Stereoselective pharmacokinetics and interconversions of flosequinan enantiomers containing chiral sulphoxide in rat

Investigating Sulfoxide-to-Sulfone Conversion as a Prodrug Strategy for a Phosphatidylinositol 4-Kinase Inhibitor in a Humanized Mouse Model of Malaria

The in vivo antimalarial efficacies of two phosphatidylinositol 4-kinase (PI4K) inhibitors, a 3,5-diaryl-2-aminopyrazine sulfoxide and its corresponding sulfone metabolite, were evaluated in the NOD-scid IL2Rγ^null (NSG) murine malaria disease model of Plasmodium falciparum infection. We hypothesized that the sulfoxide would serve as a more soluble prodrug for the sulfone, which would lead to improved drug exposure with oral dosing. Both compounds had similar efficacy (90% effective dose [ED₉₀], 0.1 mg kg^-1 of body weight) across a quadruple-dose regimen. Pharmacokinetic profiling revealed rapid sulfoxide clearance via conversion to sulfone, with sulfone identified as the major active metabolite. When the sulfoxide was dosed, the exposure of the sulfone achieved was as much as 2.9-fold higher than when the sulfone was directly dosed, thereby demonstrating that the sulfoxide served as an effective prodrug for the treatment of malaria.

Probing the stereochemistry of successive sulfoxidation of the insecticide fenamiphos in soils

Successive sulfoxidation is widely recognized as a general characteristic of the metabolism of chiral or prochiral thioethers, producing sulfoxides, and sulfones. However, information related to the stereochemistry of this process in soils is rare. In this study, the biotic transformation of the insecticide fenamiphos (a model thioether) was followed over two months in three soils, through separate incubations with fenamiphos parent, the sulfoxide intermediate (FSO), the sulfone intermediate (FSO2), and their respective stereoisomers. The results showed that the successive sulfoxidation involved oxidation of fenamiphos to FSO and subsequently to FSO2 as well as diastereomerization/enantiomerization of FSO, all of which were primarily biotic and stereoselective. The concomitant hydrolysis of fenamiphos, FSO, and FSO2 to phenols that occurred at lower rates was biotically favorable, but not stereoselective. The stereochemistry of this successive sulfoxidation transferred principally through two parallel systems, R(+)-fenamiphos → SRPR(+)-/SSPR(-)-FSO → R(+)-FSO2 and S(-)-fenamiphos → SRPS(+)-/SSPS(-)-FSO → S(-)-FSO2, between which unidirectional intersystem crossing occurred at FSO via isomeric conversions and created a system of S(-)-fenamiphos → SRPR(+)-/SSPR(-)-FSO → R(+)-FSO2. This pattern accounts for the enrichment of the intermediates SSPR(-)-/SSPS(-)-FSO and R(+)-FSO2 that are toxicologically close to the highly toxic S(-)-fenamiphos, associated with soil application of fenamiphos. Selective formation/depletion of these intermediate stereoisomers leads to dramatic variations in the ecotoxicological effects of the thioether insecticide.

Chiral inversion of RS-8359: a selective and reversible MAO-A inhibitor via oxido-reduction of keto-alcohol

RS-8359, (+/-)-4-(4-cyanoanilino)-5,6-dihydro-7-hydroxy-7H-cyclopenta[d]-pyrimidine is a selective and reversible MAO-A inhibitor. The (S)-enantiomer of RS-8359 has been demonstrated to be inverted to the (R)-enantiomer after oral administration to rats. In the current study, we investigated the chiral inversion mechanism and the properties of involved enzymes using rat liver subcellular fractions. The 7-hydroxy function of RS-8359 was oxidized at least by the two different enzymes. The cytosolic enzyme oxidized enantiospecifically the (S)-enantiomer with NADP as a cofactor. On the other hand, the microsomal enzyme catalyzed more preferentially the oxidation of the (S)-enantiomer than the (R)-enantiomer with NAD as a cofactor. With to product enantioselectivity of reduction of the 7-keto derivative, it was found that only the alcohol bearing (R)-configuration was formed by the cytosolic enzyme with NADPH and the microsomal enzyme with NADH at almost equal rate. The reduction rate was much larger than the oxidation rate of 7-hydroxy group. The results suggest that the chiral inversion might occur via an enantioselectivity of consecutive two opposing reactions, oxidation and reduction of keto-alcohol group. In this case, the direction of chiral inversion from the (S)-enantiomer to the (R)-enantiomer is governed by the enantiospecific reduction of intermediate 7-keto group to the alcohol with (R)-configuration. The enzyme responsible for the enantiospecific reduction of the 7-keto group was purified from rat liver cytosolic fractions and identified as 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD) via database search of peptide mass data obtained by nano-LC/MS/MS.

