Background: The programmed death-1 receptor (PD-1) and its ligand (PD-L1) are key therapeutic targets in the reactivation of the immune response against cancer. Avelumab* (MSB0010718C) is a fully human anti-PD-L1 IgG1 antibody being investigated in clinical trials. We report safety and clinical activity from this phase Ib dose expansion JAVELIN study (NCT01943461) of avelumab in Japanese patients (pts) with advanced gastric or gastroesophageal junction adenocarcinoma (GC/GEJ) based on level of PD-L1 expression. Methods: GC/GEJ pts (ECOG performance status 0-1 at trial entry) received avelumab 10 mg/kg Q2W by IV infusion until confirmed progression, unacceptable toxicity, or withdrawal. Tumors were assessed every 6 wks (RECIST 1.1). Best overall response rate (ORR) and progression-free survival (PFS) were evaluated. Adverse events (AEs) were graded by NCI-CTCAE v4.0. PD-L1 expression on fresh tissue samples collected up to 6 mos prior to trial was assessed by immunohistochemistry using various cutoff criteria. Results: As of Mar 11 2015, 20 pts (18 GC, 2 GEJ) were treated with a median follow-up of 6 mos. Treatment-related treatment-emergent AEs (TR-TEAEs) of any grade occurred in 18/20 pts (90%); one patient (5%) reported a grade 3 TR-TEAE (alanine aminotransferase increase). There were no treatment-related deaths. Confirmed ORR was 15.0% based on 3 partial responses (PR) and the disease control rate (PR + stable disease) was 65.0%. PD-L1 expression was evaluable in 19 pts. Pts with PD-L1+ samples (n = 5 [26.3%]; ≥ 1% cutoff) showed a 40.0% ORR compared with 7.1% ORR in pts with PD-L1− samples (n = 14 [73.7%]; p= 0.155). Median PFS was 12.3 wks (95% CI: 3.1, ne) for PD-L1+ and 11.1 wks (6.0, 12.1) for PD-L1− ( ≥ 1% cutoff). PFS rate at 12 wks was 60.0% (95% CI: 12.6, 88.2) and 32.1% (10.2, 56.9) for PD-L1+ and PD-L1− pts, respectively. Conclusions: Single agent avelumab showed an acceptable safety profile and clinical activity in GC/GEJ pts. A trend of higher ORR and higher PFS rate was observed in PD-L1+ pts compared with PD-L1− in this small cohort. Further analysis is ongoing and expansion of this cohort to 40 pts is underway. *Proposed INN. Clinical trial information: NCT01943461