Arimidex and Letrozole are both classified as non-steroidal and non-suicidal aromatase inhibitors that compete with the substrate for binding to the enzyme active site. This is very different from Aromasin (Exemestane), which is a steroidal and suicidal aromatase inhibitor that acts as a mechanism-based steroidal inhibitor that mimics the substrate, is converted by the enzyme to a reactive intermediate, and results in the inactivation of the aromatase enzyme. For ease of understanding, what this means to the layman is that Aromasin’s chemical structure resembles the traditional ‘targets’ that aromatase binds to (Testosterone, for example) and that it essentially ‘fools’ the aromatase enzyme into binding with it, only to become inhibited/deactivated. Because the binding strength is so great, this inhibition becomes permanent for the aromatase enzyme that Aromasin has become bound to. Arimidex and Letrozole, being non-suicidal aromatase inhibitors, both compete with the enzyme’s traditional ‘targets’ rather than being assured a permanent spot (which is the advantage that Aromasin has over the other two).