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

Chiral and nonchiral determination of ketoprofen in pharmaceuticals by capillary zone electrophoresis

The new method for the enantiomeric resolution of various 2-arylpropionic acids by capillary zone electrophoresis (CZE) using heptakis-tri-O-methyl-beta-cyclodextrin as chiral selector was applied to the determination of ketoprofen in different commercially-available pharmaceutical preparations. The analyte was determined under chiral and nonchiral conditions (viz. in the presence and absence of 50 mM heptakis-tri-O-methyl-beta-cyclodextrin in the background electrolyte), with significantly similar results and relative standard deviations from 1.2 to 6.5% in both cases. The limits of detection and determination for the inactive enantiomer, R-(-)-ketoprofen, were calculated to be 7.0 x 10(-7) and 1.6 x 10(-6) M, respectively. The proposed method was successfully used to determine enantiomeric purity in the drugs studied, with results comparable to those provided by the chiral HPLC method.

Enantiomeric separation and detection by high-performance liquid chromatography-mass spectrometry of 2-arylpropionic acids derivatized with benzofurazan fluorescent reagents

The enantiomneric separation and the detection of 2-arylpropionic acids after derivatization with the fluorescent reagents with a benzofurazan structure, (S)-(+)-4-(N,N- dimethylaminosulphonyl)-7-(3-aminopyrrolidin-1-yl)-2,1,3-ben zoxadiazole ((S)-DBD-Apy), (R)-(-)-4-nitro-7-(3-aminopyrrolidin-1-yl)-2,1,3- benzoxadiazole ((R)-NBD-Apy), 4-N,N-dimethylaminosulphonyl-7-piperazino-2,1,3-benzoxadi zole (DBD-PZ) and N-hydrazinoformylmethyl-N-methylamino-4,4- N,N-dimethylaminosulphonyl-2,1,3-benzoxadiazole (DBD-CO-Hz) by high-performance liquid chromatography (HPLC) and electrospray ionization mass spectrometry (ESI-MS) were examined. The diastereomeric derivatization with (S)-DBD-Apy or (R)-NBD-Apy and the separation on the reversed phase column afforded the high sensitivity. The separation on chiral stationary phase after non-chiral derivatization with DBD-PZ or DBD-CO-Hz provided less sensitivity. The signal-to-noise ratio of (S)-DBD-Apy-(S)-ketoprofen of 200:1 was observed for 12.5 picomole (pmol) injection and selected ion monitoring (SIM) of the quasi-molecular ion after splitting 1:7 before entering into the electrospray ion sources. As a result, the usefulness of these reagents for MS detection has been demonstrated.

Stereoselective pharmacokinetics and inversion of (R)- ketoprofen in healthy volunteers

The pharmacokinetics of ketoprofen enantiomers were evaluated after 25-, 50-, and 100-mg doses of (R)- ketoprofen and 100 mg of racemic ketoprofen in 25 healthy volunteers (12 male and 13 female). The fractional inversion (Finv) of (R)- ketoprofen was 8.9 +/- 3.3% using plasma data and 10.0 +/- 2.2% using urine data. There were small (< 5%) but significant differences between the enantiomers for areas under the plasma concentration-time curve (AUC) after the racemic dose (P < 0.005). Half-lives were 130-144 minutes for (R)- ketoprofen and 132-209 minutes for (S)- ketoprofen. Dose proportionality in AUC and maximum plasma concentration (Cmax) values was noted for both enantiomers. A total of 69% of the dose was recovered in the urine as (R)- and (S)- ketoprofen and conjugates. The elimination rate constant of (R)- ketoprofen was significantly different (P < 0.05) between men and women. Exposure to cyclooxygenase inhibiting (S)- ketoprofen was approximately 10% of the dose after the administration of pure (R)- ketoprofen and was independent of gender.

Preclinical enantioselective pharmacology of (R)- and (S)- ketorolac

Many of the nonsteroidal anti-inflammatory drugs (NSAIDs) are marketed as racemic mixtures, composed of (R)- and (S)- enantiomers. Racemic NSAIDs are potent cyclooxygenase (COX) inhibitors only through the action of the (S)- enantiomers, as the (R)- enantiomers do not exhibit COX inhibition. However, the (R)- enantiomer of ketoprofen exhibits potent analgesic activity and minimal ulcerogenic potential. To extend these observations, we examined the (R)- and (S)- enantiomers of RS- ketorolac, (S)- ketorolac exhibited potent COX1 and COX2 enzyme inhibition, whereas (R)- ketorolac was > 100-fold less active on both COX subtypes. Both enantiomers did not affect norepinephrine or serotonin uptake sites, and nitric oxidase or lipoxygenase activities, nor did they demonstrate any affinity for opioid receptors (mu, delta, or kappa). In experimental models, (S)- ketorolac exhibited about 10-fold greater activity than (R)- ketorolac in the murine phenylquinone writhing model. In this model, morphine sulfate was effective at much lower doses, however, and neither (R)- nor (S)- ketorolac showed any morphine-sparing effect. In the rat gait test for analgesia in the foot paw after injection of brewers yeast suspension, neither (R)- nor (S)- ketorolac affected paw volume. However, both provoked changes in gait scores, the (S)- enantiomer being 30-fold more potent than the (R)- enantiomer. A similar reduction was observed with respect to ulcerogenic potential, measured by direct microscopic changes after test conclusion. These findings suggest that (R)- ketorolac may possess analgesic activity that is independent of COX inhibition and may be associated with reduced ulcerogenic potential compared to effects exhibited by (S)- ketorolac.

Stereoselectivity and enantiomer-enantiomer interactions in the binding of ibuprofen to human serum albumin

Binding of ibuprofen (IB) enantiomers to human serum albumin (HSA) was studied using a chiral fluorescent derivatizing reagent, which enabled the measurement of IB enantiomers at a concentration as low 5 x 10(-8) M. Scatchard analyses revealed that there were two classes of binding sites for both enantiomers. For the high affinity site, the number of the binding sites was one for both enantiomers, and the binding constant of R-IB was 2.3-fold greater than that of S-IB. The difference in the affinity at the high affinity site may result in the stereoselective binding of IB enantiomers at therapeutic concentrations. It was confirmed that the high affinity site of IB enantiomers is Site II (diazepam binding site) by using site marker ligands. Also, significant enantiomer-enantiomer interactions were observed in the binding. The binding data were quantitatively analyzed and a binding model with an assumption of competitive interactions only at the high affinity site simulated the binding characteristics of IB enantiomers fairly well.

