Comparative clinical pharmacology and pharmacokinetic interactions of aromatase inhibitors

The clinical development of aromatase inhibitors (AIs) has been closely guided by clinical pharmacological investigations. During the early phases of development studies were focused on dose-related pharmacological effectiveness and specificity. More recently attention has been given to the metabolic changes which AIs elicit, with particular regard to their potential use in early breast cancer and the prophylactic setting. Pharmacological effectiveness has been studied with plasma oestrogen assays but primary oestrogens (E1 and E2) are not helpful in comparing the third generation inhibitors: anastrozole, letrozole, exemestane. All three of these compounds suppress whole body aromatisation by >96%. Most recently, we have established that significantly greater inhibition is achieved by letrozole than anastrozole at their clinically used dosages. This more complete inhibition is paralleled by significantly greater suppression of E1S.

A broad panel of endocrine investigations has indicated that these compounds have essentially complete specificity at their clinical dosages. A minor androgenic effect of exemestane is revealed by a significant suppression of sex hormone binding globulin (SHBG). Lipid and bone biomarker data are being collected in many current studies. A pharmacokinetic interaction has been established between letrozole and tamoxifen, whereby reduced circulating levels of letrozole are found with combined application. Neither anastrozole nor letrozole have any effect on plasma concentrations of tamoxifen when given in combination with it.