Enantiospecific formation of fenoprofen coenzyme A thioester in vitro

The stereospecificity of flobufen metabolism in isolated guinea pig hepatocytes

BACKGROUND

Flobufen (F) is an original nonsteroidal anti-inflammatory drug with one center of chirality. 4-Dihydroflobufen (DHF), compound with two chiral centers, is the main metabolite of F in microsomes and cytosol in all standard laboratory animals. This work describes the biotransformation of F enantiomers and DHF stereoisomers in isolated male guinea pig hepatocytes. Guinea pigs were chosen with respect to similarities in F metabolism as in Man found earlier. R-F, S-F, (2R;4S)-DHF, (2S;4R)-DHF, (2S;4S)-DHF and (2R;4R)-DHF, structurally very similar compounds, served as substrates in order to observe their interaction with enzymes. Stereospecificity of the respective enzymes was studied in vitro, using hepatocytes monolayer. Chiral HPLC using R,R-ULMO column as chiral stationary phase was used for detection and quantitation of metabolites.

RESULTS

(2R;4S)-DHF and (2S;4S)-DHF were the principle stereoisomers detected after incubation with rac-F, R-F and S-F. The ratio of (2R;4S)-DHF/(2S;4S)-DHF ranged from 1.1 to 2.4 depending on the substrate used. (2R;4S)-DHF was the major stereoisomer found after incubation with (2S;4S)-DHF and (2R;4R)-DHF. (2S;4S)-DHF was the principle stereoisomer found after incubation with (2R;4S)-DHF and (2S;4R)-DHF. Besides DHF stereoisomers, other metabolites (M-17203, UM-1 and UM-2) were also detected after incubation of hepatocytes monolayer with F. Interestingly, these metabolites were not found in incubation of all F forms and DHF with fresh liver homogenate.

CONCLUSIONS

Different activities and stereospecificities of the respective enzymes were observed for each substrate in primary culture of hepatocytes. Cell integrity is crucial for formation of secondary metabolites M-17203, UM-1 and UM-2.

Effect of chlorophenoxyacetic acid herbicides on glycine conjugation of benzoic acid

1. 2,4-Dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) (0.1-0.5 mmol/kg i.p.) delayed the disappearance of injected benzoate from blood and diminished the urinary excretion of the formed benzoylglycine, but elevated the blood levels of benzoylglycine in rat, suggesting that these herbicides interfere with both the formation and the renal transport of benzoylglycine. 2. Inhibition of the renal excretion of benzoylglycine by 2,4-D or 2,4,5-T (0.5 mmol/kg i.p.) was directly demonstrated in rat injected with benzoylglycine. 3. Inhibition of benzoylglycine formation from benzoic acid by 2,4-D or 2,4,5-T (0.5 mmol/kg i.p.) was directly demonstrated in renal pedicles-ligated rats injected with benzoate. 4. Neither 2,4-D nor 2,4,5-T influenced the hepatic concentrations of ATP, coenzyme A (CoA) or glycine; therefore, it is unlikely that they inhibit glycine conjugation of benzoic acid by diminishing the availability of co-substrates. 5. Although the chlorophenoxyacetic acids did not appear to be a substrate for the mitochondrial acyl-CoA synthetases, both 2,4-D and 2,4,5-T diminished the activity of benzoyl-CoA synthetase (but not that of benzoyl-CoA:glycine N-acyltransferase) in solubilized hepatic mitochondria. These findings suggest that 2,4-D and 2,4,5-T impair benzoylglycine formation in rat by inhibiting benzoyl-CoA synthetase.

Stereoselective and substrate-dependent inhibition of hepatic mitochondria beta-oxidation and oxidative phosphorylation by the non-steroidal anti-inflammatory drugs ibuprofen, flurbiprofen, and ketorolac

Non-steroidal anti-inflammatory drugs (NSAIDs) cause a range of adverse effects, some of which have been associated with perturbances of lipid metabolic pathways. Previous data demonstrating stereoselective formation of the CoA thioester of R-ibuprofen in particular were suggestive of possible stereoselective effects on lipid metabolism. Our aim was to characterise the relative stereoselectivity of the effects of ibuprofen, flurbiprofen, and ketorolac (0.01-1.0 mM) on both the beta-oxidation of palmitate and oxidative phosphorylation in rat hepatic mitochondria as a means of dissecting prostaglandin related from non-prostaglandin-related events. Beta-oxidation was inhibited stereoselectively by R-ibuprofen (P = 0.015), non-stereoselectively by R- and S-flurbiprofen (P = 0.002 and P = 0.004, respectively), and was essentially unaffected by either enantiomer of ketorolac. At 0.25 mM, inhibition by R-ibuprofen and both flurbiprofen enantiomers was partially reversed by increasing CoA concentrations (0-200 microM). Mitochondrial respiration was moderately inhibited by both enantiomers of ibuprofen and flurbiprofen (P < 0.01), but only by high concentrations (> or = 1 mM) of the enantiomers of ketorolac (P < 0.01). Uncoupling of oxidative phosphorylation measured as stimulation of State 4 respiration contributed to these effects. The data support interactions involving both stereoselective CoA-dependent and non-CoA-dependent mechanisms. The plasma drug concentrations required to achieve these effects are not likely to be attained in the majority of patients, although these concentrations are achievable in the gastrointestinal tract and may contribute to the well-known spectrum of adverse effects in this organ. Some patients do experience systemic adverse events which may be mediated by these mechanisms.

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.

Effect of clofibrate on the chiral inversion of ibuprofen in healthy volunteers

OBJECTIVES

To determine the influence of the hypolipidemic drug clofibrate on the stereoselective metabolism of ibuprofen in humans.

