phenotype is invariably accompanied by the acquisition of angiogenic properties (“angiogenic switch”) and vascularization of the tumor. In contrast to normal cells, which form a single layer around capillary blood vessels, multiple layers of tumor cells surround the microvasculature, effectively creating a capillary “cuff.” This dependence of tumor cells on endothelial cells, the target of angiogenic factors, may explain amplified arrest or killing of tumor cells associated with antiangiogenic therapy.7 The relationship between antiangiogenic and antitumor activity was further evaluated in studies with chemotherapy. Conventional dosing schedules for cytotoxic chemotherapy, in which maximum tolerated doses are followed by off-therapy intervals to rescue bone marrow, allow recovery of endothelial cells. This leads to tumor recurrence and increased risk of acquired drug resistance. However, cytotoxic chemotherapy administered to
From the Departments of Surgery and Cell Biology, Harvard Medical School, Boston; and the Laboratory of Surgical Research, Children’s Hospital, Boston, MA. Supported by an unrestricted educational grant from Genentech BioOncology. Address reprint requests to Judah Folkman, MD, Children’s Hospital, Hunnewell 103, 300 Longwood Ave, Boston, MA 02115. Copyright 2002, Elsevier Science (USA). All rights reserved. 0093-7754/02/2906-1604$35.00/0 doi:10.1053/sonc.2002.37263 15
16
JUDAH FOLKMAN
Table 1. Inhibitors of Angiogenesis in Various Stages of Clinical Trials in Cancer Agent
Sponsor
Mechanism
Location
Phase 1 SU6668 Angiostatin Endostatin Combretastatin ZD6474
Sugen EntreMed EntreMed Oxigene AstraZeneca
Inhibition of receptor signaling Inhibition of endothelial proliferation Inhibition of endothelial proliferation Stimulation of endothelial apoptosis Inhibition of VEGFR-2
South San Francisco, CA Rockville, MD Rockville, MD Watertown, MA Wilmington, DE
Phase 2 CAI COL-3 Squalamine TNP-470 Interleukin-12 Prinomastat
National Cancer Institute Collagenex Geneara TAP Pharmaceutical Products, Inc Wyeth Pfizer
Inhibition Inhibition Inhibition Inhibition Inhibition Inhibition
of of of of of of
calcium influx matrix metalloproteinases sodium/hydrogen ion exchange endothelial proliferation interferon-␥ matrix metalloproteinases
Bethesda, MD Newtown, PA Plymouth Meeting, PA Lake Forest, IL Madison, NJ New York, NY
Phase 3 Bevacizumab IMC-IC11 Marimastat SU5416
Genentech ImClone British Biotech Sugen
Inhibition Inhibition Inhibition Inhibition
of of of of
VEGF VEGFR-2 matrix metalloproteinases VEGFR-2
South San Francisco, CA New York, NY Oxford, UK South San Francisco, CA
Abbreviation: VEGFR, vascular endothelial growth factor receptor.
