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Metastatic castration-resistant prostate cancer (mCRPC) typically exhibits resistance to immune checkpoint inhibitors (ICIs). However, a subset of mCRPC patients displays a more immunogenic profile. This study examines efficacy and safety of dual ICI therapy in molecularly selected mCRPC. This single-arm, phase II trial included 69 molecularly selected mCRPC patients with mismatch repair deficiency (dMMR), non-synonymous tumour mutational burden ≥7.1 muts/Mb (hTMB), a BRCA2 mutation (BRCAm), or biallelic CDK12 inactivation (CDK12i). Efficacy was assessed in ICI-naïve patients (cohort A) with RECIST 1.1 (A1) and Prostate Cancer Working Group 3 (A2) measurable disease. Safety was evaluated in cohorts A and B (prior ICI monotherapy). Treatment included nivolumab 3 mg/kg and ipilimumab 1 mg/kg every 3 weeks for four cycles, followed by nivolumab 480 mg every 4 weeks for up to 1 year. The primary endpoint was disease control rate beyond 6 months (DCR > 6), aiming to surpass a DCR > 6 of 22%. Patients initiated treatment between January 2021 and February 2024. Cohort A included 65 patients. Of these, 21 had dMMR (32%), 8 had hTMB (12%), 20 had BRCAm (31%), and 16 had CDK12i (25%). DCR > 6 was achieved in 38% of patients [95% confidence interval (CI) 27% to 51%], and was highest in dMMR (81%), followed by hTMB (25%), CDK12i (19%), and BRCAm (15%). Objective response rate in cohort A was 38% (95% CI 22% to 55%) and 47% (95% CI 34% to 60%) exhibited a 50% decline in prostate-specific antigen levels. Median progression-free survival (PFS) was 4.0 months (95% CI 3.5-12.0 months) in cohort A, and 32.7 months (95% CI 21.8 months-not reached) in dMMR patients. Treatment-related adverse events (TRAEs) led to permanent discontinuation in 14 of 69 patients (20%). Grade ≥3 TRAEs occurred in 48% of patients, with diarrhoea and elevated transaminases each in 10%. There was one treatment-related death due to a bowel perforation and a second following euthanasia after grade 4 toxicity. This trial of dual ICIs in molecularly selected mCRPC met its primary endpoint, showing DCR > 6 in 38% of patients. Dual ICIs exhibited modest responses in the hTMB, BRCAm, and CDK12i subgroups, but demonstrated exceptional efficacy in dMMR. • Ipilimumab and nivolumab induced a 6-month DCR of 38% in patients with molecularly selected mCRPC. • Durable benefit was particularly high in patients with dMMR who demonstrated a median PFS of 32.7 months. • PFS was shorter in patients receiving prior taxanes, suggesting that dual ICIs should be initiated early in the disease course. [ABSTRACT FROM AUTHOR]