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 (IC(50)=2.3±0.83 μmol/l) and exemestane (IC(50)=1.4±0.42 μmol/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.


CONCLUSIONS
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.
Sun
Dongxiao
D
Molecular Epidemiology and Cancer Control, Penn State University College of Medicine, Hershey, Pennsylvania 17033, USA.
Chen
Gang
G
Dellinger
Ryan W
RW
Sharma
Arun K
AK
Lazarus
Philip
P
D000730:Androstadienes / Q000737:chemistry / Q000378:metabolism* / Q000494:pharmacology*; D001141:Aromatase / Q000378:metabolism; D047072:Aromatase Inhibitors / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D002460:Cell Line; D002851:Chromatography, High Pressure Liquid; D017353:Gene Deletion; D020719:Glucuronides / Q000378:metabolism*; D014453:Glucuronosyltransferase / Q000235:genetics*; D006031:Glycosylation / Q000187:drug effects; D006801:Humans; D007700:Kinetics; D008099:Liver / Q000187:drug effects / Q000201:enzymology; D013058:Mass Spectrometry; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology; D015778:Minor Histocompatibility Antigens
D000730:Androstadienes; D047072:Aromatase Inhibitors; D020719:Glucuronides; D015778:Minor Histocompatibility Antigens; D001141:Aromatase; D014453:Glucuronosyltransferase; C500513:UGT2B17 protein, human; C542923:17-dihydroexemestane