Stereospecific analysis of the major metabolites of ibuprofen in urine by sequential achiral-chiral high-performance liquid chromatography

A sequential achiral-chiral HPLC method has been developed for the stereospecific analysis of the two major urinary metabolites of ibuprofen, namely hydroxyibuprofen and carboxyibuprofen. Achiral analysis was carried out using a Partisil column (250x4.6 mm, 5 microm) and a mobile phase of hexane:ethanol (98.2:1.8, v/v) containing trifluoroacetic acid (TFA; 0.05%, v/v) at a flow-rate of 2.0 ml/min. The HPLC eluate containing the two metabolites was separately collected, evaporated under nitrogen and the residue dissolved in the mobile phase used for chiral chromatography. Chiral-phase analysis was carried out using a Chiralpak AD CSP (250x4.6 mm, 10 microm) with a mobile phase of hexane:ethanol (92:8, v/v) containing TFA (0.05%, v/v) at a flow-rate of 1.0 ml/min. In both assays the analytes were quantified by ultraviolet detection at a wavelength of 220 nm. Modification of the mobile-phase composition allowed the resolution of all six analytes in a single chromatographic run but with an increase in run time and consequent band broadening. The analytical method described allows the direct quantitation of the stereoisomers of both metabolites of ibuprofen in urine following the administration of therapeutic doses of the racemic drug to man.

Effects of ibuprofen enantiomers and its coenzyme A thioesters on human prostaglandin endoperoxide synthases

1. Ibuprofen enantiomers and their respective coenzyme A thioesters were tested in human platelets and blood monocytes to determine their selectivity and potency as inhibitors of cyclo-oxygenase activity of prostaglandin endoperoxide synthase-1 (PGHS-1) and PGHS-2. 2. Human blood from volunteers was drawn and allowed to clot at 37 degrees C for 1 h in the presence of increasing concentrations of the test compounds (R-ibuprofen, S-ibuprofen, R-ibuprofenoyl-CoA, S-ibuprofenoyl-CoA, NS-398). Immunoreactive (ir) thromboxane B2 (TXB2) concentrations in serum were determined by a specific EIA assay as an index of the cyclo-oxygenase activity of platelet PGHS-1. 3. Heparin-treated blood from the same donors was incubated at 37 degrees C for 24 h with the same concentrations of the test compounds in the presence of lipopolysaccharide (LPS, 10 microg ml[-1]). The contribution of PGHS-1 was suppressed by pretreatment of the volunteers with aspirin (500 mg; 48 h before venepuncture). As a measure of LPS induced PGHS-2 activity immunoreactive prostaglandin E2 (irPGE2) plasma concentrations were determined by a specific EIA assay. 4. S-ibuprofen inhibited the activity of PGHS-1 (IC50 2.1 microM) and PGHS-2 (IC50 1.6 microM) equally. R-ibuprofen inhibited PGHS-1 (IC50 34.9) less potently than S-ibuprofen and showed no inhibition of PGHS-2 up to 250 microM. By contrast R-ibuprofenoyl-CoA thioester inhibited PGE2 production from LPS-stimulated monocytes almost two orders of magnitude more potently than the generation of TXB2 (IC50 5.6 vs 219 microM). 5. Western blotting of PGHS-2 after LPS induction of blood monocytes showed a concentration-dependent inhibition of PGHS-2 protein expression by ibuprofenoyl-CoA thioesters. 6. These data confirm that S-ibuprofen represents the active entity in the racemate with respect to cyclo-oxygenase activity. More importantly the data suggest a contribution of the R-enantiomer to therapeutic effects not only by chiral inversion to S-ibuprofen but also via inhibition of induction of PGHS-2 mediated by R-ibuprofenoyl-CoA thioester. 7. The data may explain why racemic ibuprofen is ranked as one of the safest non-steroidal anti-inflammatory drugs (NSAIDs) so far determined in epidemiological studies.

Bioanalysis of p-trifluoromethylmandelic acid and Mosher's acid by chiral gas chromatography and fluorine nuclear magnetic resonance to study chiral inversion: application to rat urine samples

Methods for the nuclear magnetic resonance and gas chromatographic analysis of the enantiomers of p-trifluoromethylmandelic acid (p-TFM) and Mosher's acid (alpha-methoxy-alpha-(trifluoromethyl)phenylacetic acid) present in rat urine samples are described. Gas chromartography was performed using cyclodextrin capillary columns with both compounds analysed following derivatisation with methanolic HCl. Nuclear magnetic resonance was performed directly on the untreated urine samples following addition of beta-cyclodextrin. The methods were suitable for the determination of the individual enantiomers of the analytes in urine. Analysis of the rat urine samples indicated that the p-TFM had undergone a unidirectional enantiomeric interconversion in vivo, while the enantiomers of Mosher's acid were excreted unchanged.

Molecular cloning and expression of a 2-arylpropionyl-coenzyme A epimerase: a key enzyme in the inversion metabolism of ibuprofen

The 2-arylpropionic acid derivatives, including ibuprofen, are the most widely used anti-inflammatory analgesic cyclooxygenase inhibitors. The (-)-R-enantiomer, which is inactive in terms of cyclooxygenase inhibition, is epimerized in vivo via the 2-arylpropionyl-coenzyme A (CoA) epimerase to the cyclooxygenase-inhibiting (+)-S-enantiomer. The molecular biology of the epimerization pathway is largely unknown. To clarify this mechanism, the sequence of the 2-arylpropionyl-CoA epimerase was identified, and the enzyme cloned and expressed. A cDNA clone encoding the 2-arylpropionyl-CoA epimerase was isolated from a rat liver cDNA library. The nucleotide and the deduced amino acid sequence of this enzyme was determined. Significant amino acid sequence similarity was found between the rat epimerase and carnitine dehydratases from Caenorhabditis elegans (41%) and Escherichia coli (27%). A bacterial expression system (E. coli strain M15[pREP4]) was used to express the epimerase protein, representing up to 20-30% of the total cellular E. coli protein. The expression of the epimerase was confirmed with Western blots using specific anti-epimerase antibodies and by measuring the rate of inversion of (R)-ibuprofenoyl-CoA. Northern blot analysis revealed a prominent 1.9-kb mRNA transcript in different rat tissues. In addition to its obvious importance in drug metabolism, the homology of the epimerase with carnitine dehydratases from several species suggests that this protein, which up to now has only been characterized as having a role in drug transformation, has a function in lipid metabolism.

