The chromatographic analysis of enantiomers in drug metabolism studies

Pharmacological importance of stereochemical resolution of enantiomeric drugs

Drug enantiomers have identical properties in an achiral environment, but should be considered as different chemical compounds. This is because they often differ considerably in potency, pharmacological activity and pharmacokinetic profile, since the modules with which they interact in biological systems are also optically active. Within biological systems, the metabolism of one isomer may be via a different pathway or occur at a different rate from that of the other isomer. Preferential binding of one isomer to plasma proteins may cause differences in circulating free drug and hence alter concentrations at active sites. Interactions of both isomers may differ at the active sites through which pharmacological action is mediated. Actions and levels of activity of the stereoisomers in vivo may also differ. All the pharmacological activity may reside in a single enantiomer, whereas several possibilities exist for the other enantiomer-- it may be inactive, have a qualitatively different effect, an antagonistic effect or produce greater toxicity. Two isomers may have nearly identical qualitative pharmacological activity, qualitatively similar pharmacological activity but quantitatively different potency, or qualitatively different pharmacological activity. To avoid adverse effects and optimise the therapeutic value of enantiomeric drugs, it is necessary that methods for the resolution of racemates be evolved and devolved to determine isomeric purity, establish the effectiveness of isomers of the drug, and detect the presence of an enantiomer with lower therapeutic activity and undesirable adverse effects. Even if a drug is given as a pure enantiomer, methods to discriminate between enantiomers are required because racemisation can occur both in vitro and in vivo. Methods developed for resolution of drug enantiomers should facilitate routine testing of single isomers and their metabolites, studies of pharmacological, toxicological and clinical effectiveness, routine analysis of racemates, pure enantiomers or intermediates in manufacturing processes, and investigation of the potential for inversion of an enantiopure drug substance during the early stages of drug development and therapeutic drug monitoring.

Stereoselective pharmacokinetics of dihydropyridine calcium antagonists

Many dihydropyridine calcium antagonists are widely used for the treatment of angina and hypertension, and many more are under development. Most of these drugs have one or more chiral centre, and the pharmacological activity between the enantiomers for these drugs is known to be markedly different. First, the stereospecific assay methods for these drugs in plasma or serum are reviewed with emphasis on chiral stationary phase high-performance liquid chromatography for their determination. Next, the stereoselective pharmacokinetics of these drugs (nilvadipine, nitrendipine, felodipine, nimodipine, manidipine, benidipine and nisoldipine) in animals, healthy subjects and patients with hepatic disease is reviewed. Enantiomer-enantiomer interaction, enantiomeric inversion and the stereochemical aspects of pharmacokinetic drug interactions in these drugs are also described.

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.

Sensitive procedure for the determination of reboxetine enantiomers in human plasma by reversed-phase high-performance liquid chromatography with fluorimetric detection after chiral derivatization with (+)-1-(9-fluorenyl)ethyl chloroformate

A sensitive and selective high-performance liquid chromatographic method for the determination of reboxetine enantiomers in human plasma was developed. Although two chiral centres are present in reboxetine, its stereospecific synthesis leads to two rather than four possible enantiomers. After extraction from plasma and reaction with (+)-1-(9-fluorenyl)ethyl chloroformate, reboxetine enantiomers were separated as diastereoisomeric derivatives by reversed-phase high-performance liquid chromatography (HPLC) and determined by fluorimetric detection. The HPLC analysis time was about 90 min. The linearity, precision, accuracy and limit of quantification of the method were evaluated. No interference from blank plasma sample was observed. The suitability of the method for in vivo samples was assessed by the analysis of plasma samples obtained from a healthy male volunteer who had received a single oral dose of 4 mg of reboxetine in tablet form.

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.

The importance of stereochemistry in drug action and disposition

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

Chiral separation by high performance liquid chromatography. I. Review on indirect separation of enantiomers as diastereomeric derivatives using ultraviolet, fluorescence and electrochemical detection

The increased attention on the therapeutic implications of stereoisomerism has provided an impetus for the development of analytical methods for enantiomeric separation. The indirect method of separation of enantiomers as diastereomers using high performance liquid chromatography (HPLC) has emerged as an efficient and versatile approach. This is due mainly to the availability of numerous chiral derivatization reagents (CDRs). This article reviews CDRs useful for the development of an indirect HPLC method using ultraviolet, fluorescence and electrochemical detection. In addition, factors crucial for the development of the indirect method are discussed.