tumor-bearing mice at frequent intervals and at a total lower dose, without discontinuation of therapy, prevents drug resistance, and tumor regression is complete and durable.8 This increase in antiangiogenic and therapeutic efficacy has been attributed to greater genetic stability of vascular endothelial cells compared with tumor cells. These findings demonstrate that the efficacy of conventional chemotherapy can be improved by changing dose and schedule to optimize exposure of the microvascular endothelium in a tumor bed to relatively low doses of the drugs. In other words, the success of chemotherapy as we know it may be in part because of the endothelial dependence of chemotherapy. The angiogenic phenotype of tumors is regulated by local balance in the activities of proangiogenic and antiangiogenic factors. Tumors that have become neovascularized often express increased levels of proangiogenic proteins, such as vascular endothelial growth factor (VEGF) and others. The expression of proangiogenic proteins can be induced by several factors, including hypoxia, activation of oncogenes, or inactivation of tumor suppressor genes.9-11 The activity of proangiogenic matrix metalloproteinases is also frequently increased in vascular tumors.12-14 In other
tumors, the angiogenic switch is the result of down regulation of antiangiogenic factors.12 In most adult tissues, the balance between proangiogenic and antiangiogenic signaling favors vasculature (ie, the angiogenic switch is “off”). A similar situation often occurs in tumors as well because the secretion of proangiogenic factors is balanced by the production of antiangiogenic molecules. This leads to a dormant, nonangiogenic tumor that is restricted to microscopic size (usually less than 0.5 to 1 mm in diameter, also called “in situ” carcinoma). In some cases, however, proangiogenic activities prevail, resulting in the angiogenic switch (the “on” position), tumor vascularization, and metastatic growth. A goal of antiangiogenic therapy is to restore the switch to the “off” position and thus inhibit tumor neovascularization and growth. Two general approaches have been used in the development of antiangiogenic agents: inhibition of proangiogenic factors (eg, therapy with antiVEGF monoclonal antibody) and therapy with endogenous inhibitors of angiogenesis (eg, angiostatin, endostatin). More than 10 specific inhibitors of angiogenesis are currently in clinical development for the treatment of cancer (Table 1). The agents include
ANGIOGENESIS IN TUMOR GROWTH & METASTASIS
antibodies (eg, neutralizing anti-VEGF antibody bevacizumab [Avastin, rhuMAb-VEGF; Genentech, South San Francisco, CA]), IMC-IC11 antibody against the VEGF receptor-2, proteins (interleukin-12, endostatin, angiostatin), and a heterogeneous array of small molecules.15 Many of these agents block VEGF, either by blocking its production by a tumor cell, by blocking its receptor (or endothelial cells), or by neutralizing VEGF itself. They can be considered as “indirect” angiogenesis inhibitors, because they block a tumor cell product or its receptor. Another example of an “indirect” angiogenesis inhibitor is the antiHER2–neutralizing antibody trastuzumab, which indirectly inhibits angiogenesis, because its target (HER2) stimulates VEGF expression. ENDOGENOUS INHIBITORS OF ANGIOGENESIS: ANGIOSTATIN AND ENDOSTATIN
Clinical and preclinical studies have shown that removal of primary tumors leads to rapid growth of previously dormant micrometastases,2,16,17 suggesting that primary tumors produce soluble factor(s) that suppress the growth of small tumors at remote sites. The first of these was identified as angiostatin, a 38-kd internal fragment of plasminogen,17 which was subsequently shown to also induce dormancy and regression of tumor xenografts.18 In preclinical studies, metastatic growth of secondary tumors (induced by irradiation of a primary tumor) was inhibited by concomitant angiostatin therapy. This finding is in agreement with results of our earlier studies, which showed synergistic effects of radiation and antiangiogenic therapy with angiostatin or bevacizumab on primary tumors.19-21 Endostatin, a 20-kd C-terminal fragment of collagen XVIII produced by hemangioendothelioma cells,22 also induced tumor dormancy, and in some transplanted tumors this effect was maintained even after therapy was discontinued, without any evidence of drug resistance.23 In vivo suppression of distant metastases, despite similar production of proangiogenic and antiangiogenic factors by the primary tumors, has been attributed to the fact that the biologic halflives of angiostatin and endostatin are substantially longer than that of the proangiogenic factors. For example, the half-lives of proangiogenic factors VEGF and fibroblast growth factor are 3
17
minutes and 30 minutes, respectively, while the half-life of angiostatin is several hours. The differential half-life is thought to effectively create an elevated level of antiangiogenic activity in the systemic circulation, leading to inhibition of angiogenesis in remote microscopic metastases. Removal of the primary tumor results in a dramatic reduction in circulating antiangiogenic factors, tipping the local angiogenic balance in remote metastases toward vascularization, growth, and metastasis. Recent evidence reveals that, in patients with cancer, the antiangiogenic activity associated with elevated systemic levels of endostatin effectively counterbalances circulating levels of proangiogenic factors (eg, VEGF).24,25 Preclinical studies suggest that antiangiogenic therapy may have the greatest initial impact on micrometastases, because in general, lower doses of an angiogenesis inhibitor are required to prevent neovascularization of microscopic metastases than to regress a primary tumor. This hypothesis remains to be evaluated in clinical trials. Effective treatment of primary tumors with antiangiogenic agents will probably require significantly higher doses than those used for therapy of remote micrometastases. Furthermore, when angiogenesis inhibitors become available in the future, they may be combined with each other, or used together with other therapeutic modalities (eg, radiation, chemotherapy– especially antiangiogenic chemotherapy– or monoclonal antibody therapy). Multicenter phase 1 trials of endostatin and angiostatin are ongoing, and early results have been presented.26-30 During the dose-escalation trials, endostatin was administered by daily intravenous bolus or brief infusion, or as a continuous infusion.27 Endostatin therapy was associated with a reduction in tumor blood flow and minimal evidence of toxicity. In 12 patients who received endostatin for more than 4 months, five had stable disease (two had stable disease after 1 year of follow-up). Evidence of tumor regression was seen in three of 61 patients. The pharmacokinetic profile of endostatin is predictable, with plasma concentrations in humans comparable to those that achieved tumor growth suppression in preclinical studies.27,28 These data support the clinical potential of antiangiogenic therapy for treatment of cancer.
