Interaction of A-nor A.19-dinor, and A-homo-5α-androstane derivatives with the androgen receptor and the epididymal androgen-binding protein

The equilibrium affinity constant for rat prostate androgen receptor and epididymal androgen binding protein (ABP) has been determined for thirty-four potential progestogens. Three A-nor-, four A,19-dinor-, and one A-homo-5 alpha-androstane derivative bind to the androgen receptor (KD less than 0.5 muM). Five of these compounds also bind to ABP with an affinity of the same order of magnitude. "Anordrin" (compound 24) and "Dinordrins" (compounds 10, 14, 15, 16, 17), which are potential female contraceptives, do not bind with high affinity to the androgen receptor or to ABP. The following modifications in A-nor derivatives favour binding to the receptor as compared to ABP: 19-nor substitution (compound 1), C-18 methyl homologation (compound 5), 2 alpha-ethinylation (compound 22). One 2 alpha-allenyl A-nor derivative (compound 25) and one A-homo derivative (compound 34) bind almost exclusively to ABP. The interaction with either binding protein is decreased by oxidation or esterification of the hydroxyl group at C-17, and by addition of a 17 alpha-ethinyl group. The latter modifications are likely to increase the specificity of androstane derivatives for receptors other than androgen binding proteins, such as the progesterone receptor.