Osaterone is a steroid anti-androgen, which inhibits the effects of an excess production of male hormone (testosterone).

Osaterone acetate is a steroid chemically related to progesterone, and as such it has potent progestagen and potent anti-androgen activity. Also, the major metabolite of osaterone acetate (15β-hydroxylated - osaterone acetate) has anti-androgenic activity. Osaterone acetate inhibits the effects of an excess of male hormone (testosterone) through various mechanisms. It competitively prevents the binding of androgens to their prostatic receptors and blocks the transport of testosterone into the prostate.

No adverse effects on semen quality have been observed.

After oral administration with food in dogs, osaterone acetate is rapidly absorbed (Tmax about 2 hours) and undergoes a first-pass effect mainly in the liver. After a dose of 0.25 mg/kg/day, the mean maximum concentration (Cmax) in plasma is about 60 µg/l.

Osaterone acetate is converted to its main, 15β-hydroxylated metabolite, which is also pharmacologically active. Osaterone acetate and its metabolite are bound to plasma proteins (around 90% and 80% respectively), mainly to albumin. This binding is reversible and not affected by other substances known to specifically bind to albumin.

Osaterone is eliminated within 14 days, mainly in faeces via biliary excretion (60%) and to a lesser extent (25%) in urine. Elimination is slow with a mean half-life (T½) of about 80 hours. After repeated administration of osaterone acetate at 0.25 mg/kg/day for 7 days, the factor of accumulation is about 3- 4 without change in the rates of absorption or elimination. Fifteen days after the last administration, the mean plasma concentration is about 6.5 µg/l.