Abstract

4,4'-(hydroxymethylene)dibenzonitrile (carbinol) is the main phase 1 metabolite of letrozole, a non-steroidal aromatase inhibitor used for endocrine therapy in postmenopausal breast cancer. We elucidated the contribution of UGT isoforms on the glucuronidation of carbinol. Identification of UGT isoforms was performed using a panel of recombinant human UGT enzymes. Kinetic studies were done in recombinant human UGT2B7 and pooled human liver microsomes (HLM). LC-MS/MS method was used for detection of metabolites. To assess the impact of UGT2B7*2, we determined the carbinol glucuronidation activity using HLM as well as UGT2B7 protein expression in 148 human livers. Moreover, we analyzed plasma concentrations of 60 letrozole treated breast cancer patients. We identified UGT2B7 as the predominant UGT isoform involved in carbinol glucuronidation. In HLM and recombinant UGT2B7, we determined K_m values (9.99 and 9.56 μM) and V_max values (3,430 and 2,399 pmol/min/mg protein), respectively. In the set of 148 human livers carbinol glucuronidation activity significantly correlated with UGT2B7 protein as determined by western blotting (r_s = 0.5088, ****P < 0.0001). Neither carbinol glucuronidation activity (*1/*1: n=25, 2,434±1,018; *1/*2: n=80, 2,356±1,372; *2/*2: n=43; 2,251±1,421 pmol/min/mg protein) nor UGT2B7 protein expression were altered by UGT2B7*2 genotype. No impact of UGT2B7*2 on plasma levels of carbinol and carbinol-gluc in 60 letrozole treated patients was found. Taken together, we suggest carbinol as a novel in vitro probe substrate for UGT2B7. In vitro and in vivo data suggest a lack of influence of the UGT2B7*2 polymorphism on carbinol glucuronidation.