The metabolic chiral inversion and dispositional enantioselectivity of the 2-arylpropionic acids and their biological consequences

Synthesis and the stereoselective enzymatic hydrolysis of flurbiprofen-basic amino acid ethyl esters

Ethyl esters of flurbiprofen L-arginine (FP-Arg-OH), flurbiprofen L-lysine (FP-Lys-OH) and flurbiprofen p-guanidino-L-phenylalanine (FP-GPA-OH) were synthesized and then the release of flurbiprofen enantiomers from these derivatives in the presence of trypsin (Tp), carboxypeptidase B (CPB) and carboxypeptidase Y (CPY) were examined in order to evaluate their availability as prodrugs for flurbiprofen (FP). The ester bonds of the three racemic FP derivatives were hydrolyzed by Tp at about 3 to 20 times the rates of N-benzoyl-L-arginine ethyl ester (Bz-Arg-OEt), a specific substrate for Tp. (R)-FP was released faster than (S)-FP by either CPB or CPY from both FP-Arg-OH and FP-Lys-OH. On the other hand, FP-GPA-OH was not hydrolyzed at all by CPB and the hydrolysis rate of this compound by CPY was very slow. (S)-Flurbiprofen L-arginine ethyl ester ((S)-FP-Arg-OEt) was separated from (R)-FP-Arg-OEt by high-performance liquid chromatography. A comparison of the kinetic parameters for the tryptic hydrolysis of the two optically active FP-Arg-OEt diastereomers and those of Bz-Arg-OEt suggested that the orientation of the scissile bond in each diastereomer to the catalytic center of Tp is more favorable than that of Bz-Arg-OEt. However, no significant difference was found between the kinetic parameters for the two diastereomers, suggesting that the orientational difference between (S)-FP and (R)-FP in the diastereomers does not have any effect on the tryptic hydrolysis of the ester bond.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Drug chirality: on the mechanism of R-aryl propionic acid class NSAIDs. Epimerization in humans and the clinical implications for the use of racemates

This review summarizes and comments on the current understanding of both the biochemical and clinical implications of the epimerization of R-aryl propionic (APA) class (1) nonsteroidal anti-inflammatory agents (NSAIDs) to S-enantiomers in humans. This article focuses principally on rac-ibuprofen and its enantiomers. In the United States, five commercialized NSAIDs are APAs. Only two of them, rac-ibuprofen and rac-fenoprofen, are subject to significant epimerization in humans. The remaining three, rac-flurbiprofen, rac-ketoprofen, and S-naproxen, are not of interest in this context.

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.

Dose-dependency of flurbiprofen enantiomer pharmacokinetics in the rat

Flurbiprofen is a chiral 2-arylpropionate used clinically as a racemate. Previously a significant pharmacokinetic interaction between the enantiomers of flurbiprofen has been reported in both rats and humans. The possible mechanism for this interaction was believed to involve competitive protein binding between the enantiomers. In addition, the saturable binding of flurbiprofen enantiomers in vitro in human plasma has been demonstrated. In this study different doses of racemic flurbiprofen were administered to rats to create differing extents of competition for protein binding sites between the enantiomers. There was a statistically significant dose-dependent increase in total body clearance and volume of distribution of both the R and S enantiomers. However, there was no change in either the S/R AUC ratio or the elimination rate constants for (R)- or (S)-flurbiprofen with increasing dose. These results are consistent with the hypothesis that the increasing amount of (R)- and (S)-flurbiprofen in the body causes displacement of flurbiprofen enantiomers from their protein binding sites, resulting in their increased total body clearance and volume of distribution. Further, the data suggest that previously reported extents of R to S enantiomeric inversion for other 2-arylpropionates may not be accurate if the enantiomers exhibit nonlinear kinetics or there is a significant kinetic interaction between the enantiomers.

Pharmacokinetics and bioinversion of ibuprofen enantiomers in humans

An open, randomized, six-way crossover study was conducted in 12 healthy males to assess pharmacokinetics and bioinversion of ibuprofen enantiomers. The mean plasma terminal half-life (t1/2) of R(-)ibuprofen was 1.74 hr when intravenously infused as a racemic mixture and was 1.84 hr when intravenously infused alone. The mean t1/2 of S(+)ibuprofen was 1.77 hr when dosed as S(+)ibuprofen. Examination of values of both the absorption and disposition parameters of R(-)ibuprofen revealed that the kinetics of R(-)ibuprofen were not altered by concurrent administration of S(+)ibuprofen. In this study, there was little or no presystemic inversion of R(-)ibuprofen to its S(+)isomer. Also, 69% of the intravenous dose of R(-)ibuprofen was systemically inverted and 57.6% of the oral dose of R(-)ibuprofen lysinate was bioavailable as S(+)ibuprofen. These results indicate that the bioinversion of R(-)ibuprofen administered orally is mainly systemic. Because bioinversion of R(-)ibuprofen is not complete, S(+)ibuprofen produced higher bioavailability of S(+)ibuprofen (92.0%) than either racemic ibuprofen (70.7%) or R(-)ibuprofen (57.6%). However, bioavailability of R(-)ibuprofen (83.6%) when dosed alone was not significantly different from when dosed as racemic mixture (80.7%).

