Partial agonist

A partial agonist is a drug which binds to a receptor and activates it to an extent that is less than the endogenous agonist of the receptor. This can result in the drug having different effects at different concentrations and in the drug having mixed agonist and antagonistic effects in different tissues. For example, selective estrogen receptor modulators like tamoxifen and raloxifene are partial agonists of the estrogen receptor and have estrogenic effects in some tissues like bone and the liver and antiestrogenic effects in other tissues like the breasts and the pituitary gland. Steroidal antiandrogens like spironolactone and cyproterone acetate are very weak partial agonists of the androgen receptor which can be thought of as functional antagonists for most intents and purposes but do have the capacity to activate the receptor and produce androgenic effects under certain circumstances like in the context of prostate cancer. Their very weak partial agonist activity also appears to make them less efficacious antiandrogens than nonsteroidal antiandrogens like flutamide, bicalutamide, and enzalutamide (which are pure, or silent antagonists of the androgen receptor).