METHODS

Healthy male subjects (n = 12) ingested a dose of 400 mg pseudoracemic ibuprofen (200 mg R-ibuprofen, 160 mg S-ibuprofen, and 40 mg 13C-S-ibuprofen) on two occasions after either pretreatment with clofibrate (2 gm/day over 1 week) or no pretreatment in a randomized order.

RESULTS

When subjects were pretreated with clofibrate, clearances of R-ibuprofen and 13C-S-ibuprofen increased significantly from 55.0 and 66.4 ml/min to 186.2 and 106.7 ml/min (p < 0.01), respectively. This increase was similarly reflected in the clearance by inversion of R-ibuprofen (control, 36.0 ml/min; treated, 118.8 ml/min; p < 0.01), as well as in the clearance by noninversion (control, 19.0 ml/min; treated, 67.4 ml/min; p < 0.01). Unbound clearance values significantly increased for R-ibuprofen (control, 19.5 L/min; treated, 38.7 L/min) but not for 13C-S-ibuprofen (11.8 versus 10.6 L/min, respectively). The fractional inversion of ibuprofen calculated from the urinary metabolite data was increased after clofibrate pretreatment (clofibrate group, 66.4%; control, 53.5%; p < 0.01). However, this was not evident when fractional inversion was calculated from the plasma concentration-time data for the unmetabolized drug.

CONCLUSIONS

Clofibrate altered the stereoselective disposition of ibuprofen in healthy volunteers by increased formation of R-ibuprofenoyl-coenzyme A rather than by an effect on oxidative metabolism of ibuprofen. This interaction has potential therapeutic implications.

Stereochemistry of peroxisomal and mitochondrial beta-oxidation of alpha-methylacyl-CoAs

The stereochemistry of beta-oxidation of alpha-methyl-branched fatty acids was analyzed, in rat liver and in human cells, with (2R)- and (2S)-2-methyltetradecanoic acid as model substrates. In rat liver, formation of the alpha,beta-unsaturated compound was found to be concentrated in mitochondria while in human cells, this activity co-distributed mainly with peroxisomal marker enzymes. In both cases, the dehydrogenating enzymes were absolutely specific for the (2S)-enantiomer. In human liver, activation was some three times faster with the (2R)- than with the (2S)-isomer while in rat liver both were activated at about the same rate.

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

The chiral inversion properties of 4-(4-methylphenyl)-2-methylthiomethyl-4-oxobutanoic acid (KE-748), an active metabolite of 2-acetylthiomethyl-4-(4-methylphenyl)-4-oxobutanoic acid (KE-298), were compared with those of ibuprofen in rats. After administration of R(-)-[2 alpha-2H]KE-748, S(+)-KE-748 was present in the rat plasma, and the deuterium atoms of the S(+)-enantiomer were almost all replaced by hydrogen atoms. After administration of S(+)-[2 alpha-2H]KE-748, the deuterium content of S(+)-KE-748 in the plasma remained intact. In the in vitro study, using a cell-free system and rat liver homogenates, the chiral inversion of ibuprofen was apparent when both CoA and ATP were present; however, KE-748 was not inverted. In the study on isolated rat hepatocytes, the unidirectional chiral inversion from R(-)-to S(+)-enantiomer was observed for both ibuprofen and KE-748. When R(-)-ibuprofen was incubated with medium and long chain fatty acids (carbon chain length C6 to C16), using isolated hepatocytes, the chiral inversion decreased significantly. On the other hand, when R(-)-KE-748 was incubated with short and medium chain fatty acids (carbon chain length C3 to C8), chiral inversion was inhibited markedly. To induce hepatic microsomal long chain fatty acid CoA ligase, rats were treated with clofibric acid (CF rats). In both in vitro and in vivo experiments on CF rats, chiral inversion from R(-)-to S(+)-ibuprofen was enhanced significantly compared with that in controls, whereas the enhancement was not observed in the case of R(-)-KE-748. There was no influence of benzoic acid, a typical substrate on medium chain fatty acid CoA ligase in the mitochondrial matrix, on chiral inversion of R(-)-ibuprofen, using, isolated hepatocytes. In contrast, the chiral inversion from R(-)-to S(+)-KE-748 was strongly inhibited in the presence of benzoic acid. These results indicate that chiral inversion of R(-)-KE-748 may proceed via formation of the CoA-thioester intermediate with loss of the 2 alpha-methine proton, in a manner similar to that seem with R(-)-ibuprofen. However, the enzymes needed to form CoA-thioester of R(-)-KE-748 differ from those for R(-)-ibuprofen.

Studies on the in vitro inversion of flurbiprofen

This paper reports in vitro studies on the metabolic inversion of flurbiprofen (FL), an arylpropionic acid antiinflammatory agent (2-APA). The inversion was studied with both rac-FL and R-FL, by incubation with rat hepatic microsomes, in the presence of either CoASH and ATP or NADPH. The two isomers of the drug were separated as their (+)-(R)-1-phenylethylamides by direct phase high-performance liquid chromatography on a silica gel column with an achiral mobile phase. The inversion was more pronounced in the presence of CoASH and ATP for both the racemate and the R-isomer, which supports the key role of CoA thioesters in the metabolic inversion of profens. The inversion observed in the presence of NADPH suggests that, when the incubation is run with hepatic microsomes, a CYP450-mediated pathway is also active. In order to get more insight into the CYP450-mediated inversion pathway, we studied the effect of irradiating microsomes with a low dose of He-Ne laser radiation (0.2 J). Such irradiation caused a significant increase in inversion at all times studied and normalized the anomalous value of inversion observed at 15 min in this pathway.

