Gas chromatographic separation of optically active anti-inflammatory 2-arylpropionic acids using (+)- or (-)-amphetamine as derivatizing reagent

Enantiomers separation by capillary electrochromatography using polysaccharide-based stationary phases

The separation of chiral compounds is an interesting and important topic of research because these compounds are involved in some biological processes, fundamentally in human health. Among the various application fields where enantiomers are remarkable, drug analysis has to be considered. Most of the drugs contain enantiomers and very often one of the two isomers could be pharmacologically more active or even dangerous. Therefore, the separation of these compounds is very important. Among the different analytical techniques usually employed, capillary electrochromatography has demonstrated great capability in enantiomers resolution. The great potential of this electromigration technique stands mainly in its high efficiency due to the use of an electrosmotic flow (flat flow profile) and on the high selectivity because of the use of a stationary phase. Chiral separation can be obtained utilizing several chiral stationary phases including a polysaccharide derivative. The aim of this review paper is to summarize the main features of capillary electrochromatography and polysaccharide derivatives of chiral stationary phase. It also report examples of practical applications utilizing this approach.

Enantiomeric fraction as an indicator of pharmaceutical biotransformation during wastewater treatment and in the environment--a review

Enantioselective analysis of some pharmaceuticals during wastewater treatment has the potential to reveal significant insights regarding the effectiveness of biotransformation processes. Furthermore, enantioselective analysis of chiral pharmaceuticals in the aquatic environment may provide a useful historical record revealing the dominant source of (treated or untreated) wastewater contamination. This review of the recent scientific literature has identified only a handful of studies that have directly investigated these promising applications. However, a range of enantioselective analytical techniques are likely to be adaptable from those which have been developed within the pharmaceutical industry. These include direct enantioseparations of enantiomers on chiral stationary phases as well as indirect separations by achiral stationary phases after chiral derivatization to form pairs of physically distinguishable diastereomers. Further investigations of the patterns of enantiomeric fractionation of pharmaceuticals in wastewater and environmental samples will provide an increasingly solid understanding of the relationship between biotransformation processes and the often overlooked parameter of enantiomeric fraction.

Optimization of capillary electrophoretic enantioseparation for basic drugs with native β-CD as a chiral selector

This study presents the advantages of the 20 microm inner diameter (id) capillary for the enantioseparation of ten basic drugs with native beta-CD as the chiral selector. The apparent binding constants of each enantiomeric pair were determined to calculate the optimum beta-CD concentration ([beta-CD]opt) and the optimization was subsequently carried out. Comparison of the 20 microm id with 50 microm id were made in terms of the results obtained in the optimization and detection limits. Applying the optimum conditions for each compound, reproducible results (RSD from 0-3; n>5) were obtained for the 20 microm id capillary. Although the sensitivity is lower in the 20 microm id capillary, the LOD determined using this capillary is still found to be acceptable for the ten basic drugs studied. Enhanced resolution and faster analysis times were the main advantages observed with the use of this capillary in enantioseparation.

Chiral discrimination of multiple profens as diastereomeric (R)-(+)-1-phenylethylamides by achiral dual-column gas chromatography

Profens were converted into diastereomeric (R)-(+)-1-phenylethylamides using ethyl chloroformate and triethylamine in dichloromethane. Gas chromatographic analysis on dual-columns with different polarities provided complete enantioresolution of eight profens, facilitating chiral discrimination based on matching with retention index sets characteristic of each enantiomer. The present method was linear (r >/= 0.9992) with good precision (0.8-6.0%) and accuracy (-9.3 to 0.003%), allowing detection of trace (R)-profens in optical purity test on four (S)-profen mixture in a single run. And the method allowed simultaneous enantiomeric screening for ibuprofen enantiomers and their chiral metabolites excreted in urine following administration of racemic ibuprofen.

