Enantioselective high-performance liquid chromatographic method for the determination of baclofen in human plasma

A new analytical method for the separation and determination of R-(−)- and S-(+)- baclofen enantiomers in human plasma by high-performance liquid chromatography (HPLC) with UV detection was developed. Enantioselective resolution of the baclofen enantiomers was achieved by using teicoplanin macrocyclic antibiotic chiral stationary phase (CSP) known as Chirobiotic T with a polar ionic mobile phase (PIM) consisting of methanol: glacial acetic acid: triethylamine, 100:0.1:0.1, (v/v/v) at a flow rate of 0.5 ml min⁻¹ and UV detection set at 220 nm. The analytes of interest with S-(+)-sulpiride as the internal standard were extracted from human plasma using liquid-liquid extraction procedure with ethyl ether under alkaline condition prior to HPLC analysis. Recoveries for R-(−)- and S-(+)-baclofen enantiomers were in the ranges of 96-103% at 60-2500 ng ml⁻¹ level. Intra-day and inter-day precision calculated as %RSD was in the ranges of 1.2-5.2 and 1.3-4.3% for both enantiomers, respectively. Intra-day and inter-day accuracy calculated as percentage error were in the ranges of 1.2-3.9 and 1.1-3.9% for both enantiomers, respectively. Linear calibration curves in the concentration ranges of 20-3000 ng ml⁻¹ for each enantiomer showed correlation coefficient (r) of 0.9997. The limit of quantitation (LOQ) and limit of detection (LOD) for each enantiomer in human plasma were 20 and 10 ng ml⁻¹ (S/N=3) respectively.     

Enantioanalysis of bisoprolol in human plasma with a macrocyclic antibiotic HPLC chiral column using fluorescence detection and solid phase extraction

A sensitive, enantioselective, high-performance liquid chromatographic (HPLC) method was developed and validated to determine S-(-)- and R-(+)-bisoprolol in human plasma. Baseline resolution was achieved using the teicoplanin macrocyclic antibiotic chiral stationary phase (CSP) known as Chirobiotic T with a polar ionic mobile phase (PIM) consisting of methanol-glacial acetic acid-triethylamine (100 : 0.02 : 0.025, v/v/v) at a flow rate of 1.5 ml/min and fluorescence detection set at 275 nm for excitation and 305 nm for emission. All analyses with S-(-)-atenolol as the internal standard were conducted at ambient temperature. The assay involved the use of a solid-phase extraction procedure for human plasma samples prior to HPLC analysis. The C18 cartridge gave good recovery rates for both enantiomers without any interference. The method was validated over the range of 20-200 ng/ml for each enantiomer concentration. Recovery rates for S-(-)- and R-(+)-bisoprolol enantiomers were in the range of 95-102%. The method proved to be precise (within-run precision expressed as % RSD ranged from 1.0-6.2% and between-run precision ranged from 0.9-6.7%) and accurate (within-run accuracies expressed as percentage error ranged from 0.2-4.8% and between-run accuracies ranged from 0.3-1.7%). The limit of quantitation and limit of detection for each enantiomer in human plasma were 20 and 5 ng/ml, respectively.     

Development and Validation of a Chiral Liquid Chromatographic Assay for Enantiomeric Separation and Quantification of Verapamil in Rat Plasma: Stereoselective Pharmacokinetic Application

A novel, fast and sensitive enantioselective HPLC assay with a new core–shell isopropyl carbamate cyclofructan 6 (superficially porous particle, SPP) chiral column (LarihcShell-P, LSP) was developed and validated for the enantiomeric separation and quantification of verapamil (VER) in rat plasma. The polar organic mobile phase composed of acetonitrile/methanol/trifluoroacetic acid/triethylamine (98:2:0.05: 0.025, v/v/v/v) and a flow rate of 0.5 mL/min was applied. Fluorescence detection set at excitation/emission wavelengths 280/313 nm was used and the whole analysis process was within 3.5 min, which is 10-fold lower than the previous reported HPLC methods in the literature. Propranolol was selected as the internal standard. The S-(−)- and R-(+)-VER enantiomers with the IS were extracted from rat plasma by utilizing Waters Oasis HLB C18 solid phase extraction cartridges without interference from endogenous compounds. The developed assay was validated following the US-FDA guidelines over the concentration range of 1–450 ng/mL (r² ≥ 0.997) for each enantiomer (plasma) and the lower limit of quantification was 1 ng/mL for both isomers. The intra- and inter-day precisions were not more than 11.6% and the recoveries of S-(−)- and R-(+)-VER at all quality control levels ranged from 92.3% to 98.2%. The developed approach was successfully applied to the stereoselective pharmacokinetic study of VER enantiomers after oral administration of 10 mg/kg racemic VER to Wistar rats. It was found that S-(−)-VER established higher C_max and area under the concentration-time curve (AUC) values than the R-(+)-enantiomer. The newly developed approach is the first chiral HPLC for the enantiomeric separation and quantification of verapamil utilizing a core–shell isopropyl carbamate cyclofructan 6 chiral column in rat plasma within 3.5 min after solid phase extraction (SPE).     