Stereoselective metabolic pathways of ketoprofen in the rat: incorporation into triacylglycerols and enantiomeric inversion

The enantiomeric bioinversion of ketoprofen (KP) enantiomers and their incorporation into triacylglycerols were investigated in the rat (1) in vitro, using liver homogenates, subcellular fractions, and hepatocytes, and (2) in vivo, in different tissue samples after oral administration of the radiolabelled compounds. In liver homogenates or subcellular fractions, the enantiomer (S)-ketoprofen (S-KP) was recovered unchanged, whereas (R)-ketoprofen (R-KP) was partially converted into its Coenzyme A (CoA) thioester and inverted to S-KP. Both processes occurred mainly in the mitochondrial fraction. This supports the mechanism of inversion via stereoselective formation of CoA thioester of R-KP, already described for other non-steroidal anti-inflammatory drugs. Incorporation into triacylglycerols was detected after incubation with intact hepatocytes in the presence of added glycerol. The process was stereoselective for R-KP vs. S-KP (covalently bound radioactivity 26,742 +/- 4,665 dpm/10(6) cells vs. 6,644 +/- 3,179 dpm/10(6) cells, respectively). However, no incorporation was found in liver samples after oral administration of either R-KP or S-KP. On the contrary, in adipose tissue samples a significant and stereoselective formation of hybrid triacylglycerols was observed: 11,076 +/- 2,790 dpm.g-1 for R-KP vs. 660 +/- 268 dpm.g-1 for S-KP. The incorporated R/S ratio, higher in adipose tissue (R/S = 17) than in hepatocytes (R/S = 4), indicates that fat may be the main tissue store for the xenobiotic R-KP in rats.

Pathological and biochemical modifications of renal function in ibuprofen-induced interstitial nephritis

Use of ibuprofen in patients with asymptomatic renal failure is known to produce acute renal toxicity. One of the manifestations is interstitial nephritis of which the pathogenic mechanism remains unclear. In the present study, this nephrotoxic syndrome was induced in rabbits by giving a single dose of uranyl nitrate, followed by consecutive doses of ibuprofen. This animal model thus allowed the assessment of renal functional and pathological changes associated with ibuprofen use in renal insufficiency. In these rabbits, the major abnormality appeared to be confined to the tubulointerstitial compartment. Microscopic examinations of the renal necropsy specimens showed tubular necrosis and interstitial lymphocytic infiltration. The histological finding of lymphocytic aggregation suggests that this nephrotoxic effect stems from a cytotoxic immune reaction in the interstitium. Moreover, levels of renal 2-arylpropionyl-CoA epimerase, a key enzyme involved in the metabolic inversion of ibuprofen, showed a significant reduction, which may result from the massive destruction of the tubular cells in these animals. These results support the premise that renal insufficiency is a prerequisite factor for ibuprofen-induced interstitial nephritis.

2-Arylpropionyl-CoA epimerase: partial peptide sequences and tissue localization

The R-enantiomers of 2-arylpropionic acids (2-APAs) such as ibuprofen (IBU) exhibit the phenomenon of species- and substrate-dependent metabolic chiral inversion. Only R-enantiomers are activated to acyl-CoA-thioesters by an acyl-CoA-synthetase via an adenylate intermediate. The acyl-CoA-thioesters are substrates for an epimerase, which is responsible for chiral inversion. A 42 kDa epimerase from the cytosolic fraction of rat livers was isolated and purified to homogeneity. Polyclonal antibodies were raised against the epimerase in rabbits. The anti-epimerase antibodies were used for affinity column chromatography to separate homogeneous protein for amino acid sequence analysis. Sequence data analysis of 3 internal peptide sequences showed 50% and more homology with regions of enzymes involved in fatty acid metabolism. The polyclonal anti-epimerase antibodies were used to analyze the tissue distribution of the in guinea pigs and rats by Western blot analysis. Furthermore, the correlation of inversion enzyme activity in various tissues under comparable incubation conditions and cross-reactivity in Western blot analysis was investigated.

Moment analysis of stereoselective biliary excretion and chiral inversion of ketoprofen enantiomers in perfused rat liver

The stereoselective local disposition of ketoprofen was evaluated by the single-pass perfusion experiment following a bolus injection of R(-)- or S(+)-ketoprofen into the liver from the portal vein. The elution time profiles of enantiomers into the hepatic vein and the excretion time profiles into the bile were kinetically assessed by local moment analysis. The hepatic recovery ratios (FH) of both enantiomers were < 1%, and the mean hepatic transit times (tH) were approximately 7 s. After the injection of S-ketoprofen into the liver, the biliary excretion ratio (Fb) of total S-ketoprofen was 68% (15% S-ketoprofen and 53% glucuronide) and the mean biliary transit time (tb) of S-ketoprofen was 10 min. R-Ketoprofen inversion from S-ketoprofen was not observed in either the perfusate or in the bile. After the injection of R-ketoprofen, the Fb of total R-ketoprofen was 12% (3% R-ketoprofen and 9% glucuronide), and tb of R-ketoprofen was 8 min. The Fb of total S-ketoprofen inverted from R-ketoprofen was 24% (7% S-ketoprofen and 17% glucuronide), and the tb of inverted S-ketoprofen was 17 min. Forty-six percent of R-ketoprofen was inverted to S-ketoprofen during a single pass through the rat liver, and the mean inversion time was 7.5 min. It was concluded that the unidirectional chiral inversion of ketoprofen was stereospecific, and the hepatic uptake and biliary excretion were stereo-nonspecific.

