Abstract
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 μM) of phenacetin O-deethylation (POD) (CYP1A2), paclitaxel 6α-hydroxylation (CYP2C8), diclofenac 4′-hydroxylation (CYP2C9), (S)-mephenytoin 4′-hydroxylation (CYP2C19), bufuralol 1′-hydroxylation (CYP2D6), testosterone 6β-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 μM, respectively (kinact/KIratio of 0.0003 min-1 μM-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 μM, kinact/KIratio, 0.0004 min-1 μM-1; (R)-(+)-zileuton; kinact, 0.012 min-1, KI, 66.6 μM, kinact/KIratio, 0.0002 min-1 μM-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.
Footnotes
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↵1 Abbreviations used are: P450, cytochrome P450; AUC, area under the plasma concentration versus time curve; AUCpo(i), AUC (oral dose) in the presence of inhibitor; AUCpo(c), AUC (oral dose) in the absence of inhibitor; Cmax,ss, maximum concentration in plasma at steady state; Cave,ss, average concentration in plasma at steady state; LC/MS-MS, liquid chromatography/tandem mass spectrometry; kinact, maximal rate of enzyme inactivation; KI, inhibitor concentration that supports half the maximal rate of inactivation; [I]u, free inhibitor concentration; fu, fraction unbound in plasma; fm, fraction of dose metabolized by all cytochromes P450; fm,CYP1A2, fraction of cytochrome P450-dependent metabolism catalyzed by CYP1A2; kobs, observed rate of enzyme inactivation; ke, rate of cytochrome P450 holoenzyme degradation in the absence of inhibitor; POD, phenacetin O-deethylase; GSH, glutathione; SOD, superoxide dismutase; L-594615, 5,6-dichloro-3a,7a-dihydro-3H-benzooxazol-2-one; L-754394, N-[2(R)-hydroxy-1(S)-indanyl]-5-[2(S)-(1,1-dimethylethylaminocarbonyl)-4-[(furo[2,3-b]pyridin-5-yl)methyl]piperazin-1-yl]-4(S)-hydroxy-2(R)-phenylmethylpentanamide; Ki, inhibition constant (reversible inhibition); fm,P450, fraction of cytochrome P450-dependent metabolism catalyzed by inhibited cytochrome P450.
- Received June 13, 2003.
- Accepted August 1, 2003.
- The American Society for Pharmacology and Experimental Therapeutics
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