KRN330 is a recombinant, fully-human monoclonal antibody directed against A33, a surface differentiation antigen that is uniformly expressed in 95 % of colorectal cancers. A previous Phase 1 study of single-agent KRN330 identified a maximum tolerated dose (MTD) of 3 mg/kg q2w and preliminary evidence of clinical activity among patients with advanced and metastatic colorectal cancer (mCRC). This Phase 1/2 trial sought to assess the safety and activity of second-line KRN330 plus irinotecan in patients with mCRC. Patients with mCRC who showed disease progression after FOLFOX/CapOx received intravenous doses of KRN330 (0.5 or 1.0 mg/kg qw or q2w) plus irinotecan (180 mg/m(2)) in a standard 3 + 3 dose escalation. The MTD of KRN330 with irinotecan in 19 patients was 0.5 mg/kg qw in the Phase 1 study with gastrointestinal effects and neutropenia being the predominant dose-limiting toxicities. In the Phase 2 study, the most frequent treatment-related Grade ≥3 toxicities in 44 patients were fatigue (15.9 %), neutropenia (13.6 %), leukopenia (6.8 %), diarrhea (4.5 %), and dehydration (4.5 %). Objective response rate (ORR) was 4.5 % and disease control rate was 45.5 % for the intent-to-treat population. Median progression-free survival was 87 days (95 % CI, 43-136 days). The prespecified ORR of KRN330 plus irinotecan was not met. Further investigation of KRN330 plus other agents may be warranted.