Mechanism-based inhibition of human liver microsomal cytochrome P450 1A2 by zileuton, a 5-lipoxygenase inhibitor.
Zileuton, a 5-lipoxygenase inhibitor, was evaluated as an inhibitor of cytochrome P450 activity in human liver microsomes. In the absence of preincubation, the racemate was found to be a weak inhibitor (IC50 > 100 microM) of phenacetin O-deethylation (POD) (CYP1A2), paclitaxel 6alpha-hydroxylation (CYP2C8), diclofenac 4'-hydroxylation (CYP2C9), (S)-mephenytoin 4'-hydroxylation (CYP2C19), bufuralol 1'-hydroxylation (CYP2D6), testosterone 6beta-hydroxylation (CYP3A4), chlorzoxazone 6-hydroxylation (CYP2E1), and bupropion hydroxylation (CYP2B6). When preincubated with NADPH-fortified human liver microsomes in the absence of substrate, zileuton (racemate) was shown to inhibit POD. The effect was NADPH-, time-, and concentration-dependent, and was characterized by a kinact (maximal rate of enzyme inactivation) and apparent KI(inhibitor concentration that supports half the maximal rate of inactivation) of 0.035 min(-1) and 117 microM, respectively (kinact/KIratio of 0.0003 min-1 microM(-1)). Preincubation-dependent inhibition of POD activity was also observed with the individual (S)-(-)- and (R)-(+)-enantiomers of zileuton [(S)-(-)-zileuton; kinact, 0.037 min(-1), KI, 98.2 microM, kinact/KIratio, 0.0004 min(-1) microM(-1); (R)-(+)-zileuton; kinact, 0.012 min(-1), KI, 66.6 microM, kinact/KIratio, 0.0002 min(-1) microM(-1)]. In addition, the inhibition of CYP1A2 was not reversed in the presence of reduced glutathione, catalase, and superoxide dismutase and was refractory to dialysis. Therefore, zileuton was characterized as a mechanism-based inhibitor of human liver microsomal CYP1A2. Mechanism-based inhibition of CYP1A2 may explain why zileuton decreases the oral clearance of antipyrine, propranolol, (R)-warfarin, and theophylline, at doses that have a minimal effect on the pharmacokinetics of (S)-warfarin, phenytoin, and terfenadine.
Lu
Ping
P
Department of Drug Metabolism, WP75A-203, Merck Research Laboratories, West Point PA 19486, USA.
Schrag
Michael L
ML
Slaughter
Donald E
DE
Raab
Conrad E
CE
Shou
Magang
M
Rodrigues
A David
AD
D019388:Cytochrome P-450 CYP1A2 / Q000378:metabolism; D065609:Cytochrome P-450 CYP1A2 Inhibitors; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D006801:Humans; D006918:Hydroxyurea / Q000031:analogs & derivatives* / Q000494:pharmacology*; D016859:Lipoxygenase Inhibitors; D008862:Microsomes, Liver / Q000187:drug effects* / Q000201:enzymology
D065609:Cytochrome P-450 CYP1A2 Inhibitors; D004791:Enzyme Inhibitors; D016859:Lipoxygenase Inhibitors; D019388:Cytochrome P-450 CYP1A2; C063449:zileuton; D006918:Hydroxyurea