Background: BTC is rare and aggressive disease with heterogeneous genetic profiles. Gemcitabine plus cisplatin (GemCis) with or without durvalumab is the standard first-line therapy but further improvement in survival outcomes is needed. In a prior study (Chae HJ et al, Eur J Cancer 2019), germline or somatic mutations in DDR genes including BRCA are detected more than half of BTC patients and associated with better survival outcomes with platinum-based chemotherapy. As the potential efficacy of Olaparib in DDR-mutated cancers and synergism between PARP inhibitors and immune checkpoint inhibitors, we designed randomized phase 2 trial investigating the efficacy and safety of olaparib monotherapy vs olaparib plus durvalumab in BTC patients as maintenance therapy for BTC patients who showed durable response to first-line platinum-based chemotherapy. Methods: This is a randomized, open-label phase 2 study conducted in a single center (Asan Medical Center, Seoul, Korea). Key eligibility criteria include histologically documented locally advanced unresectable or metastatic BTC, no progression at least 16 weeks with first-line platinum-based chemotherapy, DDR mutations on targeted sequencing of tumor tissues and ctDNA, ECOG performance status 0–1, and adequate organ function. Eligible patients will be randomized 1:1 into olaparib monotherapy arm (300 mg twice daily, every 4 weeks) or olaparib plus durvalumab (1,500 mg intravenous, every 4 weeks) arm. Study treatment is continued until disease progression, unacceptable toxicity, withdrawal of consent, or death. Response evaluation is performed every 8 weeks (fixed). Stratification factors are tumor location (intrahepatic vs extrahepatic/gallbladder) and best response to platinum-based chemotherapy (CR/PR vs SD). The six-month progression-free survival rates are the primary endpoint and overall survival, progression-free survival, response rates, and safety profiles are secondary endpoints. A total of 62 patients (31 for each arm) are planned, and 23 (37%) patients (ATM [n=6], BRCA2 [n=7], CHEK2 [n=2], BRCA1 [n=1], RAD51C [n=1], and PBRM1 [n=1]) are enrolled as of Jan 2023. Clinical trial information: NCT05222971.