The Letrozole Phase 1 Metabolite Carbinol as a Novel Probe Drug for UGT2B7

Abstract

Carbinol [4,49-(hydroxymethylene)dibenzonitrile] is the main phase 1 metabolite of letrozole, a nonsteroidal aromatase inhibitor used for endocrine therapy in postmenopausal breast cancer. We elucidated the contribution of UDP-glucuronosyltransferase (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 (HLMs). A liquid chromatography-tandem mass spectrometry 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 the plasma concentrations of 60 letrozole-treated breast cancer patients. We identified UGT2B7 as the predominant UGT isoform involved in carbinol glucuronidation. In HLMs and recombinant UGT2B7, we determined K m values (9.99 and 9.56 mM) and V max values (3430 and 2399 pmol/min per milligram of 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, 2434 6 1018; *1/*2: n = 80, 2356 6 1372; *2/*2: n = 43, 2251 6 1421 pmol/min per milligram of protein) nor UGT2B7 protein expression was altered by the UGT2B7*2 genotype. No impact of UGT2B7*2 on plasma levels of carbinol and carbinol-gluc [bis(4-cyanophenyl)methyl hexopyranosiduronic acid] in 60 letrozole-treated patients was found. Taken together, these findings 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.

AM-2201 Inhibits Multiple Cytochrome P450 and Uridine 5′-Diphospho-Glucuronosyltransferase Enzyme Activities in Human Liver Microsomes

“…The inhibitory potency ( K i , 10.0 µM) of AM-2201 on UGT2B7-catalyzed naloxone 3′-glucuronidation was similar to that ( K i, 9.8 μM) of cannabidiol for UGT2B7-catalyzed ethanol glucuronidation [ 30 ], suggesting that AM-2201 may cause drug interactions with UGT2B7 substrates, such as morphine, zidovudine, efavirenz, ethanol, carbinol, JWH-018, and flurbiprofen [ 30 , 40 , 41 , 42 , 43 , 44 ].…”

Chemical Probes for Human UDP‐Glucuronosyltransferases: A Comprehensive Review

“…Carbinol, the major phase I metabolite of letrozole, has been demonstrated to be a novel in vitro probe substrate for UGT2B7, with high selectivity, high affinity, and high conversion rate in vitro (Table S6, Supporting Information). Notably, it also has a potential as an in vivo probe for clinical studies, because physiologic activity of carbinol is strongly reduced compared with letrozole . Denopamine, one of the oral β1‐adrenoceptor selective partial agonists, has been reported to be regioselectively glucuronidated by UGT2B7 in HLM and HIM.…”

“…Notably, it also has a potential as an in vivo probe for clinical studies, because physiologic activity of carbinol is strongly reduced compared with letrozole. [135] Denopamine, one of the oral β1-adrenoceptor selective partial agonists, has been reported to be regioselectively glucuronidated www.advancedsciencenews.com www.biotechnology-journal.com by UGT2B7 in HLM and HIM. Notably, the glucuronidation site of denopamine in human is the alcoholic hydroxyl group, but not the phenolic group.…”

The use of aromatase inhibitors in boys with short stature: what to know before prescribing?

“…In vitro studies have demonstrated that CYP3A4 and CYP2A6 are involved in the metabolism of letrozole, converting it to carbinol (25,26). Carbinol is then conjugated by glucuronyl transferase, more precisely by the UGT2B7 isoform ( Figure 2) (27). The drug has a high affinity for CYP2A6 and a low affinity for CYP3A4.…”