Durability of Complete Response (CR) with Atezolizumab (atezo) in Locally Advanced/metastatic Urothelial Carcinoma (Muc).

4527 Background: Atezo (anti–PD-L1) has been shown to elicit CRs in a number of mUC patients (pts) in clinical trials. We sought to describe the kinetics, durability and outcomes associated with these CRs in Ph I (PCD) and II (IMvigor210) atezo studies, each with long-term follow-up. Methods: In PCD (pre-treated mUC) and IMvigor210 (Cohort 1, cisplatin-ineligible untreated mUC; Cohort 2, platinum-treated mUC), pts received atezo per protocol (Petrylak JAMA Oncol 2018; Balar Lancet 2017; Rosenberg Lancet 2016). This post hoc analysis descriptively assessed pt disposition, time to and duration of RECIST 1.1 response and overall survival in pts with CR. Results: CR rates were 13%, 8% and 7% in PCD, IMvigor210 Cohort 1 and Cohort 2, respectively. First response was PR in most pts with CR. Median CR duration was > 3 y in PCD, not estimable (NE) in IMvigor210 Cohort 1 and > 2 y in Cohort 2 (Table). At data cutoff, all but 2, 0 and 1 pts were alive, respectively; across studies, ≥ 40% of pts with CR were on treatment. CR pts had a first response (PR/CR) by a median of 3.5 cycles. Further pt characteristics and survival outcomes will be reported. Conclusions: Across Ph I/II atezo mUC studies, CRs appeared durable (median duration > 2 y) despite small pt numbers. Most pts with CR were alive, with responses ongoing after long-term follow-up (median follow-up > 30 mo). Clinical trial information: NCT01375842, NCT02951767, NCT02108652. [Table: see text]