Background: In biliary tract cancer (BTC), there was a positive association of overall survival (OS) with high levels of tumor-infiltrating CD4+, CD8+, and Foxp3+ T lymphocytes, whereas increases in PD-1+/CD8+ tumor-infiltrating lymphocytes had a negative association with OS (Tariq NU et al. 2018). Therefore, immune-modulating treatment can be promising option to overcome a dismal prognosis in BTC patients. However, immune checkpoint inhibitor (ICI) monotherapy, including tislelizumab (anti-PD-1 monoclonal antibody), has a modest effect in advanced BTC (Desai J et al. 2020). Thus, several combination strategies to improve the anti-tumor effect of ICI are being conducted. Anti-angiogenic agent in combination with ICI is one of the promising strategies in which anti-angiogenic agent induced improved anti-tumor immune responses by increasing tumor antigen presentation and promoting lymphocyte infiltration and migration (Song Y et al. 2020). However, this combination have not been widely tested in BTC patients. Sitravatinib is a potent inhibitor of multiple receptor tyrosine kinases including Axl, MER, MET, KIT, FLT3, RET, VEGFR1-3, PDGFRα, which targets are expressed in a number of immune cell types and promote an immunosuppressive tumor microenvironment. This study aim to evaluate the efficacy of the combination strategy of sitravatinib with tislelizumab in advanced BTC and to reveal the immune-modulation through the combination of ICI and antiangiogenic agents. Methods: This study is an open-label, phase 2 study to evaluate the efficacy of sitravatinib and tislelizumab combination treatment in advanced BTC patients who have failed to first-line chemotherapy. Eligible patients have histologically proven BTC (including intrahepatic cholangiocarcinoma, extrahepatic bile duct cancer, gallbladder cancer, and ampulla of Vater cancer) and have failed one prior systemic chemotherapy. Patients who have previously received ICI can be enrolled. However, patients who have experienced unacceptable toxicity during prior ICI treatment are excluded. All patients will receive sitravatinib 120mg orally once daily in combination with tislelizumab 200mg intravenously once every 3 weeks until disease progression, unacceptable toxicity, or withdrawal of consent. The primary endpoint is disease control rate, with key secondary endpoints including overall response rate, progression-free survival, overall survival, and safety. For evaluating metabolic response, ¹⁸F-FDG PET-CT scan will be performed before treatment and at the first response evaluation. Tissue biopsies will be conducted three times in total: screening, the first response evaluation, and disease progression. Blood samples are being collected every cycles for translational biomarker studies. Clinical trial information: NCT04727996.