Use of vancomycin silica stationary phase in packed capillary electrochromatography I. Enantiomer separation of basic compounds

Chiral separation of basic compounds was achieved by using 75 or 100 microm ID fused-silica capillaries packed with a vanoomycin-modified diol silica stationary phase. The capillary was firstly packed for about 12 cm with a slurry mixture composed of diolsilica (3:1) then with the vancomycin modified diol-silica (3:1) (23 cm), and finally with diol-silica (3:1) for about 2 cm. Frits were prepared by a heating wire at the two ends of the capillary; the detector window was prepared at 8.5 cm from the end of the capillary where vancomycin was not present. The influence of the mobile phase composition (pH and concentration, organic modifier type and concentration) on the velocity of the electroosmotic flow, chiral resolution and enantioselectivity was studied. Good enantiomeric resolution was achieved for atenolol, oxprenolol, propranolol, and venlafaxine using a mobile phase composition of 100 mM ammonium acetate solution (pH 6)/water/acetonitrile (5:5:90 v/v/v) while for terbutaline a mixture of 5:15:80 v/v/v provided the best separations. The use of methanol instead of acetonitrile caused a general increase of enantiomer resolution of the studied compounds together with a reduction of efficiency and detector response. However, the combination of acetonitrile and methanol in the mobile phase (as, e.g., 10% methanol and 80% acetonitrile) allowed to improve the enantiomer resolution with satisfactory detector response.

Derivatization procedures for gas chromatographic-mass spectrometric determination of xenobiotics in biological samples, with special attention to drugs of abuse and doping agents

The development of low cost MS detectors in recent years has promoted an important increase in the applicability of GC-MS system to analyze for the presence of foreign substances in the human body. Drugs and toxic agents are in vivo metabolized in such a way that more polar compounds are usually formed. Derivatization of these metabolites is often an unavoidable requirement for gas chromatographic analysis. Application of derivatization methods in recent years has been relevant, especially for silylation, acylation, alkylation and the formation of cyclic or diastereomeric derivatives. Given the relevance of drug of abuse testing in modern toxicology, main derivatization procedures for opiates, cocaine, cannabis, amphetamines, benzodiazepines and LSD have been reviewed. Papers describing the analyses of drugs of abuse in matrixes other than blood, such as hair or sweat, have received special attention. Advances in derivatization for sports drug testing have been particularly relevant for anabolic steroids, diuretics and corticosteroids. Among the several methodologies applied, the formation of trimethylsilyl, perfluoroacyl or methylated derivatives have proved to be both versatile and extensively used. Further advances in derivatization for GC-MS applications in clinical and forensic toxicology will depend on the one hand on the degree of further use of GC-MS for routine applications and, on the other hand, on the alternative progress made for developments in LC-MS or CE-MS. Last but not least, the appearance of comprehensive libraries in which reference spectra for different derivatives of many drugs and their metabolites are collected will have an important impact on the expansion of derivatization in GC-MS for toxicological applications.

Chiral resolution of flobufen by high performance liquid chromatography and capillary electrophoresis

The enantiomeric separation of an anti-inflammatory drug flobufen was performed on various chiral columns in liquid chromatography and by reversed phase chromatography and by capillary electrophoresis with beta-cyclodextrin as a chiral selector in the mobile phase and background electrolyte, respectively. The elution order of individual enantiomers is discussed with respect to the absolute chirality of (+/-)-flobufen determined by X-ray crystallography.

Methods of analysis of chiral non-steroidal anti-inflammatory drugs

Although the analysis of the enantiomers of chiral non-steroidal anti-inflammatory drugs (NSAIDs) has been carried out for over 20 years, there often remains a deficit within the pharmaceutical and medical sciences to address this issue. Hence, despite world-wide therapeutic use of chiral NSAIDs the importance of stereoselectivity in pharmacokinetic, pharmacodynamic and pharmacological activity and disposition has often been ignored. This review presents both the general principles that allow separation of chiral NSAID enantiomers, and discusses both the advantages and disadvantages of the available chromatographic assay methods and procedures used to separately quantify NSAID enantiomers in biological matrices.

