Determination of (R)- and (S)-alprenolol and (R)- and (S)-metoprolol as their diastereomeric derivatives in human plasma by reversed-phase liquid chromatography

A high-performance liquid chromatographic method is described for the determination of (R)- and (S)-alprenolol and (R)- and (S)-metoprolol in human plasma. Separation of the enantiomers was accomplished after preparation of diastereomeric derivatives with symmetrical anhydrides of tert.-butoxycarbonyl-l-leucine followed by treatment with trifluoroacetic acetic acid at 0°C to remove the tert.-butoxycarbonyl group. The separations of the diastereomeric derivatives were performed using a reversed-phase system with μBondapak C₁₅ as support and phosphate buffer pH 3.0 with the addition of acetonitrile as the mobile phase. High stability of the chromatographic system was achieved.The reproducibilities in the determination of (R)- and (S)-alprenolol and (R)- and (S)-metoprolol in human plasma were 9.4 and 9.8% (relative standard deviation) for alprenolol and metoprolol, respectively, at drug levels of 0.5 ng/ml.In two subjects who received single oral doses of alprenolol (100-mg tablet) and metoprolol (50-mg tablet) the plasma levels of the (R)-isomers were lower than for the (S)-isomers.     

Enantioselective analysis of propafenone in plasma using a polysaccharide-based chiral stationary phase under reversed-phase conditions

We present a method for the enantioselective analysis of propafenone in human plasma for application in clinical pharmacokinetic studies. Propafenone enantiomers were resolved on a 10-μm Chiralcel OD-R column (250×4.6 mm I.D.) after solid-phase extraction using disposable solid-phase extraction tubes (RP-18). The mobile phase used for the resolution of propafenone enantiomers and the internal standard propranolol was 0.25 M sodium perchlorate (pH 4.0)–acetonitrile (60:40, v/v), at a flow-rate of 0.7 ml/min. The method showed a mean recovery of 99.9% for (S)-propafenone and 100.5% for (R)-propafenone, and the coefficients of variation obtained in the precision and accuracy study were below 10%. The proposed method presented quantitation limits of 25 ng/ml and was linear up to a concentration of 5000 ng/ml of each enantiomer.     

Direct enantiospecific HPLC bioanalysis of (R,S)-atenolol on a chiral stationary phase

The simultaneous determination of the enantiomers of the β₁-selective adrenergic antagonist atenolol in human plasma and urine is described. After an alkaline preextraction atenolol is extracted from biological material at pH 12.3 using dichloromethane/propan-2-ol. The separation of the underivatized enantiomers is achieved by high-performance liquid chromatography on a chiral stationary phase (Chiralcel OD, cellulose tris-3, 5-dimethylphenylcarbamate, coated on silica gel) with fluorimetric detection. (?)-(S)-Pindolol is used as an internal standard. The detection limits of 5 ng/ml enantiomer in plasma and 50 ng/ml enantiomer in urine are sufficient for pharmacokinetic studies after therapeutic doses. © 1993 Wiley-Liss, Inc.     

Separation of carvedilol enantiomers in very small volumes of human plasma by capillary electrophoresis with laser-induced fluorescence

A sensitive capillary electrophoretic method for the determination of carvedilol enantiomers in 100 μl of human plasma has been developed and validated. Carvedilol and the internal standard carazolol are isolated from plasma samples by liquid–liquid extraction using diethylether. A sensitive and selective detection is provided by helium–cadmium laser-induced fluorescence. The total analysis time is 17.5 min, about 30 min are needed for the sample preparation. The linearity of the assay ranges from 1.56 to 50 ng/ml per carvedilol enantiomer. The limits of quantification (LOQ) for the carvedilol enantiomers in 100 μl of human plasma are 1.56 ng/ml. The inter-day accuracy for R-carvedilol is between 95.8 and 103% (104% at LOQ) and for S-carvedilol between 97.1 and 103% (107% at LOQ); the inter-day precision values are between 3.81 and 8.64% (10.9% at LOQ) and between 5.47 and 7.86% (7.91% at LOQ) for R- and S-carvedilol, respectively. The small sample volume needed is especially advantageous for the application in clinical studies in pediatric patients. As an application of the assay concentration/time profiles of the carvedilol enantiomers in a 5-year-old patient receiving a test dose of 0.09 mg/kg carvedilol are reported.     

