GnRH Antagonist
A compound that blocks the GnRH receptor without activating it, causing immediate suppression of reproductive hormones without the initial flare seen with GnRH agonists. Approved examples include cetrorelix, ganirelix, degarelix, relugolix, and elagolix.
Technical Context
GnRH antagonists competitively occupy GnRH receptors on pituitary gonadotrophs, immediately blocking endogenous GnRH binding. This produces rapid, dose-dependent suppression of LH, FSH, and sex steroids without the initial flare seen with agonists. Injectable antagonists (cetrorelix, ganirelix — used in IVF; degarelix — used in prostate cancer) are peptides with modifications to resist degradation. Oral antagonists (relugolix for prostate cancer/fibroids; elagolix for endometriosis) are non-peptide small molecules designed from the GnRH receptor pharmacophore. The choice between agonist and antagonist depends on whether the flare phase is clinically acceptable — antagonists are preferred when immediate suppression is needed.