Clinical pharmacokinetics of tiaprofenic acid and its enantiomers

Tiaprofenic acid is a chiral nonsteroidal anti-inflammatory drug (NSAID) of the 2-arylpropionic acid (2-APA) class. A common structural feature of 2-APA NSAIDs is a sp3-hybridised tetrahedral chiral carbon heteroatom within the propionic acid side chain moiety, with the S-enantiomer possessing most of the beneficial anti-inflammatory activity. However, all tiaprofenic acid preparations to date are marketed as the racemate. Tiaprofenic acid has been suggested to exhibit limited pharmacokinetic stereoselectivity. The synovium is the proposed site of action of NSAIDs when used for musculoskeletal disorders, and substantial concentrations of tiaprofenic acid are attained in synovial fluid. Recent data suggested that possibility of stereoselective distribution of tiaprofenic acid into synovium and cartilage. Hence, data generated using non-stereospecific assays may not always be extrapolated to explain the disposition of the individual enantiomers. Tiaprofenic acid is rapidly and almost completely absorbed when given orally. The area under the plasma concentration-time curve (AUC) of tiaprofenic acid is proportional to the oral dose administered. A sustained release dosage form is available, which may be beneficial due to the short terminal phase half-life of tiaprofenic acid (3 to 6 hours). The bioavailability is the same as that with conventional rapid release preparations, although the peak plasma drug concentration is reduced and time peak is prolonged. Tiaprofenic acid binds extensively to plasma albumin. These is negligible R to S inversion upon oral administration. Tiaprofenic acid is eliminated following extensive biotransformation to glucuronide-conjugated metabolites. Approximately 60% is eliminated as conjugates excreted in urine, and little drug is eliminated unchanged. The rate of excretion of tiaprofenic acid and its conjugates may be related to renal function; accumulation of conjugates occurs in end-stage renal disease, but not in young individuals or elderly patients. Potentially clinically important drug interactions with tiaprofenic acid have been demonstrated for some anticoagulants and probenecid. Relationships between tiaprofenic acid concentrations in biological matrices and therapeutic or toxic effects have not yet been elucidated for this drug.

Identification of chiral drug isomers by capillary electrophoresis

Separation of optical isomers of compounds of pharmaceutical interest by capillary electrophoretic techniques is reviewed. The direct and indirect separation method, as well as the main resolution mechanisms and the parameters influencing the stereoselectivity are discussed considering capillary zone electrophoresis, micellar electrokinetic chromatography, isotachophoresis and electrochromatography. Several chiral selectors have been successfully used in CE for chiral separation, including cyclodextrins and their derivatives, modified crown-ethers, proteins, antibiotics, linear saccharides and chiral surfactants. Only applications in the pharmaceutical field with the most important experimental conditions are summarised in the Tables reported in this paper. The chiral analyses of drugs in real samples like biological fluids or pharmaceutical formulations are also reported.

Stereospecific high-performance liquid chromatographic assay of ketoprofen in human plasma and urine

A high-performance liquid chromatographic (HPLC) assay suitable for the analysis of the enantiomers of ketoprofen (KT), a 2-arylpropionic acid (2-APA) non-steroidal antiinflammatory drug (NSAID), in plasma and urine was developed. Following the addition of racemic fenoprofen as internal standard (I.S.), plasma containing the KT enantiomers and I.S. was extracted by liquid-liquid extraction at an acidic pH. After evaporation of the organic layer, the drug and I.S. were reconstituted in mobile phase and injected into the HPLC system. The enantiomers were separated at ambient temperature on a commercially available 250 x 4.6 mm amylose carbamate-packed chiral column (Chiralpak AD) column with hexane-isopropyl alcohol-trifluoroacetic acid (80:19.9:0.1, v/v/v) as the mobile phase pumped at 1.0 ml/min. The enantiomers of KT were quantified by ultraviolet detection with the wavelength set at 254 nm. The assay described allows for the direct quantitation of KT enantiomers without pre-column derivatization, and is suitable for clinical studies of KT enantiomers in human plasma and urine after administration of therapeutic doses.