Enantiomeric separation and quantification of pindolol in human plasma by chiral liquid chromatography/tandem mass spectrometry using staggered injection with a CTC Trio Valve system

Pindolol is a non-selective beta-adrenergic antagonist (beta-blocker) for the treatment of cardiovascular diseases such as hypertension and angina pectoris. It has one chiral center, and, therefore, two optical isomers. It was essential to develop an enantioselective assay to measure each enantiomer in human plasma. However, separation of enantiomers using chiral chromatography usually requires relatively long retention times. This can pose a problem for rapid turnaround of a large number of samples (i.e., clinical studies). In the present study, a simple and sensitive chiral liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) method was developed and validated for the determination of S-(-)- and R-(+)-pindolol in human plasma. To increase throughput, staggered sample injection was employed using a CTC Trio Valve system on a CTC HTS PAL autosampler. The method exhibited good intra- and inter-day accuracy and precision, and was linear over a dynamic range of 250 pg/mL to 250 ng/mL for each pindolol enantiomer. Intra- and inter-day accuracy ranged between 90.0-106% and 91.6-104% for both quality control (QC) samples of S-(-)- and R-(+)-pindolol, respectively. The respective intra- and inter-day precision ranged between 4.24-7.86% and 4.98-10.4%.     

Determination of Ibuprofen in human plasma by high-performance liquid chromatography: validation and application in pharmacokinetic study

A specific method for the simultaneous determination of S-(+)Ibuprofen and R-(-)Ibuprofen enantiomers in human plasma is described. Adopting a high-performance liquid chromatographic (HPLC) system with spectrofluorometer detector, the compounds were extracted from plasma in alcohol medium and were separated on C18 column, using a solution of acetonitrile-water-acetic acid-triethylamine as mobile phase. The limit of quantitation was 0.1 microg/mL for both compounds. The method was validated by intra-day assays at three concentration levels and was used in a kinetic study in healthy volunteers. During the study we carried out inter-day assays to confirm the feasibility of the method.     

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

A sensitive high-performance liquid chromatographic technique is described for the separation of R- and S-acebutolol in human plasma and urine. The procedure involves derivatization with the chiral reagent S-(+)-1-(1-naphthyl)ethyl isocyanate. The resulting diastereoisomers are quantified using normal-phase high-performance liquid chromatography with fluorescence detection (220/389 nm). Virtual baseline separation, free from interference, with achieved (resolution factor = 1.45). Excellent linearity (r greater than 0.998) was observed throughout the range 10-500 ng/l and 2-100 mg/l in plasma and urine, respectively. Inter-assay variability was less than 5% for each enantiomer at concentrations of 10 ng/ml. This method is applicable for the determination of the pharmacokinetics, in man, of acebutolol enantiomers in plasma and urine.     

Recognition and simultaneous determination of ofloxacin enantiomers by synchronization-1st derivative fluorescence spectroscopy

Under controlling pH 3, R-(+)- and S-(-)-ofloxacin (OFLX) enantiomers can be well recognized and resolved by the synchronization-1st derivative fluorescence spectroscopic techniques, and the interference from urine blank also can be eliminated. The linear dynamic ranges are 0.36-2.16 (R), 0.36-2.89 and 3.16-31.6 mug/ml (S), respectively, for determining OFLX in urine samples. The limits of detection are 0.36 mug/ml (R) and the recoveries of R-(+)- and S-(-)-OFLX in urine samples are 97-104%. Relative standard deviation is <6.6%. Pharmacokinetic study of OFLX and levofloxacin shows that R-(+)- and S-(-)-ofloxacin reach their peak concentration in urine samples after a healthy subject has taken tablets for approximately 3 and 6 h, respectively. R-(+)-OFLX can be obviously detected in 5-6 h after a healthy subject has taken tablets, indicating the transformation of S-(-)- to R-(+)-OFLX enantiomer in human body (in vitro).     

