Real-world outcomes of atezolizumab versus durvalumab in extensive-stage small cell lung cancer.

e23393 Background: Advent landmark trials, IMpower133 and CASPIAN, demonstrated improved median progression-free survival (PFS) and overall survival (OS) among those treated with chemotherapy and atezolizumab or durvalumab, respectively, over chemotherapy alone in ES-SCLC. Two retrospective single-center academic studies demonstrated improved OS with durvalumab compared to atezolizumab, and no PFS difference. In our study, we included academic and community treated patients using an intent-to-treat approach to evaluate treatment efficacy in a real-world population. Methods: Our retrospective real-world cohort study compared adults with ES-SCLC who received ≥ 1 cycle of first-line platinum-etoposide with atezolizumab or durvalumab from January 1, 2018 to December 31, 2025 at the University of Vermont Medical Center (UVM) and affiliate community centers. The study was developed, reviewed by UVM IRB, and exempt from full review, prior to data extraction. Patients identified from chart review. Demographic, clinical, treatment variables, immune-related adverse events (irAE), survival outcomes were collected via manual chart review. To reduce confounding, propensity score matching was performed on baseline labs, demographics, performance status, and treatment setting. Patients with incomplete data were excluded. Missing values were addressed using multiple imputations. Progression was determined by radiographic/clinician assessment. OS was time from first-line treatment start to death; PFS was time from treatment start to progression/death. OS and PFS were analyzed using the Kaplan Meier method and Cox proportional hazards model. Fisher’s test was used to compare baseline characteristics and irAE. Results: Of 96 patients, 78 remained after matching (39 per group). Baseline characteristics were balanced except for treatment setting, where academic practice favored durvalumab (p = 0.019). There was no significant difference in OS between atezolizumab and durvalumab (9.96 vs 11.61 months; hazard ratio HR 1.07; 95% CI 0.62–1.84; p = 0.80). PFS was also similar between groups (6.61 vs 4.41 months; HR 1.22; 95% CI 0.73–2.04; p = 0.44). Rates of irAE were comparable (n = 8 vs n = 9, p = 0.66). Conclusions: In this real-world analysis of patients with ES-SCLC, no significant difference in OS, PFS, or irAEs were observed between atezolizumab- and durvalumab-based first-line therapy. Limited by the imbalanced treatment setting, these findings support clinical equivalence between regimens and suggest treatment selection may be guided by institutional preference, access, or patient-specific factors. Larger multicenter studies are warranted to confirm these findings. Matched cohort outcomes. Outcome Atezolizumab Durvalumab Effect Estimate p-value Median OS, months 9.96 11.61 HR 1.07 (95% CI 0.62-1.84) 0.80 Mean PFS, months 6.61 4.41 HR 1.22 (95% CI 0.73–2.04) 0.435 irAEs, n 8 9 - 0.66