Background: For patients with advanced biliary tract cancers (BTCs), gemcitabine and cisplatin has been the standard-of-care first-line therapy, with a median overall survival (OS) of 11.7 months. More recently, TOPAZ-1 demonstrated a modest, but significant overall survival benefit (HR 0.80; 95% CI 0.66 – 0.97) to the addition of durvalumab to gemcitabine and cisplatin with a median OS of 12.8 months. However, for patients without targetable molecular variants, second-line and beyond options are limited, with FOLFOX demonstrating an overall response rate (ORR) of 5% and increase in median overall survival by 0.9 months compared to active symptom control. In patients with advanced BTCs, combination immunotherapy approaches with PD-1 or PD-L1 plus CTLA-4 inhibitors have been associated with improved ORRs of 10.8% - 23% compared to ORRs of 3 – 7% observed with single agent PD-1 or PD-L1 inhibitors. XmAb20717 (vudalimab) is a novel, bispecific antibody targeting PD-1 and CTLA-4 which demonstrated an ORR of 14.1% in a phase I dose expansion cohort of patients with advanced malignancies, including patients that had experienced disease progression on prior immune checkpoint inhibitors. Methods: We initiated a single-arm, phase II clinical trial with a Simon two-stage mini-max design to evaluate the efficacy of XmAb20717, in terms of ORR, in patients with advanced biliary tract cancers previously treated with gemcitabine-based chemotherapy. In the first stage, 13 patients evaluable for efficacy will be accrued. If no responses are observed in these 13 patients, the study will be stopped for futility. Otherwise, 14 additional patients evaluable for efficacy will be accrued to total 27 evaluable patients. If 4 or more responses are observed in these 27 patients, then the null hypothesis will be rejected. XmAb20717 (10 mg/kg) is administered intravenously on days 1 and 15 of a 28-day cycle. Important inclusion criteria include that patients with FGFR2 fusions, NTRK fusions, or IDH1 mutations must have received molecularly targeted therapy unless contraindicated or refused. Patients are not eligible if they have received prior immune checkpoint inhibitor therapy. Correlative studies will include longitudinal peripheral blood collection for circulating immune cell profiling. As of January, 2023, 8 patients have been enrolled with enrollment ongoing. Clinical trial information: NCT05297903.