Mechanism-based inactivation of CYP450 enzymes: a case study of lapatinib.

Mechanism-based inactivation (MBI) of CYP450 enzymes is a unique form of inhibition in which the enzymatic machinery of the victim is responsible for generation of the reactive metabolite. This precondition sets up a time-dependency for the inactivation process, a hallmark feature that characterizes all MBI. Yet, MBI itself is a complex biochemical phenomenon that operates in different modes, namely, covalent binding to apoprotein, covalent binding of the porphyrin group and also complexation of the catalytic iron.

Studies on the role of metabolic activation in tyrosine kinase inhibitor-dependent hepatotoxicity: induction of CYP3A4 enhances the cytotoxicity of lapatinib in HepaRG cells.

Idiosyncratic hepatotoxicity has been associated with the oral tyrosine kinase inhibitor lapatinib, which is used in metastatic breast cancer therapy. Lapatinib is extensively metabolized by cytochrome P450 3A4/5 to yield an O-debenzylated metabolite, which can undergo further oxidation to a reactive quinone imine. A recent clinical study reported that concomitant use of lapatinib with dexamethasone increased the incidence of hepatotoxicity in metastatic breast cancer patients treated with lapatinib, and so we hypothesized that induction of CYP3A enhances the bioactivation of lapatinib to reactive intermediates that contribute to hepatotoxicity.

Nonenzymatic free radical-catalyzed generation of 15-deoxy-Δ(12,14)-prostaglandin J₂-like compounds (deoxy-J₂-isoprostanes) in vivo.

15-Deoxy-Δ(12,14)-prostaglandin J₂ (15-d-PGJ₂) is a reactive cyclopentenone eicosanoid generated from the dehydration of cyclooxygenase-derived prostaglandin D₂ (PGD₂). This compound possesses an α,β-unsaturated carbonyl moiety that can readily adduct thiol-containing biomolecules such as glutathione and cysteine residues of proteins via the Michael addition. Due to its reactivity, 15-d-PGJ₂ is thought to modulate inflammatory and apoptotic processes and is believed to be an endogenous ligand for peroxisome proliferator-activated receptor-γ.

Development of oxidative stress by cytochrome P450 induction in rodents is selective for barbiturates and related to loss of pyridine nucleotide-dependent protective systems.

Reactive oxygen species (ROS) and oxidative stress have been considered in a variety of disease models, and cytochrome P450 (P450) enzymes have been suggested to be a source of ROS. Induction of P450s by phenobarbital (PB), beta-naphthoflavone (betaNF), or clofibrate in a mouse model increased ROS parameters in the isolated liver microsomes, but isoniazid treatment did not. However, when F(2)-isoprostanes (F(2)-IsoPs) were measured in tissues and urine, PB showed the strongest effect and betaNF had a measurable but weaker effect.

In vivo oxidative damage in rats is associated with barbiturate response but not other cytochrome P450 inducers.

Previously published studies have shown that cytochrome P450 (P450) enzyme systems can produce reactive oxygen species and suggest roles of P450s in oxidative stress. However, most of the studies have been done in vitro, and the potential link between P450 induction and in vivo oxidative damage has not been rigorously explored with validated biomarkers. Male Sprague-Dawley rats were pretreated with typical P450 inducers (beta-naphthoflavone, phenobarbital (PB), Aroclor 1254, isoniazid, pregnenolone 16alpha-carbonitrile, and clofibrate) or the general P450 inhibitor 1-aminobenztriazole; induction of P4501A, -2B, -2E, -3A, and -4A subfamily enzymes was confirmed by immunoblotting and the suppression of P450 by 1-aminobenztriazole using spectral analysis.