Study of the Combination of DKN-01 and Nivolumab in Previously Treated Patients With Advanced Biliary Tract Cancer (BTC)

Inclusion Criteria: * Histologically confirmed intra- or extrahepatic cholangiocarcinoma or gallbladder cancer * Participants must have measurable disease by CT/MRI by RECIST version 1.1 criteria * Prior chemoembolization, radiofrequency ablation, or radiation to the liver is allowed as long as the patient has measurable disease outside of the treated area or measurable progression per RECIST v1.1 at the site of the treated area. * Documented progression after ≥1 line of systemic therapy for advanced BTC. Prior adjuvant chemotherapy qualifies as this 1 line if the last cycle of adjuvant therapy was completed within 6 months of radiological progression. * Age ≥ 18 years * ECOG performance status ≤1 * Life expectancy of greater than 3 months * Participants must have normal organ and marrow function as defined below: * Absolute neutrophil count ≥1,500/mcL * Absolute lymphocyte count ≥1.0 x 10\^9/L * Platelets ≥75,000/mcL * Hemoglobin ≥ 8.0 g/dL (prior transfusions are allowed if given ≥ 7 days before testing) * Total bilirubin \< 2.0 x institutional upper limit of normal; except patients with Gilbert Syndrome who must have a total bilirubin level of \< 3.0 x ULN * AST(SGOT)/ALT(SGPT) ≤3 × institutional upper limit of normal; \< 5 x ULN in case of liver metastases * Creatinine \< 2.0 x institutional upper limit of normal OR Creatinine clearance ≥30 mL/min/1.73 m2 for participants with creatinine levels ≥ ULN * International Normalized Ratio (INR) ≤ 1.5 x ULN unl…