A phase II trial of 5-weekly S-1 plus cisplatin (CDDP) combination with trastuzumab (Tmab) for HER2-positive advanced gastric or esophagogastric junction (EGJ) cancer: WJOG7212G (T-SPACE) study.

126 Background: Five-weekly schedule of S-1 plus CDDP (5-weekly SP) is the standard first-line chemotherapy for the patients (pts) with advanced gastric cancer (AGC) in Japan. Tmab with fluoropyrimidine plus CDDP chemotherapy has shown a survival benefit for pts with HER2-positive AGC and EGJ cancer. However, little information on the efficacy and safety of 5-weekly SP combined with Tmab is available. Methods: We conducted a prospective, single arm, multicenter, phase II trial of 5-weekly SP (S-1 40-60 mg bid for 21 days plus CDDP 60 mg/m2on day 8, every 5 weeks) combined with Tmab (8 mg/kg as a loading dose then 6 mg/kg, every 3 weeks) for pts with HER2-positive (IHC 3+ or IHC 2+ and FISH positive) AGC or EGJ cancer. Primary endpoint was response rate (RR) evaluated by independent review committee, using RECIST v1.1. Secondary endpoints included progression-free survival (PFS), overall survival (OS) and safety. With power of 80% and one-sided alpha of 10%, thirty-five pts were required to reject 40% of R...