An enantiomer-enantiomer interaction of (S)- and (R)-propafenone modifies the effect of racemic drug therapy

BACKGROUND

Therapy with racemic compounds produces effects that can be attributed to both (S)- and (R)-enantiomers. Here we have tested the hypothesis that an enantiomer-enantiomer interaction would modulate the effects of treatment with a racemate, the antiarrhythmic propafenone. Previous studies have shown that while the enantiomers of propafenone exert similar sodium channel-blocking (QRS widening) effects, it is the (S)-enantiomer that produces beta-blockade; moreover, we have demonstrated recently that (R)-propafenone inhibits the metabolism of (S)-propafenone in vitro.

METHODS AND RESULTS

This single-blind, randomized study compared the effects of (R/S)-, (S)-, (R)-propafenone (150 mg q 6 hours for 4 days) and placebo on QRS duration (delta QRS) and on maximum exercise heart rate (delta HRmax), an index of beta-blockade. The clearance of (S)-propafenone was significantly lower (-55 +/- 24%, P < .001) during treatment with (R/S)-propafenone than with the (S)-enantiomer alone, and delta HRmax was significantly altered during (R/S)-propafenone (-8.8 +/- 6.6 beats per minute; P < .01) and during (S)-propafenone (-4.3 +/- 4.8 beats per minute; P < .01) but not during (R)-propafenone (-1.8 +/- 6.4 beats per minute) or placebo (0.3 +/- 7.1 beats per minute). In contrast, (R/S)-, (S)-, and (R)-propafenone all prolonged QRS compared with placebo.

CONCLUSIONS

These data indicate that (R)-propafenone impairs the disposition of (S)-propafenone in humans. As a result, the beta-blocking effects of 150 mg of racemic propafenone (75 mg of the [S]-enantiomer) were more pronounced than those of 150 mg of (S)-propafenone alone. Thus, the effects of racemic drug therapy are not necessarily those predicted by summation of the effects of the individual enantiomers.

Enantioselective hydrolysis of racemic naproxen nitrile and naproxen amide to S-naproxen by new bacterial isolates

Bacteria were enriched from soil samples with succinate as a carbon source and racemic naproxen nitrile [2-(6-methoxy-2-naphthyl)propionitrile] as sole source of nitrogen. Since naproxen nitrile was only poorly soluble in water media amended with different water-immiscible organic phases were used for the enrichments. With pristane (2,6,10,14-tetramethylpentadecane) as the organic phase two bacterial strains were isolated (strain C3II and strain MP50) which were identified as rhodococci. Cells of both strains converted naproxen nitrile via naproxen amide to naproxen. From racemic naproxen nitrile Rhodococcus sp. C3II formed S-naproxen amide and subsequently S-naproxen. Racemic naproxen amide was hydrolysed to S-naproxen. Rhodococcus sp. MP50 converted racemic naproxen nitrile predominantly to R-naproxen amide and racemic naproxen amide to S-naproxen. With both strains racemic naproxen amide was converted to S-naproxen with an enantiomeric excess > 99% at a conversion rate up to 80% of the theoretical value. In strain C3II the enzymes which hydrolysed naproxen nitrile and naproxen amide were present only at a low constitutive level. In contrast, in Rhodococcus sp. MP50 these activities were induced when grown in the presence of various nitriles.

Is the inversion from R- to S-ketoprofen concentration dependent? Investigations in rats in vivo and in vitro

The effects of dose on the pharmacokinetics of ketoprofen (KT) enantiomers were investigated in rats in vivo and in hepatoma cells in continuous culture in vitro following administration of the optically pure enantiomers and the racemate of KT. With the exception of AUC (area under the curve) no pharmacokinetic differences could be found following i.v. administration of various doses of KT enantiomers (2.5, 5 and 10 mg/kg) and of racemic KT (5, 10 and 20 mg/kg) and between single enantiomer and racemate administration in rats in vivo. Independent of the dose administered the fraction inverted was about 66%. In line with the findings in vivo good correlation between incubation concentration and AUC of R- and S-KT was found in the hepatoma cells in vitro. The ratios of AUC(S)/AUC(R) were not significantly affected by concentration after R-KT (2.5-20 micrograms/mL) and racemate incubation (5-40 micrograms/mL) in the concentration ranges investigated. However, unlike in rats in vivo enhanced inversion was observed following racemate as compared to single enantiomer incubation in vitro.

