Antiangiogenesis and VEGF/VEGFR Targeting as Part of a Combined Modality Approach to the Treatment of Cancer

Since its inception over three decades ago the field of angiogenesis research has made significant progress (1). During tumorigenesis, the process of angiogenesis, which is tightly regulated physiologically, is often markedly disordered and requires the continued production of stimulators by tumor and stromal cells in excess of inhibitors. A number of antiangiogenic agents have now been described and many of these are currently in clinical trials and several have now been approved or are pending approval for clinical use in the treatment of cancer and other angiogenesis dependent diseases. However, before antiangiogenic agents can be successfully incorporated into clinical strategies, a greater understanding of the process of angiogenesis and of the interaction between the endothelial cell and its microenvironment is required. In addition to providing nutrients and oxygen and removing catabolites, proliferating endothelial cells produce multiple growth factors that can promote tumor growth, invasion, and survival. Angiogenesis, therefore, provides both a perfusion effect and a paracrine effect to a growing tumor and tumor cells and endothelial cells can drive each other to amplify the malignant phenotype (Figure 1). These observations led Folkman to propose a two-compartment model of malignancy consisting of endothelial and tumor cell populations (2). For effective cancer therapy, it will be prudent to target both of these compartments by combining conventional therapeutic agents and modalities, such as radiation therapy and/or chemotherapy, with antiangiogenic agents and other emerging therapies. However, additional work is still needed to provide a rational basis for the combination of angiogenesis inhibitors with other modalities and to identify potential problems that might arise from these combinations so that these agents will be successfully incorporated into existing therapy in a timely and rational manner. Figure 1 Angiogenesis influences tumor growth by both paracrine and perfusion effects. Strategies that target growth factors involved in tumor angiogenesis offer great potential and one of the most widely studied targets is VEGF and its receptors (3). VEGF, initially called VPF due to its ability to increase vascular permeability, stimulates proliferation and migration of endothelial cells and plays a pivotal role in vasculogenesis, angiogenesis, and endothelial integrity and survival. When considering therapeutic antagonism of VEGF, it is important to distinguish between anti-vascular and antiangiogenic therapy. Antiangiogenic therapy is directed against the neovasculature and thus does not have a significant effect on the resting vasculature. In contrast, anti-vascular therapy targets the existing vasculature and could therefore disrupt normal vessels. VEGF antagonism is likely to have both antiangiogenic and anti-vascular effects. Although VEGF was initially thought to be specific for the vasculature, recent data demonstrates that VEGF can also play a role in multiple other processes including tumor cell survival and motility, hematopoiesis, immune function, hepatic integrity, and neurological function (4). The multiple effects of VEGF are mediated through several different receptors including the tyrosine kinase receptors VEGFR1 (flt-1), VEGFR2 (KDR, flk-1), and VEGFR3 (flt4) with differing binding specificities for each form of VEGF (3). Several approaches to block VEGF activity are under evaluation, including anti-VEGF or VEGFR antibodies and VEGFR tyrosine kinase inhibitors. Several tyrosine kinase inhibitors are now available that selectively inhibit VEGFR2 and/or other VEGFRs. Single agents that target VEGFR along with multiple other receptor tyrosine kinases (i.e. FGFR, EGFR, PDGFR) are now being developed to more broadly, but perhaps less specifically and less selectively, target cancer growth. SU11248, for example, targets VEGFRs, PDGFR, c-kit, and flt3 and has shown significant efficacy in preclinical models of cancer and in early clinical trials but was also associated with significant toxicity that prevented continuous administration. Bevacizumab, a humanized antibody against VEGF, has shown great promise and improved survival for patients with metastatic colorectal, breast, or lung cancer when used in combination with chemotherapy (5). However, although survival was improved in these studies there remains an unmet need for improved efficacy and clinical trials with VEGF inhibitors have been associated with thromboembolic and hemorrhagic complications likely due to effects upon and/or disruption of the normal vasculature. A number of potential therapeutic limitations could arise when antiangiogenic agents are used as monotherapy in the treatment of cancer including a delay in onset of activity, the