Comparative metabolism of R(-)-fenoprofen in rats and sheep

The chiral inversion of 2-arylpropionic acids occurs in many species. It is a unique reaction specific to this group of drugs. In this study R-(-)-fenoprofen (R-(-)-FPF) was used as a model compound to investigate metabolic chiral inversion in sheep in vivo and in vitro to compare the data with the results obtained in rats. Metabolic inversion in sheep was 80%. The apparent mean values of Km and Vmax of thioester formation were: 392 microM and 2.08 nmol/min/mg in sheep and 500 microM and 22 nmol/min/mg in rats. For hydroxylation, the apparent mean values were Vmax: 0.02 nmol/min/mg in rats and 0.01 nmol/min/mg in sheep. There was no correlation between in vitro thioesterification and in vivo chiral inversion in sheep as compared to rats. In sheep most of the thioester formed underwent inversion (80%) while in rats, where in vitro thioesterification was greater, in vivo inversion was less (42%). In consequence, in rats other metabolic pathways for R(-)-FPF-CoA, such as incorporation into triacylglycerols and conjugation with amino acids, may be quantitatively more important.

(-)-R-fenoprofen: formation of fenoprofenyl-coenzyme A by rat liver microsomes

The thioesterification of fenoprofen (FPF) by rat liver microsomes has been studied using an HPLC method enabling direct quantification of the FPF-CoA produced. Over the concentration range studied (5-400 microM), studies showed the participation of a single CoA ligase in the formation of FPF-CoA, in contrast with the involvement of several isozymes with different affinities, that has been found with ibuprofen (IPF). The Km for the reaction was dependent upon the presence of non-ionic detergent, a concentration of 0.05% Triton X-100 reducing the Km from 397 to 20 microM although the detergent had no effect on Vmax. The microsomal long-chain fatty acid CoA ligase was markedly enantioselective towards (-)-R-FPF and the formation of (-)-R-FP-CoA was inhibited by both the (+)-S enantiomer and palmitic acid.

Kinetic characteristics of rat liver peroxisomal nafenopin-CoA ligase

In this study we have demonstrated that rat hepatic peroxisomes catalyse the formation of nafenopin-CoA. The process is mediated by apparent high affinity (Km 6.7 microM), low capacity (Vmax 0.31 nmol/mg/min) and low affinity, high capacity isoforms. Palmitic acid (Ki 1.1 microM), R(-) ibuprofen (Ki 7.9 microM), ciprofibrate (Ki 60.2 microM) and clofibric acid (Ki 86.8 microM) competitively inhibited nafenopin-CoA formation catalysed by the apparent high affinity isoform. An antibody raised against the microsomal palmitoyl-CoA ligase inhibited the equivalent peroxisomal enzyme significantly (P < 0.001) but did not inhibit peroxisomal nafenopin-CoA ligase activity. These data suggest that nafenopin-CoA formation is catalysed by a peroxisomal CoA ligase which differs from the peroxisomal long chain fatty acid-CoA ligase in relation to its xenobiotic/antibody inhibitor profile and kinetic characteristics.

In vitro fenoprofenyl-coenzyme A thioester formation: interspecies variations

In vitro coenzyme A thioester formation from (-)-(R)-fenoprofen (FPF) and palmitic acid has been studied using liver microsomes from rat, guinea pig, sheep, and dog. In every species with both palmitic acid or (-)-(R)-fenoprofen, the Lineweaver-Burk plot was linear in the substrate concentration range used and as a consequence agrees with the involvement of only one isoenzyme (or different isoenzymes of similar Km values). The Vmax values for the thioesterification of (-)-(R)-fenoprofen present large species variations from 2.1 +/- 1.0 with sheep liver microsomes to 60.6 +/- 11 nmol/min/mg with dog liver microsomes. These values statistically significantly correlate (r = 0.94) to the Vmax values observed when palmitic acid was used as a substrate. Furthermore palmitic acid inhibited (-)-(R)-fenoprofen-CoA formation in the same extent in all animal species. The stereoselectivity of the thioesterification was also species dependent.

Inhibition of mitochondrial beta-oxidation as a mechanism of hepatotoxicity

Severe and prolonged impairment of mitochondrial beta-oxidation leads to microvesicular steatosis, and, in severe forms, to liver failure, coma and death. Impairment of mitochondrial beta-oxidation may be either genetic or acquired, and different causes may add their effects to inhibit beta-oxidation severely and trigger the syndrome. Drugs and some endogenous compounds can sequester coenzyme A and/or inhibit mitochondrial beta-oxidation enzymes (aspirin, valproic acid, tetracyclines, several 2-arylpropionate anti-inflammatory drugs, amineptine and tianeptine); they may inhibit both mitochondrial beta-oxidation and oxidative phosphorylation (endogenous bile acids, amiodarone, perhexiline and diethylaminoethoxyhexestrol), or they may impair mitochondrial DNA transcription (interferon-alpha), or decrease mitochondrial DNA replication (dideoxynucleoside analogues), while other compounds (ethanol, female sex hormones) act through a combination of different mechanisms. Any investigational molecule should be screened for such effects.

Is the formation of R-ibuprofenyl-adenylate the first stereoselective step of chiral inversion?

Coenzyme A thioester formation is reported to be the first step of chiral inversion of R-ibuprofen. In order to investigate the mechanism of this reaction adenylate derivatives of the ibuprofen enantiomers were synthesized chemically. R- and S-ibuprofenyl-adenylates as well as free acids were incubated with rat liver mitochondria in the presence of coenzyme A, MgCl2 with or without ATP. The optical antipodes formed by inversion and the coenzyme A thioester derivatives of both enantiomers were found after incubation of both R- or S-ibuprofenyl-adenylate and R-ibuprofen. By contrast, after incubation with S-ibuprofen neither R-enantiomer nor coenzyme A thioesters were detected. These experiments suggest that the formation of R-ibuprofenyl-adenylate may be the first stereoselective step of chiral inversion.