Quantification of propranolol enantiomers in small blood samples from rats by reversed-phase high-performance liquid chromatography after chiral derivatization

A high-performance liquid chromatographic (HPLC) technique is described for quantification of R(+)- and S(−)-propranolol from 100-μl rat blood samples. The procedure involves chiral derivatization with tert.-butoxycarbonyl- -leucine anhydride to form diastereomeric propranolol- -leucine derivatives which are separated on a reversed-phase HPLC column. The method as previously reported has been modified for assaying serial blood microsamples obtained from the rat for pharmacokinetic studies. An internal standard, cyclopentyldesisopropylpropranolol, has been incorporated into the assay and several derivatization parameters have been altered. Standard curves for both enantiomers were linear over a 60-fold concentration range in 100-μl samples of whole rat blood (12.5–750 ng/ml; r=0.9992 for each enantiomer). Inter- and intra-assay variability was less than 12% for each enantiomer at 25 ng/ml. No enantiomeric interference or racemization was observed as a result of the derivatization. No analytical interference was noted from endogenous components in rat blood samples. Preliminary data from two male Sprague-Dawley rats given a 2.0 mg/kg intravenous dose of racemic propranolol revealed differential disposition of the two enantiomers. R(+)-Propranolol achieved higher initial concentration but was eliminated more rapidly than S(−)-propranolol. Terminal half-lives of R(+)- and S(−)-propranolol were 19.23 and 51.95 min, respectively, in one rat, and 14.50 and 52.07 min, respectively, in the other.     

Determination of enantiomeric concentrations of a 2,5-diaryltetrahydrofuran (L-668,750), a platelet-activating factor receptor antagonist, in rat plasma using a chiral α1-acid glycoprotein high-performance liquid chromatographic column

Racemic sulfonylated 2,5-diaryltetrahydrofuran [L-668,750, (±)-trans-2-[3-methoxy-5-(2-hydroxy)ethylsulfonyl-4-n-propoxy]-phenyl-5-(3,4,5-trimethoxyphenyl)-tetrahydrofuran, I] is a potent, specific and orally active platelet-activating factor (PAF) receptor antagonist. Its (—)-(2S,5S) enantiomer [L-680,573, (S)-I] exhibited higher PAF antagonistic potency than the (+)-(2R,5R) enantiomer [L-680,574, (R)-I] in vitro and in animal models. For assay of drug concentrations in plasma of rats dosed intravenously or orally with tritium-labeled I, we have developed a high-performance liquid chromatographic (HPLC) method which directly resolved the two enantiomers. The column contained α₁-acid glycoprotein as the chiral stationary phase and was eluted with phosphate buffer, methanol and ethanol at neutral pH. The concentration of each enantiomer in the plasma was then determined by reverse isotope dilution assay. Results showed that the plasma clearance rate of the more potent (S)-I enantiomer was more than ten-fold faster than that of the (R)-I enantiomer; the enantioselective clearance resulted in nearly ten-fold higher concentrations of the latter in plasma at all time points regardless of the dosing route. This paper describes the HPLC chiral resolution method and its application in plasma analysis.     

High-performance liquid chromatographic analysis of methocarbamol enantiomers in biological fluids

Methocarbamol enantiomers in rat and human plasma were quantified using a stereospecific high-performance liquid chromatographic method. Racemic methocarbamol and internal standard, (R)-(−)-flecainide, were isolated from plasma by a single-step extraction with ethyl acetate. After derivatization with the enantiomerically pure reagent (S)-(+)-1-(1-naphthyl)ethyl isocyanate, methocarbamol diastereomers and the (R)-flecainide derivative were separated on a normal-phase silica column with a mobile phase consisting of hexane—isopropanol (95:5, v/v) at a flow-rate of 1.6 ml/min. Ultraviolet detection was carried out at a wavelength of 280 nm. The resolution factor between the diastereomers was 2.1 (α = 1.24). An excellent linearity was observed between the methocarbamol diastereomers/internal standard derivative peak-area ratios and plasma concentrations, and the intra- and inter-day coefficients of variation were always <9.8%. The lowest quantifiable concentration was 0.5 μg/ml for each enantiomer (coefficients of variation of 9.8 and 8.8% for (S)- and (R)-methocarbamol, respectively), while the limit of detection (signal-to-noise ratio 3:1) was approximately 10 ng/ml. The assay was used to study the pharmacokinetics of methocarbamol enantiomers in a rat following intravenous administration of a 120 mg/kg dose of racemic methocarbamol and to evaluate plasma and urine concentrations in a human volunteer after oral administration of a 1000-mg dose of the racemate. The method is suitable for stereoselective pharmacokinetic studies in humans as well as in animal models.     

