ô<sup>4</sup>-Tibolone (developmental code ORG-OM-38; also known as isotibolone, 7ñ-methylnorethisterone, or 7ñ-methyl-17ñ-ethynyl-19-nortestosterone) is a synthetic androgen and progestin which was never marketed. The compound is a major active metabolite of tibolone, which itself is a prodrug of ô<sup>4</sup>-tibolone along with 3ñ-hydroxytibolone and 3ò-hydroxytibolone (which, in contrast to ô<sup>4</sup>-tibolone, are estrogens). Tibolone and ô<sup>4</sup>-tibolone are thought to be responsible for the androgenic and progestogenic activity of tibolone, while 3ñ-hydroxytibolone and 3ò-hydroxytibolone are thought to be responsible for its estrogenic activity.
Isotibolone exhibited both estrogenic and progestogenic properties in several biological test systems and was found to be 10 to 40 times as potent as norethindrone in comparative assays. Tested for androgenic activity, it stimulated growth of seminal vesicles and the levator ani but was relatively inactive in stimulating prostate gland growth.