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Background: BTCs are aggressive cancers with a poor prognosis. In preclinical models, MEK inhibition modulates the tumor immune microenvironment and enhances responses to programmed death-ligand 1 (PD-L1) inhibition. We report a randomized, open-label, multicenter phase 2 trial of atezolizumab (anti-PD-L1) as monotherapy or in combination with cobimetinib (MEK inhibitor) in BTC (NCT03201458). Methods: Eligible patients had advanced BTC [intrahepatic cholangiocarcinoma (ICC), extrahepatic cholangiocarcinoma (ECC), and gallbladder cancer (GBC)], with 1-2 lines of prior therapy in the metastatic setting, measurable disease by RECIST v1.1, and ECOG performance status ≤1. Patients randomized to Arm A received atezolizumab 840 mg IV Q2w. Patients randomized to Arm B received oral cobimetinib 60 mg daily (21 days on/7 days off) plus atezolizumab 840 mg IV Q2w. The primary endpoint was progression free survival (PFS) using the Kaplan-Meier method and compared between groups under the assumption of Cox proportional hazards, stratified for primary tumor site. Secondary endpoints included objective response rate (ORR), safety and tolerability, and overall survival (OS). Results: 86 patients were enrolled at 23 centers in the United States; 77 patients were randomized and received at least one dose of study therapy (Arm A: n=37, ICC=21, ECC=7, GBC=11; Arm B: n=38, ICC=22, ECC=8, GBC=8). Median age was 63 (range 44-86), and 48 (62%) were female. The trial met its primary endpoint, with a median PFS of 3.65 months (cobimetinib+atezolizumab) vs 1.87 months (atezolizumab monotherapy) (p=0.027). OS data are not mature at the time of analysis. There was 1 PR (3.2%), 13 SD (41.9%), and 17 PD (54.8%) in the combination arm and 1 PR (2.9%), 10 SD (29.4%), and 23 PD (67.6%) in the atezolizumab monotherapy arm. Two patients in the combination arm remain on therapy 15+ months from enrollment. One patient in each treatment arm had known mismatch repair deficiency (MMRd), of whom 1 had PD as a best response and the other withdrew prior to response evaluation. Grade 3-4 treatment-related adverse events were similar in both arms, and there were no treatment-related deaths. 4 (10%) of patients receiving atezolizumab monotherapy and 8 (22.2%) receiving cobimetinib+atezolizumab discontinued therapy due to adverse events. Changes in tumor CD8, CD4, FoxP3, PDL1, and MHC expression from paired tumor biopsies will be presented at the conference. Conclusions: We report the first randomized trial of immunotherapy in BTC. The combination of atezolizumab plus cobimetinib met its primary endpoint and significantly prolonged PFS as compared to atezolizumab monotherapy in BTC. The combination of atezolizumab and cobimetinib had manageable toxicity and warrants further investigation in BTC. Citation Format: Mark Yarchoan, Leslie Cope, Robert A. Anders, Anne Noonan, Laura W. Goff, Lipika Goyal, Jill Lacy, Daneng Li, Anuj Patel, Aiwu R. He, Ghassan Abou-Alfa, Kristen Spencer, Edward Kim, Stephanie Xavier, Amanda Ruggieri, S. Lindsey Davis, Autumn McRee, Paul Kunk, Qingfeng Zhu, Andrea Wang-Gillam, Andrew Poklepovic, Helen Chen, Elad Sharon, Gregory B. Lesinski, Nilo Azad. A multicenter randomized phase 2 trial of atezolizumab as monotherapy or in combination with cobimetinib in biliary tract cancers (BTCs): A NCI Experimental Therapeutics Clinical Trials Network (ETCTN) study [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr CT043.