Chemotherapy and Pembrolizumab, Followed by Pembrolizumab and Olaparib As Firstline Therapy in Her-2 Negative Gastric/GEJ Adenocarcinoma

400 mg Pembrolizumab day 1 Q6W (max. 18 cycles)

300 mg Olaparib bid. cont. day 1 to 42 (max. 16 cycles)

Oxaliplatin 85 mg/m² 2h day 1, 15, 29 plus Leucovorin 400 mg/m² 2h day 1, 15, 29 plus 5-FU 400 mg/m² bolus, followed by 2.400 mg/m² 46h day 1, 15, 29; Q6W, 2 cycles

Oxaliplatin 130 mg/m² 2h day 1,22 plus Capecitabine 1.000 mg/m² bid. day 1-14, 22-35; Q6W, 2 cycles