Cetuximab (Erbitux) and panitumumab (Vectibix) are monoclonal antibodies directed against the EGFR. Cetuximab is a chimeric antibody, whereas panitumumab is a fully human monoclonal antibody. The EGFR receptor is overexpressed in colorectal can-
cers and leads to an increase in tumor proliferation and growth. ' ' Cetuximab received FDA approval for use in EGFR-expressing meta-static colorectal cancer in irinotecan relapsed or refractory patients. Cetuximab should be administered in combination with irinotecan, but can be used as a single agent in patients who cannot tolerate irinotecan-based chemo-therapy. Panitumumab is approved as monotherapy agent and should not be used in combination with other agents outside of clinical trials.
Both agents are well tolerated with infusion-related reactions being cetuximab's dose-limiting toxicity and rash most commonly seen with panitumumab. Patients receiving cetuximab require premedication with acetaminophen and diphenhydramine and may require modifications to their adminstration schedule or permanent discontinuation if they develop severe allergic toxicity. A skin rash and diarrhea are also commonly seen with both agents, and health care practitioners should provide counseling to patients about these adverse effects. Treatment options include common medications used to treat acne (doxycycline), topical and systemic steroids, and general skin care. Development of rash may be a surrogate marker of response and clinicians should attempt to minimize the complications of the rash prior to discontinuing therapy.41 Other toxicities common to both agents include low magnesium, calcium, and potassium levels that require checking levels and replacement therapy as clinically indicated. A rare (less than 1%) interstitial lung disease is seen with all agents that inhibit EGFR and patients should be instructed to report an2y new onset shortness of breath. Cetuximab has an initial loading dose of 400 mg/m IV infusion. Weekly doses of 250 mg/m2 are then administered starting the following week. Panitumumab is given 6 mg/kg every 2 weeks.
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