Nivolumab plus ipilimumab for chemotherapy-refractory metastatic castration-resistant prostate cancer: results from the randomized portion of the phase 2 CheckMate 650 trial.

We report on the randomized portion of the phase 2, open-label CheckMate 650 trial (NCT02985957), in which docetaxel-experienced, biologically male patients with chemotherapy-refractory metastatic castration-resistant prostate cancer were randomized 2:2:1:2 to nivolumab 3 mg /kg plus ipilimumab 1 mg /kg (n = 73), nivolumab 1 mg /kg plus ipilimumab 3 mg /kg (n = 74), ipilimumab 3 mg /kg (n = 38), or cabazitaxel (n = 74). Primary endpoints were objective response rate and radiographic progression-free survival; key secondary endpoints included overall survival, prostate-specific antigen response rate, and safety. This portion of CheckMate 650 was not designed to statistically compare between cohorts. Respectively, objective response rates (95% CI) were 9.3% (2.6-22.1), 19.5% (8.8-34.9), 4.5% (0.1-22.8), and 12.2% (4.1-26.2), and median radiographic progression-free survival (95% CI) was 3.9 (2.2-7.6), 4.2 (3.3-5.6), 3.5 (2.1-5.8), and 7.9 (5.6-9.3) months. Respective incidence of grade ≥3 treatment-related adverse events was 28.8%, 30.1%, 18.4%, and 34.7%. Preliminary post hoc biomarker analyses in patients who received treatment with any immunotherapy regimen (i.e., treated with either of the nivolumab plus ipilimumab regimens or with ipilimumab alone, n = 12) identified a transcriptional signature, derived from the most highly expressed genes across cell types within select perivascular immune niches (comprising CD31+ endothelial, CD14+HLA-DR+ myeloid, and CD4+ and CD8+ T cells), which was associated with prolonged overall survival. These results provide further evidence of antitumor activity with nivolumab plus ipilimumab in select patients with metastatic castration-resistant prostate cancer and nominate a candidate prognostic biomarker that warrants confirmation in future prospective clinical trials.