The role of coenzyme A in the biotransformation of 2-arylpropionic acids

A number of 2-arylpropionic acid non-steroidal anti-inflammatory drugs ('profens') undergo highly enantioselective inversion from the (R)- to (S)-enantiomer. The mechanism of this inversion reaction involves the initial enantioselective formation of a coenzyme A thioester followed by epimerization and finally hydrolysis to regenerate free acids. Long-chain fatty acyl-CoA synthetase appears to mediate the initial thioester formation and an epimerase of an unknown physiologic function effects the second step. The hydrolases mediating the final step are poorly defined. Available evidence suggests that the liver is quantitatively the most important tissue site of inversion but local tissue inversion may influence the pharmacological and toxicological response of a given organ. Data from isolated rat hepatocytes indicate that other xenobiotics can modulate the formation and hydrolysis of ibuprofenyl-CoA by influencing inversion pathways, non-inversion pathways or both. Interactions between xenobiotics may therefore accentuate inter-individual variability in response to 2-aryl-propionic acids. The formation of 2-arylpropionyl-CoA thioesters in vivo has the potential to disrupt numerous biochemical pathways in addition to enhancing individual exposure to the potent anti-inflammatory (S)-enantiomers.

Lack of stereoselectivity of the peroxisome proliferation induced by 2-phenylpropionic acid: evidence against a role for lipid disturbance in peroxisome proliferation

The significance of disturbances of lipid metabolism caused by xenobiotic acyl-CoAs as possible causes of peroxisomal proliferation has been studied with the enantiomers of 2-phenylpropionic acid (2-PPA), the (R)-enantiomer of which is converted to the acyl-CoA in rats while its (S)-antipode is not. rac-2-PPA (250 mg/kg/day ip x 3) was shown to be an hepatic peroxisomal proliferator in male Sprague-Dawley rats on the basis of increases in microsomal cytochrome P-450 content and lauric acid hydroxylation and hepatic CN(-)-insensitive palmitoyl-CoA oxidation, a peroxisomal marker activity, while electron microscopy revealed a rise in the peroxisome/mitochondria ratio in hepatocytes. Further studies established the dose-response relationships for these biochemical changes. The (R)- and (S)-enantiomers were administered at a dose of 50 mg/kg/day ip x 3 and both were peroxisome proliferators of very similar potency. The effects of 100 mg/kg/day ip x 3 of the racemate, a dose giving ca. 75% of maximal response, were essentially additive of those of 50 mg/kg/day ip x 3 of its two component isomers. The stereoselectivity of acyl-CoA formation from the enantiomers of 2-PPA was confirmed by their differential inhibition of microsomal palmitoyl-CoA synthesis. Taken together, these data indicate that it is very unlikely that the acyl-CoA of 2-PPA plays any role in the peroxisomal proliferation which this compound causes in the rat.

Studies on the effect of fenoprofen on the activation and oxidation of long chain and very long chain fatty acids in hepatocytes and subcellular fractions from rat liver

We studied the effect of fenoprofen on the activation of palmitic acid (C16:0), lignoceric acid (C24:0) and cerotic acid (C26:0) in microsomal and peroxisomal fractions from rat liver. Fenoprofen was found to inhibit the formation of palmitoyl-CoA in both microsomal and peroxisomal fractions whereas the formation of lignoceroyl-CoA and cerotoyl-CoA was not inhibited at all. In freshly isolated rat hepatocytes palmitic acid beta-oxidation was progressively inhibited at increasing concentrations of fenoprofen, most probably due to its inhibitory effect on palmitoyl-CoA synthetase activity. On the other hand, fenoprofen was also found to inhibit the beta-oxidation of lignoceric acid and cerotic acid in rat hepatocytes. It is shown that the acyl-CoA oxidase activity with lignoceroyl-CoA as substrate was inhibited by fenoprofen whereas the palmitoyl-CoA and pristanoyl-CoA oxidase activities were not inhibited by fenoprofen. This finding provides an explanation for the inhibitory effect of fenoprofen on lignocerate and cerotate beta-oxidation in hepatocytes.

Stereoselectivity of biliary excretion of 2-arylpropionates in rats

To examine the stereoselectivity of biliary excretion, the optically pure enanatiomers of ketoprofen (KT), ibuprofen (IBU), and flurbiprofen (FLU) were intravenously administered to normal and bile duct-cannulated rats at 10 mg/kg. The recovery of total KT in bile was significantly higher after administration of (S)-KT than after (R)-KT [90.1 +/- 3.5% vs 68.8 +/- 8.2%, n = 3, P < 0.05]. In normal rats the terminal half-life of (R)-KT was significantly shorter than that of (S)-KT after administration of (R)-KT (2.2 +/- 0.6 h vs 14.3 +/- 4.9 h, n = 3, P < 0.05). The terminal half-life of both enantiomers was significantly shorter in rats with continuous bile drainage as compared to normal rats. No significant differences in pharmacokinetic parameters could be found between both enantiomers in bile duct-cannulated animals. The total amount of IBU in bile was slightly higher after administration of (S)-IBU than after (R)-IBU administration. The percentage of (R)-IBU after (R)-IBU administration, however, was very low [(R)-IBU: 1.5 +/- 0.9%, (S)-IBU: 23.4 +/- 5.8%]. In normal rats the clearance of (R)-IBU was significantly higher as compared to (S)-IBU. Differences in pharmacokinetic parameters between normal and bile duct-cannulated rats were not statistically significant due to high interindividual variability. The total recovery of FLU, which was excreted in bile to a lower extent than either KT or IBU, also tended to be greater after S-enantiomer administration. Only small amounts of (S)-FLU could be recovered in bile after (R)-FLU administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence of multiple forms of rat liver microsomal coenzyme A ligase catalysing the formation of 2-arylpropionyl-coenzyme A thioesters