The chiral chromatographic separation of β-adrenoceptor blocking drugs

Over 100 chromatographic procedures for the separation of beta-blocker enantiomers are reviewed including a large number for the analysis of biological samples. All the principal chiral chromatographic procedures have found use, namely Chiral Mobile Phase Additives (CMPA), Chiral Derivatization Agents (CDA) and Chiral Stationary Phases (CSP). Chiral Mobile Phase Additives are less frequently employed than the other two procedures and many of the earlier methods were based on the use of CDAs. However, the recent development of sophisticated custom-made CSPs has allowed the separation of native (underivatized) analytes and this approach appears to be gaining in popularity. The beta-blockers are an extensive group of drugs and stereoselective separations have been reported for 40 different structures.

Stereoselective pharmacokinetics and inversion of suprofen enantiomers in humans

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

High-performance liquid chromatographic method for simultaneous determination of enantiomers of 5-dimethylsulphamoyl-6,7-dichloro-2-dihydrobenzofuran-2, carboxylic acid and its N-monodemethyl metabolite in monkey plasma and urine after chiral derivatization

A quantitative method for the simultaneous high-performance liquid chromatographic (HPLC) resolution and determination of the enantiomers of 5-dimethylsulphamoyl-6,7-dichloro-2,3-dihydrobenzofuran-2-carboxyl ic acid, a new diuretic, and its N-monodemethylated metabolite in monkey plasma and urine is described. The method includes diethyl ether extraction of the samples and S-(-)-alpha-methylbenzylamide derivatization of the extract, followed by reversed-phase solid-phase extraction and injection of the resulting diastereoisomers onto a reversed-phase HPLC column. Baseline separation was obtained. The assay showed linearity over the range 0.1-50 micrograms/ml of plasma and 0.25-500 microliters of urine, with a lower limit of detection of ca. 0.01 micrograms/ml for each of the enantiomers. The method is adequate for pharmacokinetic and enantioselective disposition studies of both the diuretic and its metabolite.

Simultaneous determination of disopyramide and its mono-N-dealkylated metabolite enantiomers in human plasma and urine by enantioselective high-performance liquid chromatography

Enantiomers of disopyramide (DP) and its mono-N-dealkylated metabolite (MND) were determined in human plasma and urine by enantioselective high-performance liquid chromatography using a chiral stationary-phase column. This method was precise and sensitive: the mean recoveries from plasma at a concentration of 0.5 microgram/ml were 101.1% for (+)-DP, 98.0% for (-)-DP, 94.4% for (+)-MND and 82.9% for (-)-MND; the within- and between-day coefficients of variation at the same concentration were 4.4 and 3.3% for (+)-DP, 4.7 and 4.1% for (-)-DP, 6.5 and 4.1% for (+)-MND and 7.8 and 2.4% for (-)-MND for plasma; the lower detection limits were 40 ng/ml for (+)-DP, 80 ng/ml for (-)-DP, 100 ng/ml for (-)-MND and 200 ng/ml for (+)-MND, for 0.5 ml of plasma and 0.2 ml of urine. The ultrafiltration technique was used for determination of the unbound concentration of DP enantiomers in plasma. A preliminary study of the determination of DP and MND enantiomers in plasma and urine samples from a healthy subject given racemic DP demonstrated the clinical applicability of the present method for therapeutic monitoring and pharmacokinetic studies.

Enantiomer analysis of E- and Z-10-hydroxyamitriptyline in human urine

E- and Z-10-hydroxyamitriptyline (E- and Z-10-OH-AT) are racemic alcoholic metabolites of the antidepressant amitriptyline. Their enantiomers were separated by high-performance liquid chromatography as diastereomeric derivatives using R-(+)-alpha-methoxy-alpha-trifluoromethylphenylacetyl chloride (Mosher's reagent). Although E-10-hydroxyamitriptyline excreted in patient urine in free form or as the O-glucuronide consisted primarily of the (-)-enantiomer, the N-glucuronide contained similar amounts of the two enantiomers. Z-10-OH-AT was analysed in one patient and an excess of the (+)-isomer was found in the unconjugated, total conjugated and N-glucuronidated metabolite. The specific optical rotation of (-)-E-10-OH-AT was determined.

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.

