Background: Atezo is an approved therapy for mUC based on the IMvigor210 (IM210) and IMvigor211 (IM211) phase 2 and 3 trials. The single-arm phase 3b SAUL study in a broader patient (pt) population showed consistent efficacy and safety. The value of complete response to immunotherapy is well recognized, but less is known about the benefit of partial response (PR) or stable disease (SD). We performed post hoc analyses of outcomes in pts with PR/SD in atezo trials. Methods: Pts with mUC received atezo 1200 mg q3w until disease progression (PD) or unacceptable toxicity. Baseline characteristics, efficacy and safety were analyzed in pts achieving PR or SD in IM210 cohort 2 (2nd line), IM211 (atezo and chemotherapy [CT] arms analyzed separately) and SAUL. Data cutoffs were 5/5/15, 3/13/17 and 9/16/18, respectively. Results: The analysis population included 229 PR and 583 SD pts (183 and 421, respectively, treated with atezo); ~40% of pts had 0 Bellmunt risk factors. Baseline characteristics were generally similar between trials. PD-L1 IC 2/3 was more common in PR than SD pts in atezo-treated (41% vs 26%) and CT-treated (37% vs 23%) pts. Median time to best response (PR or SD) was 2.1 mo across trials and treatments. In PR pts, disease control (DC) and overall survival (OS) durations were longer with atezo than CT (Table). Durations of DC and OS were shorter in SD than PR pts; OS was numerically longer with atezo than with CT in the SD population. Presence of more Bellmunt risk factors was associated with worse OS with CT but showed only a weak (SD pts) or no (PR pts) association in atezo-treated pts. Conclusions: Pts achieving a PR or SD have meaningful OS expectancy; pts with PR on atezo have sustained DC. Clinical trial information: NCT02108652; NCT02302807; NCT02928406.

^aFrom 1st PR/SD until 1st PD/death. NE=not estimable.