Mifepristone

Mifepristone (Mifeprex) is a competitive progesterone receptor antagonist with IC50 of 106 nM ,16 and 19 μg/ml for hog PR, HEC-1-A and Ishikawa, respectively. In the presence of progesterone, it acts as a competitive progesterone receptor antagonist (in the absence of progesterone, mifepristone acts as a partial agonist). It is a 19-nor steroid with a bulky p- (dimethylamino)phenyl substituent above the plane of the molecule at the 11β-position responsible for inducing or stabilizing an inactive receptor conformation and a hydrophobic 1-propynyl substituent below the plane of the molecule at the 17α-position that increases its progesterone receptor binding affinity.