Background: The phase III KEYNOTE-361 study examined the efficacy and safety of 1L pembro + chemo or pembro alone vs chemo for pts with advanced UC. The PFS and OS benefit of pembro + chemo vs chemo did not reach statistical significance; no further formal tesing was done. We present an exploratory analysis of OS by subsequent therapy in KEYNOTE-361 (NCT02853305) to assess how 1L and 2L therapy selection affected survival outcomes; no formal comparisons were conducted. Methods: OS was estimated for pts by whether they received subsequent therapy, and by whether subsequent therapy included an anti–PD-(L)1 agent. Results: 351 pts were randomized to pembro + chemo, 307 pts to pembro, and 352 pts to chemo. As of Apr 29, 2020, the median (range) time from randomization to data cutoff was 31.7 (22.0-42.3) mo. 124/351 pts (35%) in the pembro + chemo arm, 126/307 pts (41%) in the pembro arm, and 215/352 pts (61%) in the chemo arm received any subsequent therapy. Similar rates of subsequent therapy (pembro + chemo: 32%; pembro: 43%; chemo: 59%) were observed for pts who experienced progressive disease (PD) by blinded independent central review (BICR). A higher rate of pts (169/352 [48%]) in the chemo arm received subsequent anti–PD-(L)1 therapy than in either the pembro + chemo arm (23/351 [7%]) or pembro arm (14/307 [5%]). Due to the small pt numbers, pts in the pembro + chemo or pembro arms who received subsequent anti−PD-(L)1 were not considered further. This analysis included all pts who received 2L therapy (465/1010 pts [46%]); the rate of 2L therapy was similar in pts with PD by BICR (274/615 [45%]). Chemo agents alone or in combination, specifically carboplatin, cisplatin, docetaxel, doxorubicin, gemcitabine, and paclitaxel, were the most commonly received subsequent therapies for pts who did not receive anti–PD-(L)1 in 2L. Pts who received 1L chemo followed by subsequent anti–PD-(L)1 had longer mOS (19.1 mo [95% CI 16.2-22.2]) than pts with 1L pembro followed by 2L therapy not including an anti−PD-(L)1 agent (16.0 mo [95% CI 11.8-19.2]) (Table). Conclusions: In this exploratory analysis, favorable survival outcomes were observed for pts who received 1L chemo followed by anti–PD-(L)1 compared with pts who received 1L pembro followed by 2L therapy not including an anti–PD-(L)1 agent. These data underline the continued importance of immunotherapy as 2L therapy for advanced UC. Clinical trial information: NCT02853305. Research Sponsor: Merck & Co., Inc

^a Based on small pt number (n=23); mOS (95% CI) was 29.3 mo (24.1-34.2). ^b Based on small pt number (n=14); mOS (95% CI) was 30.8 mo (14.8-35.7).