This study has demonstrated the involvement of multiple forms of rat hepatic microsomal CoA ligases in the formation of 2-arylpropionyl-CoA thioesters. In the presence of (-)R-ibuprofen (0.1 microM-1 mM) two enzymic processes were observed, one of which exhibited enantiospecificity and apparent high affinity for the R enantiomer (Km 0.06 microM) whilst the second, a low-affinity component was non-enantiospecific. An equivalent high-affinity isoform catalysing R-flurbiprofen-CoA formation at concentrations less than 100 microM was not demonstrated. However, at higher substrate concentrations formation of both R- and S-flurbiprofenyl-CoA thioesters occurred. Marked inter-individual variation was observed in the formation of S-ibuprofen-CoA and S-flurbiprofen-CoA in the rats studied.

The effects of ibuprofen enantiomers on hepatocyte intermediary metabolism and mitochondrial respiration

In vivo and in vitro (-)R-ibuprofen is inverted to the (+)S antipode via stereoselective formation of an R-ibuprofenyl-CoA intermediate. In this study the effects of (-)R- and (+)S-ibuprofen on metabolism and respiration were studied using isolated rat hepatocytes and mitochondria. R-Ibuprofen significantly increased the lactate to pyruvate ratio, perturbed mitochondrial ketogenesis as evidenced by alterations in the beta-hydroxybutyrate to acetoacetate ratio and uncoupled mitochondrial oxidative phosphorylation. In addition, substantial dose- and time-dependent sequestration of reduced CoA (CoASH) occurred in the presence of the R enantiomer. Similarly, S-ibuprofen altered both the cytosolic and mitochondrial redox states although the magnitude of the effect was substantially less than that observed with the R enantiomer. In contrast to R-ibuprofen, S-ibuprofen did not uncouple oxidative phosphorylation or sequester hepatocyte CoASH. It is proposed that the perturbations observed in hepatocyte intermediary metabolism and mitochondrial function are attributable to a combination of the direct effects of R-ibuprofen per se and the sequestration of CoASH as R-ibuprofenyl-CoA during the process of chiral inversion. On the basis of these results, R-ibuprofen should be considered more in terms of metabolism to a reactive acyl-CoA intermediate rather than as a pro-drug for the pharmacologically active S-enantiomer.

Influence of palmitate and benzoate on the unidirectional chiral inversion of ibuprofen in isolated rat hepatocytes

The influence of benzoic acid, a typical substrate of medium-chain acyl-CoA synthetase, and of palmitic acid, a substrate of long-chain acyl-CoA synthetase, on the metabolic chiral inversion of ibuprofen was investigated in freshly isolated hepatocytes. It was shown that the conjugation of benzoid to hippuric acid does not influence the chiral inversion of ibuprofen. In contrast, palmitic acid inhibited markedly the R-to-S inversion of ibuprofen. It was concluded that this inhibition is due to competition between (R)-ibuprofen and palmitic acid for long-chain acyl-CoA synthetases.

Enantioselective induction of peroxisomal proliferation in CD-1 mice by leukotriene antagonists

The effects of a racemic leukotriene antagonist (MK-0571) and its component enantiomers (L-668,018 and L-668,019) on hepatic peroxisome proliferation were examined in mice, rats, and rhesus monkeys. Administration of racemic MK-0571 to mice resulted in increased liver weights, increased peroxisomal volume density, and a pleiotropic induction of characteristic peroxisomal and nonperoxisomal enzyme activities associated with peroxisomal proliferation. When the individual enantiomers of MK-0571 were administered to mice, a pronounced enantioselective induction of peroxisome proliferation was observed. Toxicokinetic studies showed that the levels of each enantiomer in the liver or plasma after separate administration were similar. Thus, the enantioselectivity in the induction of peroxisome proliferation could not be explained on the basis of pharmacokinetic differences between the enantiomers. The hepatic peroxisomal response of the rat to MK-0571 was greatly attenuated compared to the mouse. As has been seen with other peroxisome-proliferating agents, MK-0571 had no effect on either peroxisomal volume density or peroxisomal enzyme activity in monkeys. Due to the high degree of enantiomeric discrimination toward the induction of peroxisomal proliferation by these enantiomers, compounds of this type may prove useful as probes to examine the mechanisms by which peroxisomal proliferating agents induce their effects.

Inhibition of rat peroxisomal palmitoyl-CoA ligase by xenobiotic carboxylic acids