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.

Stereoselective pharmacokinetics of ibuprofen in rats: effect of enantiomer-enantiomer interaction in plasma protein binding

Stereoselective pharmacokinetics of ibuprofen (IB) enantiomers were studied in rats. Unidirectional conversion from R-ibuprofen (R-IB) to S-ibuprofen (S-IB) was observed following intravenous administration. S-IB concentrations in plasma following racemate administration were simulated according to a conventional compartmental model using the parameters obtained after the administration of individual enantiomers, and resulted in overestimation of S-IB concentrations. Binding of IB enantiomers measured in rat plasma was stereoselective, the binding of R-IB being more favorable than that of S-IB. Moreover, there are interactions between IB enantiomers in binding, which may cause the increase of distribution volumes of IB enantiomers in the presence of their antipodes. Hence simulated S-IB concentrations according to a conventional compartment model were significantly greater than those observed. Indeed, when the enantiomer-enantiomer interactions were taken into account, simulation of S-IB concentrations in plasma following racemate administration was in good agreement with observed values. Therefore, interactions between stereoisomers as well as dispositional stereoselectivity have to be considered when pharmacokinetics of stereoisomers after administration of the racemate are compared to those after administration of individual isomers.

Preliminary pharmacokinetic study of ibuprofen enantiomers after administration of a new oral formulation (ibuprofen arginine) to healthy male volunteers

The pharmacokinetics of ibuprofen enantiomers were investigated in a crossover study in which seven healthy male volunteers received single oral doses of 800 mg racemic ibuprofen as a soluble granular formulation (sachet) containing L-arginine (designated trade name: Spedifen), 400 mg (-)R-ibuprofen arginine or 400 mg (+)S-ibuprofen arginine. Plasma levels of both enantiomers were monitored up to 480 minutes after drug intake using an enantioselective analytical method (HPLC with ultraviolet detection) with a quantitation limit of 0.25 mg/l. Substantial inter-subject variability in the evaluated pharmacokinetic parameters was observed in the present study. After (+)S-ibuprofen arginine, the following mean pharmacokinetic parameters +/-SD were calculated for (+)S-ibuprofen: tmax 28.6 +/- 28.4 min; Cmax 36.2 +/- 7.7 mg/l; AUC 86.4 +/- 14.9 mg.h/l; t1/2 105.2 +/- 20.4 min. After (-)R-ibuprofen arginine, the following mean pharmacokinetic parameters were calculated for (+)S-ibuprofen and (-)R-ibuprofen, respectively: tmax 90.0 +/- 17.3 and 50.5 +/- 20.5 min; Cmax 9.7 +/- 3.0 and 35.3 +/- 5.0 mg/l; AUC 47.0 +/- 17.2 and 104.7 +/- 27.7 mg.h/l; t1/2 148.1 +/- 63.6 and 97.7 +/- 23.3 min. After racemic ibuprofen arginine, the following mean pharmacokinetic parameters were calculated for (+)S- and (-)R-ibuprofen, respectively: tmax 30.7 +/- 29.1 and 22.9 +/- 29.8 min; Cmax 29.9 +/- 5.6 and 25.6 +/- 4.4 mg/l; AUC 105.1 +/- 23.0 and 65.3 +/- 15.0 mg.h/l; t1/2 136.6 +/- 20.7 and 128.6 +/- 45.0 min. Tmax values of S(+)- and (-)R-ibuprofen after a single dose of 400 mg of each enantiomer did not differ significantly from the corresponding parameters obtained after a single dose of 800 mg of racemic ibuprofen arginine, indicating that the absorption rate of (-)R- and (+)S-ibuprofen is not different when the two enantiomers are administered alone or as a racemic compound. An average of 49.3 +/- 9.0% of a dose of the (-)R-ibuprofen arginine was bioinverted into its antipode during the study period (480 minutes post-dosing). The percent bioinversion during the first 30 minutes after (-)R-ibuprofen arginine intake averaged 8.1 +/- 3.9%. The mean AUC of (+)S-ibuprofen calculated after 800 mg racemic ibuprofen arginine (105.1 +/- 23.0 mg.h/l) was lower than the mean AUC value obtained by summing the AUCs of (+)S-ibuprofen after administration of 400 mg (+)S-ibuprofen arginine and 400 mg (-)R-ibuprofen arginine (133.4 +/- 26.6 mg.h/l). In conclusion, the administration of Spedifen resulted in very rapid absorption of the (+)S-isomer (eutomer) with tmax values much lower than those observed for this isomer when conventional oral solid formulations such as capsules or tablets of racemic ibuprofen are administered. This characteristic is particularly favourable in those conditions in which a very rapid analgesic effect is required.

Pharmacokinetics and pharmacodynamics of ketoprofen enantiomers in the horse

Pharmacokinetic and pharmacodynamic parameters were established for enantiomers of the non-steroidal anti-inflammatory drug (NSAID) ketoprofen (KTP), each administered separately at a dose level of 1.1 mg/kg to a group of six New Forest geldings, in a three-period cross-over study using a tissue cage model of inflammation. For both S(+)-and R(-)-KTP, penetration into tissue cage fluid (transudate) and inflamed tissue cage fluid (exudate) was rapid, and clearances from exudate and transudate were much slower than from plasma. AUC values were, therefore, higher for exudate and, to a lesser degree, transudate than for plasma. Unidirectional chiral inversion of R(-)-to S(+)-KTP was demonstrated. Administration of both enantiomers produced marked, time-dependent inhibition of synthesis of serum thromboxane B2 and exudate prostaglandin E2, indicating non-selective inhibition of cyclo-oxygenase (COX) isoenzymes COX-1 and COX-2 respectively. Administration of both enantiomers also produced partial inhibition of beta-glucuronidase release into inflammatory exudate and of bradykinin-induced skin oedema. It is suggested that, although S(+)-KTP is generally regarded as the eutomer, R(-)-KTP was probably at least as active in inhibiting bradykinin swelling. Pharmacokinetic/pharmacodynamic (PK/PD) modelling of the data could not be undertaken following R(-)-KTP administration because of chiral inversion to S(+)-KTP, but pharmacodynamic parameters, Emax, EC50, N, keO and t1/2(keO). were determined for s(+)-KTP using the sigmoidal Emax equation. PK/DP modelling provided a novel means of comparing and quantifying several biological effects of KTP and of investigating its mechanisms of action.

