Formestane

Formestane was the first selective, type I, steroidal aromatase inhibitor used in the treatment of estrogen-receptor positive breast cancer in post-menopausal women. Formestane suppresses estrogen production from anabolic steroids or prohormones. Formestane is also a prohormone of 4-hydroxytestosterone, an active steroid with weak androgenic activity and mild aromatase inhibitor activity. It is listed as a prohibited substance by the World Anti-Doping Agency for use in athletes.

Formestane has poor oral bioavailability, and thus must be administered fortnightly (bi-weekly) by intramuscular injection. Some clinical data has suggested that the clinically recommended dose of 250mg was too low. With the discovery of newer, non-steroidal and steroidal, aromatase inhibitors which were orally active and less expensive than formestane, formestane lost popularity.

Currently, formestane (categorized as an anti-estrogenic agent) is prohibited from use in sports in accordance to the regulations of the World Anti-Doping Agency. It is not US FDA approved, and the intramuscular injection form of formestane (Lentaron) which was approved in Europe has been withdrawn.