A randomized phase III trial of personalized peptide vaccination for castration-resistant prostate cancer progressing after docetaxel

First-line chemotherapy for men with metastatic castration-resistant prostate cancer (mCRPC) has been employed to improve overall survival (OS) and progression-free survival (PFS). However, several new agents for CRPC after first-line chemotherapy prolonged survival by only a few months. To develop a new treatment modality, we conducted a phase III randomized trial of personalized peptide vaccination (PPV) for human leukocyte antigen (HLA)-A24-positive patients with castration-resistant prostate cancer (CRPC) for whom docetaxel chemotherapy failed. This randomized, double-blind, placebo-controlled, phase III trial was carried out at 68 medical centers in Japan. Patients were randomly assigned at a 2:1 ratio to receive PPV or placebo. Four of 12 warehouse peptides selected based on pre-existing peptide-specific immunoglobulin G levels or the corresponding placebo were subcutaneously injected in 6 doses weekly and then bi-weekly following the maximum of 30 doses until disease progression. The primary end-point was overall survival (OS). Efficacy analyses were performed by the full analysis set. Between August 2013 and April 2016, 310 patients were randomly assigned, and 306 patients were analyzed. Baseline characteristics were balanced between groups. The estimated median OS was 16.1 months [95% confidence interval (CI), 13–18.2] with PPV and 16.9 months (95% CI, 13.1–20.4) with placebo [hazard ratio (HR), 1.04, 95% CI, 0.80–1.37; P=0.77]. Grade ≥3 adverse events were observed in 41% of both groups. The analysis of treatment arm effects among subgroups revealed lower HRs for OS in favor of the PPV arm in patients with <64% neutrophils (HR, 0.55, 95% CI, 0.33–0.93; P=0.03) or ≥26% lymphocytes (HR, 0.70, 95% CI, 0.52–0.92; P=0.02) at baseline. PPV did not prolong OS in HLA-A24-positive patients with CRPC progressing after docetaxel chemotherapy. Subgroup analysis suggested that the patients with a lower proportion of neutrophils or a higher proportion of lymphocytes at baseline can receive survival benefits from PPV treatment.