Determination of the enantiomeric impurity in S-(-)pantoprazole using high performance liquid chromatography with sulfobutylether-beta-cyclodextrin as chiral additive

A new and accurate HPLC method using sulfobutylether-beta-cyclodextrin (SBE-beta-CD) as chiral mobile phase additive (CMPA) was developed and validated for the determination of R-(+)pantoprazole in S-(-)pantoprazole. The influences of type and concentration of CD, ACN content and buffer pH of mobile phase on the resolution and retention of enantiomers were investigated. A baseline resolution of pantoprazole enantiomers was achieved on a Spherigel C18 column (150 mm x 4.6 mm, 5 microm) using ACN and 10 mM phosphate buffer (pH 2.5) containing 10 mM SBE-beta-CD (15:85 v/v) as mobile phase with a flow rate of 0.9 mL/min at 20 degrees C. The detection wavelength was set at 290 nm. The method was extensively validated in terms of accuracy, precision and linearity according to the International Conference on Harmonisation (ICH) guidelines and proved to be robust. The LOD and LOQ for R-(+)pantoprazole were 0.2 and 0.5 microg/mL, respectively, with 5 microL injection volume. A good linear relationship was obtained in the concentration range of 0.5-6.0 microg/mL with r(2) >0.999 for R-(+)pantoprazole. The percentage recovery of the R-(+)pantoprazole ranged from 92.1 to 101.2 in bulk drug of S-(-)pantoprazole. The method is capable of determining a minimum limit of 0.05% w/w of R-enantiomer in S-(-)pantoprazole bulk samples.     

HPLC microdetermination of flurbiprofen enantiomers in plasma with a glycopeptide-type chiral stationary phase column

A rapid and stereospecific HPLC micromethod to quantify flurbiprofen enantiomers was developed. Both flurbiprofen enantiomers and indomethacin, used as internal standard, were extracted with methylene chloride from 100 microL of acidified plasma. The resolution of the R- and S-forms was performed on a bonded vancomycin chiral stationary phase (Chirobiotic V) with 20% of tetrahydrofuran in ammonium nitrate (100 mM, pH 5) as mobile phase. Calibration curves were linear in the range 0.5-10 microg/mL for both enantiomers. A good accuracy (< or = 5%) was obtained for all quality controls, with intra-day and inter-day variation coefficients equal or less than 7.7%. Recovery of both enantiomers was found in the range 77.4-86.3%. The lower limit of quantitation was 0.25 microg/mL for both enantiomers, without interference of endogenous components. This validated micromethod has been successfully applied for quantifying R- flurbiprofen and S- flurbiprofen in rat plasma.     

Determination of saterinone enantiomers in plasma samples with an internal standard using a Chiralcel OD column, fractionation and reversed-phase chromatography

A specific and validated high-performance liquid chromatographic method was developed for the determination of the S-(-) and R-(+) enantiomers of saterinone. 1-[(4-cyano-1,2-dihydro-6-methyl-2-oxopyridin-5-yl)phenoxyl] -3-[4-(2- methoxyphenyl)piperazin-1-yl]propan-2-ol, in plasma at the low ng/ml level. The enantiomers of saterinone and an internal standard, 1-[(4-cyano-1,2-dihydro-6-methyl-2-oxo-pyridin-5-yl)phenoxy]-3-[4-(2- ethoxyphenyl)piperazin-1-yl]propan-2-ol, were chromatographed on a chiral Chiralcel OD stationary phase. However, the S-(-) enantiomers of saterinone and the internal standard were unresolved, as were the R-(+) enantiomers of both substances. Therefore, the two fractions were collected and each was separately resolved on an achiral Polyencap A reversed-phase column and quantified. The detection limit was 0.5 ng/ml of enantiomer, allowing the determination of plasma levels up to 36 h after oral administration of 90, 150 and 180 mg of saterinone to twelve subjects.     

