PRX302 (topsalysin) is an investigational drug to treat prostate cancer, developed by Sophiris Bio Corp.

How PRX302 works

PRX302 is a drug that can specifically trigger the death of prostate cancer cells.

It is a “recombinant protein” — a protein produced from DNA that has been intentionally altered to a specific purpose. This alteration includes the addition of a “tail” that prevents the protein from being active. This tail can be removed by prostate-specific antigen (PSA), an enzyme that is found in high quantities in prostate cancer cells, meaning that the protein is only activated in prostate cancer cells. This ensures that the drug is highly specific to prostate tissue, and guards against damaging neighboring tissues and cells.

Once the tail is removed, multiple PRX302 proteins can join together to form a pore in prostate cancer cell membranes. This pore causes the contents of the cell to leak out, resulting in cell death.

To further ensure the tissue-specificity of the drug, PRX302 is injected directly into the prostate (intraprostatic injection).

PRX302 in clinical trials

Sophiris has completed one Phase 1 and two Phase 2 clinical trials, assessing the safety and tolerability of PRX302 in patients with prostate cancer. The results of these trials suggest that the treatment is safe and well tolerated in prostate cancer patients.

The multicenter, open-label, dose-escalation Phase 1 trial (NCT00379561) monitored 24 patients treated with PRX302 in the United States. The patients responded well to the treatment and no serious adverse effects were reported.

The open-label Phase 2a trial (NCT00686088) was carried out to assess the optimal PRX302 dosage in patients with locally recurrent prostate cancer after receiving radiation therapy. The results of this trial showed that the treatment was well tolerated, and a clinical response (a decrease in PSA levels) was observed in two out of the six patients.

The second Phase 2a trial (NCT02499848) monitored 18 patients for 24 weeks after treatment with PRX302. The results, recently presented at the 32nd European Association of Urology Congress, were promising as 60 percent of patients showed a significant clinical response (a reduction in tumor size, or Gleason pattern). After six months, two of the patients had no evidence of prostate cancer remaining.

An ongoing open-label Phase 2b trial(NCT03081481) is currently recruiting participants in the United Kingdom and in the U.S. to assess the safety, tolerability, and efficacy of PRX302 in approximately 40 men with prostate cancer. Eligible patients must have a confirmed tumor that can be accessed by injection. Researchers will determine the clinical response by biopsy and multi-parametric magnetic resonance imaging (mpMRI, a diagnostic technique that can produce a picture of the prostate) after 24 weeks to assess the efficacy of the drug. The patients will be monitored for any side effects over a 26-week period.

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