Stereoselective chiral inversion of pantoprazole enantiomers after separate doses to rats

(+/-)-Pantoprazole ((+/-)-PAN), (+/-)-5-(difluoromethoxy)-2-[[3.4-dimethoxy-2-pyridinyl)methyl]sul finyl]- 1H-benzimidazole) is a chiral sulfoxide that is used clinically as a racemic mixture. The disposition kinetics of (+)-PAN and (-)-PAN given separately has been studied in rats. Serum levels of (+)- and (-)-PAN and its metabolites, pantoprazole sulfone (PAN-SO2), pantoprazole sulfide (PAN-S), 4'-O-demethyl pantoprazole sulfone (DMPAN-SO2), and 4'-O-demethyl pantoprazole sulfide (DMPAN-S) were measured by HPLC. Following single intravenous or oral administration, both enantiomers were rapidly absorbed and metabolized, resulting in similar serum concentrations, suggesting that the two enantiomers have approximately the same disposition kinetics. The major metabolite of both (+)- and (-)-PAN was PAN-SO2, while DMPAN-SO2 was also detected as a minor metabolite. Serum levels of PAN-S and DMPAN-S could not be quantified after intravenous or oral administration of either enantiomer. Significant chiral inversion occurred after intravenous and oral administration of (+)-PAN. The AUCs of (-)-PAN after intravenous and oral dosing of (+)-PAN were 36.3 and 28.1%, respectively of those of total [(+) + (-)] PAN. In contrast, the serum levels of (+)-PAN were below quantitation limits after intravenous or oral administration of (-)-PAN. Therefore, chiral inversion was observed only after administration of (+)-PAN, supporting the hypothesis that stereoselective inversion from (+)-PAN to (-)-PAN occurs in rats.

Stereoselective S-oxidation and reduction of flosequinan in rat

1. The stereoselective S-oxidation and reduction pathways of flosequinan [(+/-)-7-fluoro-1-methyl-3-methylsulphinyl-4-quinolone] in rat were investigated in vitro. 2. Cytosol from both the liver and kidney catalysed the reduction of R(+)-flosequinan (R-FSO) and S(-)-flosequinan (S-FSO) to flosequinan sulphide (FS, 7-fluoro-1-methyl-3-methylthio-4-quinolone). Flosequinan sulphone (FSO2, 7-fluoro-1-methyl-3-methylsulphonyl-4-quinolone) was not reduced to R-FSO or S-FSO. 3. Liver microsomes catalysed four different S-oxidation pathways in the presence of NADPH, namely oxidation of FS to R-FSO and S-FSO and from R-FSO and S-FSO to FSO2. The formation of R-FSO and S-FSO from FS each exhibited a biphasic kinetic pattern, indicating that at least two distinct enzymes were involved. The pathway from FS to R-FSO appeared mainly catalysed by flavin-containing monooxygenases (FMO). 4. S-oxidation of FS to R-FSO was more rapid than that of FS to S-FSO. S-oxidation of FS to either R-FSO or S-FSO in liver microsomes was more rapid than that of either R-FSO or S-FSO to FSO2. 5. Microsomes from both the kidney and lung catalysed the stereoselective S-oxidation of FS to R-FSO, and FMO was likely to have participated in these reactions.

Stereoselective sulfoxidation of the pesticide methiocarb by flavin-containing monooxygenase and cytochrome P450-dependent monooxygenases of rat liver microsomes. Anticholinesterase activity of the two sulfoxide enantiomers

Evidence based on thermal lability and enzyme inhibition data suggests that the sulfoxidation of methiocarb (an N-methylcarbamate insecticide) by rat liver microsomes is catalyzed by flavin-containing monooxygenase(s) (FMO) and by cytochrome(s) P450 (P450). In control rats, the relative proportion is ca. 50% P450:50% FMO. Stereoselective formation of methiocarb sulfoxide from the corresponding sulfide has also been examined to compare the enantioselectivity of the two different enzyme systems. Only the FMO-dependent sulfoxidation presents a high stereoselectivity with an enantiomeric excess of 88% in favor of the (A)-enantiomer. Pretreatment of rats with different P450 inducers such as phenobarbital, 3-methylcholanthrene, dexamethasone, and pyrazole did not affect, or decreased, the rate of methiocarb sulfoxidation. Stereoselectivity of the reaction was modified, mainly because of changes in the relative involvement of FMO and P450 in sulfoxidase activity in pretreated animals. The acetylcholinesterase inhibition properties of methiocarb and its main metabolites were also investigated. Racemic methiocarb sulfoxide was slightly less inhibitory (Ki = 0.216 microM-1.min-1) than methiocarb, but a 10-fold difference was observed between the bimolecular rate constants found for the two sulfoxides produced (0.054 and 0.502 microM-1.min-1 for the (A) and (B) enantiomers, respectively).

Chiral inversion of drug: role of intestinal bacteria in the stereoselective sulphoxide reduction of flosequinan

Chiral inversion at a sulphoxide position of flosequinan enantiomers [(+/-)-7-fluoro-1-methyl-3-methylsulphinyl-4-quinolone] occurred in conventional rats but not in either germ-free rats or rats treated with antibiotics after an oral administration of each enantiomer. Thus, it was postulated that the chiral inversion occurred by mechanisms mediated by intestinal bacteria. The intestinal content isolated from conventional rats reduced R(+)- and S(-)-flosequinan to produce the sulphide, while intestinal content from rats treated with antibiotics did not reduce the drug. Several strains of facultative anaerobes possessed a high flosequinan reducing activity. Escherichia coli, Klebsiella oxytoca and Klebsiella pneumoniae reduced R(+)-flosequinan to the sulphide stereoselectively. On the other hand, Enterobacter aerogenes and Micrococcus agilis exclusively reduced S(-)-flosequinan. The sulphide, which could be produced by intestinal bacteria from R(+)- and S(-)-flosequinan, was readily absorbed upon an oral administration to rats, and was oxidized fairly rapidly to R(+)- and S(-)-flosequinan and further to the sulphone form. Based on these data, it has been confirmed that chiral inversion at the sulphoxide position of flosequinan enantiomers occur via stereoselective reduction of sulphoxide by intestinal bacteria to form the sulphide, followed by oxidation of the sulphide in the body to produce R(+)- and S(-)-flosequinan.