ABSTRACT:The epidermal growth factor receptor tyrosine kinase inhibitor erlotinib (ERL) is approved for treatment of non-small-cell lung cancer. Numerous reports of ERL-associated toxicities are consistent with immune-mediated toxicity, including drug-induced hepatitis, interstitial lung disease, Stevens-Johnson syndrome, and toxic epidermal necrolysis. Although the mechanism of toxicity has not been established, we present evidence that reactive intermediates are formed during the metabolism of ERL, which can covalently conjugate to the cysteine group of the peptide-mimetic GSH. Seven ERL-GSH conjugates were identified in incubations with hepatic microsomes. Cytochrome P450 (P450)-dependent adducts are proposed to be formed via reactive epoxide and electrophilic quinone-imine intermediates. In incubations of human liver microsomes, intestinal microsomes, pulmonary microsomes, and recombinant P450s, CYP3A4 was the primary enzyme responsible for the bioactivation of ERL; however, CYP1A1, CYP1A2, CYP3A5, and CYP2D6 were capable of catalyzing the bioactivation as well. During the metabolism of ERL, CYP3A4 and CYP3A5 are irreversibly inactivated by ERL in a time-and concentration-dependent manner. Inactivation was not dependent on oxidation of the ERL alkyne group to form a reactive oxirene or ketene, as shown by synthesizing analogs where the alkyne was replaced with a cyano group. CYP1A1, CYP1A2, and CYP2D6 were not inactivated despite catalyzing the formation of ERL-GSH adducts.Erlotinib (ERL) is a reversible inhibitor of the epidermal growth factor receptor tyrosine kinase (HER1/EGFR) and was approved for the second-and third-line treatment of non-small-cell lung cancer in 2005 (Siegel-Lakhai et al., 2005). Clinical trials indicate that ERL provides a survival benefit after failure of first-or second-line chemotherapy as a single agent and in the treatment of advanced pancreatic adenocarcinomas together with chemotherapy (Tang et al., 2006;Moore et al., 2007). Although having therapeutic benefit, treatment with ERL has been associated with life-threatening adverse effects, including drug-induced hepatitis (Liu et al., 2007b;Ramanarayanan and Scarpace, 2007;Saif, 2008;Pellegrinotti et al., 2009), interstitial lung disease (Liu et al., 2007a;Makris et al., 2007), and the severe skin disorders Stevens-Johnson syndrome and toxic epidermal necrolysis (Chou et al., 2006;Lübbe et al., 2008;Bovenschen and Alkemade, 2009). In September 2008, OSI Pharmaceuticals and Genentech (www.fda.gov/downloads/safety/medwatch/safetyinformation/ safetyalertsforhumanmedicalproducts/ucm135238) reported a pharmacokinetic study of 15 patients with advanced solid tumors and moderate liver impairment. During the study, one patient died from hepatorenal syndrome and another died because of progressive liver failure, and both deaths were attributed to ERL.In humans, ERL is extensively metabolized, predominantly by CYP3A4 and to a lesser extent by CYP1A2 and the inducible isoform CYP1A1 (Ling et al., 2006;Li et al., 2007), with metabolites exc...