Targeting Vascular Endothelial Growth Factor in Colorectal Cancer

August 1, 2002

Recent trials have established the IFL combination (fluorouracil [5-FU], leucovorin, and irinotecan [CPT-11, Camptosar]) as a new standard first-line therapy for patients with metastatic colorectal cancer. Median survival for such patients treated with IFL still ranges from approximately 14 to 18 months, however, underscoring the need for new agents with novel mechanisms of action.

ABSTRACT: Recent trials have established the IFL combination (fluorouracil [5-FU], leucovorin, and irinotecan [CPT-11, Camptosar]) as a new standard first-line therapy for patients with metastatic colorectal cancer. Median survival for such patients treated with IFL still ranges from approximately 14 to 18 months, however, underscoring the need for new agents with novel mechanisms of action. Angiogenesis has become an attractive target for anticancer drug development, based on its important roles in tumor growth, invasion, and metastasis. A potent stimulus of angiogenesis is vascular endothelial growth factor (VEGF); two agents developed to inhibit VEGF activity, bevacizumab (Avastin) and SU5416, are in advanced clinical trials. Based on encouraging results in phase I and II trials with bevacizumab, a randomized trial of IFL with or without this monoclonal antibody is under way. Similarly, a randomized trial of 5-FU and leucovorin with or without the tyrosine kinase inhibitor SU5416 has recently completed accrual and results are pending. SU5416 is also being tested in a phase I/II trial combined with IFL. This article briefly reviews preclinical and clinical data leading to the current trials of these two agents in patients with colorectal cancer. [ONCOLOGY 16(Suppl 7):13-15, 2002]

Colorectal cancer is thesecond leading cause of cancer death in the United States, with approximately45,000 deaths and 130,000 newly diagnosed patients per year.[1] Recently, twotrials randomized patients to fluorouracil (5-FU)/leucovorin or IFL(5-FU/leucovorin plus irinotecan [CPT-11, Camptosar]) as first-line therapy formetastatic colorectal cancer. The primary end point of both trials was time totumor progression. One of the trials, conducted in the United States, used IFLat a weekly × 4 schedule for each 6-week treatment cycle, and also had anirinotecan-alone arm.[2] This trial resulted in a higher response rate andlonger time to tumor progression for IFL compared with 5-FU/leucovorin, and alsoa statistically significant survival benefit (14.8 vs 12.6 months, P =.04).

The other trial, conducted in Europe, allowed sites to choose one of twoinfusion schedules (once weekly or every 2 weeks) for the 5-FU/leucovorinregimen.[3] Patients were then randomly assigned to the 5-FU/leucovorin regimenalone or in conjunction with irinotecan. Results of this trial also showed ahigher response rate and longer time to tumor progression for IFL compared with5-FU/leucovorin. A survival benefit was demonstrated for IFL compared with5-FU/leucovorin as first-line therapy (17.4 vs 14.1 months, respectively; P= .031). However, median survival (14.8 to 17.4 months) obtained with IFL isstill limited. New active agents, ideally ones that target intracellularmechanisms or pathways, are still needed.

VEGF and Angiogenesis

The process of angiogenesis, or new blood vessel formation, has emerged as anovel target for development of anticancer agents. Preclinical data demonstratethat new blood vessel formation is required for tumors to grow beyond 1 to 2mm³. Laboratory analyses also demonstrate that, in addition to being criticalfor tumor growth, angiogenesis is important for invasion and metastasis.Angiogenesis is a complex, multistep process involving breakdown of theextracellular matrix, invasion of tumor cells, signaling to stimulateendothelial cell growth, and blood vessel formation. One of the most potentstimulants of angiogenesis is vascular endothelial growth factor (VEGF).

VEGF was reported to be overexpressed in 48% to 53% of colorectal cancers.[4]The same study suggested that VEGF expression correlated with progression ofdisease and appeared to be an independent prognostic factor in colorectalcancer. Another study suggested that high preoperative serum VEGF levels wereassociated with increased likelihood of recurrence in patients with resectedcolorectal malignancies.[5] Other study results have also shown correlation ofVEGF overexpression with advanced disease stage, likelihood of developingmetastases after surgery, and overall prognosis.[6-8]

The cellular receptors for VEGF are tyrosine kinases (eg, KDR or flk-1 andFLT-1) that initiate the angiogenesis process through phosphorylation cascades.When VEGF binds to its endothelial cell receptor, the intracellular tyrosinekinase portion is activated, resulting in a phosphorylation cascade thatstimulates endothelial cell proliferation and new blood vessel growth.Inhibiting VEGF effects appears to reduce angiogenesis (reduced vessel density)in vitro, and limits tumor growth in vivo.[9-11] Two agents developed to inhibitVEGF action have entered clinical trials, namely, bevacizumab (Avastin) andSU5416.