Critical issues in chiral drug analysis in biological fluids by high-performance liquid chromatography

This review article focuses on the specificities of chiral liquid chromatography, with particular emphasis on stability, stereoconversion, enantiomeric separation, recovery and drug concentration determinations. In addition, the paper presents an overview of the different steps which have to be followed for a chiral method to be validated. Sensitivity, selectivity, linearity, precision and accuracy all have to be ensured for three chemical entities, the two enantiomers and the racemate. Only accurate and precise concentrations of the parent drug and its metabolites will lead to the reliable description of their in vitro stability and in vivo body disposition.

Effect of the enantiomers of flurbiprofen, ibuprofen, and ketoprofen on intestinal permeability

Numerous studies have demonstrated that the administration of nonsteroidal anti-inflammatory drugs (NSAIDs) increases small intestinal permeability, and this has been suggested to be a prerequisite to enteropathy. It is believed that the inhibitory effect of chiral NSAIDs on the synthesis of prostaglandins and hence their efficacy and toxicity are mainly due to the S enantiomer. Using the urinary excretion of [51Cr]-EDTA, we have investigated the effects of three nonsteroidal anti-inflammatory drugs (flurbiprofen, ibuprofen, and ketoprofen) on small intestinal permeability in rats. Single doses of each NSAID were administered orally as either the racemate or the R or S enantiomer, the enantiomer dose being half that of the racemate. Each treatment caused a significant increase in intestinal permeability above that seen in untreated animals. The R enantiomers of all three NSAIDs increased small intestinal permeability significantly above base line, which was expected for (R)-ketoprofen and (R)-ibuprofen due to substantial chiral R to S inversion. The intestinal permeability for (R)-flurbiprofen, although minimal and likely due to 10% inversion, may also suggest prostaglandin-independent involvement. Furthermore, (S)-flurbiprofen, used at one-half the dose of the racemate, increased permeability to a similar magnitude as the racemate. This observation was similar to that previously reported for etodolac. A stereochemically pure enantiomer does not necessarily offer a safer alternative than its racemic form.

Isolation and characterization of rat liver microsomal R-ibuprofenoyl-CoA synthetase

Microsomal long-chain acyl-CoA synthetase (EC 6.1.2.3.) has been suggested to be involved in the stereoselective formation of the CoA thioester of ibuprofen. In this study, we demonstrated that the microsomal enzyme from rat liver responsible for palmitoyl-CoA synthesis also catalyzes the formation of R-ibuprofenoyl-CoA in a Mg(2+)- and ATP-dependent process. Long-chain acyl-CoA synthetase from rat liver microsomes was purified to homogeneity as evidenced by SDS-gel electrophoresis. Simultaneous measurements of palmitoyl-CoA and R-ibuprofenoyl-CoA formation with HPLC in various fractions and purification steps during protein isolation revealed a high correlation between both activities. The purification procedure included solubilization of the microsomes obtained from rat livers with Triton X-100 and subsequent chromatography of the 100,000 x g supernatant on blue-sepharose, hydroxyapatite, and phosphocellulose. The purified enzyme exhibited an apparent molecular weight of 72 kDa as estimated by SDS gel electrophoresis, with specific activities of 71 nmol.min-1.mg-1 protein and 901 nmol.min-1.mg-1 protein for formation of R-ibuprofenoyl-CoA and palmitoyl-CoA, respectively. Palmitoyl-CoA formation catalyzed by the purified enzyme exhibited biphasic kinetics indicative of two isoforms, a high-affinity (KM 0.13 +/- 0.11 microM), low-capacity form and a low-affinity (KM 81 +/- 11.5 microM), high-capacity form. In contrast, measurement of R-ibuprofenoyl-CoA synthesis over a concentration range from 5 to 3000 microM showed the participation of a single CoA ligase with a KM of 184 +/- 19 microM, corresponding to the low-affinity isoform of palmitoyl-CoA synthesis with a marked enantioselectivity towards the R-form of ibuprofen. R-ibuprofenoyl-CoA formation of the enzyme preparation was inhibited by palmitic acid (KI 13.5 +/- 0.5 microM) and S-ibuprofen (KI 405 +/- 10 microM). In summary, these data give strong evidence for the identity of R-ibuprofenoyl-CoA and long-chain acyl-CoA synthetase.

Enantioselective assays in comparative bioavailability studies of racemic drug formulations: nice to know or need to know?

The importance of enantiospecific assays in studying pharmacokinetic/pharmacodynamic (PK/PD) and drug-drug interactions of racemic drugs is widely recognized. Use of such assays in comparative bioavailability studies, however, remains controversial. This commentary proposes a PK/PD-based rationale for deciding whether an enantioselective assay is important in such studies. Racemic drugs are divided into three major categories: those with negligible or nonenantioselective first-pass metabolism (category I), those where the first-pass metabolism of the less-active enantiomer is predominant (category II), and those where the first-pass metabolism of the more active and/or toxic enantiomer is predominant (category III). In addressing the need for assay selectivity, a simple analogy is made between these drug categories and the protein-binding phenomenon. Enantioselective assays are not essential for category I drugs, or for category II drugs in the majority of cases. A special consideration, however, is needed for those category II drugs that undergo racemic inversion that may be influenced by the dose level and/or the residence time of the drug formulation in the gastrointestinal tract. It is with category III drugs that enantioselective assays become important, especially when metabolism, distribution, and/or elimination processes of the active or toxic enantiomer are saturable, leading to variable enantiomeric ratios in the plasma. Factors contributing to these ratio changes include routes of administration, dose level, and input rate differences. In put rate differences are particularly relevant to bioavailability evaluation of category III drugs.

