Characterization of 17-dihydroexemestane glucuronidation: potential role of the UGT2B17 deletion in exemestane pharmacogenetics

Objective Exemestane is a third-generation aromatase inhibitor used in the treatment of breast cancer in postmenopausal women. Reduction to form 17-dihydroexemestane and subsequent glucuronidation to exemestane-17-O-glucuronide is a major pathway for exemestane metabolism. The goal of this study was to analyze 17-dihydroexemestane anti-aromatase activity, characterize the 17-dihydroexemestane glucuronidation pathway, and determine whether the functional polymorphisms in active UGTs could play a role in altered 17-dihydroexemestane glucuronidation. Methods Homogenates from a HEK293 aromatase-overexpressing cell line (HEK293-aro) were used to examine exemestane versus 17-dihydroexemestane anti-aromatase activities. UGT-overexpressing cell lines and a panel (n=110) of human liver microsome (HLM) were screened for glucuronidation activity against 17-dihydroexemestane. UGT2B17 genotyping and liver mRNA expression were performed by real-time PCR. Results The inhibition of estrone formation from androst-4-ene-3,17-dione in HEK293-aro cell homogenates was similar for 17-dihydroexemestane (IC50 = 2.3 +/- 0.83 mu mol/l) and exemestane (IC50=1.4 +/- 0.42 mu mol/l). UGTs 2B17 and 1A4 were high-expression hepatic UGTs that exhibited activity against 17-dihydroexemestane, with UGT2B17 exhibiting a 17-fold higher V-max/K-M than UGT1A4. The rate of exemestane-17-O-glucuronide formation was shown to be significantly (P < 0.001) decreased (14-fold) in HLMs exhibiting the UGT2B17(*2/*2) deletion genotype versus wild-type UGT2B17(*1/*1) HLMs; a 36-fold lower V-max/K-M (P=0.023) was observed in UGT2B17(*2/*2) versus UGT2B17(*1/*1) HLMs. A significant (P<0.0001, R-2=0.72) correlation was observed between HLM exemestane-17-O-glucuronide formation and liver UGT2B17 expression. Conclusion These data suggest that 17-dihydroexemestane is an active metabolite of exemestane and that the UGT2B17 deletion polymorphism could play an important role in determining levels of excretion of 17-dihydroexemestane and overall exemestane metabolism. Pharmacogenetics and Genomics 20:575-585 (C) 2010 Wolters Kluwer Health | Lippincott Williams & Wilkins.