Efficacy of Anti-Pd(l)1 Therapy for Patients (pts) with Advanced Urothelial Carcinoma (auc) with Primary Resistance to Platinum-Based Chemotherapy (PC).

e16515 Background: PC remains standard first-line (1L) therapy for aUC. Approximately 15% of pts exhibit primary resistance (P-R) to PC and ∼25% progress by 4 months. PD(L)1 inhibitors yield objective response rates (ORR) of ∼20% in pts with progression after PC; however, it is unclear if this benefit extends to pts with P-R to PC. We examined the efficacy of anti-PD(L)1 in pts with aUC who experienced P-R to 1L PC. Methods: We conducted a multi-institutional retrospective study of pts with aUC who experienced P-R to PC and were subsequently treated with single-agent anti-PD(L)1 therapy. Eligibility included pts with unresectable or metastatic disease diagnosed after January 1, 2017. P-R to PC was defined as radiographic progression by RECISTv1.1 within 12 weeks from initiation of PC. Pts who developed metastatic disease while receiving (neo)adjuvant PC were eligible. Clinicopathologic variables were collected. ORR to anti-PD(L)1 was the primary endpoint. Secondary endpoints included time to treatment failure (TTF, defined as time from start of anti-PD(L)1 therapy to next line of therapy, hospice or death) and overall survival (OS) were estimated using Kaplan-Meier method. Multivariate (MV) analysis using Cox regression evaluating factors associated with OS was performed. Results: Overall, 42 pts were included: 74% male, median age 65 (28-90); 79% ever smokers; 21% mixed histology; 31% received definitive locoregional therapy. Metastatic sites at diagnosis of aUC included: lymph node only (19%), liver (29%), bone (38%) and lung (33%). At diagnosis of aUC, ECOG PS was 0 in 26%, 1 in 52% and unknown in 21%. 1L PC included cisplatin (76%) and carboplatin (24%) based regimens. Anti-PD(L)1 was received either 2L (98%) or 3L (2%). Overall, ORR to anti-PD(L)1 was 17%: CR (2%), PR (14%), SD (14%), PD (57%) and unknown (12%). Of the 24 pts with PD as best response to anti-PD(L)1, only 9 (38%) received subsequent therapy. Overall, median TTF was 4.2 mo (95% CI 2.8-6.7 mo) and median OS was 7.4 mo (95% CI 4.2-11.1 mo). ORR in patients with a PDL1 combined positive score ≥ 10% (n=6) was 0%: 1 SD and 5 PD. MV analysis for OS from start of anti-PD(L)1 is shown (Table). Conclusions: P-R to PC portends a poor prognosis in pts with aUC. While a subset of patients may respond to anti-PD(L)1 therapy, the majority of pts do not derive benefit. Alternative agents, e.g. antibody drug conjugates and FGFR inhibitors, and combination-therapy should be investigated for this high risk population.[Table: see text]