Determination of celiprolol and oxprenolol in human plasma by high-performance liquid chromatography and the analytical error function

Two reversed-phase HPLC methods with UV detection to quantify celiprolol and oxprenolol in human plasma are described. The analytical methods for the determination of both drugs used the same reversed-phase HPLC column, mobile phase and extraction procedure. Linearity was obtained in the ranges 15.63–1000 and 25–800 ng/ml for celiprolol and oxprenolol, respectively. Intra-day and inter-day variation was lower than 14%. After validation of the methods, analytical error functions were established as S.D. (ng/ml)=3.096+0.041C for celiprolol and S.D. (ng/ml)=8.906+8.075·10⁻⁸C³ for oxprenolol.     

Bevantolol hydrochloride (nc-1400): Absolute configuration and direct separation of the enantiomers

Synthesis of (?)-bevantolol hydrochloride from 3,4-dimethoxyphenethylamine and (S)-(+)-m-tolyl glycidyl ether derived from (R)-(?)-epichlorohydrin established the absolute configuration of the (+) and (?) enantiomer as R and S, respectively. The purity of the enantiomers was determines using a chiral cellulose column (CHIRALCEL OD®) which allowed direct separation of the enantiomers. A separation factor (α) of 4.20 and a resolution factor (Rs) of 9.21 were obtained. © 1995 Wiley-Liss, Inc.     

High-performance liquid chromatographic assay of mepivacaine enantiomers in human plasma in the nanogram per milliliter range

A method enabling quantification of R-(−)- and S-(+)-mepivacaine in human plasma in the low nanogram per milliliter range is described. The procedure involves extraction from plasma with diethyl ether, centrifugation, back-extraction into an acidified aqueous solution, washing with a mixture of pentane and isoamylalcohol, alkalinisation, followed by extraction with a mixture of n-pentane and isoamylalcohol. After evaporation of the organic phase, the residue is redissolved in the mobile phase used for the HPLC analysis, which consists of a 6.8:93.2 (v/v) isopropanol-sodium hydrogenphosphate buffer solution with the pH adjusted to 6.8 using phosphoric acid. The HPLC method has been described previously. Separation of the enantiomers is achieved with an α₁-AGP column and the UV detection wavelength is 210 nm. The minimal detectable concentration is ca. 3 ng/ml and the lower limit of quantification is 5 ng/ml for each enantiomer. For both enantiomers r is >0.9995 over the plasma enantiomeric concentration range of 10.5–1054 ng/ml.     

Enantioselective gas chromatographic assay with electron-capture detection for amlodipine in biological samples

A sensitive enantioselective gas chromatographic assay has been developed for amlodipine, 2-[(2-aminoethoxy)-methyl]-4-(2-chlorophenyl)-3-ethoxycarbonyl-5-methoxycarbonyl-6-methyl-1,4-dihydropyridine, a calcium channel blocking therapeutic agent. The assay involves conversion of the (+)-(R)- and (−)-(S)-enantiomers of amlodipine into their acyl derivatives with the chiral reagent (+)-(S)-α-methoxy-α-trifluoromethylphenylacetyl chloride (Mosher's reagent). Peak separation after chromatography of the diastereomers was larger than 85%, and the lower limit of detection in blood plasma was 0.02 ng/ml for each enantiomer. The method has been used for the measurement of amlodipine enantiomers in human, rat and dog plasma, and in various organs of the rat.     

Analysis of the enantiomers of citalopram and its demethylated metabolites using chiral liquid chromatography

A procedure using a chirobiotic V column is presented which allows separation of the enantiomers of citalopram and its two N-demethylated metabolites, and of the internal standard, alprenolol, in human plasma. Citalopram, demethylcitalopram and didemethylcitalopram, as well as the internal standard, were recovered from plasma by liquid–liquid extraction. The limits of quantification were found to be 5 ng/ml for each enantiomer of citalopram and demethylcitalopram, and 7.5 ng/ml for each enantiomer of didemethylcitalopram. Inter- and intra-day coefficients of variation varied from 2.4% to 8.6% for S- and R-citalopram, from 2.9% to 7.4% for S- and R-demethylcitalopram, and from 5.6% to 12.4% for S- and R-didemethylcitalopram. No interference was observed from endogenous compounds following the extraction of plasma samples from 10 different patients treated with citalopram. This method allows accurate quantification for each enantiomer and is, therefore, well suited for pharmacokinetic and drug interaction investigations. The presented method replaces a previously described highly sensitive and selective high-performance liquid chromatography procedure using an acetylated β-cyclobond column which, because of manufactural problems, is no longer usable for the separation of the enantiomers of citalopram and its demethylated metabolites.     

