A phase 3, randomized, double-blind, multicenter, placebo-controlled study of S-588410, a five-peptide cancer vaccine as an adjuvant therapy after curative resection in patients with esophageal squamous cell carcinoma

Background: S-588410, a cancer peptide vaccine (CPV), comprises five HLA-A*24:02-restricted peptides from five cancer-testis antigens. In a phase 2 study, S-588410 was well-tolerated and exhibited antitumor efficacy in patients with urothelial cancer. Therefore, we aimed to evaluate the efficacy, immune response, and safety of S-588410 in patients with completely resected esophageal squamous cell carcinoma (ESCC). Methods: This phase 3 study involved patients with HLA-A*24:02-positive and lymph node metastasis-positive ESCC who received neoadjuvant therapy followed by curative resection. After randomization, patients were administered S-588410 and placebo (both emulsified with Montanide™ ISA 51VG) subcutaneously. The primary endpoint was relapse-free survival (RFS). The secondary endpoints were overall survival (OS), cytotoxic T-lymphocyte (CTL) induction, and safety. Statistical significance was tested using the one-sided weighted log-rank test with the Fleming–Harrington class of weights. Results: A total of 276 patients were randomized (N = 138/group). The median RFS was 84.3 and 84.1 weeks in the S-588410 and placebo groups, respectively (P = 0.8156), whereas the median OS was 236.3 weeks and not reached, respectively (P = 0.6533). CTL induction was observed in 132/134 (98.5%) patients who received S-588410 within 12 weeks. Injection site reactions (137/140 patients [97.9%]) were the most frequent treatment-emergent adverse events in the S-588410 group. Prolonged survival was observed in S-588410-treated patients with upper thoracic ESCC, grade 3 injection-site reactions, or high CTL intensity. Conclusions: S-588410 induced immune response and had acceptable safety but failed to reach the primary endpoint. A high CTL induction rate and intensity may be critical for prolonging survival during future CPV development.