Formestane(4-Hyroxyandrostenedione) is a steroial aromatase inhibitor and a prohormone to the anabolic steroid 4-Hydroxytestosterone. It is not only effective for estrogen reduction, it is also a mild anabolic steroid. It was first patented by G.D. Searle & Co. in 1955, but it never hit the commercial market. Structurally, 4-Hydroxytestosterone is simply testosterone with a hydroxy group at the four position, making it most similar to the steroid clostebol, which has a chloro group at the four position. Formestane and Clostebol(and Halodrol/Promagnon) are very similar in effect.
4-Hydroxytestosterone is .65 times as anabolic and .25 times as androgenic as testosterone propionate. The reason that Formestane has such low androgenic activity is that the modification at the 4th position prevents DHT conversion. In vitro, 4-Hydroxytestosterone is actually a weak inhibitor of 5-alpha reductase.