Background: IDH1/2 mutations (m) occur in approximately 25% of cholangiocarcinomas (CCAs) (20% intrahepatic, < 5% extrahepatic) and result in the accumulation of R-2-hydroxyglutarate (R2HG). R2HG, suppresses homologous recombination repair and as a result IDH1/2m are hypothesized to predict PARP inhibitor sensitivity. PARP inhibition upregulates PD-L1 and the combination is thought to be synergistic. We conducted a phase II trial of olaparib with durvalumab for patients with IDH-mutated cholangiocarcinoma. Methods: Patients with advanced IDHm CCA were enrolled in this phase II open-label study (NCT03991832). Patients received olaparib 300 mg twice daily continuously and durvalumab 1500 mg IV every 4 weeks. Simon’s optimal two-stage design was used, with an interim analysis planned for efficacy. 10 patients were accrued in the first stage, with a plan for expansion if ≥2 responses were observed. The primary objective was to determine the overall response rate (RECIST 1.1) and disease control rate (DCR). Secondary objectives included progression-free survival, overall survival, and safety of the olaparib and durvalumab combination. Results: 10 patients with IDH1m were enrolled in this study between January 2020-August 2021. Median age was 63.5 years (range 49-78), and 50% were female. All patients had a baseline ECOG PS of 0-1. 40% of patients had prior surgical resections and subsequently developed recurrent disease. 20% of patients had ³2 lines of prior chemotherapy, with 90% receiving prior platinum. Median duration of treatment was 1.95 months (range 1.8-13.5). There were no treatment-related grade 3-4 toxicities. Any grade toxicities included fatigue (100%), nausea (60%), anemia (20%), hypothyroidism (20%), elevated liver enzymes (20%). We did not observe any complete or partial responses. The DCR was 30% with 3 patients demonstrating stable disease. One patient remained on treatment for 13.5 months, and was eventually taken off study due to clinical progression. The median PFS was 1.97 months (95% CI 1.73-3.93). Given that no responses were seen in the initial 10 patients, the study did not proceed to Stage 2. Conclusions: While the combination of olaparib and durvalumab was well-tolerated, this study was discontinued after Stage I due to lack of efficacy. IDH1/2m may not induce a BRCAness phenotype and further correlative studies are planned to evaluate this. Clinical trial information: NCT03991832.