Inhibition of kinesin spindle protein (KSP) is a novel mechanism for treatment of cancer with the potential to overcome limitations associated with currently employed cytotoxic agents. Herein, we describe a C2-hydroxymethyl dihydropyrrole KSP inhibitor ( 11) that circumvents hERG channel binding and poor in vivo potency, issues that limited earlier compounds from our program. However, introduction of the C2-hydroxymethyl group caused 11 to be a substrate for cellular efflux by P-glycoprotein (Pgp). Utilizing knowledge garnered from previous KSP inhibitors, we found that beta-fluorination modulated the p K a of the piperidine nitrogen and reduced Pgp efflux, but the resulting compound ( 14) generated a toxic metabolite in vivo. Incorporation of fluorine in a strategic, metabolically benign position by synthesis of an N-methyl-3-fluoro-4-(aminomethyl)piperidine urea led to compound 30 that has an optimal in vitro and metabolic profile. Compound 30 (MK-0731) was recently studied in a phase I clinical trial in patients with taxane-refractory solid tumors.

Bromobenzene (BB) hepatotoxicity is widely attributed to the alkylation of cellular proteins by chemically reactive metabolites, particularly BB-3,4-oxide. This laboratory recently reported the first conclusive evidence that BB epoxides actually do alkylate proteins; i.e., acid hydrolysates of hepatic proteins from phenobarbital-(PB-) induced BB-treated rats contain S-(o-, S-(m-, and S-(p-bromophenyl)cysteine [Weller, P.E., and Hanzlik, R.P. (1991) Chem. Res. Toxicol. 4, 17-20]. However, these three compounds account for less than 0.5% of total protein covalent binding. Bromoquinone metabolites of BB are also suspected of alkylating proteins. To search for such adducts to protein cysteinyl or methionyl residues, we heated hepatic proteins from PB-induced BB-treated rats with a two-phase mixture of 16 N KOH and CH3I ("alkaline permethylation"). Under these conditions S-alkylated residues are cleaved via elimination and the phenoxide and thiophenoxide groups on the fragments are methylated. Product analysis by 14C HPLC and GC/MS revealed o-, m-, and p-bromothioanisoles in amounts comparable to the content of S-(bromophenyl)cysteines found by acid hydrolysis (para much greater than meta, ortho). This, too, clearly implicates protein-SH alkylation by BB-2,3- and 3,4-oxides. In addition, 2,3-dimethoxy-5-bromothioanisole and another unidentified isomer were observed. However, by far the major adduct (5-6% of total covalent binding) was 2,5-dimethoxythioanisole (i.e., a debrominated adduct). When BB-d5 was administered, the latter contained mostly 3 deuterium atoms/mol. These latter results clearly show that alkylation of protein sulfur nucleophiles in vivo by quinone metabolites is 10-15 times more extensive than their alkylation by BB epoxides. After BB-d5 was administered, the bromothioanisoles and dimethoxybromothioanisoles contained 4 and 2 deuterium atoms/mol, respectively. A weighted average calculation of deuterium retention across the six major sulfur adducts agreed well with 3H/14C retention ratios determined earlier for total liver protein covalent binding of dual-labeled [3H/14C]BB, indicating that the overall pattern of BB metabolite binding to all protein nucleophiles may closely parallel that seen here specifically for protein sulfhydryl groups. The identification of a variety of specific BB-derived adducts to protein now affords the opportunity to investigate their relative contributions to the toxicity of bromobenzene.

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Zafirlukast is a leukotriene antagonist indicated for the treatment of mild to moderate asthma, but the drug has been associated with occasional idiosyncratic hepatotoxicity. Structurally, zafirlukast is similar to 3-methylindole because it contains an N-methylindole moiety that has a 3-alkyl substituent on the indole ring. The results presented here describe the metabolic activation of zafirlukast via a similar mechanism to that described for 3-methylindole. NADP(H)-dependent biotransformation of zafirlukast by hepatic microsomes from rats and humans afforded a reactive metabolite, which was detected as its GSH adduct. Mass spectrometry and NMR data indicated that the GSH adduct was formed by the addition of GSH to the methylene carbon between the indole- and methoxy-substituted phenyl rings of zafirlukast. The formation of this reactive metabolite in human liver microsomes was shown to be exclusively catalyzed by CYP3A enzymes. Evidence for in vivo metabolic activation of zafirlukast was obtained when the same GSH adduct was detected in bile of rats given an iv or oral dose of the drug. On the basis of results with model peroxidases and of the structures of product alcohols from incubations containing H2(18)O, it appeared that zafirlukast underwent dehydrogenation by two sequential one-electron oxidations. In addition, zafirlukast proved to be a mechanism-based inhibitor of CYP3A4 activity in human liver microsomes and in microsomes containing cDNA-expressed CYP3A4. The enzyme inhibitory property of zafirlukast was selective for this enzyme among all of the P450 enzymes that were tested in human liver microsomes. The inactivation was characterized by a K(I) of 13.4 microM and k(inact) of 0.026 min(-1). In summary, zafirlukast dehydrogenation to an electrophilic alpha,beta-unsaturated iminium intermediate may be associated with idiosyncratic hepatotoxicity and/or cause drug-drug interactions through inactivation of CYP3A4.

This article is available online at http://dmd.aspetjournals.org 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 (IC 50 > 100 M) of phenacetin O-deethylation (POD) (CYP1A2), paclitaxel 6␣-hydroxylation (CYP2C8), diclofenac 4-hydroxylation (CYP2C9), (S)-mephenytoin 4-hydroxylation (CYP2C19), bufuralol 1-hydroxylation (CYP2D6), testosterone 6␤-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 k inact (maximal rate of enzyme inactivation) and apparent K I (inhibitor concentration that supports half the maximal rate of inactivation) of 0.035 min ]. 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. Zileuton (Zyflo;thien-2-ylethyl)-N-hydroxyurea), a substituted hydroxamic acid, is a potent and selective 5-lipoxygenase inhibitor that has been approved for the prevention and chronic treatment of asthma (Isreal et al., 1990;Carter et al., 1991;Bell et al., 1992;Wenzel and Kamada, 1996;Dube et al., 1999). The drug contains a single chiral center (Fig. 1), is administered as a racemic mixture of (R)-(ϩ)-and (S)-(Ϫ)-enantiomers, and is characterized by a plasma half-life of about 4 h (Wong et al., 1995). As a result of the short half-life, zileuton has to be dosed four times a day (q.i.d.), and effective inhibition (70 -80%) of leukotriene B 4 biosynthesis can be attained using a 600-mg q.i.d. dosing regimen (Awni et al., 1995a).Zileuton is well absorbed, and the majority of the dose (ϳ75%) is recovered in the urine of healthy volunteers as the N-hydroxy glucuronides of both the (R)-(ϩ)-and (S)-(Ϫ)-enantiomers (Wong et al., 1995). Although N-hydroxy glucuronidation has been confirmed with human liver microsomes, zileuton is metabolized in the presence of NADPH (Machinist et al., 1995b;Sweeny and Nellans, 1995). In fact, CYP1A2 and CYP2C9 catalyze ring hydroxylation, whereas sulfoxidation is catalyzed largely by CYP3A4. Both reactions conform to Michaelis-Menten kinetics in human liver microsomes and are described by relatively high K m values (Ն 0.2 mM).Zileuton also has been shown to be a weak inhibitor of human liver microsomal CYP3A4, CYP2C9, and CYP2D6 activity in vitro (Machinist et al., 1995b). As anticipated, zileuton has a m...