Study to Evaluate Mechanisms of Acquired Resistance to Panitumumab

In Part 1, all participants will undergo a baseline tumor biopsy and will receive panitumumab with irinotecan. Participants who respond or have stable disease will continue to receive treatment until radiographically-confirmed disease progression. These participants will then undergo a second tumor biopsy and blood sampling and then proceed to Part 2 of the study. Participants with radiographically confirmed disease progression at the time of the first tumor measurement will undergo blood sampling and proceed directly to Part 2. In Part 2, participants will receive panitumumab with ganitumab. In both parts of the study, panitumumab and irinotecan (Part 1) and panitumumab and ganitumab (Part 2) will be administered every 2 weeks until disease progression, intolerability, withdrawal of consent, death, or unless otherwise indicated by the study team.