Pembrolizumab (pembro) plus olaparib in docetaxel-pretreated patients (pts) with metastatic castrate-resistant prostate cancer (mCRPC): Cohort A of the phase 1b/2 KEYNOTE-365 study

5027 Background: Individual activity with pembro or olaparib has been observed in mCRPC pts who progressed on second-generation hormone therapy (HT) and chemotherapy. Data from cohort A (pembro+olaparib) of KEYNOTE-365 (NCT02861573), a phase 1b/2 umbrella study to test combinations in mCRPC, are presented. Methods: Pts with mCRPC who progressed within 6 mo before screening, were docetaxel-pretreated (up to 1 other chemotherapy permitted) for mCRPC and had ≤2 second-generation HTs were eligible. Pts received pembro 200 mg IV Q3W+olaparib 400 mg orally twice daily. Primary end points: safety and PSA response rate (confirmed PSA decrease ≥50%). Key secondary end points: ORR per RECIST v1.1 (investigator review), disease control rate (DCR: CR+PR+SD ≥6 mo), time to PSA progression, composite response rate, rPFS, and OS. Results: Median (95% CI) follow-up was 11 (6-15) mo. 41 pts initiated treatment (median age, 69 y; visceral disease, 42%; RECIST-measurable, 68%; homologous recombination deficient detected, 0%). Efficacy is outlined in the table. Treatment-related AEs occurred in 39 (95%) pts; most frequent (≥30%) were anemia (37%), fatigue (34%), and nausea (34%). Grade 3-5 treatment-related AEs occurred in 21 (51%) pts. There were 2 deaths; 1 was treatment-related (cause unknown). Conclusions: Pembro+olaparib had activity in pts with mCRPC who were molecularly unselected and were previously treated with docetaxel and second-generation HT. The observed safety profile for the combination is consistent with individual profiles of pembro and olaparib. Clinical trial information: NCT02861573. [Table: see text]