PRX302 is currently in North American Phase II evaluation as PSA-activated pore-punching prodrug.

Kissei Pharamaceutical is paying Protox Therapeutics $3 million up front for exclusive Japanese rights to the latter’s Phase II PSA-activated prodrug, PRX302, for prostate-related diseases including BPH and prostate cancer.

Under the terms of the deal Protox will also receive a near-term milestone of $5 million and potentially up to $67 million in additional development, regulatory, and commercial milestones. Kissei will take on all costs associated with development, regulatory filings, and commercialization for PRX302 in Japan.

PRX302 is Protox’ lead candidate, developed using its PORxin platform. Currently in Phase II development in North America, the drug is an engineered version of proaerolysin, a protein secreted by Aeromonas hydrophilia. It is designed as an inactive prodrug that is activated by prostate specific antigen (PSA) into a pore-forming toxin that selectively kills the PSA-producing cells, the firm explains.

Proaerolysin features both a binding site that allows the molecule to attach to the surface of a cell and an “activation tail.” It is removal of this tail by PSA that converts the drug into its active form, Protex continues. Once bound and activated, PRX302 combines with other activated PRX302 molecules to form a mushroom-shaped structure that is able to perforate the cell membrane.

PRX302 is injected locally into the prostate. In the case of prostate cancer the aim of treatment is to reduce the size of the tumor or eliminate the tumor altogether. For BPH lower amounts of PRX302 are used to reduce the size of the prostate, thereby restoring normal urinary function.

In January 2010, Protox reported positive top-line results from a double-blinded placebo-controlled Phase IIb study evaluating PRX302 in patients with moderate to severe BPH. The trial achieved its primary clinical endpoint of a statistically significant improvement in International Prostate Symptom Score for patients treated with PRX302 compared with those  receiving placebo.

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