18
JUDAH FOLKMAN
REFERENCES 1. Folkman J: Tumor angiogenesis, in Holland JF, Frei E III, Bast RC Jr, et al (eds): Cancer Medicine (ed 5). Hamilton, Ontario, Canada, B.C. Decker, 2000, pp 132-152 2. Folkman J: Angiogenesis, in Braunwald E, Fauci AS, Kasper DL, et al (eds): Harrison’s Principles of Internal Medicine (ed 15). New York, NY, McGraw-Hill, 2001, pp 517-530 3. Black WC, Welch HG: Advances in diagnostic imaging and overestimations of disease prevalence and the benefits of therapy. N Engl J Med 328:1237-1243, 1993 4. Carmeliet P, Jain RK: Angiogenesis in cancer and other diseases. Nature 407:249-257, 2000 5. Detmar M, Velasco P, Richard L, et al: Expression of vascular endothelial growth factor induces an invasive phenotype in human squamous cell carcinomas. Am J Pathol 156: 159-167, 2000 6. Streit M, Riccardi L, Velasco P, et al: Thrombospondin-2: A potent endogenous inhibitor of tumor growth and angiogenesis. Proc Natl Acad Sci U S A 96:14888-14893, 1999 7. Folkman J, Hahnfeldt P, Hlatky L: The logic of antiangiogenic gene therapy, in Friedmann T (ed): The Development of Human Gene Therapy. Cold Spring Harbor monograph series, monograph 36. Cold Spring Harbor, NY, Cold Spring Harbor Laboratory Press, 1999, pp 527-543 8. Browder T, Butterfield CE, Kraling BM, et al: Antiangiogenic scheduling of chemotherapy improves efficacy against experimental drug-resistant cancer. Cancer Res 60:1878-1886, 2000 9. Laughner E, Taghavi P, Chiles K, et al: HER2 (neu) signaling increases the rate of hypoxia-inducible factor 1alpha (HIF-1alpha) synthesis: Novel mechanism for HIF-1-mediated vascular endothelial growth factor expression. Mol Cell Biol 21:3995-4004, 2001 10. Ravi R, Mookerjee B, Bhujwalla ZM, et al: Regulation of tumor angiogenesis by p53-induced degradation of hypoxiainducible factor 1alpha. Genes Dev 14:34-44, 2000 11. Semenza GL: HIF-1: Using two hands to flip the angiogenic switch. Cancer Metastasis Rev 19:59-65, 2000 12. Arbiser JL, Moses MA, Fernandez CA, et al: Oncogenic H-ras stimulates tumor angiogenesis by two distinct pathways. Proc Natl Acad Sci U S A 94:861-866, 1997 13. Bergers G, Brekken R, McMahon G, et al: Matrix metalloproteinase-9 triggers the angiogenic switch during carcinogenesis. Nat Cell Biol 2:737-744, 2000 14. Wang H, Keiser JA: Vascular endothelial growth factor upregulates the expression of matrix metalloproteinases in vascular smooth muscle cells: Role of flt-1. Circ Res 83:832-840, 1998 15. Rosen L: Antiangiogenic strategies and agents in clinical trials. Oncologist 5:20-27, 2000 (suppl 1) 16. Camphausen K, Moses MA, Beecken WD, et al: Radi-
ation therapy to a primary tumor accelerates metastatic growth in mice. Cancer Res 61:2207-2211, 2001 17. O’Reilly MS, Holmgren L, Shing Y, et al: Angiostatin: A novel angiogenesis inhibitor that mediates the suppression of metastases by a Lewis lung carcinoma. Cell 79:315-328, 1994 18. O’Reilly MS, Holmgren L, Chen C, et al: Angiostatin induces and sustains dormancy of human primary tumors in mice. Nat Med 2:689-692, 1996 19. Gorski DH, Mauceri HJ, Salloum RM, et al: Potentiation of the antitumor effect of ionizing radiation by brief concomitant exposures to angiostatin. Cancer Res 58:56865689, 1998 20. Gorski DH, Beckett MA, Jaskowiak NT, et al: Blockage of the vascular