Anti-tumor vaccines have demonstrated efficacy in patients with castration-resistant metastatic prostate cancer. One vaccine, Prostvac-VF®, using a heterologous prime-boost strategy with vaccinia and fowlpox viral vectors encoding PSA, is currently being evaluated in a registration phase III multinational clinical trial. The current trial was planned to assess the clinical efficacy of this vaccine in patients with castration-resistant metastatic prostate cancer receiving subsequent docetaxel chemotherapy. 10 patients with metastatic castration-resistant prostate cancer, with a predicted survival of at least 18 months, were enrolled out of a planned 144 patients. Eight of 10 patients were treated and were randomized to receive docetaxel chemotherapy alone (Arm B, n = 2) versus treatment with Prostvac-VF (days 1, 15, 29, 43, 57) followed by docetaxel (Arm A, n = 6) chemotherapy beginning at month 3. The primary endpoint of the trial was overall survival, and secondary endpoints included time to radiographic progression and immunological response. The trial was opened within the Eastern Cooperative Oncology Group, but due to slow accrual was closed by CTEP after only 10 patients were enrolled within 13 months.
Results: Presented here are the safety, clinical, and immunological results from 8 eligible patients who underwent treatment. Two of 6 patients treated on Arm A, with vaccine followed by docetaxel, had a >50% PSA response, with one of these patients experiencing a PSA decline during treatment with vaccine. Significant PSA-specific CD4+ and CD8+ T-cell responses and IgG antibody responses specific for PSA were not detected. The primary endpoint of overall survival cannot be assessed due to limited accrual. The lack of T-cell responses, even in this small cohort, suggests that further validation and development of immune biomarkers will be important for future studies. Other trials remain ongoing to evaluate the role of anti-tumor vaccination in sequence with other traditional anti-tumor therapies.
Keywords: docetaxel; phase II clinical trial; prostate cancer; vaccine.