The purpose of this study is to test the safety and efficacy of an experimental drug, OMP-305B83, when given in combination with FOLFIRI or FOLFOX. OMP-305B83 is a humanized bispecific monoclonal antibody and was developed to target cancer stem cells. Based on preclinical studies, it is believed that OMP-305B83 may block the growth of cancer stem cells and may also impair the productive growth of new blood vessels, which tumors need to grow and spread.

The study is sponsored by OncoMed Pharmaceuticals, which is referred to as OncoMed or the Sponsor.

This is an open-label, Phase 1b dose escalation and expansion study of OMP-305B83 plus FOLFIRI or FOLFOX to evaluate the safety, efficacy, and pharmacokinetics of OMP-305B83 in combination with FOLFIRI or FOLFOX in patients with metastatic Colorectal Cancer. This study consists of a screening period, a treatment period, and a post-treatment follow-up period in which patients will be followed for survival for up to 5 years. Patients will be enrolled in two stages: a dose-escalation stage and an expansion phase.

Inclusion Criteria:

• Measurable disease per response evaluation criteria in solid tumors (RECIST) v1.1
• Age >21 years
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1
• Adequate organ and marrow function
• For women of childbearing potential and men with partners of childbearing potential, agreement (by patient and/or partner) to use two effective forms of contraception from study entry through at least 6 months after the termination visit.
• Ability to understand and the willingness to sign a written informed consent document

Exclusion Criteria:

• Receiving any other investigational agents or any other anti-cancer therapy
• Receiving prior hepatic intra-arterial chemotherapy
• Known significant clinically significant gastrointestinal disease
• Patients with brain metastases (treated or untreated) leptomeningeal disease, uncontrolled seizure disorder, or active neurologic disease
• Significant intercurrent illness that will limit the patient's ability to participate in the study or may result in their death over the next 18 months
• Pregnant or nursing women
• Inability to comply with study and follow up procedure