Multicenter phase II study of combination therapy with cetuximab and S-1 in patients with KRAS exon 2 wild-type unresectable colorectal cancer previously treated with irinotecan, oxaliplatin, and fluoropyrimidines (KSCC 0901 study)

Purpose: Anti-epidermal growth factor receptor antibody therapy alone or in combination with irinotecan is recognized as a standard third-line treatment for KRAS wild-type unresectable metastatic colorectal cancer. However, in some cases, it is difficult to administer irinotecan after third-line treatment. Therefore, we examined the efficacy and safety of the combination of cetuximab and S-1 in patients with KRAS wild-type unresectable metastatic colorectal cancer who were previously treated with irinotecan, oxaliplatin, and fluoropyrimidines. Methods: The study was designed as a phase II, non-randomized, open-label, multicenter trial. Cetuximab was initially administered at 400 mg/m², followed by weekly infusion at 250 mg/m². S-1 was administered at a fixed dose of 80 mg/m² orally twice daily for 28 days followed by a 14-day break, resulting in a 6-week treatment course. The primary endpoint was progression-free survival (PFS). The secondary endpoints were the overall response rate (ORR), overall survival (OS), disease control rate (DCR), time to treatment failure, dose intensity, safety, and BRAF mutation status. Results: Thirty-seven patients were eligible. The median PFS was 5.5 months, the median OS was 13.5 months, the ORR was 29.7 %, and the DCR was 73.0 %. The relative dose intensity was 86.8 % for cetuximab and 88.1 % for S-1. Grade 3–4 adverse events that occurred in >10 % of the patient population included rash, dry skin, diarrhea, paronychia, anorexia, fatigue, mucositis, and neutropenia. Conclusions: Combination therapy with cetuximab and S-1 was effective and well tolerated in patients with irinotecan-, oxaliplatin-, and fluoropyrimidine-refractory metastatic colorectal cancer.