Clinical pharmacokinetics of flurbiprofen and its enantiomers

Flurbiprofen is a chiral nonsteroidal anti-inflammatory drug (NSAID) of the 2-arylpropionic acid class. Although it possesses a chiral centre, with the S-(+)-enantiomer possessing most of the beneficial anti-inflammatory activity, both enantiomers may possess analgesic activity and all flurbiprofen preparations to date are marketed as the racemate. Flurbiprofen exhibits stereoselectivity in its pharmacokinetics. Stereoselectivity is exhibited at the level of protein binding and metabolite formation. Hence, the data generated using nonstereoselective assays may not be used to explain the pharmacokinetics of individual enantiomers. The absorption of flurbiprofen is rapid and almost complete when given orally. The area under the plasma concentration-time curve of flurbiprofen is proportional to the dose administered to patients. Sustained release dosage forms are available, which may be beneficial due to the short terminal phase elimination half-life of conventional immediate release flurbiprofen (3 to 6 hours). They may also decrease local gastrointestinal adverse effects. Although with these preparations the peak plasma drug concentration is reduced and time taken to achieve peak concentrations is prolonged, the bioavailability is the same as that with regular release counterparts. Flurbiprofen binds extensively to plasma albumin, apparently in a stereoselective manner. Substantial concentrations of the drug are attained in synovial fluid, which is the proposed site of action of NSAIDs. There is negligible R to S inversion after oral administration. Flurbiprofen is eliminated following extensive biotransformation to glucuro-conjugated metabolites. Conjugates are excreted in urine, and approximately 20% of flurbiprofen is eliminated unchanged. The excretion of conjugates may be tied to renal function as accumulation of conjugates occurs in end-stage renal disease, but not in young individuals or elderly patients. Although flurbiprofen is excreted into breast milk, the amount of drug transferred comprises only a small fraction of the maternal exposure. Significant drug interactions have been demonstrated for aspirin (acetylsalicylic acid), coumarins and propranolol. The relationship between concentration and anti-inflammatory and analgesic effect has yet to be elucidated for this drug.

Gas chromatographic determination of enantiomers as diastereomers following pre-column derivatization and applications to pharmacokinetic studies: a review

The introduction of chiral chromatographic methods has revolutionized the art of separation and quantitation of chiral drugs in biological fluids. A large number of chiral derivatization reagents for various functional groups are available commercially. Therefore, pre-column derivatization methods have become attractive and simple for the gas chromatographic assays in biological fluids. The intent of this article is to review the pre-column chiral derivatization reagents employed in gas chromatographic separations and analyses of enantiomers. A discussion of numerous procedures necessary to develop a quantitative gas chromatographic assay method for drug enantiomers is presented. In this regard, quantitative gas chromatographic assays based on this approach have been applied to investigate the stereoselective pharmacokinetics of several chiral drugs such as fenfluramine, norfenfluramine, methylphenidate, etodolac, propranolol, suprofen and methoxyphenamine.

Gas chromatographic-mass spectrometric determination of ibuprofen enantiomers in human plasma using R(-)-2,2,2-trifluoro-1-(9-anthryl)ethanol as derivatizing reagent

A relatively rapid, inexpensive, sensitive and stereospecific gas chromatographic-mass spectrometric method was developed for the quantification of S(+) and R(-)-ibuprofen in human plasma. This method uses a commercially available internal standard and has no interference from endogenous substances nor metabolites. The method involves derivatization of ibuprofen enantiomers with optically active R(-)-2,2,2-trifluoro-1-(9-anthryl)ethanol using oxalyl chloride as the coupling reagent. The subsequently formed diastereoisomers are separated by gas chromatography and analysed by mass spectrometry using selected-ion monitoring. The assay is successfully applied to a pharmacokinetic study. The simplicity, sensitivity and precision of the method make it convenient for the quantification of ibuprofen enantiomers in biological samples.

