By far the greatest number of antiangiogenic drugs currently in clinical development target the VEGF pathway. As reviewed by Jain in this supplement, VEGF is crucial in the angiogenic process, making this pathway a logical target. VEGF belongs to a family of seven factors (Table 3). VEGF and its isoforms are the ligands for VEGF receptors (VEGFR)-1 and -2. Although VEGF binds to two receptor tyrosine kinases, VEGFR-1 (also known as Flt-1) and VEGFR-2 (also known as KDR), the angiogenic effects are primarily exerted through binding to VEGFR-2. The binding of VEGF to these receptors on endothelial cells results in receptor dimerization and activation, which stimulates signaling cascades involving phospholipase C, protein kinase C, the Src tyrosine kinases, MAP kinase, phosphatidylinositol-3-kinase (PI3K), Ras GTPase-activating protein, and the Raf-Mek-Erk pathway (reviewed in [4,5]) (Figure 1). Additionally, VEGF binds to the non- tyrosine kinase neuropilin receptors (NRP-1 and NRP-2). NRP-1 can act as a coreceptor for VEGF, and as such potentiates VEGFR-2-dependent endothelial cell mitogenesis. Recently it has also been determined that NRP-1 regulates endothelial cell adhesion to extracellular matrix proteins, independent of VEGFR-2. NRP-1 is also expressed on tumor cells and often is the sole VEGF receptor on these cells. Binding of VEGF to NRP-1 on tumor cells promotes tumor cell survival, proliferation,[ 8] and chemotaxis. The function of NRP-2 is as yet unclear but it is likely involved in lymphangiogenesis. Agents targeted toward VEGF act by antagonizing VEGF itself, blocking its receptors, inhibiting its synthesis, or inhibiting proteins in the VEGF receptor-signaling cascade. Most of these actions are targeted toward vascular endothelial cells; however, direct effects on tumor cells are possible through inhibiting interaction of VEGF with NRP-1 and with VEGFR-2 in the isolated cases in which this receptor is expressed on tumor cells (Figure 2). As reviewed by Jain in this supplement, there are two key mechanisms by which the antagonism of VEGF suppresses tumor growth and inhibits metastasis: prevention of neovascularization[10,11] and improvement of chemotherapy delivery.[ 12]
- Anti-VEGF Agents- Several agents specifically antagonize VEGF, the most notable of which is bevacizumab, a humanized monoclonal antibody to all isoforms of VEGF. Bevacizumab has been approved for use in combination with fluorouracil(Drug information on fluorouracil) (5-FU)-based chemotherapy as firstline treatment of metastatic colorectal cancer. The pivotal phase III clinical trial of bevacizumab in combination with irinotecan(Drug information on irinotecan) (Camptosar), 5-FU, and leucovorin in metastatic colorectal cancer showed greater median survival with the combination than with chemotherapy alone (20.3 vs 15.6 months; P < .001). This finding with an agent whose primary mechanism of action is antiangiogenesis has sustained optimism for this mode of anticancer therapy. Furthermore, this benefit occurred in the absence of few significant additional side effects relative to chemotherapy. Grade 3 hypertension was more common in bevacizumab-treated patients (11% vs 2.3%), but was manageable. Randomized studies with bevacizumab have shown clinical efficacy in other cancers as well. Response rates were higher and times to progression were longer with bevacizumab added to carboplatin (Paraplatin) and paclitaxel(Drug information on paclitaxel) than with chemotherapy alone in patients with advanced or recurrent non-small-cell lung cancer (NSCLC). The overall response rates in metastatic breast cancer were higher with bevacizumab added to capecitabine(Drug information on capecitabine) than with capecitabine alone. In neither of these trials was survival greater with the addition of bevacizumab, however, suggesting that earlier treatment with an anti-VEGF agent in these settings may be required. A number of recent clinical trials (eg, BOND-2) are testing combinations of bevacizumab and agents that antagonize the epidermal growth factor receptor (EGFR)[16-20]; their results are eagerly awaited. Other anti-VEGF agents include VEGF-trap, a recombinant protein that consists of the extracellular domains of human VEGFR-1 and human VEGFR-2 fused to human IgG1 Fc, and antisense oligonucleotide VEGF (VEGF-AS, Veglin).