Advances in anti-inflammatory therapy

Anti-inflammatory drugs are widely used in veterinary practice to provide symptomatic relief of acute and chronic inflammatory conditions. Whilst much is already known about the properties of corticosteroid and non-steroidal anti-inflammatory drugs, new findings on their biology and pharmacokinetics continue to emerge. These are discussed, together with some possible novel therapeutic applications. Recent evidence, suggesting that morphine-like analgesic drugs may possess anti-inflammatory activity, is additionally presented. Knowledge of the pathways of formation, actions and interactions of the diverse range of mediators responsible for the pathophysiological changes underlying the inflammatory process is also increasing. Compounds are being developed which act selectively to block the formation or actions of these mediators and the potential of such agents as anti-inflammatory drugs is discussed. Although such compounds do not, at present, have veterinary applications, when used either alone, or in combination, some may prove to be potent and effective therapeutic agents.

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.

Influence of renal function on the pharmacokinetics of diacerein after a single oral dose

The pharmacokinetics of diacetylrhein following a single oral dose of 50 mg was studied in 12 healthy volunteers and two groups of 8 patients with mild or severe renal insufficiency. Statistical analysis using a Kruskal-Wallis rank sum test showed a significant difference between the three groups for the following parameters. In severely uraemic patients, median AUC0-infinity was multiplied by a factor of about 2: 40.5 mg.h/l versus 21.3 mg.h/l in healthy subjects, P = 0.04; and t1/2 was prolonged by the same factor: 9.6 h versus 4.3 h in the control group, P = 0.003. Apparent drug availability and renal clearance assessed through urinary data decreased with renal failure, respectively: 14.5% and 0.045 l/h versus 35.4% (P = 0.01) and 0.13 l/h (P = 0.008) in healthy subjects. Amounts of glucuro- and sulpho-conjugates in urine were lower in severely uraemic patients. Intermediate values were observed for mildly uraemic patients. Other parameters: lag time, Cmax, tmax, Vss/F, urinary glucuro- to sulpho-conjugate ratios did not change significantly. Apparent total clearance of rhein was poorly correlated with creatinine clearance and this was related to a decrease of non-renal clearance of rhein in renal insufficiency. It was concluded that, from a pharmacokinetic point of view, a reduction (50%) in the initial dosage of diacerein should be considered in severe renal failure.

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.

Bidirectional chiral inversion of the enantiomers of the nonsteroidal antiinflammatory drug oxindanac in dogs

The nonsteroidal antiinflammatory drug oxindanac exists as two enantiomers, with most of its pharmacological activity residing in the (S)-isomer. The behavior of its enantiomers was investigated in dogs. Bidirectional inversion occurred in heparinised plasma and blood, with a ratio of enantiomers [S:R] of 7.3:1 being achieved at equilibrium after incubation for 24 h at 37 degrees C. There was no detectable inversion of either isomer in plasma incubated at 4 degrees C for up to 8 h or in aqueous solution at 37 degrees C for up to 36 h. Bidirectional inversion also occurred in vivo, with a ratio of plasma AUC (0 infinity)s [S:R] of 8.1:1. The ratio of enantiomers reached equilibrium within 2 hr following (S)- or rac-oxindanac, and within 8 h following (R)-oxindanac. Elimination t1/2s of the isomers were the same (R, 12.1 h, S, 13.3 h). There were no differences in the ratio of enantiomers following oral or intravenous application, suggesting that a systemic site for inversion was predominant. Although concentrations of the respective isomers were similar at equilibrium following administration of either (R)-, (S)-, or rac-oxindanac, AUC (0 infinity)s differed due to the delay in reaching equilibrium. The extent of inversion to the (S)-isomer was 100, 73.2, and 60.7% after administration of (S)-, rac-, and (R)-oxindanac, respectively. Although pharmacological activity might be equivalent at equilibrium following administration of either (R)-, (S)-, or rac-oxindanac; efficacy at early time points should be superior in the order (S) > racemate > (R).(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics of ketoprofen enantiomers in monkeys following single and multiple oral administration

Pharmacokinetic studies are reported after single oral administration of 3 mg/kg of stereochemically pure (S)-ketoprofen [(S)-KP] and (R)-ketoprofen [(R)-KP] to three male Cynomolgus monkeys and after repeated administration for 6 months of 3, 15 and 75 mg/kg/day of (S)-KP to both male and female monkeys. A high-performance liquid chromatographic (HPLC) analysis was performed without derivatization of the samples, using a chiral column. The pharmacokinetic parameters for (S)-KP after administration of (S)-KP and for (R)-KP after administration of (R)-KP were, respectively, elimination half-life 2.32 +/- 0.36 and 1.64 +/- 0.40 h; oral clearance 3.50 +/- 0.66 and 7.50 +/- 3.20 ml/min/kg; apparent volume of distribution 0.74 +/- 0.24 and 1.16 +/- 0.76 liter/kg; mean residence time 1.79 +/- 0.77 and 1.41 +/- 0.65 h; area under the concentration/time curve 14.16 +/- 2.93 and 7.31 +/- 2.98 micrograms.h/ml. Forty-nine percent unidirectional bioinversion of (R)-KP to (S)-KP was observed in this species and the pharmacokinetic parameters for the (S)-KP resulting from this inversion were also calculated. In the study of 6-month repeated administration of (S)-KP, linear pharmacokinetic behavior and no evidence of drug accumulation were observed at the three dose levels.