Stereospecific high-performance liquid chromatographic assay of ibuprofen: improved sensitivity and sample processing efficiency

A high-performance liquid chromatographic (HPLC) assay suitable for the analysis of the enantiomers of the non-steroidal anti-inflammatory drug ibuprofen (IB) in plasma was developed. Following the addition of racemic fenoprofen as internal standard (I.S.), samples are acidified and extracted with a mixture of isooctane-isopropanol (95:5, v/v). After evaporation of the organic layer, the drug and I.S. are derivatized with S-(-)-1(1-naphthyl)ethylamine (S-NEA) after addition of ethyl chloroformate as the coupling reagent. Ethanolamine is added 3 min after the addition of S-NEA to react with the excessive ethyl chloroformate. The resultant diastereomers corresponding to IB and I.S. were chromatographed at ambient temperature on a 100 mm x 4.6 mm I.D. C18 reversed-phase column using acetonitrile-water-acetic acid-triethylamine (60:40:0.1:0.02) as the mobile phase pumped at a flow-rate of 1.2 ml/min. Detection of the fluorescent chromophore was at 280 and 320 nm for excitation and emission, respectively. The suitability of the assay for clinical pharmacokinetic studies of IB was determined by the analysis of plasma samples obtained from a healthy volunteer, following administration of a single 400-mg oral dose of racemic IB.

Stereoselective disposition: enantioselective quantitation of 3,4-(methylenedioxy) methamphetamine and three of its metabolites by gas chromatography/electron capture negative ion chemical ionization mass spectrometry

A new chiral assay for 3,4-(methylenedioxy)methamphetamine (MDMA) and three of its metabolites in biological specimens is based on direct aqueous derivatization with N-heptafluorobutyryl-S-prolyl chloride, followed by capillary chromatographic separation of the diastereomeric derivatives and detection by a mass spectrometer operated in the electron capture negative ion chemical ionization mode. The assay is linear from 5 to 1000 ng ml-1 for each enantiomer and allows simultaneous quantitation of MDMA and three of its metabolites in biological specimens. Investigation of the disposition of racemic MDMA in rats and mice revealed quantitative differences in the disposition of the enantiomers of MDMA in these species; the most noteworthy result was a two-fold greater urinary excretion of the neurotoxic S-(+)-MDMA by mice. Only MDMA and 3,4-(methylenedioxy)amphetamine (MDA) enantiomers were detected at measurable concentrations in the frontal cortices and hippocampis from rats dosed with 10 mg kg-1 of racemic MDMA; in this species the enantiomeric profiles of these two compounds were similar in brain and urine.

Enantiospecific gas chromatographic-mass spectrometric procedure for the determination of ketoprofen and ibuprofen in synovial fluid and plasma: application to protein binding studies

A method for the enantiospecific quantitation of two commonly prescribed non-steroidal anti-inflammatory drugs (ketoprofen and ibuprofen) is described. The method involves formation of a mixed anhydride of the drug with ethylchloroformate and subsequent conversion to an amide by reaction with optically active amphetamine. The subsequently formed diastereomers are separated by gas chromatography-mass spectrometry using selected-ion monitoring. The assay is capable of quantifying ketoprofen (2 ng/ml) and ibuprofen (3 ng/ml) enantiomers from a 200-microliters sample of synovial fluid or plasma and is particularly suitable for protein binding studies.

