INTERACTION OF [H-3] NOMEGESTROL ACETATE WITH CYTOSOLIC PROGESTERONE RECEPTORS FROM THE RAT UTERUS

The binding characteristics of the progestin 17-alpha-acetoxy-6-methyl-19-[H-3]norpregna-4,6-diene-3,20-dione, nomegestrol acetate ([H-3]NOM-Ac) to progesterone receptors (PgRs) of uterus were determined in the rat. Scatchard plot analysis of the equilibrium binding data showed that [H-3]NOM-Ac binds to uterine PgR with a K(d) of 5.44 +/- 1.27 nM and a B(max) of 1.51 +/- 0.11 pmol/mg protein. Analysis of dissociation kinetics showed that [H-3]NOM-Ac dissociates slowly from the PgR, k - 1 = 4.9 +/- 0.5 10(-5) s-1. Competition experiments against [H-3]NOM-Ac showed the specificity of the binding with a sequence in relative affinity as follows: ORG 2058 > P > NOM-Ac > medroxyprogesterone acetate > megestrol acetate > cyproterone acetone > NOM.