[21,22] VEGF-trap inactivates all isoforms of VEGF(-A), and VEGF-AS inhibits receptor binding and expression of VEGF-A, -C, and -D. Both have been well tolerated in phase I trials. Dose-related hypertension occurred with VEGF-trap, but no dose-limiting toxicities were apparent with VEGF-AS. Both have shown early evidence of antitumor efficacy.[ 21,22]
- Anti-VEGF Receptor Agents- Of the agents that target VEGF receptors, the most numerous and the furthest along in clinical trials are the VEGF receptor tyrosine kinase inhibitors. These block or compete with the binding of ATP, inhibiting receptor phosphorylation and the subsequent intracellular signaling cascade. Phase III trials of two of these agents are planned or are under way: vatalanib (PTK787), which has activity against VEGFR-1, -2, and -3, and SU011248, which has activity against VEGFR-2 and platelet-derived growth factor receptor (PDGFR). Phase I/II trials of vatalanib in combination with 5-FU-based chemotherapy for metastatic colorectal cancer showed good tolerability at doses up to 1,250 mg/d. Grades 3 and 4 adverse events included hypertension, fatigue, ataxia, neutropenia, thrombocytopenia, and dizziness.[23,24] The partial response rates ranged from 33% to 50%, and phase III trials in this setting are ongoing. The adverse events in phase I/II trials of SU011248 included fatigue and asthenia, nausea and vomiting, diarrhea, neutropenia, thrombocytopenia, lymphopenia, and decreased left ventricular ejection fraction of > 20%.[25,26] The dose-limiting toxicity with doses greater than the recommended 50 mg/d was fatigue and asthenia. Antitumor efficacy was shown in a phase II trial of SU011248 in patients with previously treated metastatic renal cell carcinoma. There were partial responses in 24% of patients and stable disease in 46%. A phase III trial is planned. Despite the promising results with these agents, some in this class have failed in clinical trials due to poor efficacy and safety. Semaxinib (SU5416), which was active against VEGFR-1, -2, PDGFR, Flt-4, and c-kit, was associated with a high incidence of thrombotic events and showed no increase in 1-year eventfree survival in a phase II trial in patients with renal cell carcinoma. Similarly, no recent data have been reported on a related molecule, SU6668, which is active against VEGF, PDGF, and fibroblast growth factor-1 receptors. The reason for the delay or discontinuation of the development of SU6668 is not clear, but phase I trials showed self-inducing clearance, which resulted in difficulty in maintaining pharmacologically active concentrations with the maximum tolerated dose.[29-31] Other VEGF receptor-targeted agents include a raf kinase inhibitor, sorafenib(Drug information on sorafenib) (BAY 43-9006), and a chimeric monoclonal antibody to VEGFR-2, IMC-1C11, with sorafenib being the furthest along in development. In a phase II trial there was a response to sorafenib in 25 (40%) of 63 assessable patients with metastatic renal cell carcinoma. The most common grade 3 adverse events were hand-foot skin reaction, rash, fatigue, and hypertension. Good tolerability was shown with IMC-1C11 in phase I trials, but human antichimeric antibodies developed in half of the patients, and a second-generation human monoclonal antibody is being developed.
