Phase Ib/II Trial of Nal-Irinotecan and Nivolumab as Second-Line Treatment in Patients With Advanced Biliary Tract Cancer

Inclusion * Patients must have a pathologically confirmed carcinoma of the biliary tract (intra-hepatic, extra-hepatic (hilar, distal) or gall bladder) that is not eligible for curative resection, transplantation, or ablative therapies. Tumors with mixed hepatocellular and cholangiocarcinoma histology are excluded. * Patients must have received one and only one prior systemic therapy for advanced disease. Prior therapies must have not included irinotecan or PD- 1/PD-L1 antibody. Patient should have either progressed on or within 6 months of first-line systemic therapy or deemed intolerant of that therapy. * Prior surgical resection, radiation, chemoembolization, radioembolization or other local ablative therapies are permitted if completed ≥ 4 weeks prior to registration AND if patient has recovered to ≤ grade 1 toxicity. * Patients must have radiographically measurable disease (as per RECISTv1.1) in at least one site not previously treated with radiation or liver directed therapy (including bland, chemo- or radio-embolization, or ablation) either within the liver or in a metastatic lesion. * Age ≥18 years * Child-Pugh score of less than 7 * ECOG performance status of 0-1 * Ability to understand and willingness to sign IRB-approved informed consent * Available archived tissue (FFPE block or 20 unstained slides from prior core biopsy or surgery) * Must be able to tolerate CT and/or MRI with contrast * Adequate organ function (per protocol) assessed ≤2 weeks prior to registration Exclusion * Must not have received systemic steroid therapy, or any other form of immunosuppressive therapy within 14 days prior to registration. Short bursts of steroids of 5-7 days (for COPD exacerbation or other similar indication) are allowed. * No prior history of solid organ transplantation or brain metastasis (unless treated, asymptomatic and stable). * Must not have undergone a major surgical procedure \< 4 weeks prior to registration. * Must not have an active second malignancy other than non-melanoma skin cancer or cervical carcinoma in situ. Patients with history of malignancy are eligible provided primary treatment of that cancer was completed \> 1 year prior to registration and the patient is free of clinical or radiologic evidence of recurrent or progressive malignancy. * Must have no ongoing active, uncontrolled infections (afebrile for \> 48 hours off antibiotics). * Must not have received a live vaccine within 30 days of registration * Must not have a psychiatric illness, other significant medical illness, or social situation which, in the investigator's opinion, would limit compliance or ability to comply with study requirements. * Women must not be pregnant or breastfeeding since 5-fluorouracil, nal- irinotecan and/or nivolumab may harm the fetus or child. All females of childbearing potential (not surgically sterilized and between menarche and 1- year post menopause) must have a blood test to rule out pregnancy within 2 weeks prior to registration. * Women of child-bearing potential and men must agree to use 2 methods of adequate contraception (hormonal plus barrier or 2 barrier forms) OR abstinence prior to study entry, for the duration of study participation, and for 5 months (for women) and 7 months (for men) following completion of study therapy. * Participants with an active, known or suspected autoimmune disease which may affect vital organ function, or has/may require systemic immunosuppressive therapy for management are excluded. Participants with type I diabetes mellitus, hypothyroidism only requiring hormone replacement, skin disorders (such as vitiligo, psoriasis, or alopecia) not requiring systemic treatment, or conditions not expected to recur in the absence of an external trigger are permitted to enroll. * Participants with a condition requiring systemic treatment with either corticosteroids (\>10 mg daily prednisone equivalent) or other immunosuppressive medications within 14 days of registration are excluded. Inhaled, ocular, intra-articular, intra-nasal or topical steroids, and adrenal replacement steroid doses \>10 mg daily prednisone equivalent, are permitted in the absence of active autoimmune disease. * No known UGT1A1\* variants or Gilbert's syndrome * Prisoners or subjects who are involuntarily incarcerated, or compulsorily detained for treatment of either a psychiatric or physical (e.g. infectious disease) illness would be excluded. * No known hypersensitivity to 5-fluorouracil, leucovorin, irinotecan, and/or nivolumab. * Must not have ongoing bowel obstruction. * No known HIV, Hepatitis B or Hepatitis C infection that is untreated and/or with a detectable viral load. * Patients must not have uncontrolled intercurrent illness including, but not limited to, interstitial lung disease, symptomatic congestive heart failure, unstable angina pectoris, uncontrolled cardiac arrhythmia. * No known medical condition (e.g. a condition associated with uncontrolled diarrhea such as ulcerative colitis or acute diverticulitis) that, in the investigator's opinion, would increase the risk associated with study participation or interfere with the interpretation of safety results. * Patients must not be on warfarin, strong CYP3A4 inducers (such as phenytoin, phenobarbital, primidone, carbamazepine, rifampin, rifabutin, rifapentine or St. John's wort), strong CYP3A4 inhibitors (such as ketoconazole, clarithromycin, indinavir, itraconazole, lopinavir, nefazodone, nelfinavir, ritonavir, saquinavir, telaprevir, voriconazole), and strong UGT1A1 inhibitors (such as atazanavir, gemfibrozil, indinavir and ketoconazole).