Background: Outcomes remain poor for pts treated with standard-of-care chemotherapy for advanced biliary tract cancer (BTC). Hence, there is a clinical need for novel therapeutic strategies. One such emerging strategy is antagonism of mouse double minute 2 (MDM2), which is amplified in 5–8% of cases of BTC. Brigimadlin (BI 907828) is a MDM2–p53 antagonist that blocks the MDM2–p53 interaction, thereby restoring p53 activity and leading to cell-cycle arrest and apoptosis in TP53 wild-type tumors. Brigimadlin is currently being assessed in two Phase Ia/Ib dose-escalation/expansion trials in pts with advanced/metastatic solid tumors as monotherapy (NCT03449381) and in combination with an anti-PD-1 antibody, ezabenlimab (NCT03964233). Here, we present data in pts with advanced BTCs treated in these trials. Methods: Pts in the monotherapy trial received escalating doses of brigimadlin on day 1 of 21-day cycles (Q3W). Pts in the combination trial received escalating doses of brigimadlin and 240 mg ezabenlimab on day 1 Q3W (doublet); one pt also received the anti-LAG-3 antibody BI 754111 (which has since been discontinued; triplet). Results: At data cut-off (June 2023), a total of 16 pts with BTC have been enrolled (10 in the monotherapy and 6 in the combination trial). In the monotherapy trial, 9 pts received brigimadlin 45 mg Q3W and 1 received 80 mg Q3W. In the combination trial, 5 pts received 30 mg/45 mg brigimadlin doublet and 1 received 45 mg triplet. Across both trials, 7 pts with MDM2-amplified tumors achieved partial response (PR), 3 in the monotherapy trial and 4 in the combination trial. In the monotherapy trial, the responding pts had intrahepatic cholangiocarcinoma (iCC; 80 mg, 73% tumor shrinkage, progression-free survival [PFS] 13.3 months), ampullary adenocarcinoma (45 mg, 72% tumor shrinkage, PFS censored at 666 days), and extrahepatic cholangiocarcinoma (45 mg, 68% tumor shrinkage, PFS censored at 164 days). In the combination trial, 3 responding pts had iCC (pt 1, 30 mg doublet, 54% tumor shrinkage, PFS 9.6 months; pt 2, 45 mg doublet, 49% tumor shrinkage, PFS 7.5 months; pt 3, 45 mg doublet, 39% tumor shrinkage, PFS censored at 5.5 months) and 1 had gallbladder cancer (45 mg doublet, 50% tumor shrinkage, PFS 7.9 months). A further 7 pts (5 in the monotherapy trial and 2 in the combination trial) achieved stable disease (SD). In both trials, the most common any-grade treatment-related AE (TRAE) was nausea; the most common grade ≥3 TRAEs were thrombocytopenia and neutropenia. Conclusions: Brigimadlin showed a manageable safety profile and encouraging preliminary efficacy in pts with BTC, with 7 PRs and 7 SDs in 16 pts. A Phase IIa/IIb trial of brigimadlin in patients with MDM2-amplified, TP53 wild-type BTC and other solid tumors is ongoing (Brightline-2). Clinical trial information: NCT03449381 and NCT03964233.