* Castration achieved with 1,000 mg, 1,500 mg doses
* Phase 2 studies to start next year
By Deena Beasley
LOS ANGELES, Sept 15 (Reuters) - Higher doses of an experimental prostate cancer drug being developed by GTx Inc GTXI.O were shown in an early-stage trial to induce temporary medical castration in healthy volunteers, the company said on Tuesday.
GTx aims to begin two mid-stage trials of the drug early next year. One is to be in patients diagnosed with advanced prostate cancer and another in those who have failed to respond to injectable drugs, like Lupron, used to treat the cancer by reducing the amount of testosterone made by the body.
Most types of prostate cancer need the male hormone testosterone to grow and spread.
The trial with 60 healthy volunteers found that medical castration was achieved in subjects given either 1,000 mg or 1,500 mg of the drug, called GTx-758, but not a lower dose of 600 mg.
The company said the percentage of treatment-compliant subjects receiving 1,500 mg who achieved medical castration was comparable to rates seen with treatments like Lupron. Castration was seen in these subjects within three weeks.
No flares in testosterone levels were observed, and no serious side effects were reported.
GTx also said the drug increased levels of sex hormone binding globulin, a protein that binds to testosterone, thus reducing levels of free testosterone — the form of the hormone prostate cancer cells use for growth.
In the 1970s, advanced prostate cancer was treated with a drug called diethylstilbestrol (DES), a synthetic form of estrogen that suppresses the production and effects of testosterone.
DES works well, but can cause dangerous side effects such as blood clots, according to GTx Chief Executive Officer Mitchell Steiner. GTx-758 is an oral compound designed like DES to bind to estrogen receptors in the brain, but not to receptors on blood platelets, he explained.
As a selective estrogen receptor alpha agonist, GTx-758 has the potential to achieve medical castration by inhibition of the pituitary and hypothalamus without bone loss or hot flashes, the company says.
“The big game changer was going from surgery to a shot,” Steiner said, referring to the fact that surgical castration has become a less common treatment for prostate cancer. “Now we could go from a shot to an oral drug with fewer side effects.”
The American Cancer Society projects that nearly 218,000 Americans will be diagnosed with prostate cancer this year.
GTx said it expects to report full results from the trial at upcoming medical meetings.
Reporting by Deena Beasley