Epidermal Growth Factor Receptor Monoclonal Antibodies for the Treatment of Metastatic Colorectal Cancer

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Treatment of metastatic colorectal disease has evolved over the last decade. Two epidermal growth factor receptor (EGFR) monoclonal antibodies— cetuximab and panitumumab—have been developed in an effort to provide yet another therapeutic option. The EGFR is a transmembrane glycoprotein, expressed constitutively throughout the body and found on many epithelial tissues. The monoclonal antibodies bind to and inhibit the activation of the receptor in the body. This inhibition prevents tumor cell growth, angiogenesis, invasion, and metastasis, and induces apoptosis. Cetuximab and panitumumab exhibit nonlinear pharmacokinetics. Both monoclonal antibodies are approved for the treatment of refractory metastatic colorectal cancer. Cetuximab in combination with irinotecan has significantly better response rates and progression-free survival compared with those of cetuximab or irinotecan alone. Cetuximab and panitumumab as monotherapy have shown significantly better response rates and progression-free survival compared with best supportive care in patients refractory to irinotecan and oxaliplatin. In the Cetuximab Combined with Irinotecan in First Line Therapy for Metastatic Colorectal Cancer (CRYSTAL) trial, treatment-naïve patients received cetuximab in combination with the chemotherapy regimen infusional leucovorin, fluorouracil, and irinotecan (FOLFIRI) or FOLFIRI alone; the difference in progression-free survival was statistically significant but suggested only a modest benefit over FOLFIRI alone (8.9 vs 8 mo, p=0.036). Results of a preplanned analysis of the first 231 events in the Panitumumab Advanced Colorectal Cancer Evaluation (PACCE) trial favored the control group (chemotherapy regimen with folinic acid [leucovorin], fluorouracil, and oxaliplatin [FOLFOX] plus bevacizumab) instead of the control group plus panitumumab. For clinical consideration, many trials have shown that the intensity or absence of EGFR expression is not a clinically significant predictor of outcomes. Development and intensity of a rash are suggested to be a positive predictor of outcomes in patients. The most common adverse events of EGFR monoclonal antibody therapy are rash, diarrhea, and hypomagnesemia. Other serious but not common adverse events include hypersensitivity reactions and pulmonary toxicity. The availability of EGFR monoclonal antibodies has provided another weapon in the arsenal to treat refractory metastatic colorectal cancer. They have shown safety and efficacy in combination with other chemotherapy regimens and as monotherapy; however, their use as metastatic colorectal cancer therapy needs to be further explored.

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