endothelial growth factor stress response increases the antitumor effects of ionizing radiation. Cancer Res 59:3374-3378, 1999 21. Mauceri HJ, Hanna NN, Beckett MA, et al: Combined effects of angiostatin and ionizing radiation in antitumour therapy. Nature 394:287-291, 1998 22. O’Reilly MS, Boehm T, Shing Y, et al: Endostatin: An endogenous inhibitor of angiogenesis and tumor growth. Cell 88:277-285, 1997 23. Boehm T, Folkman J, Browder T, et al: Antiangiogenic therapy of experimental cancer does not induce acquired drug resistance. Nature 390:404-407, 1997 24. Feldman AL, Tamarkin L, Paciotti GF, et al: Serum endostatin levels are elevated and correlate with serum vascular endothelial growth factor levels in patients with stage IV clear cell renal cancer. Clin Cancer Res 6:4628-4634, 2000 25. Feldman AL, Pak H, Yang JC, et al: Serum endostatin levels are elevated in patients with soft tissue sarcoma. Cancer 91:1525-1529, 2001 26. DeMoraes TED, Fogler WE, Grant D, et al: Recombinant human angiostatin (rhA): A phase I clinical trial assessing safety, pharmocokinetics (PK) and pharmacodynamics (PD). Proc Am Soc Clin Oncol 20:3a, 2001 (abstr 10) 27. Eder JP, Clark JW, Supko JG, et al: A phase I pharmacokinetic and pharmacodynamic trial of recombinant human endostatin. Proc Am Soc Clin Oncol 20:70a, 2001 (abstr 275) 28. Fogler WE, Song M, Supko JG, et al: Recombinant human endostatin demonstrates consistent and predictable pharmacokinetics following intravenous bolus administration to cancer patients. Proc Am Soc Clin Oncol 20:69a, 2001 (abstr 274) 29. Herbst RS, Tran HT, Mullani NA, et al: Phase I clinical trial of recombinant human endostatin (rHE) in patients (pts) with solid tumors: Pharmacokinetic (PK), safety and efficacy analysis using surrogate endpoints of tissue and radiologic response. Proc Am Soc Clin Oncol 20:3a, 2001 (abstr 9) 30. Thomas JP, Schiller J, Lee F, et al: A phase I pharmacokinetic and pharmacodynamic study of recombinant human endostatin. Proc Am Soc Clin Oncol 20:70a, 2001 (abstr 276)
T
HE GROWTH of new capillary blood vessels, or angiogenesis, is required for tumor growth and metastasis, and thus constitutes an important control point in the progression of cancer. For this reason, tumor angiogenesis has become the focus of extensive investigation,1,2 and its inhibition is emerging as a rational and potentially valuable new approach to cancer therapy. Antiangiogenic therapies in development target various factors implicated in tumor angiogenesis, and all of these therapies inhibit endothelial cell proliferation (or induce endothelial cell apoptosis) and migration into the tumor bed, which is the initial step in capillary formation. Antiangiogenic agents may be particularly effective in the context of combination therapy because they enhance the delivery and therapeutic efficacy of treatment modalities that directly target cancer cells (eg, radiation, chemotherapy, monoclonal antibody therapy). THE ROLE OF VASCULATURE IN CANCER GROWTH
Autopsy studies in accident victims show that the incidence of in situ tumors, particularly those Seminars in Oncology, Vol 29, No 6, Suppl 16 (December), 2002: pp 15-18