Improved high-performance liquid chromatographic assay method for the enantiomers of ibuprofen

A rapid, inexpensive and sensitive high-performance liquid chromatographic method for the quantitation of ibuprofen enantiomers from a variety of biological fluids is reported. This method uses a commercially available internal standard and has significantly less interference from endogenous co-extracted solutes than do previously reported methods. The method involves the acid extraction of drug and internal standard [(+/-)-fenoprofen] from the biological fluid with isooctane-isopropanol (95:5) followed by evaporation and derivatization with ethylchloroformate and R-(+)-alpha-phenylethylamine. Excellent linearity was observed between the peak-area ratio and enantiomer concentration (r > 0.99) over a concentration range of 0.25-50 micrograms/ml. This method is suitable for the quantitation of ibuprofen from single-dose pharmacokinetic studies involving either rats or humans.

Mirror images: the analysis of pharmaceutical enantiomers

Pharmaceutical enantiomers often exhibit different pharmacodynamic and pharmacokinetic properties. Stereospecific chromatographic assays are available to separate these stereoisomers. Therapeutic agents often contain chemical functional groups (e.g. amino, hydroxyl, carbonyl, and carboxylic acid). These can be reacted with enantiomerically pure reagents to give diastereoisomers suitable for analysis on achiral gas chromatographic (GC) and high performance liquid chromatographic (HPLC) columns. Alternatively, derivatized or underivatized drugs may be resolved on chiral chromatographic phases. A wide variety of GC (e.g. amino acid, cyclodextrin, and metal-complex) and HPLC (mobile phase additive, crown ether, pi-pi interaction and related phases, protein, cyclodextrin, polysaccharide, methacrylate and amide polymer, and ligand exchange) columns are commercially available. This article reviews the chromatographic separation of enantiomers.

Determination of methamphetamine enantiomer ratios in urine by gas chromatography-mass spectrometry

Analysis of the enantiomers of methamphetamine and its metabolite amphetamine is an extremely important process for a number of scientific disciplines. From studies of biological activity and mechanisms through determination of precursor molecules in a criminal investigation are all served by this analytical procedure. Utilization of gas chromatography-mass spectrometry with chiral derivatizing reagents is the most common chiral procedure and produces excellent results. Of the chiral derivatizing reagents available, the most widely used is trifluoroacetyl-l-prolyl chloride (TPC). Utilization of other derivatives require either synthesis by the analyst or have not shown themselves to provide as good a separation as did the TPC reagent. Use of chiral stationary phases yield good results but the disadvantages of temperature limits of these columns and the narrow use to which the columns can be put has limited their utilization. A significant utility of the chiral stationary phase is the ability to determine the purity of a chiral derivatizing reagent. Even if not used for routine analysis of enantiomers, utilization of this procedure can determine the purity of a reagent such as TPC and allow for accurate calculation of actual amounts of each enantiomer. This can be estimated using chiral derivatives on an achiral column, but it is limited to the extent that it is not able to differentiate enantiomeric impurity in the reagent versus the drug itself. Description of chromatographic procedures primarily focusing on gas chromatographic-mass spectrometric techniques but also including liquid chromatographic techniques along with examples of extraction and derivatization procedures is the focus of this review.

High-performance liquid chromatographic determination of the enantiomers of flecainide in human plasma and urine

