Panitumumab (Vectibix, Amgen) was approved by the FDA for the treatment of patients with EGFR-expressing, metastatic colorectal carcinoma with disease progression on or following fluoropyrimidine-, oxaliplatin-, or irinotecan-containing regimens. The NCCN Colon Cancer panel added panitumumab as alternate option to cetuximab after first or second progression on previous therapy. This addition came with a recommendation that patients should not be excluded from therapy on the basis of EGFR results. NCCN does not recommend using either of these two EGFR blockers after the other. This recommendation is based on a lack of studies or literature showing effectiveness for such use and  the similar mechanism of action of the two drugs. Because the mechanism of action is similar, a patient was failing on one of the drugs would presumably also failed the other one. Prescribing information states specifically that Vectibix is not appropriate for use after Erbitux.

NCCN.ORG, colon cancer

Vectbix, Prescribing Information