'Formestane(Lentaron(R)) is a second generation selective aromatase inhibitor with an IC50 of 80 nM. Formestane(Lentaron(R)) is used in the treatment of estrogen-receptor positive breast cancer in post-menopausal women. Formestane(Lentaron(R)) is often used to suppress estrogen production from anabolic steroids or prohormones. Formestane(Lentaron(R)) also acts as a prohormone to 4-hydroxytestosterone, an active steroid which displays weak androgenic activity in addition to acting as a mild aromatase inhibitor. Formestane has poor oral bioavailability and as such is no longer popular as many orally active aromatase inhibitors have been identified.'