Synthesis and biochemical evaluation of inhibitors of estrogen biosynthesis

The synthesis and biochemical evaluation of various C19-steroidal derivatives as inhibitors of estrogen biosynthesis are described. Steroids with substitutions on the A or B ring were synthesized by Michael addition of various thiol reagents to appropriate dienone intermediates. An in vitro assay employing the microsomal fraction isolated from human term placenta was used to evaluate aromatase inhibitory properties. Agents exhibiting high inhibitory activity were further evaluated in inital velocity studies (low product formation) to determine apparent Ki values. Several 7alpha-substituted androst-4-ene-3,17-diones were effective competitive inhibitors and have apparent Ki values equal to or less than the apparent Km of 0.063 microM for the substrate androstenedione.