5009 Background: Preclinical and phase 1 results suggest PI3K/mTOR pathway inhibition may enhance androgen receptor inhibition. We report the results of a double-blind, placebo-controlled, randomized Phase 1b/2 study of ENZ±LY (a dual PI3K/mTOR inhibitor) in pts with mCRPC who progressed on abiraterone. Methods: Phase 1b pts received single-agent LY 200 mg twice daily (BID) for 1 wk prior to starting LY+ENZ. Phase 2 pts were randomized 1:1 to 160 mg daily ENZ with PL or 200 mg BID LY on a 28-d cycle. The primary objective was progression-free survival (PFS: serological, radiographic [rPFS], or death) by PCWG2 criteria. Secondary objectives were rPFS, safety, decline in PSA, and PK. Exploratory biomarker analyses included outcomes by presence of androgen receptor variant 7 (AR-V7). 92 primary PFS events were needed for the study to have at least 80% power at one-sided alpha=0.20. Results: LY+ENZ was tolerable during Phase 1b with 1 dose limiting toxicity observed in 13 enrolled pts. Mean LY exposures remained in an efficacious range despite a 30% average decrease when combined with ENZ. In Phase 2, 129 pts were randomized to LY+ENZ (N=65) and PL+ENZ (N=64) (Table). Median PCWG2-PFS was 3.7 mos (LY+ENZ) vs 2.9 mos (PL+ENZ) (HR 0.66, 95% CI 0.43, 0.99; p-value 0.0208). Conclusions: Combination LY+ENZ had a clinically manageable safety profile. The primary end-point of PCWG2-PFS was met and is supported by a clinically meaningful delay in rPFS in AR-V7 negative pts. The biomarker data provide important insights to inform future development strategies. Clinical trial information: NCT02407054. [Table: see text]