Moment analysis of stereoselective enterohepatic circulation and unidirectional chiral inversion of ketoprofen enantiomers in rat

The stereoselective enterohepatic circulation (EHC) and the synchronous chiral inversion of ketoprofen enantiomer in rat were evaluated by moment analysis based on the recirculatory concept. (R)-(-)- and (S)-(+)-ketoprofen were independently administered into rats, and the plasma and bile concentrations of both enantiomers were determined by a column-switching HPLC. (S)-Ketoprofen was generated by the chiral inversion from (R)-ketoprofen, whereas (R)-ketoprofen was not generated from (S)-ketoprofen. Within 30 min after intravenous administrations, the plasma time courses of R- and S-enantiomers were almost the same between rats with laparotomy and those with bile-duct cannula. After 30 min, the plasma concentrations in rats with laparotomy were significantly higher than those in rats with bile-duct cannula. The Laplace-transformed equations for stereoselective EHC and the synchronous chiral inversion were derived by means of the transfer function method on the basis of the recirculatory theory. The global moments (AUC and MRT) which were derived directly from the transformed equations were related to the local moments for the single EHC. The recirculation ratios of (R)- and (S)-ketoprofen for the single EHC were estimated to be 15.4% and 63.6%, respectively. The absorption ratios of (R)- and (S)-ketoprofen for the absorption process from the gastrointestinal tract into the systemic circulation were 87.0% and 83.8%, respectively. The biliary excretion rations of (R)- and (S)-ketoprofen for the disposition process through the systemic circulation into the bile were 17.7% and 75.8%, respectively. The chiral inversion ratio from (R)-ketoprofen into (S)-ketoprofen was 59.5%. The complicated disposition of ketoprofen, i.e., the simultaneous EHC and chiral inversion, was able to be analyzed by a moment method in a simple way.

Enantioselective disposition of albuterol in humans

The study of enantioselective disposition of chiral drugs is important to provide a rationale of plasma concentration-effect relationships, which are often misleading when based on total drug concentration. It is also important when considering new dosage routes or formulations in order to optimize therapeutic plasma concentrations of the active enantiomer. Improvements in the sensitivity and selectivity of biological assays coupled with the developments in chiral analysis have made it possible to study the enantioselective disposition of drugs. Although valuable pharmacokinetic data were obtained for the beta 2-agonists by nonenantioselective methodology, more recent chiral studies have revealed the existence of extensive enantioselectivity in the disposition of these agents. The most significant features of the enantioselective disposition of albuterol are the relatively rapid plasma clearance and low bioavailability of the eutomer. Although this in itself does not necessarily justify the development of a single enantiomer formulation, the implications of the high levels of distomer after i.v. and oral dosing await clarification. Similarly, more work is required to elucidate the consequences of the major difference in disposition between albuterol and terbutaline in humans through both in vivo and in vitro studies of the mechanisms giving rise to this phenomenon. The enantioselective disposition of the other clinically used beta 2-agonists, such as fenoterol, formoterol, and salmeterol also needs to be characterized. The metabolism of the majority of beta 2-agonists is generally by conjugation to give one major metabolite. The situation is therefore uncomplicated by multiple metabolic pathways, which may differ in the extent and direction of their enantioselectivity. Many beta 2-agonists are excreted largely unchanged in the urine making studies of urinary excretion accessible without the requirement for very sensitive assays. The realization that the enantiomers of beta 2-agonists previously thought of as "inactive" may be associated with toxic effects is a further compelling reason to study the enantioselective pharmacokinetics of this class of drugs. In addition, the role of enantiomers in producing side effects, such as tremor and reduction in renal function, needs to be reassessed. The beta 2-agonists can be looked on as textbook examples of the inherent danger of ignoring chirality in the study of pharmacokinetics and pharmacodynamics. The growing body of information on the enantioselective disposition of beta 2-agonists in humans will enhance the rational use of these drugs in the future management of patients.

Evaluation of carprofen in calves using a tissue cage model of inflammation

The arylpropionate anti-inflammatory drug, carprofen, was administered intravenously as the racemate at a dose rate of 0.7 mg kg-1 body weight to six Friesian bull calves aged 16-17 weeks. Anti-inflammatory and pharmacokinetic properties were investigated using a tissue cage model of inflammation based on intracaveal injection of the mild irritant, carrageenin. Carprofen displayed enantioselective pharmacokinetics, with the R(-) enantiomer predominating in plasma at all measuring times. Elimination half-life and mean residence time were shorter and volume of distribution and clearance were greater for the S(+) than for the R(-) enantiomer. Penetration of both enantiomers into transudate (non-stimulated tissue cage) was poor but penetration into exudate (carrageenin-stimulated tissue cage) was good. Carprofen failed to reduce exudate concentration of prostaglandin E2 and the reductions in 12-hydroxyeicosatetraenoic acid were non-significant at most sampling times. The long elimination half-life of both R(-) and S(+) carprofen enantiomers and their ready penetration into and slow clearance from inflammatory exudate indicate that the drug is likely to have a long duration of action in calves. The mechanism of action is unknown but it is unlikely to involve inhibition of either cyclooxygenase or 12-lipoxygenase.

Pharmacodynamics and chiral pharmacokinetics of carprofen in calves

The non-steroidal anti-inflammatory drug, carprofen, was administered intravenously as the racemate at a dose rate of 0.7 mg kg-1 to six Friesian bull calves aged 8-10 weeks. Anti-inflammatory properties were indicated by attenuation of temperature rise at sites of intradermal injection of the irritants, carrageenin and dextran, but responses were not statistically significant at most recording times. Carrageenin- and dextran-induced swelling were not significantly reduced by carprofen. Carprofen reduced ex vivo serum thromboxane B2 synthesis but this effect was also not significant at most sampling times. Enantioselective pharmacokinetics of carprofen was demonstrated, plasma concentrations of the R(-) enantiomer predominating at all sampling times. It is concluded that inhibition of cyclo-oxygenase is unlikely to be the sole mechanism of action of carprofen in calves.

Pharmacodynamics and enantioselective pharmacokinetics of carprofen in the cat

The pharmacodynamics and enantioselective pharmacokinetics of the arylpropionic acid non-steroidal anti-inflammatory drug, carprofen, were investigated in cats after administration of the racemic mixture (rac-carprofen) at dose rates ranging from 0.7 to 4.0 mg kg-1 intravenously and subcutaneously. A low dose of rac-carprofen (0.7 mg kg-1) partially inhibited the rise in skin temperature at a site of acute inflammation but had no effect on the ex vivo synthesis of serum thromboxane (Tx) B2. A higher dose (4.0 mg kg-1) inhibited oedematous swelling, although the response was statistically significant at only one time, and also reduced the ex vivo synthesis of serum TxB2 for 12 hours after intravenous injection or 24 hours after subcutaneous injection. The main features of carprofen pharmacokinetics were a low distribution volume, a relatively long elimination half-life, the predominance of the R(-) enantiomer and a bioavailability (after subcutaneous dosing) of 100 per cent and 92 per cent, respectively, after doses of 0.7 and 4.0 mg kg-1. On the basis of these data, it is suggested that a dose of 4.0 mg kg-1 by both intravenous and subcutaneous routes should be evaluated in clinical subjects.

