Multicenter Phase I Trial of a DNA Vaccine Encoding the Androgen Receptor Ligand-binding Domain (pTVG-AR, MVI-118) in Patients with Metastatic Prostate Cancer

Purpose: Preclinical studies demonstrated that a DNA vaccine (pTVG-AR, MVI-118) encoding the androgen receptor ligand-binding domain (AR LBD) augmented antigen-specific CD8(+) T cells, delayed prostate cancer progression and emergence of castration-resistant disease, and prolonged survival of tumor-bearing mice. This vaccine was evaluated in a multicenter phase I trial. Patients and Methods: Patients with metastatic castrationsensitive prostate cancer(mCSPC) who had recently begun androgen deprivation therapy were randomly assigned to receivepTVG-AR on one of two treatment schedules over one year, and with or without GM-CSF as a vaccine adjuvant. Patients were followedfor 18months. Primary objectives were safety and immune response. Secondary objectives included median time to PSA progression, and 18-month PSA-PFS (PPFS). Results: Forty patients were enrolled at three centers. Twentyseven patients completed treatment and 18 months of follow-up. Eleven patients (28%) had a PSA progression event before the 18-month time point. No grade 3 or 4 adverse events were observed. Of 30 patients with samples available for immune analysis, 14 (47%) developed Th1-type immunity to the AR LBD, as determined by IFNg and/or granzyme B ELISPOT. Persistent IFNg immune responses were observed irrespective of GM-CSF adjuvant. Patients who developed T-cell immunity had a significantly prolonged PPFS compared with patients without immunity (HR = 0.01; 95% CI, 0.0-0.21; P = 0.003). Conclusions: pTVG-AR was safe and immunologically active in patients with mCSPC. Association between immunity and PPFS suggests that treatment may delay the time to castration resistance, consistent with preclinical findings, and will be prospectively evaluated in future trials.