of the breast, prostate, and thyroid gland, is substantially higher than the documented prevalence of cancer,3 indicating that tumor metastasis is a relatively infrequent phenomenon. It has been proposed that the low metastatic activity of in situ tumors may be related to the fact that they have not acquired an angiogenic phenotype (ability to recruit vasculature), because tumor growth and metastasis have been shown to require increased expression of angiogenic factors and increased vascularization.4-6 In other words, although in situ tumors may replicate rapidly, their growth and metastatic properties are severely restricted by the absence of adequate blood supply. THE ANGIOGENIC SWITCH
The change from a quiescent to an invasive phenotype is invariably accompanied by the acquisition of angiogenic properties (“angiogenic switch”) and vascularization of the tumor. In contrast to normal cells, which form a single layer around capillary blood vessels, multiple layers of tumor cells surround the microvasculature, effectively creating a capillary “cuff.” This dependence of tumor cells on endothelial cells, the target of angiogenic factors, may explain amplified arrest or killing of tumor cells associated with antiangiogenic therapy.7 The relationship between antiangiogenic and antitumor activity was further evaluated in studies with chemotherapy. Conventional dosing schedules for cytotoxic chemotherapy, in which maximum tolerated doses are followed by off-therapy intervals to rescue bone marrow, allow recovery of endothelial cells. This leads to tumor recurrence and increased risk of acquired drug resistance. However, cytotoxic chemotherapy administered to
From the Departments of Surgery and Cell Biology, Harvard Medical School, Boston; and the Laboratory of Surgical Research, Children’s Hospital, Boston, MA. Supported by an unrestricted educational grant from Genentech BioOncology. Address reprint requests to Judah Folkman, MD, Children’s Hospital, Hunnewell 103, 300 Longwood Ave, Boston, MA 02115. Copyright 2002, Elsevier Science (USA). All rights reserved. 0093-7754/02/2906-1604$35.00/0 doi:10.1053/sonc.2002.37263 15
16
JUDAH FOLKMAN
Table 1. Inhibitors of Angiogenesis in Various Stages of Clinical Trials in Cancer Agent
Sponsor
Mechanism
Location
Phase 1 SU6668 Angiostatin Endostatin Combretastatin ZD6474
Sugen EntreMed EntreMed Oxigene AstraZeneca
Inhibition of receptor signaling Inhibition of endothelial proliferation Inhibition of endothelial proliferation Stimulation of endothelial apoptosis Inhibition of VEGFR-2
South San Francisco, CA Rockville, MD Rockville, MD Watertown, MA Wilmington, DE
Phase 2 CAI COL-3 Squalamine TNP-470 Interleukin-12 Prinomastat
National Cancer Institute Collagenex Geneara TAP Pharmaceutical Products, Inc Wyeth Pfizer
Inhibition Inhibition Inhibition Inhibition Inhibition Inhibition
of of of of of of
calcium influx matrix metalloproteinases sodium/hydrogen ion exchange endothelial proliferation interferon-␥ matrix metalloproteinases
Bethesda, MD Newtown, PA Plymouth Meeting, PA Lake Forest, IL Madison, NJ New York, NY
Phase 3 Bevacizumab IMC-IC11 Marimastat SU5416
Genentech ImClone British Biotech Sugen
Inhibition Inhibition Inhibition Inhibition
of of of of
VEGF VEGFR-2 matrix metalloproteinases VEGFR-2
South San Francisco, CA New York, NY Oxford, UK South San Francisco, CA
Abbreviation: VEGFR, vascular endothelial growth factor receptor.
