pTVG-HP DNA Vaccine Shows Long-Term Survival Benefit in Recurrent Prostate Cancer

Long-term follow-up from a randomized phase 2 trial suggests that the DNA vaccine pTVG-HP (MVI-816) may significantly extend overall survival in men with PSA-recurrent, non-metastatic, castration-sensitive prostate cancer. The findings are particularly notable because the vaccine did not improve the study’s original primary endpoint of 2-year metastasis-free survival.

The trial enrolled 99 men whose prostate cancer had returned biochemically after definitive treatment. All had a PSA doubling time of less than 12 months, but no metastases detectable on conventional CT or bone scans. Participants received either pTVG-HP, which encodes the prostate cancer antigen prostatic acid phosphatase (PAP), together with GM-CSF, or GM-CSF alone. Treatment was administered over two years.

The original endpoint was negative: 2-year metastasis-free survival was 41.8% with pTVG-HP versus 42.3% with GM-CSF alone (P = .97). However, the later overall survival analysis showed median survival of 13.4 years in the vaccine arm compared with 8.6 years in the control arm. The hazard ratio for death was 0.42, corresponding to an apparent 58% relative reduction in mortality risk.

Additional analyses pointed in the same direction. In a 59-patient cohort with complete long-term follow-up, adjusted hazard ratios for overall survival remained favorable at approximately 0.56–0.60 after accounting for baseline factors, androgen-deprivation therapy, and subsequent treatment. The vaccine was also associated with numerically longer metastasis-free survival, delayed initiation of ADT, and a longer interval from ADT to treatment for castration-resistant disease.

The results may reflect a distinctive feature of cancer vaccines: they may influence the long-term behavior of micrometastatic disease without producing rapid radiographic responses. A similar disconnect between progression measures and overall survival has been observed with sipuleucel-T, another prostate cancer immunotherapy targeting PAP.

Nevertheless, the findings are exploratory. The survival analysis was not planned as the original primary endpoint, the trial was not prospectively powered for overall survival, and complete long-term data were available mainly from one participating site. Subsequent treatments were also heterogeneous and did not reflect current standard approaches involving intensified androgen-receptor pathway inhibition.

pTVG-HP is now being investigated in combination with PD-1 inhibitors such as pembrolizumab and nivolumab. These studies have demonstrated immune activation and signs of disease control, but definitive evidence of clinical benefit remains unavailable. A larger, prospectively powered trial with contemporary treatment and complete long-term follow-up will be needed to confirm whether the striking survival difference represents a true vaccine effect.

Source.