A new HPLC method with fluorescence detection for the determination of memantine in human plasma

A sensitive, selective, simple and fast HPLC method based on the formation of derivative with fluorescamine was developed for the determination of memantine (ME) in human plasma. Separation was achieved on a CN column (200 mm×4.6 mm) using acetonitrile-10 mM orthophosphoric acid containing 1 mL/L triethylamine (45:55, v/v) at a flow rate of 1 mL/min. Emission and excitation wavelengths were 480 and 380 nm, respectively. Amantadine was used as an internal standard. Calibration graphs were rectilinear over the range of 1.0-100.0 ng/mL. Limit of detection and limit of quantification were found to be 0.3 and 1.0 ng/mL, respectively. Intra-day and inter-day relative standard deviation values were found to be <2.03%. Average recovery was also found to be around 94%. Proposed method was applied for the pharmacokinetic study in a healthy volunteer after a single oral administration of 20 mg of ME.     

A stereospecific high-performance liquid chromatographic assay for the determination of ketoconazole enantiomers in rat plasma

A stereospecific high-performance liquid chromatographic assay was developed for the quantitation of ketoconazole enantiomers (KTZ) in rat plasma. After protein precipitation of 100 microL plasma using acetonitrile, a wash step was performed using hexane. The supernatant was removed and KTZ enantiomers and amiodarone, the internal standard, were extracted using liquid-liquid extraction with tert-butyl methyl ether. After transfer and evaporation of the organic layer, the residue was reconstituted in mobile phase and injected into the HPLC through a chiral column. The mobile phase consisted of hexane:ethanol:2-propanol with diethyl amine, pumped at 1.5 mL/min. All components eluted within 18 min. KTZ enantiomers were baseline resolved and peaks were symmetrical in appearance with no interferences. Calibration curves were linear over the range 62.5-5000 ng/mL of enantiomer. The intraday and interday CV% assessments were 相似文献   

Chiral analysis of baclofen by alpha-cyclodextrin-modified capillary electrophoresis and laser-induced fluorescence detection

An enantioselective method for baclofen (4-amino-3-p-chlorophenylbutyric acid) based on capillary electrophoresis (CE) separation and laser-induced fluorescence (LIF) detection has been developed. Naphthalene-2,3-dicarboxaldehyde (NDA) was used for precolumn derivatization of the nonfluorescent drug. alpha-Cyclodextrin (alpha-CD) was included in the buffer as a chiral selector for the separation of NDA-labeled S-(+)- and R-(-)-baclofen. Optimal resolution and detection were obtained with an electrophoretic buffer of 50 mM sodium borate (pH 9.5) containing 7 mM alpha-CD and a He-Cd laser (lambda ex = 442 nm, lambda em = 500 nm). Combined with a simple cleanup procedure, this method can be applied to the analysis of baclofen enantiomers in human plasma. The relative standard deviation (RSD) values on peak areas of a plasma sample containing 1.0 microM racemic baclofen were 6.4 and 4.9% (n = 8) for the S-(+)- and R-(-)-enantiomer, respectively. The RSD value on migration times of both enantiomers was 0.5% (n = 8). Calibration graphs for S-(+)- and R-(-)-baclofen in plasma showed a good linearity (r > or = 0.999) in the concentration range of 0.1-2.0 microM. The limit of detection of baclofen in plasma was about 10 ng/mL.     

Stereoselective determination of demethyl- and didemethyl-citalopram in rat plasma and brain tissue by liquid chromatography with fluorescence detection using precolumn derivatization

A rapid and sensitive HPLC enantioselective method with fluorescence detection was developed to determine (-)-(R) and (+)-(S) enantiomers of the metabolites of citalopram, demethyl- and didemethyl-citalopram in plasma and brain tissue. This assay involves pre-column chiral derivatization with (-)-(R)-1-(1-naphthyl)ethyl isocyanate followed by separation on a normal-phase silica column. The developed liquid-liquid extraction procedure permits quantitative determination of analytes with recoveries ranged between 81 and 88% with intra- and inter-day relative standard deviations less than 10.5%. Linearity was obtained over the concentration range 5-1000 ng/mL and 100-10,000 ng/g for spiked drug-free plasma and brain tissue, respectively, with detection limits lower than 2.1 ng/mL and 42.8 ng/g.     