tumor-bearing mice at frequent intervals and at a total lower dose, without discontinuation of therapy, prevents drug resistance, and tumor regression is complete and durable.8 This increase in antiangiogenic and therapeutic efficacy has been attributed to greater genetic stability of vascular endothelial cells compared with tumor cells. These findings demonstrate that the efficacy of conventional chemotherapy can be improved by changing dose and schedule to optimize exposure of the microvascular endothelium in a tumor bed to relatively low doses of the drugs. In other words, the success of chemotherapy as we know it may be in part because of the endothelial dependence of chemotherapy. The angiogenic phenotype of tumors is regulated by local balance in the activities of proangiogenic and antiangiogenic factors. Tumors that have become neovascularized often express increased levels of proangiogenic proteins, such as vascular endothelial growth factor (VEGF) and others. The expression of proangiogenic proteins can be induced by several factors, including hypoxia, activation of oncogenes, or inactivation of tumor suppressor genes.9-11 The activity of proangiogenic matrix metalloproteinases is also frequently increased in vascular tumors.12-14 In other
tumors, the angiogenic switch is the result of down regulation of antiangiogenic factors.12 In most adult tissues, the balance between proangiogenic and antiangiogenic signaling favors vasculature (ie, the angiogenic switch is “off”). A similar situation often occurs in tumors as well because the secretion of proangiogenic factors is balanced by the production of antiangiogenic molecules. This leads to a dormant, nonangiogenic tumor that is restricted to microscopic size (usually less than 0.5 to 1 mm in diameter, also called “in situ” carcinoma). In some cases, however, proangiogenic activities prevail, resulting in the angiogenic switch (the “on” position), tumor vascularization, and metastatic growth. A goal of antiangiogenic therapy is to restore the switch to the “off” position and thus inhibit tumor neovascularization and growth. Two general approaches have been used in the development of antiangiogenic agents: inhibition of proangiogenic factors (eg, therapy with antiVEGF monoclonal antibody) and therapy with endogenous inhibitors of angiogenesis (eg, angiostatin, endostatin). More than 10 specific inhibitors of angiogenesis are currently in clinical development for the treatment of cancer (Table 1). The agents include
ANGIOGENESIS IN TUMOR GROWTH & METASTASIS
antibodies (eg, neutralizing anti-VEGF antibody bevacizumab [Avastin, rhuMAb-VEGF; Genentech, South San Francisco, CA]), IMC-IC11 antibody against the VEGF receptor-2, proteins (interleukin-12, endostatin, angiostatin), and a heterogeneous array of small molecules.15 Many of these agents block VEGF, either by blocking its production by a tumor cell, by blocking its receptor (or endothelial cells), or by neutralizing VEGF itself. They can be considered as “indirect” angiogenesis inhibitors, because they block a tumor cell product or its receptor. Another example of an “indirect” angiogenesis inhibitor is the antiHER2–neutralizing antibody trastuzumab, which indirectly inhibits angiogenesis, because its target (HER2) stimulates VEGF expression. ENDOGENOUS INHIBITORS OF ANGIOGENESIS: ANGIOSTATIN AND ENDOSTATIN
Clinical and preclinical studies have shown that removal of primary tumors leads to rapid growth of previously dormant micrometastases,2,16,17 suggesting that primary tumors produce soluble factor(s) that suppress the growth of small tumors at remote sites. The first of these was identified as angiostatin, a 38-kd internal fragment of plasminogen,17 which was subsequently shown to also induce dormancy and regression of tumor xenografts.18 In preclinical studies, metastatic growth of secondary tumors (induced by irradiation of a primary tumor) was inhibited by concomitant angiostatin therapy. This finding is in agreement with results of our earlier studies, which showed synergistic effects of radiation and antiangiogenic therapy with angiostatin or bevacizumab on primary tumors.19-21 