Watson Pharmaceuticals has received FDA approval of Trelstar (triptorelin pamoate for injectable suspension) 22.5 mg, the first and only six-month intramuscular gonadotropin releasing hormone (GnRH) agonist for the palliative treatment of advanced prostate cancer.

This new formulation maintains mean testosterone levels similar those achieved with surgical castration. Trelstar 22.5 mg is expected to launch in May, according to Watson.

Trelstar 22.5 mg releases triptorelin pamoate over a prolonged period, suppressing testosterone for six months. Trelstar was first approved in the United States in 2000 and is currently available in one-month (3.75 mg) and three-month (11.25 mg) formulations.

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Trelstar is administered via the Mixject delivery system, which uses a 21-gauge needle, thus offering easy administration with a low incidence of injection site pain.

The approval is based on a 12-month phase 3 study. During the study, Trelstar 22.5 mg produced a mean testosterone serum level of 12.8 ng/dL from month 2 through month 12, well below castration levels associated with androgen deprivation therapy, according to the Morristown, N.J.-based company. Median PSA levels were reduced by 96.4% at the end of the study.

The most common reported adverse events associated with the use of the product included hot flushes (71.7% of subjects), erectile dysfunction (10%) and testicular atrophy (7.5%).