Bevacizumab

Murine monoclonal antibodies have been developed to inhibit VEGF in a varietyof tumor models. However, murine antibodies can readily induce human antimurineantibody (HAMA) responses. Bevacizumab, a recombinant humanized monoclonalantibody to VEGF, was designed with a human IgG1 framework and a murine VEGF-bindingportion. In the first phase I trial of single-agent bevacizumab, 25 patientswere treated on five dose levels ranging from 0.1 to 10 mg/kg.[12] While nograde 3 or 4 toxicities were clearly related to therapy, there were two episodesof serious bleeding from tumor that were not clearly related to therapy. Nopartial or complete responses occurred, but one patient had a minor response and12 experienced stable disease during the 70-day study period. No patientdeveloped antibodies to bevacizumab.

Subsequently, in a phase Ib trial, 12 patients were assigned to one of threetreatment arms (four patients per arm). Treatments included bevacizumab combinedwith either doxorubicin, carboplatin (Paraplatin)/paclitaxel, or5-FU/leucovorin.[13] Results showed that bevacizumab could be safely combinedwith three chemotherapy regimens. One patient with colorectal cancer respondedto 5-FU/leucovorin plus bevacizumab treatment.

A subsequent randomized phase II trial evaluated the efficacy andtolerability of two different doses of bevacizumab in conjunction with 5-FU andleucovorin. A total of 104 patients were randomly assigned to receive 5-FU andleucovorin, or 5-FU and leucovorin with either 5 or 10 mg/kg of bevacizumab.[14]Randomized phase II trials are designed to evaluate each arm individually andcomparisons between treatment arms have to be viewed cautiously. Nonetheless,time to disease progression was longer than expected in patients receiving 5mg/kg bevacizumab than in those receiving 5-FU and leucovorin alone (9.2 vs 5.2months, respectively, when independent review facility results were included;time to disease progression was 7.2 months in patients receiving 10 mg/kgbevacizumab). These data led to a randomized trial that is currently testing IFLplus placebo vs IFL plus bevacizumab in patients with metastatic colorectalcancer.

Inhibition ofthe Flk-1 (KDR) Receptor With SU5416

The tyrosine kinase inhibitor SU5416 blocks the kinase activity of the Flk-1receptor for VEGF. This small molecule appears to inhibit tyrosine kinase atlevels of 40 nM.[15] In animal models of liver metastasis, SU5416 decreased thenumber of liver metastases by 48% and microvessel formation by 42%. SU5416administration increased the rate of apoptosis 3-fold in tumor cells and 19-foldin endothelial cells.[16] Two phase I trials of twice-weekly SU5416 wereperformed. One study established a recommended phase II dose of 145 mg/m² twiceweekly, with dose-limiting toxicities of headache and emesis.[17]Pharmacokinetic analysis demonstrated that at the 145 mg/m² dose level, an areaunder the concentration-time curve of 0 to 24 for SU5416 was similar to thatproducing "effective tumor growth inhibition" in animal studies.[18]

Another phase I trial using the twice-weekly dosing schedule showed thatemesis and headache were dose-related, but a maximum tolerated dose had not beenreached at the time of report.[19] This trial also assessed vascularpermeability by using dynamic contrast magnetic resonance imaging. Preliminaryanalysis suggested that vascular permeability increased in patients with stabledisease. Results from both of these phase I trials suggested that SU5416 caninduce its own clearance and that the half-life is short.

SU5416 was then assessed in a phase II trial of patients who had received nomore than two previous therapies for advanced colorectal cancer.[20] Preliminaryfindings for the first 15 enrolled patients suggested that 7 patients had stabledisease. A phase I/II trial evaluated SU5416 combined with 5-FU andleucovorin,[21] using the two most common 5-FU plus leucovorin schedules: daily× 5 every 4 to 5 weeks (Mayo Clinic regimen) and weekly × 6 every 8 weeks (RoswellPark regimen). The regimens were well tolerated overall; fewer side effects wereseen in patients receiving the Roswell Park regimen. Responses were seen and thelong survival time in this study was promising.

Vanderbilt University initiated a phase I/II trial of SU5416 plus IFL.[22]SU5416 was given as a 3-hour infusion twice weekly throughout the cycle.Irinotecan, 5-FU, and leucovorin were administered weekly for 4 weeks followedby a 2-week rest (6-week cycles). Eligible patients had not received previoustreatment for colorectal cancer (previous adjuvant therapy was allowed providingat least 1 year had lapsed since the last dose). However, further development ofthis agent has been halted since the early results of a trial in patients withcolorectal carcinoma were not promising.

Conclusions

Two new VEGF inhibitors, SU5416 and bevacizumab, have shown intriguing signsof antitumor activity in vitro and in the clinic. SU5416 and bevacizumab maypotentially enhance the effects of current standard chemotherapy regimens forcolorectal cancer (eg, IFL), possibly prolonging time to tumor progression andsurvival; however, results of several ongoing and planned trials are needed todetermine the roles of these agents in this disease. Bevacizumab is currentlybeing investigated in a randomized trial comparing IFL to IFL plus bevacizumab.Several other trials are assessing bevacizumab, including an Eastern CooperativeOncology Group study of second-line therapy that includes a single-agentbevacizumab arm. These trials will help define the role of bevacizumab in first-and/or second-line treatment of colorectal cancer. Further development of SU5416has been halted, but other tyrosine kinase inhibitors of VEGF-mediatedangiogenesis are under development.

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