Prostvac is an investigational drug candidate for the treatment of prostate cancer, being developed by the biotechnology company Bavarian Nordic. Prostvac is specifically being investigated for the treatment of asymptomatic or minimally symptomatic metastatic castration-resistant prostate cancer (mCRPC).

How Prostvac works

Prostvac is an “immunotherapy,” meaning that it works by encouraging the body’s own immune system to recognize and kill cancer cells. The immune system can detect and destroy abnormal cells, such as cancer cells, through a specific type of white blood cells (or lymphocytes) called cytotoxic T lymphocytes (CTL).

CTLs identify targets based on a substance called an antigen. CTLs are generated against new antigens produced by cells they destroy.

Prostvac is a modified pox-virus containing the PSA (prostate-specific antigen) gene, along with a group of three other genes (collectively called TRICOM) that can stimulate the immune system. When Prostvac is injected, the cells infected by the virus start to produce PSA and the proteins encoded by TRICOM. CTLs immediately recognize and destroy the infected cells, which generates more CTLs that target other cells producing PSA.

PSA usually is produced in high levels by prostate cancer cells, making the treatment specific to the cancer tissue. To compound this, once the CTLs target the tumor cells they can detect further cancer-specific antigens.

Prostvac in clinical trials

A Phase 3 clinical trial called PROSPECT (NCT01322490) testing Prostvac has finished recruiting participants and is ongoing at more than 200 sites across 15 countries. This multicenter, randomized, double-blind, placebo-controlled study aims to assess the effectiveness of Prostvac on its own and in combination with GM-CSF at prolonging survival in men with prostate cancer, compared to a placebo.

GM-CSF (sargramostin, or Leukine) is included in clinical trials alongside immunotherapy treatments, as it encourages the body to produce more white blood cells. This will result in more CTLs to attack the cancer cells.

The PROSPECT trial is being conducted under a Special Protocol Assessment (SPA) from the U.S. Food and Drug Administration (FDA), confirming that the overall design of the trial meets the FDA standards should it be used to gain approval.

Four Phase 1, and four Phase 2 clinical trials testing Prostvac already have been completed, with nine more Phase 2 trials still ongoing. The studies have suggested that Prostvac is safe and well-tolerated in patients, with the most common adverse effect observed being a reaction at the site of injection. Other less common side effects include fatigue, fever, and nausea.

The results from these trials have been promising, and have been published in a number of scientific journals. For example, the results of a randomized, double-blind, placebo-controlled Phase 2 trial (NCT00078585), published in the Journal of Clinical Oncology, demonstrated a significant improvement in the survival of men treated with Prostvac compared to placebo. On average, men treated with Prostvac survived 8.5 months longer than those treated with placebo and had a 44 percent reduction in death rate overall.

Bavarian Nordic also intends to assess the benefit of Prostvac in combination with other treatments, such as immune checkpoint inhibitors. It is thought that by combining these two treatments anti-cancer activity could be enhanced.

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