Building on the Foundation of 5-FU to Treat Metastatic Colorectal Cancer

Cathy Eng, MD
Assistant Professor
Department of GI Medical Oncology
The University of Texas
M.D. Anderson Cancer Center
Houston, Texas

Pohl and colleagues have provided a concise overview of current treatment options for metastatic colorectal cancer (mCRC). However, the authors do not provide personal insights as to what direction this burgeoning field will take next. Within the past decade, an abundance of treatment options have emerged. With all available therapies, the median overall survival is reportedly 31.8 months.[1] Yet the 5-year overall survival of a patient with surgically unresectable disease is < 10%, and the duration of treatment is indefinite.

Recent therapeutic development has focused primarily on biologic therapy, which is thought to result in fewer toxicities but has largely remained unproven, since combined treatment with chemotherapy continues to provide the most effective approach for the mCRC patient. Furthermore, single-agent or combined biologic therapy alone has no greater efficacy and may in fact result in significant toxicities.[2] Hence, treatment for the mCRC patient often carries a risk of toxicity, financial burden, and possible psychological strain. We need to identify key predictive and prognostic markers to optimally benefit our patients.

Bevacizumab, FOLFOX, and XELOX
Two signal transduction pathways mentioned by the authors involve the vascular endothelial growth factor (VEGF) and the epidermal growth factor receptor (EGFR). Pohl and colleagues highlight the role of the VEGF inhibitor bevacizumab (Avastin) in first- and second-line bevacizumab-naive patients and discuss the original registration trials of AVF2107 and ECOG 3200.[3,4] They briefly mention the results of NO16966 but do not attempt to explain the confounding data.[5,6]

The NO16966 study was originally created as a 2 × 2 international phase III trial to establish noninferiority of FOLFOX (leucovorin, fluorouracil [5-FU], oxaliplatin [Eloxatin]) vs XELOX (capecitabine [Xeloda], oxaliplatin). With the subsequent US Food and Drug Administration (FDA) approval of bevacizumab, NO16966 was amended to be a 2 × 2 placebo-controlled trial with primary endpoints of noninferiority for FOLFOX vs XELOX and superiority of bevacizumab when added to chemotherapy; each primary endpoint was fulfilled. However, when evaluating the benefit of bevacizumab to the specific chemotherapy regimen, the hazard ratio (HR) for progression-free survival was superior only for the XELOX regimen (7.4 vs 9.3 months; HR = 0.77 [97.5% confidence interval (CI) = 0.63–0.94]; P = .0026) in comparison to the FOLFOX regimen (8.6 vs 9.4 months; HR = 0.89 [97.5% CI = 0.73–1.08]; P = .1871). The preliminary findings from this trial resulted in significant concerns about bevacizumab in combination with FOLFOX.

The authors suggest that the response rate and overall survival are consequences of the discontinuation of all chemotherapy, but further elaboration is needed. Other reasons account for this outcome: First, the superior hazard ratio for XELOX/bevacizumab may be a result of the longer progression-free survival in the control arm of FOLFOX vs XELOX resulting in a greater measurable difference. Second, only 29% of bevacizumab patients received treatment until disease progression. Final details of NO16966 are to be published at a later date.

Other Anti-VEGF Therapies
Anti-VEGF therapies discussed by the authors include the oral tyrosine kinase inhibitors (TKI) sunitinib (Sutent) and vatalanib. Sunitinib is a multitargeted TKI currently in phase I/ II development for the treatment of mCRC. Pohl and coauthors state that the reason for the efficacy of sunitinib in metastatic renal cell carcinoma (mRCC), as opposed to less impressive results in mCRC, is unknown. However, they fail to mention the known association between anti-VEGF therapy and mRCC. The transcription factor known as hypoxia-inducible factor (HIF-1α) is normally degraded by the von Hippel-Landau (VHL) protein. However, in the presence of the VHL mutation, HIF-1α degradation is inhibited, resulting in increased VEGF expression, which is likely to account for the single-agent activity of sunitinib and bevacizumab.

The authors suggest that there is a future role for the oral VEGF receptor inhibitor vatalanib in mCRC. Furthermore, high levels of lactate dehydrogenase (LDH) correlated with progression-free survival. However, vatalanib has been extensively evaluated in the first- and second-line settings, failing to fulfill the primary endpoint of progression-free survival. Furthermore, correlation with elevated LDH and progression-free survival was noted on post ad hoc analysis. Elevated LDH levels are nonspecific and not classically viewed as a prognostic indicator in mCRC. Hence, it is unlikely that vatalanib will have a future role in mCRC.