GTx says selective estrogen receptor agonist induces medical castration

According to a Reuters report based on a recent media release from GTx, the investigational agent GTx-758 is able to induce temporary medical castration in healthy volunteers, making this agent yet another possible candidate for the hormonal management of progressive prostate cancer.

Data from this Phase II trial will be presented at appropriate scientific meetings in the relatively near future. In the meantime, GTx is planning two trials of GTx-758 that are scheduled to start enrolling patients early in 2011. The first of these two trials will be in patients diagnosed with advanced prostate cancer; the second will be in men newly diagnosed with advanced prostate cancer who do not respond well to initial LHRH therapies. GTx-758 is an oral drug (like the antiandrogens) as opposed to an injectable agent (like the LHRH agonists).

The initial Phase I trial showed that medical castration was achieved within 3 weeks in a significant percentage of 60 volunteers given either 1,000 mg or 1,500 mg of GTx-758, but not a lower dose of 600 mg. There was no testosterone “flare” associated with onset of therapy and there were no significant adverse effects reported — but this is a small number of patients treated with GTx-758 for a maximum of 56 days.

GTx says that GTx-758 increased levels of sex hormone binding globulin, which is a protein that binds to testosterone, thus reducing levels of free testosterone — the hormone that prostate cancer cells use most commonly to stimulate growth. They have also stated that GTx-758 acts as a selective estrogen receptor agonist, and has the potential to achieve medical castration by inhibition of the pituitary and hypothalamus without effects on bone density or induction of hot flashes.

Obviously there is a considerable way to go before we will know that GTx-758 is an effective and safe option for the treatment of progressive prostate cancer, but this does represent the first possibility in yet another new class of drugs for the management of progressive disease.

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