Although the complete biological evaluation of many of the foregoing hormone analogues is still in progress, certain observations in this regard may be summarized briefly. As discussed in more detail elsewhere (p. 6), studies with monofunctional steroids emphasize the unique importance of the 17β-hydroxyl group for the physiological activity of androstane compounds (54). The 17β-hydroxyl group (but not a 3-hydroxyl or 3-ketone group) endows the hydrocarbon, androstane, with full androgenic-type activity in the accessory sexual tissues of the female rat (54) and moderate activity in the male. Whereas androstan-17β-ol and 17-methylandrostan-17β-ol show the same type of action on rat uterus and vaginal epithelium as does testosterone, introduction of unsaturated groupings at certain locations in the molecule shifts the action toward estrogenic activity (keratini-zation of vagina). Thus, 17-ethynylandrostan-17β-ol, in 50 to 100 µg. doses, elicits the typical response seen with estradiol or stdbestrol, whereas 3-deoxyestradiol (1,3,5 [io]-estratrien-17β-ol) and 3-deoxyestrone are estrogens at dose levels of about 10 Mg. (55). In contrast to the inactive 3-sub-stituted monofunctional steroids of the androstane series, 17-deoxyestra-diol, which possesses the phenolic A-ring, is estrogenic at a dose level of I Mg- (55).

In androstane compounds bearing a 17β-hydroxyl group, the size of the substituent at position 3 is a limiting factor in determining biological activity. Androstan-17β-ol, 3β-fluoroandrostan-17β-ol, 3-methylene andro-stan-17β-ol, and 3-ketoandrostan-17β-ol (dihydrotestosterone) are all as active as testosterone in stimulating the growth of certain secondary sexual tissues of the female rat; but 3/3-chloroandrostan-17β-ol and 3β-methyl-androstan-17β-ol are inactive. 3-Methyl-2-androsten-17β-ol likewise shows no primary hormonal activity in the male or female rat but is an inhibitor of pituitary gonadotrophin in the parabiotic rat assay.

In the usual difunctional androgenic steroids, 17-keto compounds generally are somewhat less potent than the corresponding 17β-hydroxysteroids, but they still show a significant degree of activity. In the monofunctional steroids, this difference appears to be gready magnified, inasmuch as androstan-17-one and 3 β-fluoroandrostan-17-one are inactive. In marked contrast to the 17 esters of testosterone and dihydrotestosterone, the acetate esters of the monofunctional steroids, androstan-17β-ol and 3β-fluoro-androstan-17β-ol, are hormonally inactive. Moreover, 2a-methylandro-stan-17β-ol is inactive, in contrast to 2a-methyldihydrotestosterone, which is a potent androgen in both male and female rats.

Introduction of fluorine in position 21 of progesterone was found to increase progestational activity fivefold (185), whereas 17-fluoroproges-terone is somewhat more active than progesterone itself. Not untd the preparation of the 2a-fluorosteroids did we realize our basic objective of eliminating primary hormonal activity by the introduction of fluorine adjacent to an oxygen function while still retaining secondary and anticancer actions. 2a-Fluorodihydrotestosterone (2a-fluoro-17β-hydroxyan-drostan-3-one) shows practically no androgenic activity, but in the rat it is strongly anabolic and shows the ability of the potent androgens to inhibit the growth of mammary tumors (145). Moreover, 2a-fluorodihy-drotestosterone effects a marked prolongation of the life of dystrophic mice (R. M. Dowben, personal communication).