In vitro binding of 16-methylated C18 and C19 steroid derivatives to the androgen receptor

The binding of androgens and structurally related analogues to the androgen receptor was studied. The in vitro experiments were carried out with cytosol of castrated rat prostate, using [3H]R1881 (methyltrienolone) as radioligand. The binding parameters measured were Kd = 1.25 x 10(-10) M and Bmax = 111 fmol (mg protein)-1. Ligand specificity was confirmed by competition experiments with known androgen, oestrogen and progestogen ligands. The receptor binding of substituted steroids was studied. The RBAs (relative binding affinities) of our recently synthetized 16-alkyl steroids were low. The only exception was the 17 beta-hydroxy-16 beta-methylestr-4-en-3-one, which exhibited the remarkable RBA of 22.9%.