Stereospecific determination of tiaprofenic acid in plasma: problems with drug degradation

A sensitive stereospecific high-performance liquid chromatographic assay for the quantification of tiaprofenic acid in human plasma was developed. The procedure involved extraction of tiaprofenic acid from acidified plasma into hexanediethyl ether (8:2, v/v). Stereospecific separation was achieved with a prepacked alpha1-acid glycoprotein column without derivatization. The mobile phase consisted of 2% 2-propanol in 0.01 M phosphate buffer, pH 6.5. Tiaprofenic acid was detected at 317 nm. The limit of quantification was found to be 25 ng/ml for each enantiomer using a 0.5 ml plasma sample. The assay was reproducible and accurate to be applied to the stereoselective pharmacokinetic analysis of tiaprofenic acid in plasma. Because of photoinstability of tiaprofenic acid plasma sampling and sample extraction should be performed under light protection.

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.

Preclinical and clinical development of dexketoprofen

Dexketoprofen trometamol is a water-soluble salt of the dextrorotatory enantiomer of the nonsteroidal anti-inflammatory drug (NSAID) ketoprofen. Racemic ketoprofen is used as an analgesic and an anti-inflammatory agent, and is one of the most potent in vitro inhibitors of prostaglandin synthesis. This effect is due to the S(+)-enantiomer (dexketoprofen), while the R(-)-enantiomer is devoid of such activity. The pharmacokinetic profile of ketoprofen and its enantiomers was assessed in several animals species and in human volunteers. In humans, the relative bioavailability of oral dexketoprofen trometamol (12.5 and 25 mg, respectively) is similar to that of oral racemic ketoprofen (25 and 50 mg, respectively), as measured in all cases by the area under the concentration-time curve values for S(+)-ketoprofen. Dexketoprofen trometamol, given as a tablet, is rapidly absorbed, with a time to maximum plasma concentration (tmax) of between 0.25 and 0.75 hours, whereas the tmax for the S-enantiomer after the racemic drug, administered as tablets or capsules prepared with the free acid, is between 0.5 and 3 hours. Peak plasma concentrations of 1.4 and 3.1 mg/L are reached after administration of dexketoprofen trometamol 12.5 and 25 mg, respectively. From 70 to 80% of the administered dose is recovered in the urine during the first 12 hours, mainly as the acyl-glucuronoconjugated parent drug. No R(-)-ketoprofen is found in the urine after administration of dexketoprofen [S(+)-ketoprofen], confirming the absence of bioinversion of the S(+)-enantiomer in humans. in animal studies, the anti-inflammatory potency of dexketoprofen was always equivalent to that demonstrated by twice the dose of ketoprofen. Similarly, animal studies showed a high analgesic potency for dexketoprofen trometamol. The R(-)-enantiomer demonstrated a much lower potency, its analgesic action being apparent only in conditions where the metabolic bioinversion to the S(+)-enantiomer was significant. The gastric ulcerogenic effect of dexketoprofen at various oral doses (1.5 to 6 mg/kg) in the rat do not differ from those of the corresponding double doses (3 to 12 mg/kg) of racemic ketoprofen. Repeated (5-day) oral administration of dexketoprofen as the trometamol salt causes less gastric ulceration than was observed after the acid form of both dexketoprofen and the racemate. In addition, single dose dexketoprofen as the free acid at 10 to 20 mg/kg does not show a significant intestinal ulcerogenic effect in rats, while racemic ketoprofen 20 or 40 mg/kg is clearly ulcerogenic to the small intestine. The analgesic efficacy of oral dexketoprofen trometamol 10 to 20 mg is superior to that of placebo and similar to that of ibuprofen 400 mg in patients with moderate to serve pain after third molar extraction. The time to onset of pain relief appeared to be shorter in patients treated with dexketoprofen trometamol than in those treated with ibuprofen 400 mg. Dexketoprofen trometamol was well tolerated, with a reported incidence of adverse events similar to that of placebo.

Disposition of CS-670, a novel nonsteroidal anit-inflammatory drug, and its metabolites in healthy human volunteers

CS-670, a novel nonsteroidal anti-inflammatory drug, is a racemic prodrug. Plasma concentrations and urinary excretion of CS-670 and its metabolites were determined in experimental subjects after oral administration at a single 120 mg dose. CS-670 and four metabolites, the saturated ketone (M-A), unsaturated-alcohol (M-B), cis-alcohol (M-C), and trans-alcohol (M-D), were quantitated by GC-MS. The major metabolites in human plasma were M-B, M-C, and M-D and their terminal half-lives (t1/2) were 0.9, 2.6, and 1.2 h, respectively. The total recovery in the urine was 26% of the dose, but unchanged CS-670 accounted for less than 2% over a 48 h period. In addition, the absolute configurations of the metabolites were examined by HPLC after derivatization with chiral reagents. It was found that the configuration of the propionic acid moiety of the metabolites, M-B, M-C, and M-D, in human plasma, was rapidly inverted from (-)-(R) to the (+)-(S) configuration in stereoselective biotransformation. Furthermore, the configurations of the 1'- and 2'-carbons of M-C and M-D, were found to be (1'R, 2'S) and (1'R, 2'S), respectively. These results show that CS-670 is readily biotransformed by chiral inversion of the 2-arylpropionic acid moiety and stereoselective reduction of the alpha, beta-unsaturated ketone moiety in humans.

Production of R-(-)-Ketoprofen from an Amide Compound by Comamonas acidovorans KPO-2771-4

R-(-)-2-(3(prm1)-Benzoylphenyl)propionic acid [R-(-)-ketoprofen] was produced from racemic 2-(3(prm1)-benzoylphenyl)propionamide (keto-amide) by the isolated bacterial strain Comamonas acidovorans KPO-2771-4. Sodium fumarate as the carbon source and 2-azacyclononanone or isobutyronitrile as the enhancer in the culture medium were effective for bacterial growth and the enhancement of R-(-)-ketoprofen-producing activity. R-(-)-Ketoprofen produced from the keto-amide by resting cells was present in 99% enantiomeric exess. C. acidovorans KPO-2771-4 has an R-enantioselective amidase for keto-amide because the purified amidase from the bacterium hydrolyzed keto-amide, producing optically pure R-ketoprofen and ammonia.

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.

