pTVG-HP Vaccine May Help Patients With Recurrent Prostate Cancer Live Longer

Patients with early, non-spreading prostate cancer that was starting to grow again despite hormone therapy lived longer after receiving an experimental vaccine called pTVG-HP, also known as MVI-816, according to long-term follow-up results from a phase 2 study. The research was conducted at the University of Wisconsin Carbone Cancer Center, University of California, San Francisco, and Johns Hopkins University.

Unlike vaccines that prevent infections, pTVG-HP is designed to train a patient's own immune system to recognize and attack prostate cancer cells. It targets a protein called prostatic acid phosphatase (PAP), which is found on these cells. In this study, patients treated with the vaccine had a median overall survival, meaning the length of time patients lived after starting treatment, of 13.4 years, compared with 8.6 years for patients who received a comparison treatment of GM-CSF alone, an immune-stimulating substance often paired with vaccines.

What Data Support the Long-Term Survival Findings?

The study, registered as NCT01341652, originally aimed to find out whether the vaccine could delay the cancer from spreading in patients whose prostate-specific antigen (PSA), a blood marker used to track prostate cancer, was rising quickly even though they were on hormone therapy that lowers testosterone. This type of cancer, which still responds to hormone therapy but is starting to progress, is called castration-sensitive prostate cancer.

When researchers followed patients for a longer period than originally planned, they found a survival benefit that had not been visible in the study's original results. Patients who received pTVG-HP lived a median of 13.4 years, compared with 8.6 years for those who received GM-CSF alone. Put another way, patients who received the vaccine had about a 58% lower risk of dying during the study period than patients who received GM-CSF alone. Looking only at patients treated at a single site, 59 patients, the pattern held: those who received the vaccine had about a 40% lower risk of dying than those who received GM-CSF alone. Patients who received the vaccine also went longer before needing to start stronger treatment for cancer that no longer responds to hormone therapy, a median of 4.7 years, compared with 2.1 years for those who received GM-CSF alone.

These long-term survival results are different from what the study first reported. At the original two-year checkpoint, the same share of patients in each group had cancer that had not spread, 41.8% of those who received the vaccine compared with 42.3% of those who received GM-CSF alone, a result that did not favor the vaccine. The typical time before the cancer spread was also similar between groups, 18.9 months for the vaccine group compared with 18.3 months for the comparison group, a gap researchers did not consider meaningful.

One group of patients did stand out even in the earlier results, those whose PSA levels were rising especially fast before treatment, doubling in less than three months. Among these 21 patients, those who received the vaccine went a median of 12.0 months before their cancer spread, compared with 6.1 months for those who received GM-CSF alone. In this smaller group, patients who received GM-CSF alone were reported to have more than four times the risk of their cancer spreading compared with those who received the vaccine.

Researchers also found immune cells trained to recognize PAP in patients from both groups, including cells known to help coordinate a strong immune response. In a smaller group of 31 patients, bone scans that measure cancer activity in the bones showed activity rising 50% from three to six months in patients who received GM-CSF alone, while it dropped 23% in patients who received the vaccine, a pattern researchers said may point to the vaccine affecting cancer cells in the bone too small to detect on standard scans.

What Were the Trial Details?

The phase 2 study enrolled 99 patients with castration-sensitive prostate cancer whose PSA was doubling in less than 12 months. Patients were randomly assigned to receive either the pTVG-HP vaccine together with GM-CSF, or GM-CSF alone. Both were given as shots under the skin every two weeks for the first three months, then every three months for up to two years. The long-term survival results reflect 97 of these patients, 48 who received the vaccine and 49 who received GM-CSF alone.

What Side Effects Were Reported?

The available reports on these results did not include specific information about side effects patients experienced during this study. Researchers noted that the vaccine's safety had been studied in an earlier, separate group of patients with recurrent, non-spreading prostate cancer. Additional studies combining pTVG-HP with a different type of immunotherapy, known as PD-1 blockade, are currently underway.

References

1. "Phase II Trial of a DNA Vaccine Encoding Prostatic Acid Phosphatase (pTVG-HP [MVI-816]) in Patients With Progressive, Nonmetastatic, Castration-Sensitive Prostate Cancer" by Dr. Douglas G. McNeel, et al., Journal of Clinical Oncology.

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