Single agent olaparib in advanced oesophago-gastric cancer: Primary results from the SOlar clinical phase II single arm trial.

427 Background: A subset of pts with advanced oesophago-gastric adenocarcinoma (OGA) are characterised by deficient DNA damage repair (DDR) providing a rationale for PARP inhibition (PARPi) in these tumours. Potential activity was observed in the phase 3 gastric cancer trial with Olaparib + paclitaxel, however the efficacy of Olaparib monotherapy and response biomarkers have not been established. Herein we report the primary clinical results from the SOlar trial. Methods: SOlar is a prospective, open label, single arm phase II trial using a Simon’s two stage optimal design, evaluating the efficacy and safety of Olaparib 300mg twice daily in pts with locally advanced/metastatic OGA who had previously progressed on ≥1 line of therapy in the advanced setting. Pts with measurable disease by RECIST v1.1 amenable to mandatory baseline biopsy were included. The primary end point was disease control rate (DCR) at 8 weeks from Olaparib initiation by RECIST v1.1. In this two-stage optimal design, if ≥5/27 pts had disease control in stage 1, a further 27 pts were to be recruited in stage 2. If ≥12/54 evaluable pts achieved disease control, Olaparib would warrant further investigation. Other key endpoints include; overall response rate, progression free survival and overall survival, and translational biomarker analyses (NCT03829345). Results: Between July 2019 - February 2024, 55 pts with locally advanced/metastatic OGA were enrolled. 51 pts received Olaparib and 50 were evaluable for the primary endpoint (5 withdrew). The median age was 61yrs (range: 54-67), 7 (14%) were HER2 -positive and 2 had a known germline BRCA mutation. 19 (38%) pts had received ≥3 lines of therapy and 22 (44%) pts had received prior immunotherapy for OGA. 30 (60%) pts had objective responses to prior platinum chemotherapy (CR + PR). The DCR at 8-weeks was 28% (91% one-sided confidence interval lower bound: 19.4) with 14/50 evaluable pts having stable disease. No objective response was observed. 36/51 (71%) treated pts experienced a treatment-related adverse event (TRAE) of any grade, and 9 (18%) experienced a grade ≥3 TRAE (most common being anaemia, fatigue, vomiting). Two serious adverse events were reported as treatment-related: definitely related grade 1 pneumonitis, and possibly related grade 4 lung infection. Conclusions: In SOlar, Olaparib met its primary endpoint with 28% DCR in this heavily pre-treated group. No new safety concerns were observed. Ongoing translational analyses include; homologous repair deficiency scoring, whole exome sequencing, RNA expression, serial ctDNA, and multiplex immunofluorescent assessment of the tumour micro-environment, with a view to identify a subgroup who might benefit from PARPi to support ongoing PARPi combinatorial studies in advanced OGA. Clinical trial information: NCT03829345 .