Formestanes (lentaron) is a type I steroidal aromatase inhibitor (AI). It was developed for use in treatment of estrogen-receptor-positive breast cancer in post-menopausal women. For medical purposes, it is made in intramuscular depot form and injected. It is not used in oral form due to poor bioavailability, which makes the oral version.

Supplement companies, with a bodybuilder customer base, have made it in topical form. It was available as part of gel stacks, which are rubbed onto the skin with no need for injection. That allowed transdermal delivery, bypassing the liver. In case you are wondering, the reason I use "was" is because formestanes is now illegal and a scheduled drug.

Formestanes is a second generation, irreversible, aromatase inhibitor. It inhibits the aromatase enzyme responsible for converting androgens to estrogens, thereby preventing estrogen production. Breast cancer may be estrogen sensitive or insensitive. A majority of breast cancers are estrogen sensitive. Estrogen sensitive breast cancer cells depend on estrogen for viability. Thus removal of estrogen from the body can be an effective treatment for hormone sensitive breast cancers. Formestanes has been targeted specifically for the treatment of postmenopausal women. Unlike premenopausal women who produce most estrogen in the ovaries, postmenopausal women produce most estrogen in peripheral tissues with the help of the aromatase enzyme. Formestanes, an aromatase inhibitor, can thus help to decrease the local production of estrogen by blocking the aromatase enzyme in peripheral tissues (ie. Adispose tissue of the breast) to treat hormone sensitive breast cancer.