The chemistry and biochemistry of aromatase inhibitors.

Abstract

Aromatase is the last enzyme in estrogenesis, and as such is of central importance in estrogen metabolic and reproductive processes. Four central postulates of aromatase active site topology were elaborated: 1. The C-19 cavity postulate predicted that a cavity exists in aromatase. Steroids synthesized to investigate this postulate have confirmed that a 6A deep pocket exists to accommodate C-19 androgen derivatives. 2. The C-3 chelation postulate suggested that chelating groups at the C-3 position might constitute a new generation of aromatase inhibitors and cause aromatase inactivation. Steroidal derivatives with C-3 ethylenedioxy and C-3 thioketal groups have shown this approach to be invalid. 3. The C-5 nucleophile postulate led to investigate the mechanism of action of known C-4, C-5 and C-6 pharmacophores via C-4,5 and C-5,6 androgen epoxides. No inactivation of the enzyme was reported in the presence of these epoxy-steroids, thus weakening contemporary aromatase mechanistic theory involving mechanisms relying on an incoming C-4 nucleophile. Topological analysis and computer simulations of known pharmacophoric patterns led to the biochemical investigations of over 80 steroidal test compounds, many exhibiting potent competitive and some inactivating kinetic profiles. The synthesis of hitherto unreported androgen derivatives was described. 4. Bioorganic simulations of the aromatase mechanism have led to a novel approach to the aromatase mechanism of action. These approaches are based on altered A-ring reactivity via thioketal or hemi-thioketal enzyme intermediates. The chemistry of postulated active site generated analogues was investigated and the resulting rearrangement products lend credence to mechanisms requiring altered reactivities during aromatase oxidations.