Study of interaction of carprofen and its enantiomers with human serum albumin--II. Stereoselective site-to-site displacement of carprofen by ibuprofen

The site-to-site displacement of carprofen, a site II-specific drug, bound to human serum albumin (HSA) by ibuprofen, another site II-specific drug, was qualitatively and quantitatively studied by circular dichroism (CD) and equilibrium dialysis (ED). Carprofen gives rise to different CD spectra at lower (1:1) and higher (3:1) molar ratios to HSA, indicating different mechanisms for the binding of this drug to its high and low affinity sites on HSA. Ibuprofen at a 5:1 molar ratio to HSA displaces carprofen at a molar ratio of 1:1 to HSA from its high affinity binding site (site II) to its low affinity site (site I), as shown by production of the CD spectrum similar to that obtained in the case of the carprofen-HSA complex at a molar ratio 3:1. As revealed by the ED experiments, the free fraction of carprofen at a molar ratio of 1:2 to HSA (2 x 10(-5) M) was not initially increased by the addition of ibuprofen at a lower concentration, but at a higher concentration (6 x 10(-5) M), the free fraction was increased by only 90%. When site I was sufficiently blocked by a site I-specific drug like warfarin or phenylbutazone (6 x 10(-5) M), there was about a 4-fold increase in the free fraction of carprofen caused by ibuprofen. This site-to-site displacement demonstrated by carprofen was found to be stereospecific as indicated by the highest interaction between the S(+)-enantiomers of carprofen and ibuprofen. Moreover, the displacement of carprofen occurred at the azapropazone region rather than the warfarin region of site I on HSA.

Influence of renal function on the pharmacokinetics of diacerein after a single oral dose

The pharmacokinetics of diacerein (a new anti-inflammatory analgesic antipyretic drug) following a single oral dose of 50 mg was studied in 12 healthy volunteers and two groups of eight patients with mild or severe renal insufficiency. Statistical analysis using a Kruskal-Wallis rank sum test showed a significant difference between the three groups for the following parameters. In severely uraemic patients, median AUC0-infinity was multiplied by a factor of ca 2: 40.5 mg h/l versus 21.3 in healthy subjects, P = 0.04; and t1/2 was prolonged by the same factor: 9.6 h versus 4.3 in the control group, P = 0.003. Apparent drug availability and renal clearance assessed through urinary data decreased with renal failure, respectively: 14.5% and 0.045 l/h versus 35.4% (P = 0.01) and 0.13 l/h (P = 0.008) in healthy subjects. Amounts of glucuro and sulfo conjugates in urine were lower in severely uraemic patients. Intermediate values were observed for mildly uraemic patients. Other parameters: lag-time, Cmax, tmax, Vss/F, urinary glucuro- to sulphoconjugate ratios did not change significantly. Apparent total clearance of rhein was poorly correlated with creatinine clearance and this was related to a decrease of non-renal clearance of rhein in renal insufficiency. It was concluded that, from a pharmacokinetic point of view, a reduction (50%) in the maintenance dosage of diacerein should be considered in severe renal failure.

Stereoselective glucuronidation of (R)- and (S)-naproxen by recombinant rat phenol UDP-glucuronosyltransferase (UGT1A1) and its human orthologue

Recombinant rat phenol UDP-glucuronosyltransferase (UGT1A1) conjugates (R)-naproxen at a much higher rate (> 17-fold) than its (S)-enantiomer, substantiating previous findings on stereoselective glucuronidation of racemic naproxen. In contrast, the recombinant human orthologue conjugated both enantiomers at equal rates. In line with high constitutive expression of UGT1A1 in extrahepatic tissues, a high R/S ratio of naproxen glucuronidation was found in rat testes, intestine, lung and kidney. The results demonstrate that (R)-naproxen represents a stereoselective substrate of rat UGT1A1, but not of the human orthologous UGT1A1.