ATP-dependent coenzyme A (CoA) ligases catalyse the formation of the acyl-CoA thioesters of xenobiotic carboxylic acids and the formation of xenobiotic-CoAs has been implicated as being a causative factor in peroxisomal proliferation. In this study we have demonstrated using rat liver peroxisomes that the formation of palmitoyl-CoA is inhibited by a variety of xenobiotic carboxylic acids. Palmitoyl-CoA formation exhibited biphasic kinetics indicative of two isoforms, a high affinity (Km1 2.3 microM) low capacity form and a low affinity (Km2 831 microM) high capacity form. These forms were differentially inhibited by a range of xenobiotics. However, it would appear that the low affinity component may not contribute to any major extent to the formation of xenobiotic-CoAs in vivo. At a concentration of 1 mM, greater than 20% inhibition of the high affinity form was observed with the 2-arylpropionates, ibuprofen, naproxen, benoxaprofen, fenoprofen, indoprofen, ketoprofen, tiaprofenic acid and cicloprofen, the hypolipidaemics, nafenopin and ciprofibrate, and the herbicides, silvex and 2,4,5-trichlorophenoxyacetate. Valproic acid, clofibric acid, salicylic acid and 2,4-dichlorophenoxy-acetate were non-inhibitory at all concentrations studied (0.1-2.5 mM). Analysis of the type of inhibition established that only nafenopin (Ki 430 microM) and ciprofibrate (Ki 97 microM) were competitive inhibitors of palmitoyl-CoA formation suggesting that they bind at the active site and thus potentially function as alternative substrates for the peroxisomal ligase. Notably, clofibric acid which has previously been shown to form clofibroyl-CoA in peroxisomes did not interact with the palmitoyl-CoA ligase thereby suggesting that activation is mediated via an alternative peroxisomal CoA ligase. In addition, the xenobiotic inhibitors of the peroxisomal palmitoyl-CoA ligase differed from those previously reported for the equivalent microsomal enzyme suggesting that the organellar forms may be functionally distinct. This study establishes that numerous xenobiotic carboxylic acids interact with the peroxisomal palmitoyl-CoA ligase; however, it would appear that relatively few function as alternative substrates. The toxicological ramifications of peroxisomally mediated xenobiotic-CoA formation and the identification of other peroxisomal xenobiotic-CoA ligase(s) remain to be elucidated.

Inhibition kinetics of hepatic microsomal long chain fatty acid-CoA ligase by 2-arylpropionic acid non-steroidal anti-inflammatory drugs

Microsomal long chain fatty acid CoA ligase (EC 6.2.1.3) has been implicated in the formation of CoA thioesters of xenobiotics containing a carboxylic acid moiety. In this study we have demonstrated that the microsomal enzyme from rat liver exhibits biphasic kinetics for the formation of palmitoyl-CoA, i.e. there are high affinity low capacity Kmhigh, 1.6 microM, Vmaxhigh, 12.9 nmol/mg/min) and low affinity high capacity (Kmlow, 506 microM, Vmaxlow, 58.3 nmol/mg/min) components. Inhibition of the high affinity isoform was studied using the R and S enantiomers of ibuprofen, fenoprofen, ketoprofen and naproxen. The high affinity component of palmitoyl-CoA formation was competitively inhibited by R-fenoprofen (Ki 15.4 microM) while R-ibuprofen exhibited mixed inhibition kinetics. In contrast the R and S enantiomers of ketoprofen and naproxen were non-competitive inhibitors. This diversity of inhibition kinetics observed argues in favour of a binding site in addition to the catalytic site. A competitive interaction with the high affinity form correlated with literature evidence of enantiospecific chiral inversion and "hybrid" triglyceride formation for the R enantiomers of fenoprofen and ibuprofen. Paradoxically, R-ketoprofen which is extensively inverted in rats was a non-competitive inhibitor of palmitoyl-CoA formation by the high affinity isoform suggesting that it may not act as an alternate substrate. The results of this study clearly indicate that formation of R-2-arylpropionate-CoAs is not fully explained by interaction with the high affinity isoform of a microsomal long chain (palmitoyl) CoA ligase and therefore the involvement of other isoforms cannot be discounted.

The effect of the enantiomers of ibuprofen and flurbiprofen on the beta-oxidation of palmitate in the rat

The effects of the enantiomers of ibuprofen (0.25 and 0.50 mmol/kg b.w.) and flurbiprofen (0.01, 0.03, and 0.06 mmol/kg b.w.) on the beta-oxidation of palmitate were investigated in the rat. The mean cumulative exhalation of 14CO2 after ip administration of [U-14C]palmitic acid was significantly reduced over 6 h by ibuprofen at the higher dose but not at the lower dose for either enantiomer. There was no difference between the enantiomers, the reduction over 6 h being 31.3 and 33.0% for (R)- and (S)-ibuprofen, respectively. There was also a significant inhibition of beta-oxidation by flurbiprofen at all 3 doses. Again, there was no stereoselectivity evident in this inhibition. Flurbiprofen was much more potent than ibuprofen in eliciting this effect, the 0.01mmol/kg dose giving a similar reduction in beta-oxidation as observed for the 0.50 mmol/kg dose of ibuprofen. The data support the hypothesis that inhibition of the in vivo beta-oxidation of palmitate by ibuprofen and flurbiprofen is primarily via a nonstereoselective noncoenzyme A-dependent mechanism.

Chiral inversion of 2-arylpropionic acid non-steroidal anti-inflammatory drugs--II. Racemization and hydrolysis of (R)- and (S)-ibuprofen-CoA thioesters

The inversion of 2-arylpropionic acids (2-APAs) has become the subject of much attention. It is a unique reaction specific to this group of drugs. Inversion proceeds via stereoselective activation of the R-enantiomer to its CoA thioester whereby it is then racemized and hydrolysed to release free drug. The racemization and hydrolysis processes have been examined in this study using chemically synthesized CoA thioesters of the ibuprofen enantiomers and in vitro models employing rat liver homogenate and the mitochondrial and microsomal fractions as the source of the 'racemase' enzymes. Rat liver homogenate mediated the racemization and hydrolysis of both (R)- and (S)-ibuprofen-CoA thioesters. The rat liver mitochondrial fraction similarly mediated racemization and hydrolysis of both CoA thioesters. There was less racemase activity in the rat liver microsomal fraction and the data indicated that this fraction may contain two hydrolases which act separately on the (R)- and (S)-ibuprofen-CoA thioesters. The data are further evidence that the stereoselectivity of the CoA synthetase controls the overall stereoselectivity of inversion.