High-performance liquid chromatographic determination of thiopentene enantiomers in sheep plasma

An HPLC method was developed to determine the plasma concentrations of R(+)- and S(−)-thiopentone for pharmacokinetic studies in sheep. The method required separation of the thiopentone enantiomers from the corresponding pentobarbitone enantiomers which are usually present as metabolites of thiopentone. Phenylbutazone was used as an internal standard. After acidification, the plasma samples were extracted with a mixture of ether and hexane (2:8). The solvent was evaporated to dryness and the residues were reconstituted with sodium hydroxide solution (pH 10). The samples were chromatographed on a 100 mm × 4 mm I.D.. Chiral AGP-CSP column. The mobile phase was 4.5% 2-propanol in 0.1 M phosphate buffer (pH 6.2) with a flow-rate of 0.9 ml/min. This gave k′ values of 1.92, 2.92, 5.71, 9.30 and 11.98 for R(+)-pentobarbitone, S(−)-pentobarbitone, R(+)-thiopentone, S(−)-thiopentone, and phenylbutazone, respectively. At detection wavelength of 287 nm, the limit of quantitation was 5 ng/ml for R(+)-thiopentone and 6 ng/ml for S(−)-thiopentone. The inter-day coefficients of variation at concentrations of 0.02, 0.1 and 8 μg/ml were, respectively, 4.8, 4.4 and 3.5% for R(+)-thiopentone and, respectively, 5.0, 4.3 and 3.9% for S(−)-thiopentone (n = 6 each enantiomer). At the same concentrations, the intra-day coefficients of variation from six sets of replicates (measured over six days) were, respectively, 8.0, 8.0 and 8.8% for R(+)-thiopentene and 8.8, 7.4 and 9.6% for S(−)-thiopentone. Linearity over the standard range, 0.01–40 μg/ml, was shown by correlation coefficients> 0.998. This method has proven suitable for pharmacokinetic studies of thiopentone enantiomers after administration of rac-thiopentone in human plasma also and would be suitable for pharmacokinetic studies of the pentobarbitone eantiomers.     

High-performance liquid chromatographic determination of (S)- and (R)-propanolol in human plasma and urine with a chiral β-cyclodextrin bonded phase

The determination of propanolol enantiomers in microsamples of human plasma and urine by HPLC using a chiral stationary phase is described. After extraction from 200 μl of plasma or urine with racemic alprenolol as internal standard (I.S.), the enantiomers are separated on a β-cyclodextrin column with a polar organic mobile phase and determined by fluorescence detection. The retention times of I.S. and propranolol enantiomers are about 12–13 min and 16–18 min, respectively. Peak resolutions are 1.4 for I.S. and 2.2 for propranol. The use of alprenolol as I.S. improves significantly the coefficients of variation (C.V.: 0.6–4.2%). Sensitivity is approximately 1.5 ng/ml per propranolol enantiomer. The assay is applied to pharmacokinetic studies of racemic propranolol in human biological fluids. The (S)-propranolol levels are always higher than the (R)-antipode concentrations in plasma and urine.     

Influence of Gasoline Inhalation on the Enantioselective Pharmacokinetics of Fluoxetine in Rats

Fluoxetine is used clinically as a racemic mixture of (+)‐(S) and (–)‐(R) enantiomers for the treatment of depression. CYP2D6 catalyzes the metabolism of both fluoxetine enantiomers. We aimed to evaluate whether exposure to gasoline results in CYP2D inhibition. Male Wistar rats exposed to filtered air (n = 36; control group) or to 600 ppm of gasoline (n = 36) in a nose‐only inhalation exposure chamber for 6 weeks (6 h/day, 5 days/week) received a single oral 10‐mg/kg dose of racemic fluoxetine. Fluoxetine enantiomers in plasma samples were analyzed by a validated analytical method using LC‐MS/MS. The separation of fluoxetine enantiomers was performed in a Chirobiotic V column using as the mobile phase a mixture of ethanol:ammonium acetate 15 mM. Higher plasma concentrations of the (+)‐(S)‐fluoxetine enantiomer were found in the control group (enantiomeric ratio AUC_{(+)‐(S)/(–)‐(R)} = 1.68). In animals exposed to gasoline, we observed an increase in AUC_0‐∞ for both enantiomers, with a sharper increase seen for the (–)‐(R)‐fluoxetine enantiomer (enantiomeric ratio AUC_{(+)‐(S)/(–)‐(R)} = 1.07), resulting in a loss of enantioselectivity. Exposure to gasoline was found to result in the loss of enantioselectivity of fluoxetine, with the predominant reduction occurring in the clearance of the (–)‐(R)‐fluoxetine enantiomer (55% vs. 30%). Chirality 25:206–210, 2013. © 2013 Wiley Periodicals, Inc.     

