Background: Panitumumab with fluorouracil, leucovorin, and irinotecan (FOLFIRI) in second-line chemotherapy increased objective response rate and prolonged progression-free survival (PFS) versus FOLFIRI alone in patients with wild-type (WT) KRAS metastatic colorectal cancer (mCRC) (Peeters et al, J Clin Oncol 2010). This trial (UMIN000004659) evaluated tolerability and efficacy of combination therapy with irinotecan and S-1, an oral fluoropyrimidine (IRIS) plus panitumumab as second-line chemotherapy in patients with WT KRASmCRC. Methods: Main inclusion criteria were: patients with WT KRAS mCRC refractory to one prior chemotherapy regimen for mCRC, ECOG PS 0-2, and age ≥20 years. Patients received panitumumab (6 mg/m²) and irinotecan (100 mg/m²) on days 1 and 15 and S-1 (40-60 mg according to body surface area) twice daily for 2 weeks, repeated every 4 weeks. The primary endpoint was completion rate of protocol therapy (CRT). The secondary endpoints were response rate (RR), progression-free survival (PFS), and overall survival (OS). Results: Thirty-seven patients were enrolled in 9 centers. The overall CRT was 62.2% (23/37). Most frequent grade 3/4 toxicities were: skin rash (24%), diarrhea (16%), and appetite loss (11%). The overall RR was 32.4% (12/37). Of these, four patients underwent conversion surgery. Median PFS and OS were 9.5 months (95% CI: 3.5-15.4 months) and 20.1 months (95% CI: 16.7-23.2 months), respectively. Conclusions: IRIS plus panitumumab has acceptable toxicity profile and promising efficacy in patients with previously treated WT KRAS mCRC. This regimen can be an additional treatment option for second-line chemotherapy in WT KRAS mCRC. Clinical trial information: UMIN000004659.