Phase II study of pembrolizumab in docetaxel-pretreated patients with metastatic castration-resistant prostate cancer (mCRPC): Updated follow-up of cohorts (C) one to three from KEYNOTE-199.

Abstract

e17584 Background: Pembrolizumab monotherapy has shown antitumor activity and acceptable safety in patients with mCRPC previously treated with a next-generation hormone agent (NHA) and docetaxel. Presented herein are updated results from KEYNOTE-199 (NCT02787005), a multicohort phase 2 study, in patients with RECIST-measurable PD-L1+ disease (C1), RECIST-measurable, PD-L1− disease (C2), and bone-predominant disease, irrespective of PD-L1 (C3). Methods: Patients who previously received ≥1 NHA and 1 or 2 chemotherapies, including docetaxel, received pembrolizumab 200 mg Q3W for 35 cycles or until progression/toxicity. The primary end point was ORR per RECIST v1.1 by blinded independent central review (BICR). Key secondary end points were DCR, PSA response rate (≥50% decrease from baseline), time to PSA progression, rPFS, OS, DOR, and safety. rPFS and OS were evaluated using the Kaplan-Meier method. Results: Of 258 treated patients (C1, 133; C2, 67; C3, 58), 6 completed therapy (C1, 4; C3, 2) and 252 discontinued (C1, 129; C2, 67; C3, 56), primarily due to progression (C1, 106; C2, 61; C3, 45). Median (range) time from enrollment to data cutoff was 31.3 mo (26.7-34.7) in C1, 30.6 mo (28.0-34.1) in C2, and 32.6 mo (27.4-34.4) in C3. Efficacy results are displayed in the table. ORR (95% CI) for patients with measurable disease was 6% (2.6-11.5) in C1 and 3% (0.4-10.4) in C2. Of 10 responders, 6 had a DOR ≥18 mo. Median time to PSA progression was 4 mo across cohorts. Any grade treatment-related AEs (TRAEs) occurred in 57-60% of patients across C1-3. Grade ≥3 TRAEs occurred in 16% of patients in C1, 15% in C2, and 17% in C3; 1 patient in each cohort died of a TRAE (C1, sepsis; C2, unknown; C3, immune-related pneumonitis). Conclusions: Pembrolizumab monotherapy was well tolerated and showed durable, antitumor activity and disease control with survival up to 24 mo in 3 cohorts of docetaxel and NHA-pretreated patients with RECIST-measurable or bone-predominant mCRPC. Clinical trial information: NCT02787005 . [Table: see text]