Background: Gemcitabine, cisplatin and durvalumab followed by maintenance durvalumab is standard of care first-line (1L) therapy for patients (pts) with advanced BTC. In this trial, we investigated the combination of a PARP inhibitor with anti-PD1 antibody in absence of progression after at least 4 months of standard 1L platinum-based chemotherapy. Methods: Pts with advanced BTC, ECOG PS 0-1 and absence of progression on 1L platinum-based therapy were enrolled on a phase 2 trial to receive rucaparib 600 mg PO BID in combination with nivolumab 240 mg IV Q2 weeks every 28 days for up to 2 years. The primary endpoint was 4-month progression-free survival (PFS) rate from the date of first study treatment with an alternative hypothesis of 85%. Secondary endpoints included median PFS and overall survival (OS) from first study treatment (PFS1, OS1) as well as from start date of first-line chemotherapy (PFS2, OS2), best overall response, and safety. Results: 31 pts were enrolled of which 14 (45%) were women, 25 (80.6%) Caucasian and 24 (77.64%) with ECOG 1. Primary disease site was intrahepatic, extrahepatic hilar and distal, and gallbladder cancer in 21 (67.7%), 4 (12.9%), 1 (3.2%) and 4 (12.9%), respectively. 1L platinum-based treatment included gemcitabine/cisplatin based in 26 (83.9%), FOLFOX in 3 (9.7%) and carboplatin/paclitaxel in 1 (3.2%) pts. Mean (range) cycles of rucaparib and nivolumab were 6.75 (1-24) and 5.9 (1-24). The median follow-up time is 28.7 months using reverse censoring methodology. PFS rate at 4 months was 54.8% (95% CI, 36-70.3), comparable to the 63% null estimate based on ABC-02 trial. Median PFS1 and OS1 are estimated to be 4.6 months (95% CI, 3.7-6.2) and 15.9 months (95% CI, 9.8-24.1). Median PFS2 and OS2 calculated from date of 1L chemotherapy are estimated as 9.9 months (95% CI, 8.3-11.3) and 21.4 months (95% CI, 14.8-26.7). Two (6.4%) pts had partial response (with IDH R132G and KRAS G12D) as best response with stable disease in an additional 22 (71%) pts. Grade 3-5 treatment-related adverse events (TRAEs) occurred in 15 (48.4%) pts, including 1 patient who died on therapy due to possibly-related immune-related myocarditis. The most common grade 2 or greater TRAEs included fatigue (9, 29.0%), anemia (7, 22.6%), neutropenia (6, 19.3%), elevated AST (5, 16.1%) and elevated ALT (4, 12.9%). Reasons for treatment discontinuation included death (2), progression (27), adverse event (1) while 1 patient completed 2 years of study treatment. Conclusions: The primary endpoint of PFS rate at 4 months was not met but the preliminary measures of PFS1 and OS1 are much longer than expected, and hypothesis generating for rational combination of PARP inhibitor with maintenance immunotherapy in this rare cancer. Exploratory endpoint of association of genomic analysis with efficacy is ongoing and will be discussed. Clinical trial information: NCT03639935.