Background: Second-line treatment options for patients with biliary tract cancers (BTC) are limited. Lenvatinib, an anti-angiogenic multikinase inhibitor, in combination with the programmed death-1 immune checkpoint inhibitor pembrolizumab, has demonstrated promising antitumor activity with a manageable safety profile in patients with select advanced solid tumors. LEAP-005 (NCT03797326) is evaluating the efficacy and safety of lenvatinib plus pembrolizumab in patients with previously treated advanced solid tumors; here we present results from the BTC cohort of LEAP-005. Methods: In this nonrandomized, open-label, phase II study, eligible patients were aged ≥18 years with histologically or cytologically documented advanced (metastatic and/or unresectable) BTC with disease progression after 1 prior line of therapy, measurable disease per RECIST v1.1, ECOG PS of 0‒1, and tissue sample evaluable for PD-L1 expression. Patients received lenvatinib 20 mg once daily plus pembrolizumab 200 mg Q3W for up to 35 cycles (approximately 2 years) or until confirmed disease progression, unacceptable toxicity, or withdrawal of consent. Treatment with lenvatinib could continue beyond 2 years in patients experiencing clinical benefit. Primary endpoints were ORR (per RECIST v1.1 by blinded independent central review) and safety. Secondary endpoints were the disease control rate (DCR; comprising CR, PR, and SD), duration of response (DOR), PFS, and OS. Tumor imaging was performed Q9W from treatment initiation for 54 weeks, then Q12W to week 102, and Q24W thereafter. Results: 31 patients were enrolled in the BTC cohort (ECOG PS 1, 55%; 84% ex-US). As of April 10, 2020, median time from first dose to data cutoff (DCO) was 9.5 months (range, 3.1‒11.9), with 16 patients on treatment at DCO. There were 3 (10%) PRs and 18 (58%) SDs. ORR was 10% (95% CI, 2‒26), and DCR was 68% (95% CI, 49‒83). Median DOR was 5.3 months (range, 2.1+ to 6.2). Median PFS was 6.1 months (95% CI, 2.1‒6.4). Median OS was 8.6 months (95% CI, 5.6 to NR). Treatment-related AEs occurred in 30 patients (97%), including 15 (48%) who had grade 3‒4 AEs; there were no treatment-related deaths. 2 (6%) discontinued treatment due to treatment-related AEs (myocarditis, pyrexia; n = 1 each). The most frequent treatment-related AEs were hypertension (42%), dysphonia (39%), diarrhea (32%), fatigue (32%), and nausea (32%). 14 patients (45%) had immune-mediated AEs and 1 patient (3%) had an infusion-related reaction. Conclusions: Lenvatinib plus pembrolizumab demonstrated encouraging efficacy and manageable toxicity in patients with advanced BTC who had received 1 line of prior therapy. Based on these data, enrollment in the BTC cohort has been expanded to 100 patients. Clinical trial information: NCT03797326