Determination of darusentan enantiomers in rat plasma by enantioselective liquid chromatography with tandem mass spectrometry using cellulose-based chiral stationary phase

A sensitive, specific and rapid liquid chromatography-mass spectrometry (LC-MS/MS) method has been developed and validated for enantioselective determination of darusentan enantiomers, orally active potent endothelin-A receptor antagonist, in rat plasma. The plasma samples were pretreated by protein precipitation with methanol and baseline chromatographic separation was performed on a Chiralcel OD-RH column with a mobile phase consisting of acetonitrile/water/formic acid (50:50:0.1, v/v/v) at a flow rate of 0.5 mL/min. The detection was accomplished by multiple-reaction monitoring (MRM) scanning via electrospray ionization (ESI) source operating in the negative ionization mode. The calibration curve was linear over the investigated concentration from 0.500 to 2500 ng/mL (r≥0.995) for each enantiomer using 50 μL of rat plasma. The lower limit of quantitation (LLOQ) for each enantiomer was 0.500 ng/mL. The intra- and inter-day precisions were not more than 10.2% and the accuracy was within the range from -5.4 to 6.3% for darusentan enantiomers. No chiral inversion was observed during the plasma preparation, storage and analysis. The method proved adequate for enantioselective pharmacokinetic studies of darusentan enantiomers after oral administration of three different doses of racemic darusentan.     

Enantiospecific separation and quantitation of mephenytoin and its metabolites nirvanol and 4'-hydroxymephenytoin in human plasma and urine by liquid chromatography/tandem mass spectrometry

A sensitive method using enantiospecific liquid chromatography/tandem mass spectrometry detection for the quantitation of S- and R-mephenytoin as well as its metabolites S- and R-nirvanol and S- and R-4'-hydroxymephenytoin in plasma and urine has been developed and validated. Plasma samples were prepared by protein precipitation with acetonitrile, while urine samples were diluted twice with the mobile phase before injection. The analytes were then separated on a chiral alpha(1)-acid glycoprotein (AGP) column and thereafter detected, using electrospray ionization tandem mass spectrometry. In plasma, the lower limit of quantification (LLOQ) was 1 ng/mL for S- and R-4'-hydroxymephenytoin and S-nirvanol and 3 ng/mL for R-nirvanol and S- and R-mephenytoin. In urine, the LLOQ was 3 ng/mL for all compounds. Resulting plasma and urine intra-day precision values (CV) were <12.4% and <6.4%, respectively, while plasma and urine accuracy values were 87.2-108.3% and 98.9-104.8% of the nominal values, respectively. The method was validated for plasma in the concentration ranges 1-500 ng/mL for S- and R-4'-hydroxymephenytoin, 1-1000 ng/mL for S-nirvanol, and 3-1500 ng/mL for R-nirvanol and S- and R-mephenytoin. The validated concentration range in urine was 3-5000 ng/mL for all compounds. By using this method, the metabolic activities of two human drug-metabolizing enzymes, cytochrome P450 (CYP) 2C19 and CYP2B6, were simultaneously characterized.     

高效液相色谱-串联质谱法分析毛发中甲基苯丙胺和苯丙胺手性对映异构体

建立了高效液相色谱-串联质谱（HPLC-MS/MS）分析毛发中甲基苯丙胺与苯丙胺对映异构体的手性分离方法。采用SUPELCO Astec CHIROBIOTIC^® V2手性液相色谱柱,以甲醇-含0.1%（v/v）甲酸的20 mmol/L乙酸铵水溶液（99：1,v/v）为流动相进行手性分离。结果表明,甲醇高温水浴超声法能较好地提取苯丙胺类化合物,且峰形较好（拖尾因子>0.95）。S-（+）-甲基苯丙胺、R-（-）-甲基苯丙胺、S-（+）-苯丙胺和R-（-）-苯丙胺在15~300 ng/mg范围内线性关系良好,相关系数均大于0.99;甲基苯丙胺和苯丙胺的检出限分别为0.1 ng/mg和0.15 ng/mg,定量限分别为0.4 ng/mg和0.5 ng/mg;日内精密度均≤6.8%,日间精密度均≤11.4%。采用所建方法对50余嫌疑人毛发进行手性分析,检出单一S-（+）-甲基苯丙胺和S-（+）-苯丙胺的占70%,同时检出S-（+）-甲基苯丙胺、R-（-）-甲基苯丙胺、S-（+）-苯丙胺和R-（-）-苯丙胺的占18%。该法简单快速,精密度好,可为实际法医毒物鉴定案例中的毛发手性分析提供技术支持与科学依据。     