Pharmacokinetics and pharmacodynamics of ketoprofen in calves applying PK/PD modelling

The pharmacokinetics (PK) and pharmacodynamics (PD) of ketoprofen (KTP) were studied in calves following intravenous administration of the drug racemate at a dose rate of 3 mg/kg. To evaluate the anti-inflammatory properties of KTP, a model of acute inflammation, consisting of surgically implanted subcutaneous tissue cages stimulated by intracaveal injection of carrageenan, was used. No differences were observed between disposition curves of KTP enantiomers in plasma, exudate or transudate. This indicates that in calves KTP pharmacokinetics is not enantioselective. S(+)- and R(-)- KTP each had a short elimination half-life (t1/2 beta) of 0.42 +/- 0.08 h and 0.42 +/- 0.09 h, respectively. The volume of distribution (Vd) was low, values of 0.20 +/- 0.06 L/kg and 0.22 +/- 0.06 L/kg being obtained for R(-) and S(+)KTP, respectively. Body clearance (ClB) was high, correlating with the short elimination half-life, 0.33 +/- 0.03 L/kg/h [R(-)KTP] and 0.32 +/- 0.04 L/kg/h [S(+)-KTP]. KTP pharmacodynamics was evaluated by determining the effects on serum thromboxane (TxB2), exudate prostaglandin (PGE2), leukotriene (LTB4) and beta-glucuronidase (beta-glu) and bradykinin (BK)-induced oedematous swelling. Effect-concentration inter-relationships were analysed by PK/PD modelling. KTP did not affect exudate LTB4, but inhibition of the other variables was statistically significant. The mean EC50 values for inhibition of serum TxB2, exudate PGE2 and beta-glu and BK-induced swelling were 0.118, 0.086, 0.06 and 0.00029 microgram/mL, respectively. These data indicate that KTP exerted an inhibitory action, not only as expected, on eicosanoid (TxB2 and PGE2) synthesis but also on exudate beta-glu and BK-induced oedema. The EC50 values for these actions indicate that they are likely to contribute to the overall anti-inflammatory effects of KTP in calves. However, claims that KTP inhibits 5-lipoxygenase and thereby blocks the production of inflammatory mediators such as LTB4 were not substantiated. PK/PD modelling has proved to be a useful tool for analysing the in vivo pharmacodynamics of KTP and for providing new approaches to elucidating its mechanism(s) of action.

Permeability of pure enantiomers of ketorolac through human cadaver skin

The permeability of pure enantiomers of ketorolac acid, a potent non-narcotic analgesic, through human cadaver skin was evaluated. The melting temperature of each enantiomer was 20 degrees C higher than that of the racemic compound. As expected, the solubility of the racemic compound in water and isopropyl alcohol/water/isopropyl myristate (IPA/water/IPM, 50:50:1.5) was roughly 2 times higher than that of the enantiomers. The permeability of the enantiomers through poly(ethylenevinyl acetate) (EVA) synthetic membrane and human cadaver skin was determined with a side-by-side diffusion cell. The skin flux of the racemic compound was about 1.5 times higher than those of the enantiomers. On the other hand, no significant differences in the intrinsic permeability coefficient of the racemic compound and the enantiomers in the EVA membrane and human cadaver skin was observed. An excellent agreement between the predicted and experimental flux ratio of the racemic compound and enantiomer in the EVA membrane and cadaver skin was observed. The IPA/water/IPM (50:50:1.5) provided the highest in vitro skin flux of the S enantiomer among the three vehicle formulations studied. The skin flux of the active pure S enantiomer was ca. 34% higher than that of the impure S enantiomer in the racemic mixture. Furthermore, about 14% intersubject variability in the in vitro skin flux of the S enantiomer was observed. The required skin flux of the S enantiomer as calculated from the pharmacokinetic parameters was about 32 micrograms/cm2/h from a 25 cm2 transdermal patch, which was readily achievable from the IPA/water/IPM (50:50:1.5) ternary vehicle system.

Stereoselective disposition of ibuprofen in patients with compromised renal haemodynamics

1. The primary aim of this study was to assess the impact of renal haemodynamics on the pharmacokinetic behaviour of ibuprofen enantiomers. Thirty-two patients and ten age-matched healthy volunteers participated in this study. These patients had at least one of the following risk factors for cardiovascular disorders: hypertension, diabetes mellitus, hyperlipidaemia and hyperuricaemia with or without consequent complications such as coronary artery disease, congestive heart failure, cerebral vascular disease, and chronic renal failure. Renal function in these patients was thus characterized by unstable renal haemodynamics that might render them susceptible to ibuprofen-incurred renal damage. 2. Each subject received a single oral dose of 800 mg of racemic ibuprofen. The pharmacokinetic parameters of (S)- and (R)-ibuprofen, t 1/2(S), t 1/2(R), AUC(S), AUC(R), V/F(R), and CL/F(R), for each individual were determined from respective plasma concentration-time curves. To assess the effect of individual clinical conditions on the disposition of ibuprofen enantiomers, the arithmetic means of these pharmacokinetic parameters for each disease group were compared with those of the healthy volunteers by a t-test. 3. All of these disease groups showed elevated AUC(S) and higher (S)/(R) AUC ratios. These disease states along with gender and age were analyzed by multiple linear regression to discern significant factors for elevating AUC(S). Of these, advanced age (P = 0.02) and hypertension (P = 0.03) were identified as independent factors contributing to AUC(S) increase in this population. Thus, patients with these two clinical conditions are at particular risk from the adverse renal effect of ibuprofen.

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.

Pharmacokinetics of ketoprofen in healthy horses and horses with acute synovitis

The pharmacokinetic properties of a single intravenous dose of ketoprofen (2.2 mg/kg) in plasma and synovial fluid were compared in four healthy animals and four horses with experimentally induced acute synovitis. Synovitis was induced by the injection of a 1% solution of sterile carrageenan into the left intercarpal joint. Ketoprofen was administered at the same time as carrageenan infection. The plasma disposition followed a biexponential equation or a two-compartment model in most horses. The plasma harmonic mean half-life in healthy horses (0.88 h) was longer than in horses with synovitis (0.55 h). Synovial fluid concentrations of ketoprofen in healthy horses approximated those in plasma by 3 h post-dose. In horses with synovitis, synovial fluid concentrations approximated plasma concentrations by 1 h. Synovial fluid concentrations of ketoprofen in horses with synovitis were 6.5 times higher than those in healthy horses at 1 h. The area under the synovial fluid concentration curve for horses with synovitis was greater than in healthy horses. These data suggest that the inflamed joint serves as a site of sequestration for ketoprofen. Furthermore, these results indicate that plasma pharmacokinetics may be altered by inflammation in a peripheral compartments such as the joint.