Endostatin, a 20-kd C-terminal fragment of collagen XVIII produced by hemangioendothelioma cells,22 also induced tumor dormancy, and in some transplanted tumors this effect was maintained even after therapy was discontinued, without any evidence of drug resistance.23 In vivo suppression of distant metastases, despite similar production of proangiogenic and antiangiogenic factors by the primary tumors, has been attributed to the fact that the biologic halflives of angiostatin and endostatin are substantially longer than that of the proangiogenic factors. For example, the half-lives of proangiogenic factors VEGF and fibroblast growth factor are 3
17
minutes and 30 minutes, respectively, while the half-life of angiostatin is several hours. The differential half-life is thought to effectively create an elevated level of antiangiogenic activity in the systemic circulation, leading to inhibition of angiogenesis in remote microscopic metastases. Removal of the primary tumor results in a dramatic reduction in circulating antiangiogenic factors, tipping the local angiogenic balance in remote metastases toward vascularization, growth, and metastasis. Recent evidence reveals that, in patients with cancer, the antiangiogenic activity associated with elevated systemic levels of endostatin effectively counterbalances circulating levels of proangiogenic factors (eg, VEGF).24,25 Preclinical studies suggest that antiangiogenic therapy may have the greatest initial impact on micrometastases, because in general, lower doses of an angiogenesis inhibitor are required to prevent neovascularization of microscopic metastases than to regress a primary tumor. This hypothesis remains to be evaluated in clinical trials. Effective treatment of primary tumors with antiangiogenic agents will probably require significantly higher doses than those used for therapy of remote micrometastases. Furthermore, when angiogenesis inhibitors become available in the future, they may be combined with each other, or used together with other therapeutic modalities (eg, radiation, chemotherapy– especially antiangiogenic chemotherapy– or monoclonal antibody therapy). Multicenter phase 1 trials of endostatin and angiostatin are ongoing, and early results have been presented.26-30 During the dose-escalation trials, endostatin was administered by daily intravenous bolus or brief infusion, or as a continuous infusion.27 Endostatin therapy was associated with a reduction in tumor blood flow and minimal evidence of toxicity. In 12 patients who received endostatin for more than 4 months, five had stable disease (two had stable disease after 1 year of follow-up). Evidence of tumor regression was seen in three of 61 patients. The pharmacokinetic profile of endostatin is predictable, with plasma concentrations in humans comparable to those that achieved tumor growth suppression in preclinical studies.27,28 These data support the clinical potential of antiangiogenic therapy for treatment of cancer.
18
JUDAH FOLKMAN
REFERENCES 1. Folkman J: Tumor angiogenesis, in Holland JF, Frei E III, Bast RC Jr, et al (eds): Cancer Medicine (ed 5). Hamilton, Ontario, Canada, B.C. Decker, 2000, pp 132-152 2. Folkman J: Angiogenesis, in Braunwald E, Fauci AS, Kasper DL, et al (eds): Harrison’s Principles of Internal Medicine (ed 15). New York, NY, McGraw-Hill, 2001, pp 517-530 3. Black WC, Welch HG: Advances in diagnostic imaging and overestimations of disease prevalence and the benefits of therapy. N Engl J Med 328:1237-1243, 1993 4. Carmeliet P, Jain RK: Angiogenesis in cancer and other diseases. Nature 407:249-257, 2000 5. Detmar M, Velasco P, Richard L, et al: Expression of vascular endothelial growth factor induces an invasive phenotype in human squamous cell carcinomas. Am J Pathol 156: 159-167, 2000 6. Streit M, Riccardi L, Velasco P, et al: Thrombospondin-2: A potent endogenous inhibitor of tumor growth and angiogenesis. Proc Natl Acad Sci U S A 96:14888-14893, 1999 7. Folkman J, Hahnfeldt P, Hlatky L: The logic of antiangiogenic gene therapy, in Friedmann T (ed): The Development of Human Gene Therapy. Cold Spring Harbor monograph series, monograph 36. Cold