Background: Olaparib is a PARP inhibitor that has shown activity in relapsed gastric cancer (GC) when combined with chemotherapy. MEDIOLA assesses the efficacy and safety of a chemo-free combination of olaparib and durvalumab, an anti-programmed cell death ligand-1 (PD-L1) agent in patients (pts) with solid tumors, including relapsed GC (NCT02734004). Methods: Eligible pts had GC that relapsed following platinum-containing therapy. Pts received olaparib 300 mg PO BID for a 4-wk run-in to allow for serial biopsies, followed by combination olaparib and durvalumab (1.5 g IV q 4 wks) until progressive disease (PD) as measured by RECIST 1.1. Tumor measurements were taken at baseline, 4 wks and q 8 wks thereafter. Primary endpoints were disease control rate (DCR) at 12 wks, safety, and tolerability. The secondary endpoints included DCR at 28 wks, objective response rate (ORR), duration of response (DoR), progression-free survival, and OS. Biomarker endpoints included PD-L1 expression and evaluation of tumor infiltrating lymphocytes. Bayesian predictive probability design was used for statistical analysis. Results: Forty pts were included in the safety and 39 in the efficacy analysis. Among 39 pts, median age was 57 yrs (range 28–77). Nineteen pts (48%) had a grade 3 AE; three pts (8%) a grade 4 AE. Most common AEs were anemia, lipase increase, fatigue, dysphagia, hyponatremia, and alkaline phosphatase increase. Ten pts (25%) had immune-mediated AEs, most commonly rash. The ORR was 10%; two pts had complete response; two had partial response. Median DoR was 11.1 months. DCR at 12 wks was 26%. Further efficacy and biomarker data will be presented. Conclusions: The combination of olaparib and durvalumab was tolerable, with no unexpected AEs. All responses occurred after the addition of durvalumab and were durable, suggesting synergistic treatment effect of the combination in some pts. Furthermore, several pts with early PD showed unexpectedly long survival. DCR at 12 wks was below the target (70%) due to a high rate of early PDs following the olaparib run-in. To address the early treatment failures, an upfront addition of more aggressive therapies to the combination should be explored. Clinical trial information: NCT02734004