Steroid 5alpha-reductase is a system of two isozymes (5alphaR-1 and 5alphaR-2) which catalyzes the NADPH-dependent reduction of testosterone to dihydrotestosterone in many androgen sensitive tissues and which is related to several human endocrine diseases such as benign prostatic hyperplasia (BPH), prostatic cancer, acne, alopecia, pattern baldness in men and hirsutism in women. The discovery of new potent and selective 5alphaR inhibitors is thus of great interest for pharmaceutical treatment of these diseases. The synthesis of a novel class of inhibitors for human 5alphaR-1 and 5alphaR-2, having the 19-nor-10-azasteroid skeleton, is described. The inhibitory potency of the 19-nor-10-azasteroids was determined in homogenates of human hypertrophic prostates toward 5alphaR-2 and in DU-145 human prostatic adenocarcinoma cells toward 5alphaR-1, in comparison with finasteride (IC50 = 3 nM for 5alphaR-2 and approximately 42 nM for 5alphaR-1), a drug which is currently used for BPH treatment. The inhibition potency was dependent on the type of substituent at position 17 and on the presence and position of the unsaturation in the A and C rings. delta9(11)-19-Nor-10-azaandrost-4-ene-3,17-dione (or 10-azaestra-4,9(11)-diene-3,17-dione) (4a) and 19-nor-10-azaandrost-4-ene-3,17-dione (5) were weak inhibitors of 5alphaR-2 (IC50 = 4.6 and 4.4 microM, respectively) but more potent inhibitors of 5alphaR-1 (IC50 = 263 and 299 nM, respectively), whereas 19-nor-10-aza-5alpha-androstane-3,17-dione (7) was inactive for both the isoenzymes. The best result was achieved with the 9:1 mixture of delta9(11)- and delta8(9)-17beta-(N-tert-butylcarbamoyl)-19-nor-10-aza-4- androsten-3-one (10a,b) which was a good inhibitor of 5alphaR-1 and 5alphaR-2 (IC50 = 127 and 122 nM, respectively), with a potency very close to that of finasteride. The results of ab initio calculations suggest that the inhibition potency of 19-nor-10-azasteroids could be directly related to the nucleophilicity of the carbonyl group in the 3-position.

19-Nor-10-azasteroids are a new class of 5α-reductase inhibitors whose activity depends on the presence of the bridgehead N-10 atom conjugated with the 4-en-3-one moiety in the A ring. The X-ray structure of 19-nor-10-azasteroid 1 has been determined and it is compared with the X-ray structure of testosterone. A complete conformational analysis of these compounds has been performed, determining the number and energy of the possible conformers, as well as the molecular flexibility of the 10-azasteroidal skeleton. Thus, MM2* molecular mechanics calculations and AM1 semiempirical energy refinements revealed that 19-nor-10-azasteroids 1−3 have four possible conformations with very small energy differences and that they are very flexible molecules. The conformational analysis has been extended to testosterone (4), which also showed conformational flexibility, with three different conformations, and to 6-azasteroid 5 and 4-azasteroid 6, for which only two thermally accessible conformations have been found. Compared to 19-nor-10-azasteroids 1−3, azasteroids 5 and 6 appear to be more rigid structures. By a best fit analysis of all conformers of 1−5 with the global minimum of testosterone (4-I) it has been found that the lowest energy conformers of 1, 3, and 5 are very close to the structure of 4-I, and among the conformers of 2, the best similarity has been observed for the highest energy conformer 2-IV.

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