Inhibition of androgen receptor binding by natural and synthetic steroids in cultured human genital skin fibroblasts

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Abstract
The ability of various natural and synthetic steroids (some of which are widely used in clinical practice) to compete with dihydrotestosterone receptor binding in human genital skin fibroblasts was studied. Binding was assessed in fibroblast monolayers after incubation for 1 h at 37 °C with 2 nM3H-dihydrotestosterone in the presence or absence of increasing concentrations of the steroid to be tested. Inhibition constants (Ki) were determined as the concentration of competitor-required for 50% inhibition of3H-dihydrotestosterone binding. In addition, relative binding activity (RBA) of each test compound was calculated. Each competitor was tested in at least two different cell strains. The concentrations of unlabeled methyltrienolone (a synthetic nonmetabolizable androgen) and dihydrotestosterone for 50% inhibition of3H-dihydrotestosterone binding were in the same order of magnitude, namely, 2 nM (2.2 respectively, 2.4 nM), whereas the affinity of testosterone was approximately one-fifth that of dihydrotestosterone. Other potent competitors for dihydrotestosterone binding were three progestins (norgestrel, gestoden, and medroxyprogesterone acetate) which have Ki values similar to testosterone. An order of magnitude lower Ki values (around 10−7M) were found for the androgen 17α-propylmesterolone, the antiandrogen cyproterone acetate, and the progestin norethisterone acetate. Binding affinities of all other steroids to the androgen receptor were markedly lower and showed the following order of potency: estrogens (estradiol, ethinyl estradiol, diethylstilbestrol) > glucocorticoids as well as aromatase inhibitors and potassium canrenoate. We conclude that (a) among the compounds tested some progestins are very potent in their ability to interact with human skin fibroblast receptors and thus may affect endogenous androgen action; (b) estrogens are relatively weak androgen receptor binders; and (c) this receptor assay in combination with pharmacokinetic and metabolic studies appears to be a useful screening test to evaluate the potency of various steroids for androgen and antiandrogen therapy.