Background:TAS-118 (S-1 and leucovorin) + oxaliplatin (L-OHP) improved overall survival (OS) compared to S-1 + cisplatin for patients (pts) with advanced gastric cancer (GC) (Kang, Lancet Oncol. 2020). This study investigated the feasibility of peri (pre and post)-operative (op) chemotherapy (chemo) with TAS-118 ± L-OHP in pts with locally advanced resectable GC. While it was reported that pre-op TAS-118 + L-OHP followed by D2 gastrectomy was well tolerated and showed promising efficay (Takahari, ASCO-GI. 2020), the recommended post-op chemo regimen, TAS-118 or TAS-118 + L-OHP, has yet to be determined. Methods:Eligible pts with GC of clinical T3-4N1-3M0 were enrolled. The protocol treatment consisted of pre-op chemo with 4 courses of TAS-118 (40-60 mg/body, orally, twice daily, 7 days) + L-OHP (85 mg/m², intravenously, day 1) in a 2-week cycle, and gastrectomy with D2 lymphadenectomy, followed by post-op chemo with 12 courses of TAS-118 (step 1) and 8 courses of TAS-118 + L-OHP (step 2). Step 2 was started if the dose-limiting toxicity (DLT) occurred in < 6 of 10 pts in step 1. Up to 20 pts were included in the analysis of feasibility after a recommended regimen was determined. Results:Between December 2016 and February 2019, 45 pts were enrolled. The numbers of pts with cT3/4a and cN1/2/3 were 13/32 and 25/17/3, respectively. Excluding 14 pts (4 achieving pathological complete response, 4 not satisfying the criteria for post-op chemo, 3 physician judgement or pt withdrawal, 2 progressive disease, 1 adverse event [AE]), 31 pts (11/20 in step 1/2) received the post-op chemo. No DLT was observed in either step. The post-op chemo completion rate was 90.9% (95% CI, 63.6-99.5) in step 1 and 80.0% (95% CI, 59.9-92.9) in step 2. The median relative dose intensity of TAS-118 in step 1 was 83.3%, and those of TAS-118 and L-OHP in step 2 were 69.9% and 74.3%, respectively. One pt in step 2 discontinued post-op chemo due to AE. Grade ³ 3 AEs observed in ≥ 10% of pts were weight loss in both step 1 and step 2 (2 in each), and hypokalemia (n = 3) and neutropenia (n = 2) in step 2. At 1-year follow-up after the last pt was enrolled, recurrence-free survival and OS rates were 91.1% (95% CI, 78.0-96.6) and 100%, respectively at 12 months, and 69.1% (95% CI, 49.6-82.3) and 95.5% (95% CI, 71.9-99.3), respectively at 24 months. Conclusions:Taken together with the feasibility and efficacy of pre-op chemo, peri-op chemo with TAS-118 + L-OHP with D2 gastrectomy was well tolerated and showed promising efficacy. Clinical trial information: UMIN000024688.