A stereospecific high-performance liquid chromatographic method for the determination of (R,S)-flecainide acetate [(R,S)-N-(2-piperidylmethyl)-2,5-bis-(2,2,2-trifluoroethoxy)benzam ide acetate] in human plasma and urine is described. After addition of the internal standard [IS; (R,S)-N-(2-piperidylmethyl)-2,3-bis(2,2,2-trifluoroethoxy)- benzamide hydrochloride], a single-step extraction of alkalinized samples was performed with distilled diethyl ether. The organic layer was evaporated and the drug and IS were derivatized with 1-[(4-nitrophenyl)sulfonyl]-L-propyl chloride at 80 degrees C for 2 h. The diastereomeric derivatives of flecainide and IS were chromatographed on a C18 reversed-phase column with a mobile phase consisting of acetonitrile: water:triethylamine (45:55:0.2) at a flow rate of 1 mL/min. Flecainide diastereomers were separated with a resolution factor of 1.25 and detected by UV spectroscopy at a wavelength of 280 nm. An excellent linearity was observed between the peak area ratios (flecainide derivatives:IS) and plasma concentrations, and the intra- and interday coefficients of variation were always less than 9.8%. The lowest quantifiable concentration was set at 50 ng/mL for each enantiomer (CV of 4.9 and 4.4%), while the lowest limit of detection (signal:noise, 3:1) was on the order of a few nanograms. The assay was used to study the pharmacokinetics of flecainide enantiomers in a patient receiving (R,S)-flecainide therapy. The steady-state plasma time courses for the enantiomers were found to be parallel, but the difference between (R)- and (S)-flecainide concentrations was significant. The urinary excretion data were consistent with the plasma results. The method is suitable for therapeutic monitoring of flecainide enantiomers and for stereoselective pharmacokinetic studies in humans.

Determination of the enantiomers of suprofen and [2H3]suprofen in plasma by capillary gas chromatography-mass spectrometry

A method for the stereoselective assay of the (+)- and (-)-enantiomers of suprofen and [2H3]suprofen in human plasma was developed using gas chromatography-mass spectrometry-selected-ion monitoring. (+/-)-[2H7]Suprofen was used as an internal standard. The method involved diethyl ether extraction and chiral derivatization with S-(-)-1-(naphthyl)ethylamine to form diastereomeric amide. The diastereoisomers were separated on a capillary gas chromatograph-mass spectrometer. Quantitation was achieved by selected-ion monitoring of the quasi-molecular ions of the diastereoisomers. The sensitivity, specificity, accuracy and reproducibility of the method were demonstrated to be satisfactory for application to pharmacokinetic studies of suprofen enantiomers.

Application of a stereospecific high-performance liquid chromatography assay to a pharmacokinetic study of etodolac enantiomers in humans

An HPLC assay suitable for pharmacokinetic analysis of enantiomers of etodolac [(+/-)-1,8-diethyl-1,3,4,9-tetrahydropyrano[3,4-b] indole-1-acetic acid] was developed. Following addition of internal standard (IS), (+/-)-2-(4-benzoylphenyl)butyric acid, the constituents were extracted from the specimen into a mixture of isooctane:isopropanol (95:5). The organic layer was evaporated and the drug and IS were sequentially derivatized with ethyl chloroformate and iota(-)-alpha-phenylethylamine. The diastereoisomers thus formed were extracted and chromatographed on a normal-phase column, with a mobile phase consisting of hexane:ethyl acetate:isopropanol (85:15:0.2) at a flow rate of 2 mL/min. The etodolac diastereoisomers were separated with a resolution factor of 6.4 and detected at a wavelength of 280 nm. Excellent linear relationships were found between the peak area ratios (etodolac:IS) and the plasma and urine concentrations (0.2-20 mg/L), with intra- and interday variations of less than 10.1%. The assay was applied to a preliminary pharmacokinetic study following seven repeated oral administrations of 200 mg/12 h of racemic etodolac to two healthy subjects. The plasma concentrations of the active S-(+)-enantiomer were considerably less than those of the inactive antipode (AUC S:R, 2.5:30.9 mg.L-1.h-1) due to a greater volume of distribution of the latter (S, 101 and 135 L versus R, 24 and 17 L). Considerable concentrations of conjugated enantiomers were also found in plasma (AUC conjugated: intact: S, 1.1; R, 0.23).