Stereoselective analysis of fenoprofen and its metabolites

Reversed-phase high-performance liquid chromatographic assays have been developed to quantitate simultaneously fenoprofen and its major metabolites as well as to distinguish between their R- and S- enantiomers following a single oral dose of 600 mg racemic fenoprofen to healthy volunteers. The compounds are extracted from plasma (after precipitation of plasma protein) or assayed directly in diluted urine samples employing a gradient solution on a C18 column and ultraviolet detection. Two internal standards, ketoprofen and flunoxaprofen, are used to allow measurement of very low (0.05 microgram/ml) and high (70 microgram/ml) concentrations in each sample. R- and S-fenoprofen glucuronides can be separated directly; the 4'-hydroxyfenoprofen conjugates are measured via an indirect method by comparing the concentration of 4'-hydroxyfenoprofen before and after hydrolysis. The R- and S-enantiomers of both parent and 4'-hydroxy metabolite are derivatized with L-leucinamide via an ethyl chloroformate intermediate and subsequently analyzed on a C18 column. Concentrations of metabolites found in plasma were low when compared to parent drug. The S/R ratio of fenoprofen in plasma always exceeds 1 and increases with time after dosage while the S/R ratio of its 4'-hydroxy metabolite remains almost unchanged at 1.1. R-Fenoprofen glucuronide disappears rapidly from plasma as compared to its S-antipode; a less pronounced difference is noted between R- and S-4'-hydroxyfenoprofen conjugates. Fenoprofen is almost completely excreted as its S-acyl glucuronides; the renal clearance of unchanged drug is very low.

Stereoselective arylpropionyl-CoA thioester formation in vitro

The inversion from R- to S-enantiomer that occurs for some arylpropionic acids may have both toxicological and therapeutic implications. To characterize some properties of this inversion, arylpropionyl-CoA thioester formation was studied in rat tissue homogenates and subcellular fractions for the enantiomers of fenoprofen, ibuprofen, and flurbiprofen. Thioesters were formed from (R)-fenoprofen (64%) and (R)-ibuprofen (33%) but not from the corresponding S-enantiomers or the enantiomers of flurbiprofen. This correlates with the extensive inversion of fenoprofen and ibuprofen and lack of inversion of flurbiprofen in vivo. Subcellular fractions from rat liver showed thioester formation to occur in mitochondria and microsomes but not cytosol. Once formed, the thioesters were readily racemized by whole rat liver homogenate, mitochondria, and cytosol, but only partially inverted (S:R = 0.3) in microsomes. Thioester formation from fenoprofen and ibuprofen was studied in tissue homogenate obtained from liver, diaphragm, kidney, lung, skeletal muscle, smooth muscle, fat, caecum, and intestines. The liver was at least 50-fold more efficient than the other tissues studied and would be expected to be a major organ of enantiomeric inversion. Our data support the hypothesis that R- to S-enantiomeric inversion of arylpropionic acids proceeds via the stereoselective formation of CoA thioesters followed by enzymatic racemization and hydrolysis of the thioesters to regenerate free acid.

Enantiomers in arthritic disorders

Drugs which have a center of asymmetry are often administered as an equal mixture of the two possible enantiomeric forms i.e. a racemate. However, there are frequently large pharmacodynamic and pharmacokinetic differences between enantiomers. Consequently, it is possible that while one enantiomer mediates the antiinflammatory or antirheumatic action, the other enantiomer, although adding little to the efficacy of the drug, may contribute to its adverse effects. Asymmetric drugs may also serve as sensitive pharmacological probes of the mechanisms underlying the action of drugs and the inflammatory processes which they modulate. These concepts are the focus for this review.

Chiral inversion of 2-arylpropionic acid non-steroidal anti-inflammatory drugs--1. In vitro studies of ibuprofen and flurbiprofen

The mechanism of inversion of the enantiomers of 2-arylpropionic acids was investigated in vitro using tissue homogenates. Crude rat liver homogenate was shown to mediate the inversion of R to S-ibuprofen, but not inversion of the S to the R-enantiomer. Inversion required CoA and ATP as cofactors. In contrast, R-ibuprofen was not inverted by homogenates of kidney or small intestine and there was no inversion of the enantiomers of flurbiprofen by any of these tissue homogenates. Long-chain acyl-CoA synthetase was partially purified from rat liver microsomes and bound to Matrex Gel Red A. R-Ibuprofen was shown to be a substrate for this enzyme while S-ibuprofen and R and S-flurbiprofen were not substrates. These data are consistent with the hypothesis that the stereospecificity of inversion is controlled by the acyl-CoA synthetase. R-Ibuprofen-CoA did not racemize in either buffer solution (pH 7.4) or human plasma consistent with the hypothesis that racemization of the CoA thioesters is mediated enzymatically.

Chirality: pharmacokinetics and pharmacodynamics in 3 dimensions

1. Biological macromolecules are able to distinguish between enantiomeric substrates. A three-point interaction between the drug enantiomers and the macromolecule (Easson-Stedman hypothesis) can frequently account for this selectivity. 2. Significant pharmacodynamic differences between enantiomers are more the rule than the exception. 3. Pharmacokinetic differences between enantiomers are, in general, not as great as the pharmacodynamic differences. However, stereoselective protein binding, metabolism and renal clearance are still very important aspects of understanding drug disposition and the time course of drug action. 4. There may be pharmacokinetic and pharmacodynamic enantiomer-enantiomer interactions. Consequently, the activity and disposition of a racemic drug may not be the simple sum of the activities and disposition of the individual enantiomers. 5. Enantiomers have been used as sensitive 3-dimensional probes to establish structure-activity relationships, to provide insights into genetic polymorphism of drug metabolism, and to provide insights into other aspects of drug disposition. 6. A need for a 3-dimensional understanding of pharmacodynamics and pharmacokinetics is implicit in the asymmetric nature of biological environments.