Production of S-(+)-2-phenylpropionic acid from (R,S)-2-phenylpropionitrile by the combination of nitrile hydratase and stereoselective amidase in Rhodococcus equi TG328

A new soil isolate, tentatively identified as Rhodococcus equi TG328, was found to be effective in the production of S-(+)-2-phenylpropionic acid from (R,S)-2-phenylpropionitrile. The conversion is catalysed by two enzymes. First, a nitrile hydratase converts the (R,S)-nitrile to (R,S)-2-phenylpropionamide. Second, a stereoselective amidase converts the S-(+)-amide to S-(+)-2-phenylpropionic acid. Conditions for optimal enzyme production and accumulation of S-(+)-2-phenylpropionic acid by resting cells were studied. The reaction of resting cells for 30 h at 10 degrees C with (R,S)-2-phenylpropionitrile resulted in the production of 100 g of S-(+)-2-phenylpropionic acid per litre of reaction mixture. The enantiometric excess of the purified S-(+)-2-phenylpropionic acid was 99.4%. The amount of S-(+)-2-phenylpropionic acid accumulated was enhanced by lower reaction temperatures. In addition, unreacted R-(-)-2-phenylpropionamide with 99.0% enantiometric excess was isolated.

Disposition of enantiomers of sulpiride in humans and rats

Pharmacokinetics of sulpiride enantiomers after intravenous administration of (+/-)-, (+)-, and (-)-sulpiride was examined in humans and rats. Pharmacokinetics profiles were similar in (+)- and (-)-enantiomers after intravenous administration of (+/-)-sulpiride. Metabolic inversion at a chiral centre was not observed after intravenous administration of each enantiomer in rats.

Variability in the stereoselective disposition of ibuprofen in patients with rheumatoid arthritis

1. Patients suffering from rheumatoid arthritis received oral doses of 600 mg racemic ibuprofen (n = 25; RAC) or 400 mg (S)-ibuprofen (n = 25; S-IBU) in a double-blind, randomized parallel-group study. 2. The pharmacokinetic parameters of (S)-ibuprofen were not statistically different between treatments (P > 0.05). Comparing (S)- and (R)-ibuprofen within the group receiving the racemate significantly higher Cmax (20.3 +/- 5.3 vs 17.7 +/- 4.4 micrograms ml-1; P < 0.02; 95% confidence interval for differences (CI): 0.5-4.6), AUC (86.2 +/- 23.5 vs 67.6 +/- 26.6 micrograms ml-1 h; P < 0.001; CI: 9.5-27.6), mean residence time (4.5 +/- 1.1 vs 4.1 +/- 1.2 h; P < 0.01; CI: 0.1-0.6) and renal clearance (0.8 +/- 0.6 vs 0.0 +/- 0.0 ml min-1; P < 0.001; CI: 0.5-1.0) values were observed for the (S)-enantiomer. 3. No difference was found (P > 0.05) between treatments in the percentage of the dose recovered in the urine as (R)- or (S)-ibuprofen plus metabolites (S-IBU: 80.2 +/- 8.47 vs RAC: 74.1 +/- 14.0%). 4. Interindividual variation in the pharmacokinetics of (S)-ibuprofen following administration of the racemate was similar to that following the administration of the single isomer suggesting that chiral inversion is not a major factor contributing to variability in the disposition of this drug.

Stereoselective cyclooxygenase inhibition in cellular models by the enantiomers of ketoprofen

The pharmacological activity of rac-ketoprofen and its enantiomers was investigated in vitro using different cellular models. The effect of these compounds on arachidonic acid metabolism was assessed by measuring the inhibition of prostanoid generation under the action of several agonists. Thus, we have evaluated the inhibition of (1) thromboxane B2 synthesis in rabbit platelets and human polymorphonuclear leukocytes (PMNs), (2) prostaglandin E2 synthesis in three cultured cells, namely human umbilical vein endothelial cells (HUVEC), human keratinocytes, and mouse macrophage-like P388D1 cells. The IC50 values found for (+)-(S)-ketoprofen were in the range between 0.1 nM and 0.8 microM, being slightly lower in all models than those found for rac-ketoprofen (0.4 nM-3 microM). On the other hand (-)-(R)-ketoprofen showed inhibition of cyclooxygenase only at concentrations two or three orders of magnitude higher than those required for the (+)-(S) enantiomer. These results, obtained with cell types of relevance for inflammatory processes and with compounds of high optical purity, demonstrate that the prostanoid biosynthesis inhibition caused by the drug rac-ketoprofen is exclusively due to its dextrorotatory enantiomer.

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)

Microbial metabolism of 2-arylpropionic acids: chiral inversion of ibuprofen and 2-phenylpropionic acid

The metabolism of (R,S)-ibuprofen has been investigated in 24 microbial cultures. Of these Cunninghamella elegans, Mucor hiemalis, and Verticillium lecanii catalyzed the oxidation of the drug to 2-[4-(2-hydroxy-2-methylpropyl)phenyl]propionic acid, a known mammalian metabolite. The extent of metabolism was greatest with V. lecanii, with some 47% of the substrate being consumed over a 7-day incubation period. Enantiomeric analysis indicated stereoselective metabolism of (R)-ibuprofen, the enantiomeric composition of the residual substrate being R/S = 0.25. Following a preparative scale incubation of (R,S)-ibuprofen with V. lecanii, in which the reaction was allowed to go to completion, the metabolite was found to be predominantly of the S-configuration (S/R = 2.1), suggesting that chiral inversion of either the drug and/or the metabolite had taken place. Analysis of extracts following incubation of (R,S)-, (R)-, and (S)-2-phenylpropionic acid with V. lecanii, for 21 days, indicated that chiral inversion of the (R)-enantiomer to its optical antipode had taken place. The results of these investigations indicate that microorganisms, in addition to mammals, are able to mediate the chiral inversion of 2-arylpropionic acids. This observation may have implications for the preparation of optically pure 2-arylpropionic acids.

