Background: Lack of an effective second-line treatment of patients (pts) with advanced cholangiocarcinoma (CC) necessitates the development of new therapies. Preclinical studies suggest CC susceptibility to PARP inhibition (PARPi): ERCC1 is underexpressed in 74% of CCs, and olaparib is selective for ERCC1 deficiency, profoundly inhibiting DNA repair. Additionally, PARPi exploits IDH mutation-related DNA damage repair deficiency, which is found in about 25% of CCs. Unfortunately, PARPi also upregulates PD-1-PD-L1 receptor-ligand binding, which attenuates anticancer immunity and counteracts the efficacy of PARPi. However, this can be prevented by PD-1 inhibition—blockade of PD-1-PD-L1 acts to re-sensitize cancer cells to T-cell killing. Hence, we hypothesize that the combination of olaparib and pembrolizumab will produce a durable anti-tumor response against CC by synergistically inducing DNA damage and increasing immune response. Methods: Thirty-six pts with advanced CC, who either failed to respond to or progressed on first-line therapy, will be enrolled to receive olaparib (300 mg PO bid) daily plus pembrolizumab (200 mg IV Q3 weeks) for 12 months, unless unacceptable toxicities or cancer progression occur, in which cases therapy will cease. MRI or CT tumor assessment will occur just before therapy, every 6 weeks for the first 6 months of therapy, and then every 9 weeks for the next 6 months of therapy (total, 12 months). Three tumor biopsies will be collected: at baseline; at week 4; and at time of progression. In each biopsy, ERCC1, PD-1, and PD-L1 expression, IDH1/2 mutation status, and immune cell (CD3 and CD8) response will be assessed. The total study duration will be 20-36 months. The primary endpoint will be overall response rate; the secondary endpoints will be PFS, OS, duration of response, and safety and tolerability. It is hypothesized that in pts with advanced CC, second-line therapy with olaparib plus pembrolizumab will improve the response rate from 17.5% to 35%, as well as increase PFS and OS compared to cytotoxic chemotherapy. Study enrollment began in Q1 2020. NCI number pending at time of abstract submission. Clinical trial information: pending at time of submission.