title: Mechanism-based inactivation of cytochrome P450 3A4 by lapatinib.
abstract: Fatalities stemming from hepatotoxicity associated with the clinical use of lapatinib (Tykerb), an oral dual tyrosine kinase inhibitor (ErbB-1 and ErbB-2) used in the treatment of metastatic breast cancer, have been reported. We investigated the inhibition of CYP3A4 by lapatinib as a possible cause of its idiosyncratic toxicity. Inhibition of CYP3A4 was time-, concentration-, and NADPH-dependent, with k(inact) = 0.0202 min(-1) and K(i) = 1.709 μM. The partition ratio was approximately 50.9. Addition of GSH did not affect the rate of inactivation. Testosterone protected CYP3A4 from inactivation by lapatinib. The characteristic Soret peak associated with a metabolite-intermediate complex was not observed for lapatinib during spectral difference scanning. However, reduced carbon monoxide (CO)-difference spectroscopy did reveal a 43% loss of the spectrally detectable CYP3A4-CO complex in the presence of lapatinib. Incubation of either lapatinib or its dealkylated metabolite with human liver microsomes in the presence of GSH resulted in the formation of a reactive metabolite (RM)-GSH adduct derived from the O-dealkylated metabolite of lapatinib. In addition, coincubation of lapatinib with ketoconazole inhibited the formation of the RM-GSH adduct. In conclusion, we demonstrated for the first time that lapatinib is a mechanism-based inactivator of CYP3A4, most likely via the formation and further oxidation of its O-dealkylated metabolite to a quinoneimine that covalently modifies the CYP3A4 apoprotein and/or heme moiety.
authors.affiliation: Department of Pharmacy, Faculty of Science, National University of Singapore, 18 Science Drive 4, Singapore 117543, Singapore.
authors.forename: Woon Chien
authors.initials: WC
authors.lastname: Teng
authors.forename: Jing Wen
authors.initials: JW
authors.lastname: Oh
authors.forename: Lee Sun
authors.initials: LS
authors.lastname: New
authors.forename: Michelle D
authors.initials: MD
authors.lastname: Wahlin
authors.forename: Sidney D
authors.initials: SD
authors.lastname: Nelson
authors.forename: Han Kiat
authors.initials: HK
authors.lastname: Ho
authors.forename: Eric Chun Yong
authors.initials: EC
authors.lastname: Chan
mesh_terms: D051544:Cytochrome P-450 CYP3A / Q000378:metabolism*; D065692:Cytochrome P-450 CYP3A Inhibitors; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D006801:Humans; D000077341:Lapatinib; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology; D011799:Quinazolines / Q000737:chemistry / Q000378:metabolism* / Q000494:pharmacology*
chemical_list: D065692:Cytochrome P-450 CYP3A Inhibitors; D004791:Enzyme Inhibitors; D011799:Quinazolines; D000077341:Lapatinib; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human