Simultaneous determination of disopyramide and mono-N-dealkyldisopyramide enantiomers in human plasma by capillary electrophoresis

In this paper, a rapid method for the enantioselective analysis of the antiarrhythmic drug disopyramide and its main metabolite mono-N-dealkyldisopyramide in human plasma by capillary electrophoresis employing the cyclodextrin-modified electrokinetic chromatography mode is described. Sample clean-up was carried out by alkalinization with sodium hydroxide followed by liquid-liquid extraction with toluene. The complete enantioselective analysis was performed within less than 5 min using 20 mmol/L sodium acetate buffer, pH 5.0, containing 0.2% w/v sulfated beta-cyclodextrin as chiral selector. A 40 cm uncoated fused-silica capillary was used for the analysis, performed at a voltage of 15 kV and at 20 degrees C. The calibration curves were linear over the concentration range of 62.5-1850 ng/mL and 125-1850 ng/mL for each enantiomer of disopyramide and mono-N-dealkyldisopyramide. The mean recoveries for disopyramide and mono-N-dealkyldisopyramide enantiomers were up to 87 and 69%, respectively. All four enantiomers studied could be quantified at three different concentrations (200, 400 and 600 ng/mL) with coefficient of variation and % relative error not higher than 15%. The quantitation limit was 62.5 ng/mL for (+)-(S)-and (-)-(R)-disopyramide and (-)-(R)-mono-N-dealkyldisopyramide and 125 ng/mL for (+)-(S)-mono-N-dealkyldisopyramide, using 1 mL of human plasma.     

Stereoselective determination of R(-)- and S(+)-MK-571, a leukotriene D4 antagonist, in human plasma by chiral high-performance liquid chromatography.

A stereoselective high-performance liquid chromatographic method that utilizes fluorescence detection was developed for the selective and sensitive quantification of R(-)- and S(+)-enantiomers of MK-571 (1), a potent and specific leukotriene D4 antagonist, in human plasma. Racemic 1 was isolated from the acidified plasma using solid-phase extraction and the resulting residue was successfully reacted with isobutyl chloroformate and R(+)-1-(1-naphthyl)ethylamine in triethylamine-acetonitrile medium to form the diastereomer of each enantiomer. A structural analogue of 1 was used as internal standard. The derivatized sample was dissolved in 1,1,2-trichlorotrifluoroethane and an aliquot was chromatographed on a (R)-urea chiral column using a mobile phase containing 89% triethylamine-pentane (3:1000, v/v), 10% 2-propanol, and 1% acetonitrile at a flow-rate of 1.5 ml/min. The fluorescence response (excitation wavelength, 350 nm; emission wavelength, 410 nm) was linear (r2 greater than 0.999) for concentrations of enantiomers of 1 from 0.05 micrograms/ml, the lowest quantitation limit, up to 2.5 micrograms/ml. Intra-day coefficients of variation at 0.05 microgram/ml were 2.4% for the R(-)-isomer and 2.0% for S(+)-isomer. The corresponding inter-day coefficients of variation for R(-)- and S(+)-1 were 2.6 and 3.6%, respectively. The utility of the methodology was established by analysis of plasma samples from male volunteers receiving single intravenous and oral doses of racemic 1.     

Determination of the enantiomeric composition of ibuprofen in human plasma by high-performance liquid chromatography

A normal-phase high-performance liquid chromatographic method, using a hexane-ethyl acetate solvent system, for the determination of the enantiomeric composition of ibuprofen in human plasma is described. The method is based on the resolution of the diastereoisomeric amides formed on reaction of the ibuprofen enantiomers with S-1-(naphthen-1-yl)ethylamine using p-chlorophenoxy-acetic acid as internal standard. The application of the method for the determination of the enantiomeric composition of ibuprofen in human plasma following the repeated oral administration of the drug to two volunteers is reported. The plasma concentrations of the S-(+) enantiomer were always greater than that of the R-(-), the ratio of the areas under the enantiomer plasma concentration-time curves (S/R) being 1.8 and 1.6.