The importance of stereochemistry in drug action and disposition

Many biologically active synthetic drugs contain chiral centers, although they are used as racemic mixtures. Enantiomers are hard to distinguish in the chemical laboratory but are readily discriminated in the body and differ in their biological activities and disposition. The pharmacokinetic profiles of enantiomers can be variable, especially for drugs with a first-pass effect and enantioselective pharmacokinetic monitoring should be carried out. Ultimately, whether to exploit a racemate or a single enantiomer in therapy is a multi-faceted decision to which drug disposition data have important contributions to make.

Pharmacokinetics of ibuprofen enantiomers after single and repeated doses in man

The pharmacokinetic parameters of ibuprofen enantiomers after a single 600 mg dose and repeated 3 x 400 mg doses of Nurofen were determined in 12 healthy volunteers. Terminal half-lives were similar for both enantiomers, but plasma levels of S-ibuprofen were higher than those of R-ibuprofen, due to the chiral inversion and differences in distribution and metabolism. Comparison of maximal concentrations and areas under the concentration vs time curves between the first and last doses for each enantiomer indicated linear pharmacokinetics with no time-dependency. A large inter-individual variability in the ratio of S- to R-ibuprofen average concentrations at steady-state was observed (mean +/- SD 1.89 +/- 0.89) and probably accounts for the known lack of correlation between racemic ibuprofen concentrations and therapeutic efficacy.

The in vitro metabolic inversion of R(-) to S(+) indoprofen

The paper reports a study on the metabolic inversion of indoprofen (2-[4-(2-isoindolinyl-1-one)-phenyl]-propionic acid) following incubation of the drug with liver microsomes from non-induced and phenobarbital-induced rats. The enantiomeric composition of the drug was determined after different incubation times of the racemate and the individual isomers. The S(+)/R(-) ratio was evaluated by densitometry following HPTLC separation of the R(+)-1-phenylethylamides. After incubation of the racemate and the individual isomers, no detectable amounts of indoprofen catabolites were extracted from the acidified incubation mixture. An appreciable enrichment in the S(+) enantiomer was observed after incubation of both racemate and R(-)-indoprofen; the S(+)/R(-) ratio reached a maximum after 1 h. Values were higher in the case of induction. After incubation of S(+)-indoprofen, a small but statistically significant decrease of the S(+)/R(-) ratio was observed. The increase of the S(+)-isomer concentration observed following incubation of R(-)-indoprofen can be ascribed to metabolic inversion by phenobarbital-inducible liver enzymes.

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.

Metabolic chiral inversion of 2-arylpropionates in rat H4IIE and human Hep G2 hepatoma cells. Relationship to in vivo metabolism

The inversion of 2-arylpropionic acids (2-APAs) has been investigated in vitro using rat H4IIE and human Hep G2 hepatoma cells in continuous culture. The effect of substrate concentration (15-150 micrograms/mL), cell density (1.5-12 x 10(6) cells/dish) and serum content of the culture medium (0-20%) on inversion was examined in rat H4IIE hepatoma cells using R-ibuprofen as model compound. Increasing R-ibuprofen concentrations and decreasing serum content of the medium resulted in increased inversion whereas variation of cell density had no effect. Furthermore, rat H4IIE and human Hep G2 hepatoma cells were incubated with the individual enantiomers of ibuprofen, ketoprofen and flurbiprofen under optimized culture conditions (serum-free culture medium). The elimination rate constants (kel) and fractions inverted (Fi) were determined. Although inversion occurred slowly in the tumor cells and thus long incubation periods (120 hr) were required, the hepatoma cells were nevertheless able to mimic qualitatively the species and substance specificity of inversion of 2-APAs as observed in vivo.

