Differences between polymorphic variants of UDP-glucuronosyltransferases (UGTs) active against the major tamoxifen metabolite, endoxifen: Potential impact on individual response of TAM treatment.

B99

Tamoxifen (TAM) is widely used in the treatment and prevention of breast cancer. trans-4-hydroxy-tamoxifen (4-OH-TAM) and 4-hydroxy-N-desmethyl-TAM (endoxifen) are the major active metabolites of TAM, exhibiting 100-fold higher anti-estrogenic activity than TAM itself, with endoxifen concentrations reaching plasma concentrations that are 6-12-fold higher than that of 4-OH-TAM. One of the major mechanisms of elimination of TAM and its metabolites is by glucuronidation. In the present study, characterization of endoxifen glucuronidation by individual UDP-glucuronosyltransferases (UGTs) and relevant UGT polymorphic variants was determined. In vitro assays using human liver microsomes demonstrated that both the trans and cis isomers of endoxifen were catalyzed solely by O-glucuronidation. Of the UGT-over-expressing cell lines tested, the highest overall activity as determined by kinetic analysis (V_max/K_m) were UGTs 1A10>1A8>2B7>1A3≈1A9>1A1≈1A7 for trans-endoxifen, and 1A10>1A9>2B7>1A7>1A8>1A3>1A1 for cis-endoxifen. Although antibodies against UGTs 2B15 and 2B17 were unavailable, both enzymes demonstrated high levels of activity towards cis-endoxifen as determined by K_m analysis. Similar to that observed in liver microsomes, all individual UGTs catalyzed endoxifen-O-glucuronide formation. No detectable activity was observed for UGTs 1A4, 1A6, 2B4, 2B10 and 2B11 against either endoxifen isomer. The UGT1A10^139Lys and UGT2B7^268Tyr variants exhibited significantly (p<0.02) lower glucuronidating activities as determined by in vitro kinetic analysis (V_max/K_m) against endoxifen as compared to their wild type UGT1A10^139Glu and UGT2B7^268His counterparts, an effect observed against both endoxifen isomers. These results suggest that several UGTs may play an important role in the metabolism of endoxifen, that the UGT1A10^139Lys and UGT2B7^268Tyr variants could significantly alter endoxifen clearance rates in vivo, and that UGT variants could potentially play an important role on overall patient response to TAM.