746 Background: HGCSG0902 is the multicenter phase II study to investigate the safety and efficacy of IRIS/Cet as second-line treatment in patients with KRAS wild type mCRC. RR was 33.3% (95% CI, 20.8%-45.9%), therefore primary endpoint was met (Muto et al. ESMO 2014 ). Here we report an exploratory analysis of outcomes based on administration interval of Cet (every week [EW] vs bi-weekly [BW]). Methods: Eligibility includes histologically confirmed colorectal cancer, previously received oxaliplatin-contained chemotherapy, PS: 0-1, EGFR positive and KRAS WT. Pts. received S-1 80-120 mg/day p.o. on days 1-14 and irinotecan 100 mg/m2 on days 1 and 15 repeated every 28 days. Cet was administrated 400 mg/m2 as loading dose and continued 250 mg/m2 every week or 500 mg/m2 bi-weekly. The primary endpoint was RR and the secondary endpoints were disease control rate, PFS, OS and safety. To compare with EW and BW, Fisher’s exact test was used in terms of patient characteristics, AE, RR, and Log-rank test was used in terms of PFS, OS, and TTF. Results: Between March 2010 and September 2013, 58 pts were enrolled. One patient was not administered (57 pts were safety analysis set), and 3 pts were ineligible (54 pts were efficacy analysis set). Based on each physician choice, 34 pts. of EW and 23 of BW were included in the full safety analysis set. RR was 34.4% in the EW group and 31.8% in the BW group (p=1.000). Median PFS was 4.2 months (95% CI 3.5−4.9) in the EW group and 6.1 months (4.1-8.1) in the BW group (HR 0.752, 95% CI 0.413−1.372, p=0.350). The most common non-hematological adverse events of grade 3 or higher were diarrhea (23.5% in the EW group vs. 52.2% in the BW group: p=0.005) and stomatitis (2.9% in the EW group vs. 30.4% in the BW group: p=0.046), these were significantly more common in the BW group. Conclusions: IRIS/Cet appeared to be highly effective with RR and PFS in the both treatment schedule, and also had met the primary endpoint. Diarrhea and stomatitis were significantly more common in the BW group. Therefore, in case of treatment with IRIS, cetuximab should be administered weekly. Clinical trial information: 000004882.