Effective androgen therapy requires either the administration of testosterone in a slowly absorbed form (dermal patches or micronised oral preparation) or the oral or parenteral administration of chemically modified analogues. Such chemical modifications either retard the rate of absorption or catabolism so as to sustain effective blood levels, or enhance the androgenic potency of each molecule so that physiologic effects can be achieved at a lower blood level.
Three types of modification have proved useful (Fig. 11.47): esterification of the 17b-hydroxyl group; alkylation at the 17a position; and modification of the ring structure, particularly substitutions at the 2, 9 and 11 positions. Most drugs contain combinations of ring-structure alterations and either 17a-alkylation or esterification of the 17-hydroxyl group.
