Abstract
O-Dealkylation of the tyrosine kinase inhibitor lapatinib by cytochrome P450 3A enzymes is implicated in the development of lapatinib-induced hepatotoxicity. Conjugative metabolism of the O-debenzylated phenolic metabolite of lapatinib (M1) via glucuronidation and sulfation is thought to be a major detoxication pathway for lapatinib in preclinical species (rat and dog), limiting formation of the quinoneimine reactive metabolite. Glucuronidation of M1 by human recombinant UDP-glucuronosyltransferases (UGTs) has been reported in vitro; however, the relative UGT enzyme contributions are unknown, and the inter-species differences in the conjugation vs. bioactivation pathways of M1 have not been fully elucidated. In the present study, reaction phenotyping experiments using human recombinant UGT enzymes and enzyme-selective chemical inhibitors demonstrated that UGT1A1 was the major hepatic UGT enzyme involved in lapatinib M1 glucuronidation. Formation of the M1-glucuronide by human liver microsomes from UGT1A1-genotyped donors was significantly correlated with UGT1A1 activity, as measured by 17β-estradiol 3-glucuronidation (R2 = 0.90). Inter-species differences were found in the biotransformation of M1 in human, rat, and dog liver microsomal and S9 fractions via glucuronidation, sulfation, aldehyde oxidase-mediated oxidation, and bioactivation to the quinoneimine, trapped as GSH conjugate. Moreover, we demonstrated the sequential metabolism of lapatinib in primary human hepatocytes to the M1-glucuronide, M1-sulfate, and quinoneimine-GSH conjugate. M1 glucuronidation highly correlated with the rates of M1 formation, suggesting that O-dealkylation may be the rate-limiting step in lapatinib biotransformation. Inter-individual variability in the formation and clearance pathways of lapatinib M1 likely influences the hepatic exposure to reactive metabolites and may affect the risk for hepatotoxicity.
Significance Statement We used an integrated approach to examine the inter-individual and inter-species differences in detoxication vs. bioactivation pathways of lapatinib, which is associated with idiosyncratic hepatotoxicity. In addition to P450-mediated bioactivation, we report that multiple non-P450 pathways are involved in the biotransformation of the primary phenolic metabolite of lapatinib in vitro, including glucuronidation, sulfation, and aldehyde oxidase-mediated oxidation. UGT1A1 was identified as the major hepatic enzyme involved in lapatinib M1 glucuronidation, which may limit hepatic exposure to the potentially toxic quinoneimine.
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