AIFM and aromasin are suicidal inhibitors, they bind to the enzyme "taking it out of the game" irreversibly, letro and dex are competitive inhibitors, the bind to the enzyme at a different site, the enzyme is still active, though not able to convert only while bound which is reversible as blood levels drop these enzymes are free to convert again. Also what you get is active enzyme floating around thats not doing its job but still may be binding to other co-factors and sites. This can lead to upregulation of aromatase enzyme production. though this is poorly understood (and could be due to a completely different mechanism that elucidated above) and has only been shown in vitro at this time. (though a number of Anabolic Androgenic Steroids (AAS) users report estrogen rebound with both letro and dex)
in vitro study
Clin Breast Cancer. 2000 Sep;1 Suppl 1:S68-73. Links
Comparison of in vitro exemestane activity versus other antiaromatase agents.Soudon J.
Pharmacell, Paris, France.
pharm2@jupiter.chu-stlouis.fr
Anastrozole, letrozole, and exemestane are the most selective and potent oral antiaromatase agents currently available. However, in vitro and in vivo studies comparing these agents are lacking. Anastrozole and letrozole are reversible, competitive nonsteroidal type II inhibitors, whereas exemestane is an irreversible steroidal type I inactivator. The study was conducted to determine the impact of this characteristic on in vitro residual aromatase activity and protein levels after incubation of JEG-3 cells with aminoglutethimide (a type II inhibitor), anastrozole, exemestane, or letrozole. Aromatase activity was measured after various incubation times with each antiaromatase agent at a concentration 10 times higher than IC50 (concentration giving 50% inhibition).
Only exemestane induced a residual inhibition of aromatase activity after its removal, without any change in the aromatase protein level.
Aromatase activity increased after preincubation of JEG-3 cells with either aminoglutethimide or anastrozole without any change in the aromatase protein level.
The aromatase protein level increased rapidly when cells were incubated with letrozole and aromatase activity inhibition disappeared immediately after removal of the drug. The breakthrough effects in aromatase activity or protein levels observed after treatment with reversible inhibitors may be a factor in therapeutic failure with these agents. These results suggest a possible advantage for exemestane because it is the only clinically available oral irreversible aromatase inactivator.