Background: Biliary tract cancers (BTCs) are aggressive malignancies with limited treatment options. The addition of durvalumab or pembrolizumab to cisplatin–gemcitabine improves overall survival (OS), but validated biomarkers of benefit remain limited. Immune-mediated adverse events (imAEs) have been proposed as treatment-emergent clinical correlates of immune activation. Methods: We retrospectively analyzed an international cohort of patients with locally advanced or metastatic BTC treated with first-line cisplatin–gemcitabine plus durvalumab. Primary endpoints were OS and progression-free survival (PFS). The principal exposure was the occurrence of any imAE. A post hoc complete-case Cox model was adjusted for age, sex, primary tumor site, prior surgery, ECOG performance status, disease stage, albumin, bilirubin, and neutrophil-to-lymphocyte ratio (NLR); analyses of individual AEs were exploratory. Exact AE onset dates were not consistently available, precluding a reliable formal time-dependent exposure analysis. Results: A total of 1358 patients were included; all received durvalumab, and 276 (20.3%) developed at least one imAE. In the fully baseline-adjusted model, any imAE remained associated with a lower risk of death (adjusted HR 0.72, 95% CI 0.57–0.92; p = 0.008) and progression or death (adjusted HR 0.63, 95% CI 0.52–0.77; p < 0.001). In exploratory baseline-adjusted analyses of individual imAEs, other imAEs remained associated with OS (HR 0.39, 95% CI 0.24–0.62; p < 0.001) and PFS (HR 0.56, 95% CI 0.40–0.77; p < 0.001), while hypothyroidism remained associated with PFS only (HR 0.65, 95% CI 0.43–0.97; p = 0.034); rash and hyperthyroidism were not independently associated after full baseline adjustment. In AE-focused exploratory models, decreased appetite and hyponatremia were associated with worse outcomes, neutropenia with better outcomes, and ALT elevation with worse PFS. Baseline corticosteroid use was uncommon (n = 21), and all steroid analyses are vulnerable to confounding by indication and exposure-timing bias. Conclusions: Treatment-emergent imAEs may represent potential prognostic markers or clinical correlates of benefit, but they are not validated surrogate endpoints. Prospective studies with precise AE-onset capture are required.

Introduction: Currently, no conclusive evidence exists regarding the association between biliary tract cancer (BTC) and other primary malignancies. Our study aims to explore a potential correlation be