ABSTRACT
PURPOSE: Despite the success of immune checkpoint inhibitors (ICIs) across various cancers, their efficacy in metastatic castration-resistant prostate cancer (mCRPC) is modest, except for a subset of patients who experience significant, yet unpredictable, benefits. DNA repair defects (DRD) are associated with higher neoantigen load, which may predict response. Our study explored the potential of DRD for enhanced responsiveness to the ICI nivolumab. METHODS: We conducted a phase II, multi-center, single-arm trial evaluating nivolumab in post-docetaxel mCRPC patients. DRD was assessed using circulating tumor DNA (ctDNA). The primary endpoint was PSA50 response. Secondary endpoints included objective response rate (ORR), radiographic progression-free survival (rPFS), and overall survival (OS). Also, exploratory comprehensive genomic profiling was performed via whole-exome sequencing (WES) of tumor samples and matched normal tissue, alongside PD-L1 expression evaluation. RESULTS: Among the 38 enrolled patients, DRD was identifiable in 30.5% (11/36) through ctDNA and/or WES analysis. The overall PSA50 response rate was 10.5% (4/38). PSA50 response and ORR did not significantly differ between patients with and without DRD (18.2% vs. 8%; p = 0.57 and 50% vs. 17.6%, p= 0.27, respectively). Median PSA-PFS (1.9 vs. 2.8 months, p=0.52) and rPFS (3.4 vs. 5.5 months, p=0.7) were not statistically different between patients with and without DRD. Grade ≥ 3 adverse events were reported in 47.3% of participants. CONCLUSION: Nivolumab has clinical activity in a subset of mCRPC patients, however, DRD do not predicted response.These results highlight the necessity of identifying new biomarkers to more accurately determine mCRPC patients who might respond to ICIs.