Murine teratology and pharmacokinetics of the enantiomers of sodium 2-ethylhexanoate

A mouse model for the induction of exencephaly with sodium (+/-)-2-ethylhexanoate has been developed using multiple administration regimes. With three consecutive administrations at one-half-day intervals, the most sensitive time to induce exencephaly was Gestational Days 8-9. Using the racemic substance it was determined that the SWV strain was more sensitive to the induction of exencephaly than the C57BL/6NCrlBR strain. The enantiomers of 2-ethylhexanoic acid were separated via preparative HPLC to greater than 99.8% optical purity, and greater than 99% purity according to a gas chromatographic analysis. It was demonstrated that the (R)-enantiomer is a more potent teratogen than the (S)-enantiomer for the induction of exencephaly as well as malformations of other organ systems. Pharmacokinetic analyses for each of the enantiomers were performed in maternal plasma, maternal muscle, and embryo. The pharmacokinetics showed that the peak concentration (Cmax) for both enantiomers in the three compartments was approximately equivalent and was attained within 15 min following the third administration. The area under the concentration versus time curve values for the two enantiomers were approximately 10% higher for the (R)-antipode because of a slightly slower elimination of this compound. There was negligible (or no) racemization of the two enantiomers in the biological samples. The results suggest that teratologic differences in the enantiomers of sodium 2-ethylhexanoate are not due to differences in the concentrations of these antipodes in the embryo, but more likely result from the specific interaction of the enantiomers with chiral molecules in the embryo.

Stereoselective high-performance liquid chromatographic determination of flurbiprofen in human plasma

An enantioselective high-performance liquid chromatographic assay for the quantitation of the enantiomers of flurbiprofen in human plasma is described. The procedure involved extraction of flurbiprofen from acidified plasma into hexane-diethyl ether (8:2, v/v). Stereoselective separation was achieved with a prepacked alpha 1-acid glycoprotein column without any derivatization procedure. A second assay using a conventional reversed-phase column to determine racemic flurbiprofen is also described. The detection wavelength was set at 246 nm. The limit of quantification was found to be 50 ng/ml for both assays. The method was demonstrated to be sufficiently sensitive for stereoselective pharmacokinetic studies of flurbiprofen.

Application of antibody-mediated extraction for the stereoselective determination of the active metabolite of loxoprofen in human and rat plasma

Antibody-mediated extraction followed by chiral high-performance liquid chromatography (HPLC) was applied to stereoselective determination in human and rat plasma of the active metabolite [(2S,1'R,2'S)-trans-alcohol] with three chiral centers of Loxoprofen, a 2-arylpropionic acid antiinflammatory agent after oral administration. Antiserum against the (1'R,2'S)-cyclopentanol moiety was obtained from a rabbit immunized with bovine serum albumin conjugate linked to the propionic acid moiety, in which another chiral center is located. Then, the immunoglobulin G purified by a protein A column was coupled to BrCN-activated Sepharose 4B. Plasma samples were applied to the immobilized antibody column. After washing the column to remove unrequired stereoisomers, a mixture of two diastereomers whose configurations were 1'R,2'S in the cyclopentanol moiety was extracted with 95% methanol. The solvent was evaporated and the residue was derivatized with (+)-(R)-1-(1-naphthyl)ethylamine as a chiral reagent to separate the diastereomers by HPLC. This combined analytical method showed the stereoselective metabolism of Loxoprofen in human, that is, 64% of the total amount of four trans-alcohol stereoisomers was in the 2S,1'R,2'S form, which is the active metabolite. This phenomenon was also observed in rats given Loxoprofen and its (2S)- and (2R)-isomers, and is explained by stereoselective ketone reduction of Loxoprofen to the (1'R,2'S)-trans-alcohol and inversion from 2R to 2S in the propionic acid moiety. Antibody-mediated extraction should be a selective and simple clean-up method for determining haptens with complicated structures, combined with an appropriate analytical method.

Formation of glycine conjugate and (-)-(R)-enantiomer from (+)-(S)-2-phenylpropionic acid suggesting the formation of the CoA thioester intermediate of (+)-(S)-enantiomer in dogs

It has been proposed that the chiral inversion of the 2-arylpropionic acids is due to the stereospecific formation of the (-)-R-profenyl-CoA thioesters which are putative intermediates in the inversion. Accordingly, amino acid conjugation, for which the CoA thioesters are obligate intermediates, should be restricted to those optical forms which give rise to the (-)-R-profenyl-CoA, i.e., the racemates and the (-)-(R)-isomers. We have examined this problem in dogs with respect to 2-phenylpropionic acid(2-PPA). Regardless of the optical configuration of 2-phenylpropionic acid administered, the glycine conjugate was the major urinary metabolite and this was shown to be exclusively the (+)-(S)-enantiomer by chiral HPLC. Both (-)-(R)- and (+)-(S)-2-phenylpropionic acid were present in plasma after the administration of either antipode, and further evidence of the chiral inversion of both enantiomers was provided by the presence of some 25% of the opposite enantiomer in the free 2-phenylpropionic acid and its glucuronide excreted in urine after administration of (-)-(R)- and (+)-(S)-2-phenylpropionic acid. The (+)-(S)-enantiomer underwent chiral inversion to the (-)-(R)-antipode when incubated with dog hepatocytes. These data suggests that both enantiomers of 2-phenylpropionic acid are substrates for canine hepatic acyl CoA ligase(s) and thus undergo chiral inversion, but that the CoA thioester of only (+)-(S)-2-phenylpropionic acid is a substrate for the glycine N-acyl transferase. These studies are presently being extended to the structure and species specificity of the reverse inversion and amino acid conjugation of profen NSAIDs.

