Kinetic analysis of the binding of nomegestrol acetate to the progesterone receptors in rat uterus by competition studies.

The characteristics of binding (Kinetic and equilibrium binding analysis) of nomegestrol acetate (NOM, 17 alpha-acetoxy-6 alpha-methyl-19-nor-pregna-4.6-diene-3.20-dione) to the progesterone receptor (PgR) in rat uterine cytosolic fraction were determined in comparison to progesterone (P), to fully appreciate the amplitude and specificity of the induced biological response. Since an appropriate radio-labelled form of this steroid molecule was not available, competition studies were performed against the synthetic progestin: [3H]-Organon 2058 [( 3H]-ORG). This allowed a direct comparison between the unlabelled forms of NOM and P, the kinetic constants of which were respectively: Inhibition constant (Ki): 22.8 and 34.3 nM; Association rate constant (k1): 0.39 X 10(3) and 0.21 X 10(3) M-1.s-1; Dissociation rate constant (k-1): 1.81 X 10(-5) and 2.16 X 10(-5) s-1. These results are much more informative than the mere determination of relative binding affinities which only reflect the specificity of the PgR. It was concluded that NOM behaves like the natural hormone in the cytosol of rat uterus.