need for a prolonged treatment course, an inability to eradicate microscopic disease, and the potential for resistance to therapy. Although drug resistance may not be a general property of angiogenesis inhibitors, resistance could emerge to agents that act indirectly or agents that target a single proangiogenic factor such as VEGF. In addition, most advanced malignant tumors produce multiple angiogenic factors and targeting only one may not be adequate for complete tumor control. It may therefore be prudent to combine antiangiogenic agents and other emerging therapies with chemotherapy and radiotherapy. It had long been assumed that an angiogenesis inhibitor would impair the effect of ionizing radiation by inducing tumor hypoxia. However, Teicher and colleagues (6) observed that antiangiogenic therapy given in combination with the angiogenesis inhibitors TNP-470 and minocycline, a weak inhibitor of metalloproteinase activity, improved tumor oxygenation and the anti-tumor effect of radiation therapy. More recently, a number of agents with antiangiogenic activity have been used in combination with concurrent radiation therapy for the treatment of cancer. In studies conducted by Milas and colleagues (7), an enhancement of the response to radiation therapy was observed when it was combined with a selective inhibitor of the cyclooxygenase-2 enzyme providing the basis for clinical trials. In patients with poor prognosis lung cancer, celecoxib was administered concurrently with thoracic radiotherapy. The treatment resulted in actuarial local progression-free survival of 66.0% at 1 year and 42.2% at 2 years (8). However, given the cardiac toxicity that has now been observed with this class of agents, further assessment and additional studies are required. Enhancement of tumor response to radiation therapy has been demonstrated when it is combined with anti-VEGF agents (9) even for tumors that were markedly hypoxic with an increase in tumor growth delay and an augmentation of tumor curability in the combined modality groups. More recently, ZD6474, a small molecule inhibitor of VEGFR2 with additional activity against EGFR, was combined with radiation therapy in the treatment of tumor xenografts. Two combination schedules were examined with ZD6474 given before each dose of radiation (concurrent schedule) or given 30 minutes after the last dose of radiotherapy (sequential schedule) (10). The growth delay induced using the concurrent schedule was greater than that induced by ZD6474 or radiation treatment alone but the sequential schedule maximally enhanced tumor growth delay. In a brain tumor model, VEGFR2 blockade induced normalization of the tumor vasculature that was associated with an interval characterized by an increase in tumor oxygenation. During the normalization window, but not before or after it, VEGFR2 blockade increased pericyte coverage of brain tumor vessels and combined radiotherapy during this period was associated with the optimal anti-tumor efficacy (11). The enhancement of the effect of radiation therapy by antiangiogenic therapy may also be dependent upon the tumor microenvironment. Lund et al (12) treated mice with glioblastoma xenografts implanted into the thigh or intracranially with TNP-470 and/or radiation therapy. For the thigh tumors, a significant enhancement of the anti-tumor effect of each modality was seen in the combination group. However, no enhancement was observed for intracranial tumors. It is tempting to speculate that differences in the capillary beds and microenvironment of the brain and the subcutaneous tissues of the thigh may have contributed to the differences in response. Recently, the concurrent administration of irradiation, chemotherapy, and the antiangiogenic agent SU11657, a small molecule inhibitor of multiple receptor tyrosine kinases including VEGFR2, PDGFR, and c-kit, was studied in the treatment of A431 tumor xenografts (13). Combination therapy was superior to all single and dual combinations. When taken together, preclinical and clinical studies of angiogenesis inhibition demonstrate the potential for combining antiangiogenic agents with conventional therapeutics in the clinical setting. Antiangiogenic therapy will need to be integrated with existing and emerging therapies for treating cancer and may need to be optimized on a patient-specific basis. However, conventional strategies for monitoring anti-cancer therapies may not apply for antiangiogenic agents and novel surrogates must be developed and validated (Figure 2). Clinical trials of antiangiogenic agents combined with other therapies need to be designed not only to determine if the agents are safe and have evidence of efficacy but also to validate both invasive and non-invasive surrogates of response. Figure 2 Development of surrogates for response to antiangiogenic therapy as part of the combined modality treatment of cancer.