Received for publication March 10, 2008.
Revised July 2, 2008.
Accepted for publication July 29, 2008.

Oxidative In Vitro Metabolism of Liquiritigenin, a Bioactive Compound Isolated from the Chinese Herbal Selective Estrogen -Receptor Agonist MF101

Abstract

Liquiritigenin (2,3-dihydro-7-hydroxy-2-(4-hydroxyphenyl)-(S)-4H-1-benzopyran-4-one) is one of the major active compounds of MF101, a drug currently in clinical trials for the treatment of hot flashes and night sweats in postmenopausal women. MF101 is a selective estrogen receptor agonist, but does not activate the estrogen receptor . Incubation with pooled human liver microsomes yielded a single metabolite. Its structure was elucidated using MS/MS in combination with analysis of the fragmentation patterns. The metabolite resulted from the loss of two hydrogens and rearrangement to the stable 7,4'-dihydroxyflavone. The structure was also confirmed by comparison to authentic standard material. Maximum apparent reaction velocity (V_max) and Michaelis-Menten constant (K_m) for the formation of 7,4'-dihydroxyflavone were 32.5 nmol/g protein/min and 128 µmol/L, respectively. After correction for protein binding (free fraction F_u= 0.84), the apparent CL_int for 7,4'-dihydroxyflavone formation was 0.3 mL/g/min. Liquiritigenin was almost exclusively metabolized by CYP3A enzymes. Comparison of liquiritigenin metabolism in human liver microsomes isolated from 16 individuals showed 9.5-fold variability in metabolite formation (3.4 to 32.2 nmol/g protein/min). An estrogen receptor luciferase assay indicated that the metabolite was a 3-fold more potent activator of the estrogen receptor than the parent compound and did not activate the estrogen receptor .

Key words: analytical pharmacology/toxicology, CYP inhibition, cytochrome P450 catalyzed oxidations, drug clearance, human CYP enzymes, kinetics, liver microsomes, mass spectrometry, metabolite identification, metabolite kinetics