(-)-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.

Role of cytochrome P450 2C9 and an allelic variant in the 4'-hydroxylation of (R)- and (S)-flurbiprofen

Flurbiprofen is a chiral non-steroidal anti-inflammatory drug used in the treatment of pain or inflammation. The primary routes of biotransformation for (R)- and (S)-flurbiprofen are oxidation (presumably cytochrome P450) and conjugation. To date, the specific cytochrome P450 (P450) involved in the oxidative metabolism of this compound (specifically 4'-hydroxylation) has not been elucidated. Experiments were conducted to characterize the kinetic parameters (Km and Vmax) for the 4'-hydroxylation of (R)- and (S)-flurbiprofen in human liver microsomes, to determine if enantiomeric interactions occur when both enantiomers are present, and to identify the specific P450 form(s) involved in this reaction. In human liver microsomes, the Km and Vmax (mean +/- SD) for (R)-4'-hydroxy-flurbiprofen formation were 3.1 +/- 0.8 microM and 305 +/- 168 pmol.min-1.mg protein)-1, respectively. In comparison, the Km and Vmax (mean +/- SD) for (S)-4'-hydroxy-flurbiprofen formation were 1.9 +/- 0.4 microM and 343 +/- 196 pmol.min-1.mg protein-1, respectively. Enantiomeric interaction studies revealed a decrease in Km and Vmax for both enantiomers and an apparent loss of stereoselectivity. Racemic-warfarin, tolbutamide, alpha-naphthoflavone and erythromycin were studied as potential inhibitors of this process. The estimated Ki values for the inhibition of (R)- and (S)-4'-hydroxy-flurbiprofen formation by racemic-warfarin were 2.2 and 4.7 microM. This reaction was also inhibited by tolbutamide. In contrast, erythromycin and alpha-naphthoflavone had no appreciable effect on 4'-hydroxy-flurbiprofen formation. cDNA-expression of individual forms was used to determine which P450 was involved in 4'-hydroxy-flurbiprofen formation. P450 2C9 and an allelic variant (R144C) readily catalyzed the formation of 4'-hydroxy-flurbiprofen. P450 1A2 was also active albeit with a turnover rate 1/140th that of P450 2C9R144C (P450s 2C8, 2E1 and 3A4 were not active toward either enantiomer). The results of these studies indicate that the enantiomers of flurbiprofen may exhibit stereoselectivity with respect to enzyme affinity but have roughly equal maximum formation velocities. Additionally, these two enantiomers may compete for the enzyme resulting in lower maximum velocities for both enantiomers. Finally, of those P450 forms examined, only P450 2C9 and an allelic variant catalyzed the 4'-hydroxylation of both (R)- and (S)-flurbiprofen.

Stereochemical determinants of the nature and consequences of drug metabolism

Enantiomeric discrimination in drug disposition depends on the mechanism of the process under consideration. Absorption, distribution and excretion are generally passive processes which do not differentiate between enantiomers, but enzymic metabolism and protein binding, to plasma or tissue proteins, can show a high degree of stereoselectivity. In terms of metabolism, chiral discrimination occurs at both substrate and product levels, giving rise to five distinct stereochemical courses for drug metabolism, namely (i) prochiral-->chiral, (ii) chiral-->chiral, (iii) chiral-->diastereoisomer, (iv) chiral-->non-chiral and (v) chiral inversion. As a result, the metabolic and pharmacokinetic profiles of enantiomers after administration of racemic drugs can be very variable, so that the exposure to the two enantiomers may be very different. There now an enormous number of examples of each of these possibilities. The net result of the interaction of the stereoselectivities of these various processes can obscure the fact that one (or more) shows a marked stereoselectivity. This is particularly the case for metabolism: while the ratios of the total plasma clearance of the enantiomers of a wide range of drugs never exceed 2, individual metabolic pathways often show much greater stereoselectivity. This is particularly evident for those high-affinity, low-capacity enzyme systems which exhibit genetic polymorphism, namely the human cytochromes P450 2C18 and 2D6. This review provides an introduction to the stereoselectivity of drug metabolism.

Pharmacokinetics and pharmacodynamics of ketoprofen enantiomers in calves

The pharmacokinetics (PK) and pharmacodynamics (PD) of (S)- and (R)-ketoprofen (KTP) enantiomers were studied in calves after intravenous administration of each enantiomer at a dose of 1.5 mg/kg. Pharmacodynamic properties were evaluated using a model of acute inflammation, comprising subcutaneously implanted tissue cages stimulated by intracaveal injection of carrageenan. Chiral inversion of (R)-KTP to the (S)-antipode occurred. The R:S ratio in plasma was 33:1 5 min after administration, decreasing to 1:1 at 8 h. The calculated extent of inversion was 31 +/- 7%. The R:S ratio in inflammatory exudate was of the order 3:1 at all the sampling times and the ratio in transudate was approximately 2:1 for 6 h, declining to 1:1 at 30 h. Only (S)-KTP was detected in biological fluids after administration of this enantiomer. Elimination half-life was longer for the (S) (2.19 h) than the (R)-enantiomer (1.30 h) and volume of distribution was also somewhat higher for the (S)-enantiomer. Body clearance values were 0.119 l/kg/h for (S)-KTP and 0.151 l/kg/h for the (R)-antipode. For (R)-KTP effects obtained were considered as a hybrid, since they potentially reflect the actions of both enantiomers. Concentrations of LTB4 and the cytokines interleukin-1, interleukin-6, and tumor necrosis factor alpha, in exudate were not significantly affected by either (R)- or (S)-KTP treatments. Inhibition of ex vivo thromboxane B2 (TxB2) synthesis, exudate prostaglandin E2 (PGE2) synthesis, beta-glucuronidase release (beta-glu), and bradykinin-induced skin swelling was significant in both treated groups. PK/PD modelling was applied to the (S)-KTP treatment only. EC50 values for inhibition of serum TxB2, exudate PGE2 and beta-glu and BK-induced swelling were 0.047, 0.042, 0.101, and 0.038 microgram/ml, respectively. It is concluded that the low EC50 values for inhibition of TxB2 and PGE2 by (S)-KTP are likely to explain the effects produced by (R)-KTP administration, since concentrations of (S)-KTP in exudate of these calves following chiral inversion were at least 5 times higher than the EC50 at all sampling times. The data for beta-glu and bradykinin-induced swelling inhibition indicate possible inhibitory actions of (R)-KTP as well as (S)-KTP.