Rapid and sensitive high-performance liquid chromatographic assay of tiaprofenic acid enantiomers in human plasma and urine

A sensitive stereospecific high-performance liquid chromatographic assay of tiaprofenic acid (TA) enantiomers in human plasma and urine was developed. The biological specimens are acidified, and the drug and internal standard, (+/-)-2-(4-benzoylphenyl)butyric acid, extracted with an isooctane-isopropanol (95:5) mixture (plasma) or chloroform (urine), followed by sequential reaction of the enantiomers with trichloroethyl chloroformate and L-leucinamide. The reactions were complete at ambient temperature in less than 3 min. The diastereoisomers of TA and internal standard were then extracted into chloroform. The organic layer was evaporated, and the reconstituted residue chromatographed at ambient temperature on a C18 reversed-phase column with a mobile phase consisting of 0.06 M monopotassium phosphate-acetonitrile-triethylamine (65:35:0.02) at a flow-rate of 1 ml/min. The TA diastereoisomers were detected at 310 nm, free of interfering peaks, with a resolution factor of 2.1. Within the examined plasma and urine enantiomeric concentration ranges of 0.2-20 and 10-100 mg/l, respectively, an excellent linear relationship was obtained between the peak-area ratios and the corresponding concentrations. The assay was reproducible and sufficiently accurate to be applied to the stereoselective pharmacokinetic analysis of TA enantiomers in plasma and urine following administration of therapeutic doses of the drug.

Pharmacokinetics of ibuprofen enantiomers in humans following oral administration of tablets with different absorption rates

Ibuprofen (IB) is a racemic drug and is administered as such. While activity is due mainly to the S enantiomer, pharmacokinetic interpretations, as well as criteria to assess the bioequivalence of IB formulations, are based on measurements of the total (S + R) drug concentrations. IB enantiomers possess different disposition properties mainly as a result of R-to-S isomeric bioinversion. Inversion is maximal during the absorption phase, suggesting, perhaps, involvement of a presystemic process. This concept was evaluated in healthy subjects by crossover administration of four IB tablets having different absorption rates. The plasma concentrations of the individual isomers were measured using a stereospecific gas chromatographic assay. Differences among the products were insignificant with respect to the extent to the absorption. The S:R concentration ratios rose for 4 to 6 hr and then remained relatively unchanged. This observation was consistent with equal terminal t1/2 values for the enantiomers. There were significant differences between the peak times (Tmax) of the products. The S:R ratios of the concentrations at Tmax of S and AUC also differed; significant positive correlations were found between Tmax and the S:R ratios of Cmax. Thus the extent of R-to-S inversion, and hence the potency of a racemic dose of IB, may be absorption rate dependent.

Pharmacokinetics of ketoprofen enantiomers in healthy subjects following single and multiple doses

Ketoprofen (KT; m-benzoylhydratropic acid), a 2-arylpropionic acid (2-APA) nonsteroidal anti-inflammatory drug (NSAID), is marketed and used as a racemic mixture. Although generally the activity of 2-APAs is suggested to be mainly due to the S-enantiomer, information on KT pharmacokinetics is based on measurement of total concentrations of S- and R-enantiomers. In this work, using a crossover fashion, the pharmacokinetics of KT enantiomers following single (50 mg, po) and then multiple (50 mg, q6h for 3 d) doses were delineated in eight healthy subjects. A sensitive stereospecific HPLC assay was used to measure KT enantiomers in plasma and urine, and conjugated KT enantiomers in urine. There were no significant differences between the pharmacokinetic indices calculated after single and multiple administration of KT. In plasma, small but significant differences were found between concentrations of the enantiomers (mean S:R ratios of 0.81 +/- 0.19 after single and 0.87 +/- 0.11 after repeated doses). Negligible amounts of unchanged KT enantiomers were found in urine. More than 80% of the given doses was found in urine as conjugated S- and R-KT, the predominant enantiomer being S-KT (mean S:R ratios of 1.19 +/- 0.05 after single and 1.17 +/- 0.05 after repeated doses). No significant difference between the elimination t1/2 of the enantiomers was observed. It is suggested that stereoselective conjugation followed by preferential biliary excretion of the conjugated R-KT enantiomer is responsible for the observed stereoselectivity in the pharmacokinetics of the drug.