Oxendolone (INN, USAN, JAN) (brand name Prostetin; former developmental code name TSAA-291), also known as 16β-ethyl-19-nortestosterone or 16β-ethyl-17β-hydroxy-4-estren-3-one, is a steroidal antiandrogen and progestin of the 19-nortestosterone group that has been marketed in Japan by Takeda for the treatment of benign prostatic hyperplasia (BPH) since 1981. It binds to the androgen receptor (AR) (Ki = 320 nM) and progesterone receptor (Ki = 20 nM) and acts as a weak but clinically relevant inhibitor of 5α-reductase (IC50 = 1.4 μM). The binding affinity of oxendolone for the AR is far lower than that of cyproterone acetate. At the AR, oxendolone is not a silent antagonist but is rather predominantly antagonistic with weak agonistic activity; for this reason, it has been described as a sele