Biweekly S-1 plus oxaliplatin (SOX) reintroduction in previously treated metastatic colorectal cancer patients (ORION 2 study): a phase II study to evaluate the efficacy and safety Abstract Background The reintroduction of oxaliplatin as a third-or-later-line regimen has been a promising option for patients with metastatic colorectal cancer (mCRC) who previously received chemotherapy including oxaliplatin. In this single-arm phase II study, we evaluated the efficacy of biweekly SOX, which is the combination of oxaliplatin reintroduction and S-1, as a third-or-later-line treatment. Methods Patients with mCRC who had previously received prior chemotherapy including oxaliplatin and irinotecan and were planned to receive the reintroduction of oxaliplatin were enrolled. Oxaliplatin (85 mg/m2) with/without bevacizumab (5 mg/kg) was given intravenously on day 1. Oral S-1 was administered on day 2–8 at a dose of 40–60 mg (calculated according to the body surface area) twice a day. Cycles were repeated every 2 weeks. The primary endpoint was the progression-free survival (PFS); our hypothesis was that the median PFS would be 3.5 months with a minimum threshold above 2.0 months. The secondary endpoints included the adverse events (AEs), response rate and overall survival (OS). Results A total of 41 patients from 12 institutes were enrolled. The median PFS and OS survival were 3.3 months (95% confidence interval [CI] 2.7–4.2) and 10.1 months (8.3–14.6), and response rate and disease control rate were 10.0% and 65.0%, respectively. Grade 3 AEs included thrombocytopenia (5.0%), anorexia (5.0%), pneumonia (5.0%) and fatigue (5.0%). There were no cases of grade 4 AEs or treatment-related death. Conclusion Biweekly SOX regimen with reintroduction of oxaliplatin could be exploitable as the third- and/or later-line treatments for patients with mCRC.