Pulmonary inversion of 2-arylpropionic acids: influence of protein binding

The possible contribution of pulmonary metabolism to the putative first-pass metabolism of 2-arylpropionic acid nonsteroidal antiinflammatory drugs has not been documented. Isolated perfused rabbit lungs, perfused with 4.5% bovine serum albumin or 5% dextran, were used to study the pulmonary elimination of (R)- and rac-ibuprofen, fenoprofen, and flurbiprofen. In the absence of protein binding, ibuprofen was metabolized via inversion and other pathways, whereas fenoprofen metabolism was essentially restricted to inversion of the (R)-enantiomer; fraction inverted (+/- SE) was 0.37 +/- 0.05 for (R)-ibuprofen and 0.85 +/- 0.03 for (R)-fenoprofen. In the presence of protein, neither ibuprofen nor fenoprofen was metabolized. Flurbiprofen did not undergo pulmonary elimination under any condition studied. This study illustrates that even though a tissue is capable of metabolism, particularly inversion of 2-arylpropionics, the quantitative importance of such elimination pathways may be minimal in the presence of the high degree of protein binding that is characteristic of these drugs.

Effects of pure enantiomers of flurbiprofen in comparison to racemic flurbiprofen on eicosanoid release from various rat organs ex vivo

The effects of oral treatment of rats with pure enantiomers of flurbiprofen in comparison to racemic flurbiprofen on ex vivo release of eicosanoids from gastric mucosa, jejunum, lung, brain and clotting whole blood were investigated. With the S(+) enantiomer and the racemate dose-dependent inhibition of release of cyclooxygenase products of arachidonate metabolism in all tissues tested was observed, while release of leukotriene (LT) C4 was inhibited in gastric mucosa, but not in jejunum and lung. On the other hand, the R(-) enantiomer inhibited cyclooxygenase in the various tissues less potently and to a variable degree with no significant effect in the jejunum. The R(-) enantiomer had no effect on LTC4 release from any of the tissues investigated. Furthermore, the effect of a high dose of 25 mg/kg of the S(+) enantiomer on release of cyclooxygenase products from the various tissues was much longer lasting than that of an identical dose of the R(-) enantiomer. Stereoselective pharmacokinetics of the flurbiprofen enantiomers and/or organ specific cyclooxygenase activities could underly these results. The more potent cyclooxygenase inhibition by the S(+) enantiomer correlates with its higher anti-inflammatory activity and gastrointestinal toxicity. On the other hand, both enantiomers have been shown previously to be almost equally effective analgesics. Inhibition of brain cyclooxygenase might contribute to this effect.

Stereoselective pharmacokinetics and inversion of suprofen enantiomers in humans

The stereoselective pharmacokinetics of suprofen enantiomers has been studied in humans by means of stable isotope-labeled pseudoracemate-diastereomer methodology. After a single oral dose of a near equimolar mixture of unlabeled-(R)-(-)- and [2H3]-(S)-(+)-suprofen [or unlabeled-(S)- and [2H3]-(R)-suprofen] to three healthy male subjects, the plasma concentrations of drug were determined by a stereospecific gas chromatography-mass spectrometry method. Racemic [2H7]suprofen was used as an internal standard. The method involved chiral derivatization with (S)-(-)-1-(naphthyl)ethylamine to form the diastereomeric amide. The plasma concentrations were consistently higher for the (R)-isomer than the (S)-isomer. No significant difference in the elimination half-life of the enantiomers was observed. An average of 6.8% of an administered dose of the (R)-isomer was stereospecifically inverted to the (S)-isomer. There was no measurable inversion of the (S)- to (R)-isomer. The present stable isotope-labeled pseudoracemate-diastereomer methodology has made it possible to evaluate the pharmacokinetics of each enantiomer, including the estimation of chiral inversion after administration of the racemic mixture.

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.

Chiral stereoisomeric molecules in the treatment of arthritis

Pharmacokinetic and pharmacodynamic properties of drugs and their ultimate therapeutic effects are often significantly influenced by interactions between the geometry of host receptors, host enzymes, and the three-dimensional structure of drugs. Drug molecules that are mirror images of each other are chiral stereoisomers, and such chiral isomer compounds are commonly used as therapeutic agents by rheumatologists either as racemates (mixtures of chiral isomers) or as pure stereoisomers. Understanding and using such stereoisomeric drugs may lead to lower risks of drug toxicity, better therapeutic indices, and newer approaches for the treatment of articular disorders. A review of the properties of these special isomers is presented, and their therapeutic advantages are discussed.