Background: The FOLFOXIRI regimen (irinotecan, oxaliplatin, 5-fluorouracil and leucovorin) improves the response rate and overall survival compared to FOLFIRI in pts with metastatic colorectal cancer (mCRC), and addition of cetuximab to chemotherapy increases efficacy in pts with RAS wild-type mCRC. We conducted a phase 1 study of FOLFOXIRI plus cetuximab to determine the maximum-tolerated dose (MTD) and recommended dose (RD) and to assess its safety and efficacy in Japanese pts with RAS wild-type mCRC. Methods: Main eligibility criteria were: histologically confirmed colorectal adenocarcinoma; KRAS and NRAS wild-type status; measurable metastatic disease according to Response Evaluation Criteria in Solid Tumors version 1.1; age 20-74 years; Eastern Cooperative Oncology Group performance status 0 or 1. Pts with UDP-glucuronosyltransferase 1A1*6/*6, *28/*28 and *6/*28 genotypes were excluded. Pts received an escalating dose of intravenous irinotecan (100, 120, and 150 mg/m² in the dose levels 0, 1, and 2, respectively) and a fixed dose of intravenous oxaliplatin (85 mg/m²), continuous infusion 5-fluorouracil (2400 mg/m²) plus l-leucovorin (200 mg/m²), and cetuximab (an initial dose of 400 mg/m² followed by 250 mg/m² per week). Results: A total of 9 Japanese pts were treated (3 and 9 in the dose levels 1 and 2, respectively). No patients experienced a dose-limiting toxicity (the MTD was not reached), and the dose level 2 (irinotecan 150 mg/m²) was established as the RD. With a median 8 cycles per patient, the most common grade 3 or 4 adverse events included neutropenia (44%), paronychia (22%), and acne-like rash (11%). No febrile neutropenia and treatment-related death were observed. Among 9 pts, 1 pt had complete response, 7 pts had partial response, and 1 pt had progressive disease, for an overall response rate of 89%. As of September 24, 2017, median progression free survival was 14.0 (95% CI 7.2-20.7) months. Conclusions: The combination of cetuximab and FOLFOXIRI has shown a favorable toxicity profile and promising antitumor activity in pts with RAS wild-type mCRC. Clinical trial information: UMIN000018217.