Background: Patients (pts) with advanced biliary tract cancers (BTC) have poor prognosis despite systemic chemotherapy, and treatment beyond first-line platinum doublet remains investigational. The immunomodulatory properties of conventional cytotoxic therapy, particularly in regard to the upregulation of PD-L1 expression rendering tumor cells more sensitive to T cell-mediated lysis and neoantigen production, rapid emergence of chemotherapy resistance, and known modest efficacy of single agent PD-1 antibody in BTC provide a rationale for combining chemotherapy and immunotherapy. This multi-center, phase Ib/II, single-arm study is designed to investigate the role of nal-irinotecan, 5-FU and leucovorin in combination with nivolumab as second-line therapy in pts with advanced BTC. Methods: Key eligibility criteria include histologically confirmed advanced, unresectable biliary carcinoma (intra- or extra-hepatic and gallbladder) with progression or intolerance of first-line systemic therapy (excluding irinotecan and PD-1/PD-L1 antibody), measurable disease per RECIST v1.1, ECOG PS 0-1, Child Pugh A or B7, and absence of autoimmune disease or chronic steroid use. Primary objective of the phase Ib portion is to determine the recommended phase 2 dose, and of the phase II portion is to evaluate the median progression-free survival. Secondary objectives include evaluation of objective response rate per immune related (ir)RECIST, median OS and safety in this patient population. Exploratory objectives include identification of biomarker predictors of response and mechanisms of resistance through serial biopsies and blood collection (pre, on and post therapy), including sequential whole exome/transcriptomic analysis and immune cell subset analysis (tissue and blood). Therapy includes nal-irinotecan 70 mg/m², leucovorin 200 (dose level -1) or 400 mg/m² (dose level 0), 5-fluouracil 2400 mg/m² IV over 46 hours, and nivolumab 240 mg on day 1 every 2 weeks for 6 months. In the absence of disease progression, pts may continue therapy for up to 2 years. Accrual goal is 30 evaluable pts. Using a null hypothesis value of median PFS of 2.9 months, and an alternative hypothesis of 5.0 months, this ongoing study has > 80% power, with a two-sided alpha of 0.05 to identify treatment efficacy of study arm. Clinical trial information: NCT03785873