Background: In preliminary analyses from the randomized phase 2, open-label CheckMate 650 trial, nivolumab (NIVO) 1 mg/kg (N1) plus ipilimumab (IPI) 3 mg/kg (I3) Q3W × 4 doses showed clinical activity in patients (pts) with post-chemotherapy (post-CT) mCRPC, particularly those with high tumor mutational burden (TMB), but early toxicity contributed to treatment discontinuations. Here, we report results from pts with post-CT mCRPC receiving alternative NIVO+IPI dosing regimens vs IPI alone vs cabazitaxel (CABA) in CheckMate 650. Methods: Newly enrolled pts previously treated with docetaxel for mCRPC were randomized 2:2:1:2 to cohorts D1 (NIVO 3 mg/kg [N3] + IPI 1 mg/kg [I1] Q3W × 4 doses then NIVO 480 mg Q4W), D2 (N1 Q3W × 8 doses + I3 Q6W × 4 doses then NIVO 480 mg Q4W), D3 (I3 Q3W × 4 doses), or D4 (CABA 20 or 25 mg/m² Q3W + prednisone 10 mg × 10 doses). Crossover from cohorts D3 and D4 to cohort D1 was allowed after radiographic progressive disease. Outcomes included safety, objective response rate (ORR), prostate-specific antigen response rate (PSA-RR; confirmed PSA decline ≥ 50% from baseline), radiographic progression-free survival (rPFS) per blinded independent central review (BICR), and overall survival (OS). Associations between efficacy and TMB were assessed. Results: Overall, 259 pts were randomized (D1, n = 73; D2, n = 74; D3, n = 38; D4, n = 74). Median (range) follow-up for OS was 23.3 (6.0–31.5) months. In the NIVO+IPI cohorts (D1 and D2), median duration of therapy was 2.8 and 2.4 months; the median number of IPI doses received was 4 and 2, and 15% and 26% of pts discontinued due to study drug toxicity, respectively. Two pts died due to study drug toxicity (1 each in cohorts D1 and D2). Key safety and efficacy data are shown. Several pts in cohorts D1 and D2 showed notable reductions (75-100%) in tumor size and PSA. Approximately one-third of pts in cohorts D3 and D4 crossed over to D1. Based on preliminary analyses in small numbers of evaluable pts, there was no clear and consistent association between efficacy and tissue or blood TMB in the NIVO+IPI cohorts (D1 and D2). Conclusions: These results further support the clinical activity of NIVO+IPI in select pts with post-CT mCRPC. Detailed evaluations of the characteristics of responders to NIVO+IPI, including more expansive biomarker analyses, are warranted. Clinical trial information: NCT02985957.

^aAll treated pts. ^bPts with baseline measurable disease. ^cPts with baseline and ≥ 1 postbaseline PSA assessment. ^dAll randomized pts. NE, not estimable.