Exemestane potency is unchanged by common nonsynonymous polymorphisms in CYP19A1: results of a novel anti‐aromatase activity assay examining exemestane and its derivatives

Exemestane (EXE) treats estrogen receptor positive (ER+) breast cancer in postmenopausal women by inhibiting the estrogen‐synthesizing cytochrome P450 CYP19A1. Variability in the severity and incidence of side effects as well as overall drug efficacy may be partially explained by genetic factors, including nonsynonymous variation in CYP19A1, also known as aromatase. The present study identified phase I EXE metabolites in human liver microsomes (HLM) and investigated mechanisms that may alter the extent of systemic estrogen deprivation in EXE‐treated women with breast cancer, including whether functional polymorphisms in aromatase cause differential inhibition by EXE and whether EXE metabolites possess anti‐aromatase activity. The potency of EXE and ten of its derivatives was measured with HEK293‐overexpressed wild type aromatase (CYP19A1*1) using a rapid novel UPLC tandem mass spectrometry method. Of the ten compounds assayed, five were poor inhibitors (IC (50) ˃ 50 μmol/L) of wild type aromatase while five others, including the major metabolite, 17β‐dihydroexemestane (17β‐DHE), exhibited moderate potency, with IC (50) values ranging between 1.2 and 7.1 μmol/L. The anti‐aromatase activity of EXE was also tested with two common allozymes, aromatase(Thr201Met) (CYP19A1*3) and aromatase(Arg264Cys) (CYP19A1*4). Differential inhibition of variant aromatase is unlikely to account for variable clinical outcomes as EXE‐mediated inhibition of aromatase(Thr201Met) (IC (50) = 0.86 ± 0.12 μmol/L) and aromatase(Arg264Cys) (IC (50) = 1.7 ± 0.65 μmol/L) did not significantly differ from wild type (IC (50) = 0.92 ± 0.17 μmol/L). Although less potent than the parent drug, these results suggest that active metabolites may contribute to the therapeutic mechanism of EXE.