Carboplatin-based chemotherapy (C) versus immunotherapy (IO) in metastatic urothelial carcinoma (mUC).

e16003 Background: There are limited data comparing first-line (1L) C versus IO in cisplatin-ineligible mUC patients. The primary evidence guiding treatment decisions was a May 2018 Food and Drug Administration (FDA) safety alert based on early review of two ongoing phase III trials, reporting shorter survival in PD-L1 negative patients receiving IO relative to chemotherapy. Final results from these trials are unknown and may not be applicable to a real-world cohort. Methods: We conducted a retrospective cohort study of mUC patients receiving 1L C or IO from January 1, 2011 to May 18, 2018 using the Flatiron Health electronic health record-derived database. The primary outcome was overall survival (OS) from start of 1L treatment to date of death. We compared 12- and 36-month OS, and hazard ratios before and after 12 months. Propensity score-based inverse probability of treatment weighting (IPTW) was used to address confounding in Kaplan-Meier (KM) and Cox multivariable regression model estimates of comparative-effectiveness. Results: Of 2,243 patients, 562 received IO and 1,681 received C. Baseline characteristics were balanced between groups, with few exceptions (Table). In the first 12 months, IO was associated with an increased hazard of death compared to C (HR 1.38, 95% CI 1.16-1.66). Among patients who survived one year, survival was improved with IO compared to C (HR 0.50, 95% CI 0.27-0.92). Conclusions: In routine clinical practice, 1L IO was associated with inferior 12-month OS relative to C, but superior OS beyond 12 months. Clinicians and patients should carefully consider how to balance the early benefit of C against the late benefit of IO. Currently pending trial results will contribute additional evidence to inform treatment decision making. [Table: see text]