Stereoselective determination of flecainide in human plasma by high-performance liquid chromatography with fluorescence detection

Enantiomers of a drug may differ in their pharmacological activities or their disposition constants. We now describe a stereoselective analytical method for the determination of the antiarrhythmic agent flecainide in plasma. The resolution of the enantiomers is achieved by high-performance liquid chromatography (HPLC) on a normal phase silica column following derivatization with the optically active reagent (-)-methyl chloroformate. The eluting diastereoisomers are monitored by fluorescence detection at an excitation wavelength of 305 nm and an emission wavelength of 340 nm. The limit of sensitivity for the assay is as low as 2.5 ng/mL for each enantiomer using 1 mL of plasma. A new liquid-liquid extraction procedure with high recovery (greater than 95%) and high selectivity is also reported. The intra- and interassay coefficient of variation for replicated analysis of spiked plasma samples is less than 4.0% and 7.0%, respectively. The method is suitable for single and multiple dose pharmacokinetic studies in healthy volunteers or in patients.

Resolution of enantiomers of the antiarrhythmic drug encainide and its major metabolites by chiral derivatization and high-performance liquid chromatography

Commercially available chiral columns were unable to provide adequate resolution of enantiomers of the antiarrhythmic drug encainide or its major metabolites. The homochiral derivatizing agent, (-)-menthyl chloroformate, was found to react at the tertiary piperidine nitrogen of racemic encainide providing two menthyl carbamate diastereomers. The individual diastereomers could be separated with baseline resolution on normal-phase high-performance liquid chromatography on a silica column. Structures of the derivatives were confirmed by electron impact mass spectrometry and 1H NMR spectroscopy. The method was adapted for the chiral analysis of the major metabolites of encainide. The limit of sensitivity for racemic encainide was 10 ng on column and it was possible to detect a mixture containing (+)- and (-)-encainide in a ratio of 1:99. Preliminary studies indicated that (-)-encainide was O-demethylated to a greater extent than the (+)-enantiomer by rat liver microsomes.

Enantioselective drug analysis: problems and resolutions

1. With the increasing appreciation that the enantiomers of a chiral drug can differ pharmacokinetically and/or pharmacodynamically, there is considerable interest in methods for the resolution and quantification of enantiomers. 2. Enantiomers possess identical physical and chemical properties in a symmetrical environment and, therefore, their resolution requires the introduction of an asymmetric or chiral environment allowing diastereomeric interactions. This can be achieved using a number of chromatographic techniques, of which the most developed and widely used is high-performance liquid chromatography (HPLC). 3. Resolution and quantification of enantiomers can be performed using HPLC by either converting the enantiomers to covalent diastereomers prior to chromatography or introducing a chiral environment to the chromatographic system, thereby allowing temporary diastereomeric interactions. 4. Antibodies are chiral molecules which can bind the enantiomers of a chiral drug in a differential manner. This is the basis of enantioselective immunoassay, which is a promising technique for the enantioselective analysis of drugs in biological fluids. 5. Each of the methods available has its limitations, advantages and potential applications in the pharmaceutical industry.

Species differences in the chirality of the carbonyl reduction of [14C] fenofibrate in laboratory animals and humans

The prochiral carbonyl group of fenofibrate (isopropyl 2-[4-(4-chlorobenzoyl)phenoxy]-2-methyl propionate) is reduced during its metabolism giving rise to a chiral secondary alcohol, "reduced fenofibric acid." Chiral and diastereomeric HPLC methods have been developed for the determination of its enantiomeric composition and these have been applied to the measurement of the "reduced fenofibric acid" enantiomers in urine of rats, guinea pigs, dogs, and human volunteers given [14C]fenofibrate. In the three animal species, the reduction is markedly enantioselective for the (-)-isomer, the enantiomeric ratios (-/+) being 95:5. This was not due to differences in the excretion of the enantiomers, since when racemic "reduced fenofibric acid" was given to rats it was recovered in the urine with the same enantiomeric composition as the dose form. In humans the ratio was 52:48 showing the lack of stereoselectivity of reduction in this species.

Direct high-performance liquid chromatographic resolution of dihydropyridine enantiomers

A direct high-performance liquid chromatographic enantioseparation on a chiral stationary phase using a protein as chiral selector was achieved for some dihydropyridines with different structural groups (amine, amide, acid and hydroxyl groups). The protein involved was alpha 1-acid glycoprotein and the work was mainly carried out with an EnantioPac column. An improved version of this chiral column (Chiral-AGP) was compared with the previous one and gave interesting results.

