Background: Biliary tract cancers are rare with the majority of pts diagnosed with metastatic disease at initial presentation. Up to 20% of pts will have peritoneal metastases, resulting in symptoms of ascites, abdominal pain, and potential bowel obstruction. Despite advances in standard of care systemic treatments such as combination gemcitabine, cisplatin, and durvalumab (gem/cis/durva), penetration to peritoneal metastases may be poor. While addition of nab-paclitaxel to gem/cis increased objective response in a phase 2 study (1), SWOG 1815 did not demonstrate a significant improvement in survival with the triplet combination compared to gem/cis (2), possibly due to added toxicity. PIPAC directly delivers treatment to the peritoneal space, which could potentially reduce toxicity and improve efficacy. We aim to investigate the safety of PIPAC nab-paclitaxel plus systemic gem/cis/durva in biliary tract cancer pts with peritoneal metastases. Methods: This is an investigator-initiated, non-randomized phase 1 trial enrolling up to 12 evaluable pts using a modified phase 1 queue 3+3 design. Eligible pts must have treatment-naïve histologically or cytologically confirmed intra/extra-hepatic cholangiocarcinoma or gallbladder cancer with documented metastatic disease and visible peritoneal disease on imaging or laparoscopy. Gem/cis (800/25 mg/m²) will be administered on Days 1 and 8 of 3-week cycles; durva (1500 mg) will be administered on Day 1. Pts will receive PIPAC nab-paclitaxel (90 mg/m²) on Day 3 of Cycles 1, 3, and 5. If dose-limiting toxicities (DLTs) occur, nab-paclitaxel may be reduced to 70 mg/m²; the DLT window for each pt is 42 days after the first PIPAC. After eight 3-week cycles, durva (1500 mg) will be continued on Day 1 of 4-week cycles until disease progression or unacceptable toxicity. The primary endpoint is treatment-related adverse events (AEs) graded by NCI Common Terminology Criteria for Adverse Events version (v) 5.0. AEs will be monitored during treatment and for 4 weeks after the last dose of treatment with any agent. Secondary endpoints include objective response per Response Evaluation Criteria in Solid Tumors v1.1, peritoneal regression grading score at each PIPAC, peritoneal carcinomatosis index at time of laparoscopy, post-operative surgical complications, progression-free/overall survival, PIPAC technical failure rate, EQ-5D-5L and MD Anderson Symptom Inventory responses before/during treatment, and number of daily steps taken before/during treatment. Exploratory endpoints include characterization of sub-clonal tumor evolution/tumor microenvironment changes and PIPAC nab-paclitaxel pharmacokinetics/pharmacodynamics. Enrollment began 6/2023 and is ongoing. 1. Shroff et al, JAMA Oncol 2019. 2. Shroff et al, JCO 2023 LBA490. Clinical trial information: NCT05285358.