Biochem/physiol Actions Trilostane is an inhibitor of 3 β-hydroxysteroid dehydrogenase (3-β-HSD or delta 5-delta 4-isomerase), an essential enzyme for the biosynthesis of all classes of hormonal steroids. It has been used in the treatment of Cushing′s syndrome for stopping the production of cortisol, and is currently approved for dogs in the US, but is still a human drug in the UK and other countries. It is being investigated as a possible treatment for both breast cancer and prostate cancer to prevent the synthesis of estrogens and androgens from endogenous precursors. It has also been used to inhibit endogenous production of progesterone in research studies.