Spring Harbor, NY, Cold Spring Harbor Laboratory Press, 1999, pp 527-543 8. Browder T, Butterfield CE, Kraling BM, et al: Antiangiogenic scheduling of chemotherapy improves efficacy against experimental drug-resistant cancer. Cancer Res 60:1878-1886, 2000 9. Laughner E, Taghavi P, Chiles K, et al: HER2 (neu) signaling increases the rate of hypoxia-inducible factor 1alpha (HIF-1alpha) synthesis: Novel mechanism for HIF-1-mediated vascular endothelial growth factor expression. Mol Cell Biol 21:3995-4004, 2001 10. Ravi R, Mookerjee B, Bhujwalla ZM, et al: Regulation of tumor angiogenesis by p53-induced degradation of hypoxiainducible factor 1alpha. Genes Dev 14:34-44, 2000 11. Semenza GL: HIF-1: Using two hands to flip the angiogenic switch. Cancer Metastasis Rev 19:59-65, 2000 12. Arbiser JL, Moses MA, Fernandez CA, et al: Oncogenic H-ras stimulates tumor angiogenesis by two distinct pathways. Proc Natl Acad Sci U S A 94:861-866, 1997 13. Bergers G, Brekken R, McMahon G, et al: Matrix metalloproteinase-9 triggers the angiogenic switch during carcinogenesis. Nat Cell Biol 2:737-744, 2000 14. Wang H, Keiser JA: Vascular endothelial growth factor upregulates the expression of matrix metalloproteinases in vascular smooth muscle cells: Role of flt-1. Circ Res 83:832-840, 1998 15. Rosen L: Antiangiogenic strategies and agents in clinical trials. Oncologist 5:20-27, 2000 (suppl 1) 16. Camphausen K, Moses MA, Beecken WD, et al: Radi-
ation therapy to a primary tumor accelerates metastatic growth in mice. Cancer Res 61:2207-2211, 2001 17. O’Reilly MS, Holmgren L, Shing Y, et al: Angiostatin: A novel angiogenesis inhibitor that mediates the suppression of metastases by a Lewis lung carcinoma. Cell 79:315-328, 1994 18. O’Reilly MS, Holmgren L, Chen C, et al: Angiostatin induces and sustains dormancy of human primary tumors in mice. Nat Med 2:689-692, 1996 19. Gorski DH, Mauceri HJ, Salloum RM, et al: Potentiation of the antitumor effect of ionizing radiation by brief concomitant exposures to angiostatin. Cancer Res 58:56865689, 1998 20. Gorski DH, Beckett MA, Jaskowiak NT, et al: Blockage of the vascular endothelial growth factor stress response increases the antitumor effects of ionizing radiation. Cancer Res 59:3374-3378, 1999 21. Mauceri HJ, Hanna NN, Beckett MA, et al: Combined effects of angiostatin and ionizing radiation in antitumour therapy. Nature 394:287-291, 1998 22. O’Reilly MS, Boehm T, Shing Y, et al: Endostatin: An endogenous inhibitor of angiogenesis and tumor growth. Cell 88:277-285, 1997 23. Boehm T, Folkman J, Browder T, et al: Antiangiogenic therapy of experimental cancer does not induce acquired drug resistance. Nature 390:404-407, 1997 24. Feldman AL, Tamarkin L, Paciotti GF, et al: Serum endostatin levels are elevated and correlate with serum vascular endothelial growth factor levels in patients with stage IV clear cell renal cancer. Clin Cancer Res 6:4628-4634, 2000 25. Feldman AL, Pak H, Yang JC, et al: Serum endostatin levels are elevated in patients with soft tissue sarcoma. Cancer 91:1525-1529, 2001 26. DeMoraes TED, Fogler WE, Grant D, et al: Recombinant human angiostatin (rhA): A phase I clinical trial assessing safety, pharmocokinetics (PK) and pharmacodynamics (PD). Proc Am Soc Clin Oncol 20:3a, 2001 (abstr 10) 27. Eder JP, Clark JW, Supko JG, et al: A phase I pharmacokinetic and pharmacodynamic trial of recombinant human endostatin. Proc Am Soc Clin Oncol 20:70a, 2001 (abstr 275) 28. Fogler WE, Song M, Supko JG, et al: Recombinant human endostatin demonstrates consistent and predictable pharmacokinetics following intravenous bolus administration to cancer patients. Proc Am Soc Clin Oncol 20:69a, 2001 (abstr 274) 29. Herbst RS, Tran HT, Mullani NA, et al: Phase I clinical trial of recombinant human endostatin (rHE) in patients (pts) with solid tumors: Pharmacokinetic (PK), safety and efficacy analysis using surrogate endpoints of tissue and radiologic response. Proc Am Soc Clin Oncol 20:3a, 2001 (abstr 9) 30. Thomas JP, Schiller J, Lee F, et al: A phase I pharmacokinetic and pharmacodynamic study of recombinant human endostatin. Proc Am Soc Clin Oncol 20:70a, 2001 (abstr 276)