This is a Phase Ib, open-label, multicenter, dose-escalation study designed to assess if PRO95780 in combination with two different irinotecan-containing regimens is safe and tolerable in patients with metastatic colorectal cancer (mCRC) who have progressed following, or cannot tolerate, first-line therapy with 5-fluorouracil-, oxaliplatin-, and bevacizumab-containing regimens. This study will also make a preliminary assessment of the anti-tumor activity of PRO95780 in combination with irinotecan and cetuximab or the FOLFIRI regimen plus bevacizumab.