Mechanistic studies on metabolic chiral inversion of 4-(4-methylphenyl)-2-methylthiomethyl-4-oxobutanoic acid (KE-748), an active metabolite of the new anti-rheumatic agent 2-acetylthiomethyl-4-(4-methylphenyl)-4-oxobutanoic acid (KE-298), in rats

Reduction of flobufen in pig hepatocytes: effect of pig breed (domestic, wild) and castration

Knowledge of the biotransformation processes of veterinary drugs and food supplements in food-producing animals is increasingly important. Residual levels of parent compounds or their metabolites in food of animal origin may differ with the breed, breeding conditions, and gender of animals. The nonsteroidal antiinflammatory drug flobufen, 4-(2',4'-difluorobiphenyl-4-yl)-2-methyl-4-oxobutanoic acid (racemic or its individual enantiomers) was used as a model to evaluate differences in activity, stereoselectivity, and stereospecificity of reductases in primary cultures of hepatocytes from intact male or castrated male domestic pigs (Sus scrofa domestica) or male wild pig (Sus scrofa scrofa). Time-dependent consumption of flobufen enantiomers and formation of dihydroflobufen (DHF) diastereoisomers as their principal metabolites in hepatocytes were measured using chiral HPLC. Flobufen reduction in hepatocytes from all three experimental groups of animals was stereoselective ((+)-R-flobufen was predominantly metabolized) and stereospecific (2R;4S-DHF and 2S;4S-DHF diastereoisomers were preferentially formed). Flobufen reductases activity in male domestic pigs was 30 times higher compared to castrated pigs. Flobufen reductases activity was similar in domestic and wild pigs. The stereospecificity and stereoselectivity of DHF production did not significantly differ with breed or castration of animal. Chiral inversion of flobufen enantiomers was also studied and differences between castrated and intact male pigs were seen.

Biotransformation of flobufen enantiomers in ruminant hepatocytes and subcellular fractions

Flobufen (F), a new antiinflammatory drug, has one chiral and one prochiral center in its structure. Reduction of rac-F, the principal biotransformation pathway, leads to the formation of four diastereoisomers of 4-dihydroflobufen (DHF). F was chosen as a model substrate for interspecies comparison of activity, stereospecificity, and stereoselectivity of biotransformation enzymes in fallow bucks, red deer stags, and roe bucks in vitro. Formation of F metabolites was examined in hepatocyte suspension and in subcellular fractions of liver homogenate. (+)-R-F, (-)-S-F and rac-F were used as substrates. After incubation of substrates, the amounts and ratios of DHF diastereoisomers and F enantiomers were assessed by HPLC, with (R,R)-ULMO and terguride-bonded columns. Considerable interspecies differences in stereoselectivity and stereospecificity of F reductases were found at the cellular and subcellular levels, although these ruminants are closely related. Chiral inversion of F enantiomers to their antipodes was detected in vitro in all ruminants tested, but individual species also differed in the direction and rate of this inversion.

Stereoselective disposition of tiaprofenic acid enantiomers in rats

The pharmacokinetics and metabolic chiral inversion of the S(+)- and R(-)-enantiomers of tiaprofenic acid (S-TIA, R-TIA) were assessed in vivo in rats, and in addition the biochemistry of inversion was investigated in vitro in rat liver homogenates. Drug enantiomer concentrations in plasma were investigated following administration of S-TIA and R-TIA (i.p. 3 and 9 mg/kg) over 24 hr. Plasma concentrations of TIA enantiomers were determined by stereospecific HPLC analysis. After administration of R-TIA it was found that 1) there was a time delay of peak S-TIA plasma concentrations, 2) S-TIA concentrations exceeded R-TIA concentrations from approximately 2 hr after dosing, 3) Cmax and AUC(0-infinity) for S-TIA were greater than for R-TIA following administration of S-TIA, and 4) inversion was bidirectional but favored inversion of R-TIA to S-TIA. Bidirectional inversion was also observed when TIA enantiomers were incubated with liver homogenates up to 24 hr. However, the rate of inversion favored transformation of the R-enantiomer to the S-enantiomer. In conclusion, stereoselective pharmacokinetics of R- and S-TIA were observed in rats and bidirectional inversion in rat liver homogenates has been demonstrated for the first time. Chiral inversion of TIA may involve metabolic routes different from those associated with inversion of other 2-arylpropionic acids such as ibuprofen.

Role of hepatic fatty acid:coenzyme A ligases in the metabolism of xenobiotic carboxylic acids

1. Formation of acyl-coenzymes (Co)A occurs as an obligatory step in the metabolism of a variety of endogenous substrates, including fatty acids. The reaction is catalysed by ATP-dependent acid:CoA ligases (EC 6.2.1.1-2.1.3; AMP forming), classified on the basis of their ability to conjugate saturated fatty acids of differing chain lengths, short (C2-C4), medium (C4-C12) and long (C10-C22). The enzymes are located in various cell compartments (cytosol, smooth endoplasmic reticulum, mitochondria and peroxisomes) and exhibit wide tissue distribution, with highest activity associated with liver and adipose tissue. 2. Formation of acyl-CoA is not unique to endogenous substrates, but also occurs as an obligatory step in the metabolism of some xenobiotic carboxylic acids. The mitochondrial medium-chain CoA ligase is principally associated with metabolism via amino acid conjugation and activates substrates such as benzoic and salicylic acids. Although amino acid conjugation was previously considered an a priori route of metabolism for xenobiotic-CoA, it is now recognized that these highly reactive and potentially toxic intermediates function as alternative substrates in pathways of intermediary metabolism, particularly those associated with lipid biosyntheses. 3. In addition to a role in fatty acid metabolism, the hepatic microsomal and peroxisomal long-chain-CoA-ligases have been implicated in the formation of the acyl-CoA thioesters of a variety of hypolipidaemic and peroxisome proliferating agents (e.g. clofibric acid) and of the R(-)-enantiomers of the commonly used 2-arylpropionic acid non-steroidal anti-inflammatory drugs (e.g. ibuprofen). In vitro kinetic studies using rat hepatic microsomes and peroxisomes have alluded to the possibility of xenobiotic-CoA ligase multiplicity. Although cDNA encoding a long-chain ligase have been isolated from rat and human liver, there is currently no molecular evidence of multiple isoforms. The gene has been localized to chromosome 4 and homology searches have revealed a significant similarity with enzymes of the luciferase family. 4. Increasing recognition that formation of a CoA conjugate increases chemical reactivity of xenobiotic carboxylic acids has led to an awareness that the relative activity, substrate specificity and intracellular location of the xenobiotic-CoA ligases may explain differences in toxicity. 5. Continued characterization of the human xenobiotic-CoA ligases in terms of substrate/inhibitor profiles and regulation, will allow a greater understanding of the role of these enzymes in the metabolism of carboxylic acids.