Study to Evaluate Mechanisms of Acquired Resistance to Panitumumab

This study is designed to evaluate the mechanism(s) of resistance to the anti-epidermal growth factor receptor (EGFR) antibody panitumumab given in combination with irinotecan in metastatic colorectal carcinoma (mCRC) patients with wild-type Kirsten rat sarcoma-2 virus oncogene (KRAS) tumor status at the time of initial diagnosis.