Semi-preparative chromatographic purification of the enantiomers S-(−)-amlodipine and R-(+)-amlodipine

Pharmacokinetic studies of optically pure compounds after single enantiomer administration are becoming increasingly important. The process of racemization in vivo can diminish all expected advantages of single enantiomer treatment. Amlodipine, one of the calcium channel blockers, currently used in therapy as a racemate, is one of such drugs under study. In order to administer single enantiomers of amlodipine to healthy volunteers both were chromatographically purified and characterised. The two optical isomers of amlodipine, active S-(−)- and non-active R-(+)-amlodipine, were purified using chromatographic procedure adopted from the analytical separation. Enantiomers were successfully converted to benzenesulphonic salt without any racemization. All semi-preparative purifications were monitored with complementary analytical methods, HPLC and CE, along with the determination of optical activity so that the final product was sufficiently defined for further in vivo studies. The analytical method developed for the determination of plasma concentrations of each enantiomer of amlodipine in these studies is also briefly described.     

Steady‐State Plasma Concentration of Donepezil Enantiomers and Its Stereoselective Metabolism and Transport In Vitro

The aim of the present study was to elucidate the differences in the plasma concentration of two enantiomers of donepezil in Chinese patients with Alzheimer's disease (AD) and investigate in vitro stereoselective metabolism and transport. Donepezil enantiomers were separated and determined by LC‐MS/MS using D5‐donepezil as an internal standard on a Sepax Chiralomix SB‐5 column. In vitro stereoselective metabolism and transport of donepezil were investigated in human liver microsomes and MDCKII‐MDR1 cell monolayer. Pre‐dose (Css‐min) plasma concentrations were determined in 52 patients. The mean plasma level of (R)‐donepezil was 14.94 ng/ml and that of (S)‐donepezil was 23.37 ng/ml. One patient's plasma concentration of (R)‐donepezil was higher than (S)‐donepezil and the ratio is 1.51. The mean plasma levels of (S)‐donepezil were found to be higher than those of (R)‐donepezil in 51 patients and the ratio of plasma (R)‐ to (S)‐donepezil varies from 0.34 to 0.85. In the in vitro microsomal system, (R)‐donepezil degraded faster than (S)‐donepezil. V_max of (R)‐donepezil was significantly higher than (S)‐donepezil. The P‐gp inhibition experiment shown that the P_app of the two enantiomers was higher than 200 and the efflux ratios were 1.11 and 0.99. The results of the P‐gp inhibition identification experiment showed IC50 values of 35.5 and 20.4 μM, respectively, for the two enantiomers. The results indicate that donepezil exhibits stereoselective hepatic metabolism that may explain the differences in the steady‐state plasma concentrations observed. Neither (R)‐ nor (S)‐donepezil was a P‐gp substance and the two enantiomers are highly permeable through the blood–brain barrier. Chirality 25:498–505, 2013. © 2013 Wiley Periodicals, Inc.     

Modified gas chromatographic–mass spectrometric assay for the determination of gacyclidine enantiomers in human plasma