Determination of the S(+)- and R(-)-enantiomers of baclofen in plasma and urine by gas chromatography using a chiral fused-silica capillary column and an electron-capture detector

A sensitive and enantiospecific gas chromatographic method for the determination of the S(+)- and R(-)-enantiomers of baclofen (I and II) in plasma and urine has been developed and validated. The method is based on the complete resolution of the derivatized enantiomers on a chiral fused-silica capillary column. The hydrochloride salt of a (-)-fluoro analogue of baclofen (III.HCl) was used as the internal standard in plasma, the hydrochloride salt of a (+)-fluoro analogue of baclofen (IV.HCl) as the internal standard in urine. Rapid and convenient isolation of the compounds was achieved using reversed-phase Bond-Elut C18 columns. After elution, the compounds were converted into isobutyl esters and purified by base-specific solvent extraction. The isobutyl esters were then N-acylated with heptafluorobutyric anhydride. The derivatives were quantitated after separation on the chiral column using electron-capture detection. The analysis of spiked plasma and urine samples demonstrated the good accuracy and precision of the method, with limits of quantitation of 25 nmol/l for I and II in plasma and of 2 mumol/l for I and II in urine. The method appears to be suitable for use in pharmacokinetic studies of the enantiomers in plasma and urine from animals and man after administration of the racemic baclofen.

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

The 2-arylpropionic acids are currently an important group of non-steroidal anti-inflammatory agents. They contain a chiral centre, and in vitro studies on inhibition of prostaglandin synthesis show that their activity resides almost exclusively in the S(+)-isomers. However, this stereoselectivity of action is not manifest in vivo, due to the thus-far-unique unidirectional metabolic inversion of the chiral centre from the inactive R(-)-isomers to the S(+)-antipodes. Available evidence strongly suggests that this reaction proceeds via the formation of the acyl CoA thioesters of the 2-arylpropionates, but the participation of enzyme(s) in the inversion process remains uncertain. Although the chiral inversion is seemingly a general feature of the fate of 2-arylpropionates, there do occur important combinations of acid and species where the reaction is not extant. The stereochemistry of the chiral centre of these acids also influences other aspects of their disposition, including the oxidative metabolism of the aryl/arylakyl moiety, glucuronidation of the -COOH group and plasma protein binding, and the importance of certain of these becomes more evident when renal function is impaired. The biological consequences of the metabolic chiral inversion and enantioselective disposition of the 2-arylpropionates have been summarized in terms of their implications for the development and use of safer and more effective drugs of this class.

Stereoselective disposition of flupentixol: influence on steady-state plasma concentrations in schizophrenic patients

Steady-state plasma concentrations of cis(Z)-flupentixol (active principle) and trans(E)-flupentixol (inactive) were measured in 41 patients at least on one occasion. Results indicate that concentrations of the trans-isomer are significatively higher. This demonstrates that the two isomers are not handled in the same way by the organism. This may be relevant if plasma level monitoring is performed using non-specific analytical methods.

Determination of (+)- and (-)-nilvadipine in human plasma using chiral stationary-phase liquid chromatography and gas chromatography-mass spectrometry, and a preliminary pharmacokinetic study in humans

A stereoselective and sensitive method for the determination of nilvadipine, a new dihydropyridine calcium antagonist, in human plasma was developed. An internal standard, the deuterated analogue of racemic nilvadipine, was added to the plasma and extracted with an n-hexane:ethyl acetate (92.5:7.5) mixture under alkaline conditions. Each enantiomer in the extract was separated on a chiral stationary-phase column (Chiralpak OT(+)) for HPLC, and the effluents containing the respective isomers were collected. Each effluent was analyzed by fused-silica capillary column GC-electron capture negative ion chemical ionization MS. The mass spectrometer was set to monitor the molecular anions of nilvadipine and the internal standard. Calibration curves were linear for concentrations of each enantiomer from 0.025 to 10 ng/mL. The mean intra- and interassay precisions, as estimated by RSD, were less than 6% for each enantiomer. Assay suitability was assessed in a pharmacokinetic study in which four subjects were given a 6-mg oral dose of racemic nilvadipine. The t1/2 values of the two enantiomers were similar, but the AUC values of the more potent (+)-enantiomer were 2.4-3.6 times higher than those of its optical antipode.

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.