- Other VEGF-Targeted Agents- Other agents that act on the VEGF pathway include AE-941 (Neovastat) and enzastaurin (LY317615). AE-941 is a derivative of shark cartilage that prevents VEGF from binding to its receptors and also inhibits matrix metalloproteinase (MMP) activity. AE-941 did not meet its primary end point, longer median overall survival, in a phase III trial in patients with renal cell carcinoma, but significantly longer survival was seen in the subgroup of healthier patients with clear cell histology and only one metastatic site (26.3 months with AE-941 vs 12.6 months with placebo; P = .02). The agent is also very well tolerated.[35,36] Its development continues, and a phase III trial in NSCLC is ongoing. Enzastaurin is an inhibitor of the protein kinase C-β isozyme, which is activated by VEGF and potentiates the effects of VEGF. It is currently in phase I trials. Agents That Do Not Target the VEGF Pathway
- Antiangiogenic Factors- Proteins that mimic naturally occurring antiangiogenic factors have been developed. Those on which phase II data are available include recombinant human angiostatin and endostatin. Preliminary phase II data on angiostatin in combination with paclitaxel and carboplatin(Drug information on carboplatin) in patients with NSCLC show partial responses in 39% and stable disease in 39% of 23 patients.[ 39] The duration of response was 8 months or longer in four of these patients. The most common adverse event was rash, in 92% of patients (grade 1 in 75%). There were few adverse events in phase I trials of angiostatin monotherapy, mainly grade 1 or 2 injection-site reactions.[ 40,41] Hemorrhage in central nervous system metastases occurred in two patients, and deep-vein thrombosis in two. Long-term treatment (>1 year in five patients) resulted in no additional treatment-related adverse events. A phase II study of endostatin in patients with advanced neuroendocrine tumors showed stable disease in 62% of patients. The only severe adverse event was myocardial infarction in a patient with prior coronary artery disease.
- Integrin-Targeted Agents- Several molecules have been designed to target αv integrins. Integrins are heterodimeric cell-surface receptors that play an important role in endothelial cell migration, proliferation, and survival.[ 43] Two molecules are in phase II trials: MEDI-522 (Vitaxin) and cilengitide (EMD 121974). MEDI- 522 is a humanized monoclonal antibody to αvβ3. Phase I trials showed an absence of serious adverse events other than infusion-related reactions.[ 44,45] Cilengitide is a peptide that contains the Arg-Gly-Asp (RGD) sequence. This sequence is present on integrins and is a binding epitope that is shared by many integrin ligands. Thus, RGD-containing peptides are thought to antagonize integrin-mediated actions. Phase I trials showed no dose-limiting toxicities.[43,46] Phase II trials of both MEDI-522 and cilengitide are ongoing.
- Other Agents- A number of agents aimed at other targets have also been developed. These include the MMP inhibitor BMS-275291, the heparanase inhibitor PI-88, and a group of agents referred to as vascular- disruptive agents that attack existing tumor vasculature rather than prevent the formation of new vessels. BMS-275291 is a nonhydroxamate MMP inhibitor that has been designed to inhibit only MMPs that are not sheddases. It is thought that the inhibition of sheddases contributes to the arthrotoxicity associated with older hydroxamate MMP inhibitors. Grade 2 or higher musculoskeletal symptoms developed in approximately a third of the patients in two phase II trials.[48,49] The drug was administered in both trials with paclitaxel, which is well known for causing arthralgias; however, the trials were randomized and the trial in breast cancer was halted early because of greater toxicity in the BMS-275291 group. The trial in NSCLC, on the other hand, found no difference in arthrotoxicity between those receiving BMS-275291 and those receiving placebo (both along with paclitaxel and carboplatin), and patient accrual to a phase III trial is ongoing. PI-88 inhibits heparanase, an enzyme that causes the release of proangiogenic factors from the extracellular matrix. Good tolerability and evidence of efficacy with PI-88 in phase I trials has led to continued study.[50,51] A phase II trial in multiple myeloma showed disease stabilization in 39% of patients. Phase II trials in melanoma, liver cancer, and NSCLC are ongoing. Four small-molecule vascular-disruptive agents are in clinical trials. Three of these, combretastatin A4, ZD6126, and AVE8062, cause the destruction of tumor vasculature through tubulin depolymerization of proliferating endothelial cells. The fourth, 5,6-dimethylxanthenone-4-acetic acid, is a flavonoid that induces the release of tumor necrosis factor from intratumoral macrophages. Combretastatin A4 is the furthest along in development, with phase II trials ongoing. The most common dose-limiting adverse event in phase I trials was tumor pain.[53,54] Others were acute coronary syndrome, ataxia, and motor neuropathy.
Several investigational antiangiogenic drugs are aimed at targets other than the VEGF pathway, including endogenous antiangiogenic factors, integrin inhibitors, enzyme inhibitors, and vascular-disruptive agents. None of these, however, have yet advanced to phase III clinical trials. Those in phase II trials are discussed here.