A modified method for the determination of gacyclidine enantiomers in human plasma by GC–MS with selected-ion monitoring using the deuterated derivative of gacyclidine (d₃-gacyclidine) as internal standard was developed. Following a single-step liquid–liquid extraction with hexane, drug enantiomers were separated on a chiral fused-silica capillary column (CP-Chirasil-Dex; Chrompack). The fragment ion, m/z 266, was selected for monitoring d₃-gacyclidine (retention times of 35.2 and 35.6 min for the (+)- and (−)-enantiomer, respectively) whereas the fragment ion, m/z 263, was selected for quantitation of gacyclidine (retention times of 35.4 and 35.9 min for the (+)- and (−)-enantiomer, respectively). The limit of quantitation for each enantiomer was 0.3 ng/ml, using 1 ml of sample, with a relative standard deviation (RSD) <14% and a signal-to-noise ratio of 5. The extraction recovery of both gacyclidine enantiomers from human plasma was about 75%. The calibration curves were linear (r²>0.996) over the working range of 0.312 to 20 ng/ml. Within- and between-day RSD were <9% at 5, 10 and 20 ng/ml, and <16% at 0.312, 0.625, 1.25 and 2.5 ng/ml. Intraday and interday bias were less than 11% for both enantiomers. The chromatographic behavior of d₃-gacyclidine remained satisfactory even after more than 500 injections. Applicability of this specific and stereoselective assay is demonstrated for a clinical pharmacokinetic study with racemic gacyclidine.     

Correlation between R/S enantiomer ratio of lansoprazole and CYP2C19 activity after single oral and enteral administration

The purpose of this study was to investigate whether CYP2C19 activity can be estimated from plasma concentrations of lansoprazole enantiomers 4 h (C_4h) after single administration by oral and enteral routes. Sixty‐nine subjects, 22 homozygous extensive metabolizers (homEMs), 32 heterozygous EMs (hetEMs), and 15 poor metabolizers (PMs), participated in the study. After a single oral or enteral dose of racemic lansoprazole (30 mg), plasma concentrations of lansoprazole enantiomers were measured 4 h postdose. The R/S ratio of lansoprazole at 4 h differed significantly among the three groups (P < 0.0001) regardless of the administration route. The R/S ratio of lansoprazole in CYP2C19 PMs ranged from 3.0 to 13.7, whereas in homEMs and hetEMs the ratio ranged from 8.6 to 90 and 2.1 to 122, respectively. The relationship between (S)‐lansoprazole concentration and R/S ratio of lansoprazole at C_4h is given by the following formula: log₁₀ [R/S ratio] = 2.2 – 0.64 × log₁₀ [C_4h of (S)‐lansoprazole] (r = 0.867, P < 0.0001). Thus, phenotyping CYP2C19 using the R/S enantiomer ratio of lansoprazole seems unlikely. However, to obtain a pharmacological effect similar to that in CYP2C19 PMs, we can presume that lansoprazole has a sufficient effect in the patient with an R/S enantiomer ratio at 4 h ≤ 13.70 and (S)‐lansoprazole concentration at 4 h ≥ 50 ng/ml. Chirality 2010. © 2009 Wiley‐Liss, Inc.     

Stereoselective Determination of Metoprolol and its Metabolite α‐Hydroxymetoprolol in Plasma by LC‐MS/MS: Application to Pharmacokinetics during Pregnancy

Metoprolol is available for clinical use as a racemic mixture. The S‐(?)‐metoprolol enantiomer is the one expressing higher activity in the blockade of the β₁‐adrenergic receptor. The α‐hydroxymetoprolol metabolite also has activity in the blockade of the β₁‐adrenergic receptor. The present study describes the development and validation of a stereoselective method for sequential analysis of metoprolol and of α‐hydroxymetoprolol in plasma using high‐performance liquid chromatography with tandem mass spectrometry (LC‐MS/MS). 1‐ml aliquots of plasma were extracted with dichloromethane : diisopropyl ether (1:1, v/v). Metoprolol enantiomers and α‐hydroxymetoprolol isomers were separated on a Chiralpak AD column (Daicel Chemical Industries, New York, NY, USA) and quantitated by LC‐MS/MS. The limit of quantitation obtained was 0.2 ng of each metoprolol enantiomer/ml plasma and 0.1 ng/ml of each α‐hydroxymetoprolol isomer/ml plasma. The method was applied to the study of kinetic disposition of metoprolol in plasma samples collected up to 24 h after the administration of a single oral dose of 100‐mg metoprolol tartrate to a hypertensive parturient with a gestational age of 42 weeks. The clinical study showed that the metoprolol pharmakokinetics is enantioselective, with the observation of higher area under the curve (AUC)^0?∞ values for S‐(?)‐metoprolol (AUC_S‐(?)/AUC_R‐(+) = 1.81) and the favoring of the formation of the new chiral center 1′R of α‐hydroxymetoprolol (AUC^0?∞_1′R/1′S = 2.78). Chirality, 25:1–7, 2013. © 2012 Wiley Periodicals, Inc.