This trial investigated the safety and efficacy of B4 + P in mUC....Among B2+ pts ORR was 57.5% (95% CI 42-74, 8CR, 15 PR), median OS 15.3 mo (95% CI 12.0- NE), PFS 6.5 mo (95% CI 26-12.8), DOR 27 mo (range 10.4, NE)….B4 + P is well tolerated and may have synergistic efficacy in mUC. B2 was expressed in the majority of pts and predicted for improved tx outcome.