the study groups and pre-specified sub-stratifications (cT2N0M0, urothelial cell carcinoma (UCC), gemcitabine-cisplatin (GC) & dose- dense methotrexate-vinblastine-doxorubicin-cisplatin (ddMVAC)) and applied logistic regression modeling. Cox proportional hazards regression compared overall survival. RESULTS: Of 406 patients included, 42 intentionally had 3 cycles of NAC. Of the remaining 364, 140 (38.5%) were unable to complete 4 cycles due to one or more toxicities (renal insufficiency (26.7%), thrombocytopenia (25.6%), neutropenia (24.4%), and consti- tutional symptoms (23.3%) were most common). Patients with major deviations had similar pT0 rates across stratifications (30.5% vs. 27.6% cT2NanyM0; 33.1% vs. 30.0% restricted to cT2N0M0þUCCþGC/ ddMVAC) and no difference in survival (HR 1.08 (95%CI 0.69-1.70), p[0.74) compared to patients completing 4 NAC cycles. Patients receiving 1-2 cycles appeared to have lower response. CONCLUSIONS: Almost 40% of patients intended to receive 4 cycles of NAC before cystectomy will have a major deviation precluding complete therapy. However, most complete 3 cycles of therapy and still derive similar benefits in pathologic response and survival. Source of Funding: N/A MP41-12 AVELUMAB FIRST-LINE MAINTENANCE PLUS BEST SUPPORTIVE CARE VS BEST SUPPORTIVE CARE ALONE FOR ADVANCED UROTHELIAL CARCINOMA: ANALYSIS OF TIME TO END OF NEXT-LINE THERAPY IN JAVELIN BLADDER 100 Petros Grivas*, Seattle, WA; Se Hoon Park, Seoul, Korea, Republic of Korea; Eric Voog, Le Mans, France; Evgeny Kopyltsov, Omsk, Russian Federation; Howard Gurney, Sydney, Australia; David Queiroz Borges Muniz, Sao Paulo, Brazil; Fr ed eric Rolland, Nantes, France; Anne Birgitte Als, Aarhus, Denmark; Bego~ na P Valderrama, Seville, Spain; Jing Wang, Cambridge, MA; Nuno Costa, Porto Salvo, Portugal; Robert J Laliberte, Cambridge, MA; Alessandra di Pietro, Milano, Italy; Thomas Powles, London, United Kingdom; Joaquim Bellmunt, Boston, MA INTRODUCTION AND OBJECTIVE: Avelumab first-line (1L) maintenance is approved in various countries for patients (pts) with advanced urothelial carcinoma (UC) that has not progressed with 1L platinum-based chemotherapy based on significantly prolonged overall survival (OS) seen with avelumabþbest supportive care (BSC) vs BSC alone in the phase 3 JAVELIN Bladder 100 trial. OS was prolonged despite the more frequent use of subsequent anticancer therapy in the BSC alone arm (42.3% in the avelumabþBSC arm vs 61.7% in the BSC alone arm), most commonly with immune checkpoint inhibitors (6.3% vs 43.7%, respectively). To further characterize the efficacy of avelumab 1L maintenance, we report a post hoc analysis of the time to end of next-line therapy (for any reason) in the randomized trial population. METHODS: In JAVELIN Bladder 100 (NCT02603432), eligible pts had unresectable locally advanced or metastatic UC without pro- gression with 4-6 cycles of 1L gemcitabineþeither cisplatin or carbo- platin. The primary endpoint was OS from randomization, assessed in 2 populations: all pts and pts with PD-L1þ tumors (Ventana SP263). In this exploratory analysis, time from randomization until end of next-line treatment received after progression (due to death or discontinuation) was assessed. RESULTS: A total of 700 pts were randomized 1:1 to avelumab 1L maintenanceþBSC or BSC alone. Among all randomized pts, time to end of next-line therapy was prolonged in the avelumabþBSC arm vs the BSC alone arm (Table). Time to end of next-line therapy was also longer in the avelumabþBSC arm vs the BSC alone arm in pts with PD-L1þ tumors (n[358) or PD-L1 tumors (n[270). CONCLUSIONS: Pts who received avelumab 1L main- tenanceþBSC had prolonged time to end of next-line treatment vs those who received BSC alone, irrespective of PD-L1 status. These data provide further evidence of the efficacy of a maintenance approach with avelumab in pts with advanced UC that has not progressed with 1L platinum-based chemotherapy. Source of Funding: Funded by Pfizer as part of an alliance between Merck KGaA, Darmstadt, Germany and Pfizer MP41-13 AVELUMAB FIRST-LINE MAINTENANCE FOR ADVANCED UROTHELIAL CARCINOMA: ANALYSIS OF CLINICAL AND GENOMIC SUBGROUPS FROM THE JAVELIN BLADDER 100 TRIAL Thomas Powles*, London, United Kingdom; Daniel P Petrylak, New Haven, CT; Se Hoon Park, Seoul, Korea, Republic of Korea; Srikala S Sridhar, Toronto, Canada; Claudia Caserta, Terni, Italy; Antoine Thiery-Vuillemin, Besançon, France; Hyo Jin Lee, Daejeon, Korea, Republic of Korea; Joaquim Bellmunt, Boston, MA; Yoshiaki Yamamoto, Ube, Yamaguchi, Japan; Jeanny B Aragon-Ching, Fairfax, VA; Bo Huang, Groton, CT; Keith Ching, New York, NY; Craig Davis, La Jolla, CA; Alessandra di Pietro, Milano, Italy; Yohann Loriot, Villejuif, France; Petros Grivas, Seattle, WA INTRODUCTION AND OBJECTIVE: In the phase 3 JAVELIN Bladder 100 trial, avelumab first-line (1L) maintenanceþbest supportive care (BSC) significantly prolonged overall survival (OS) vs BSC alone in patients (pts) with advanced urothelial carcinoma (UC) that had not progressed on 1L platinum-based chemotherapy (HR, 0.69 [95% CI: 0.56, 0.86; 1-sided p[0.0005]). We report post hoc analyses in previously unreported clinical and genomic subgroups. METHODS: In JAVELIN Bladder 100 (NCT02603432), eligible pts had unresectable locally advanced or metastatic UC without pro- gression after 4-6 cycles of 1L gemcitabineþcisplatin or carboplatin, and were randomized to receive avelumabþBSC (n[350) or BSC alone (n[350). The primary endpoint was OS, in all randomized pts and pts with PD-L1þ tumors (Ventana SP263 assay). In this exploratory analysis, we analyzed OS in disease stage and site subgroups, in pts with PD-L1þ tumors who received 1L gemcitabineþcarboplatin, and in genomic subtypes (RNAseq whole-transcriptome profiling of tumor tissue) defined using data from The Cancer Genome Atlas (TCGA 2017). Interaction tests were not performed. RESULTS: Prolonged OS was observed in the avelumabþBSC arm vs the BSC alone arm in pts with upper or lower tract tumors, metastatic or locally advanced (LA) and unresectable disease (prior to chemotherapy), and lymph node-only disease post-chemotherapy Vol. 206, No. 3S, Supplement, Sunday, September 12, 2021 THE JOURNAL OF UROLOGY Ò e765 Copyright © 2021 American Urological Association Education and Research, Inc. Unauthorized reproduction of this article is prohibited.