Stereoselectivity in clinical pharmacokinetics and drug development

Macrochirality of Self-Assembled and Co-assembled Supramolecular Structures of a Pair of Enantiomeric Peptides

Although macrochirality of peptides’ supramolecular structures has been found to play important roles in biological activities, how macrochirality is determined by the molecular chirality of the constituted amino acids is still unclear. Here, two chiral peptides, Ac-^LK^LH^LH^LQ^LK^LL^LV^LF^LF^LA^LK-NH₂ (KK-11) and Ac-^DK^DH^DH^DQ^DK^DL ^DV^DF^DF^DA^DK-NH₂ (KKd-11), which were composed entirely of either L- or D-amino acids, were designed for studying the chiral characteristics of the supramolecular microstructures. It was found that monocomponent KK-11 or KKd-11 self-assembled into right- or left-handed helical nanofibrils, respectively. However, when they co-assembled with concentration ratios varied from 1:9 to 9:1, achiral nanowire-like structures were formed. Both circular dichroism and Fourier transform infrared spectra indicated that the secondary structures changed when the peptides co-assembled. MD simulations indicated that KK-11 or KKd-11 exhibited a strong propensity to self-assemble into right-handed or left-handed nanofibrils, respectively. However, when KK-11 and KKd-11 were both presented in a solution, they had a higher probability to co-assemble instead of self-sort. MD simulations indicated that, in their mixtures, they formed nanowires without handedness feature, a good agreement with experimental observation. Our results shed light on the molecular mechanisms of the macrochirality of peptide supramolecular microstructures.

Determination of flumequine enantiomers and 7-hydroxyflumequine in water and sediment by chiral HPLC coupled with hybrid quadrupole-time-of-flight mass spectrometer

A liquid chromatography-tandem mass spectrometric (LC-MS/MS) method for simultaneous enantiomeric analysis of flumequine and its metabolite 7-hydroxyflumequine in water and sediment had been developed based on the separation method. Sediment samples were extracted with ACN and EDTA-Mcllvaine buffer solution (40:60, v/v) then were enriched and cleaned-up by Cleanert PEP solid-phase extraction cartridges. The extract solvent, solid cartridges, mobile phase ratios, and chiral separation column were all optimized to reach high sensitivity and selectivity, good peak shape, and satisfactory resolution. The results showed that the calibration curves of flumequine enantiomers and 7-hydroxyflumequine were linear in the range of 1.0 to 200.0 µg/L with correlation coefficients of 0.9822–0.9988, the mean recoveries for both the enantiomers ranged from 69.9–84.6% with relative standard deviations (RSDs) being 13.1% or below. The limits of detection (LODs) for both flumequine enantiomers were 2.5 µg/L and 5.0 µg/kg in water and sediment samples, whereas the limits of quantification (LOQs) were 8.0 µg/L and 15.0 µg/kg, respectively. While the LODs for 7-hydroxyflumequine were 3.2 µg/L in water samples and 7.0 µg/kg in sediment samples. The proposed method will be extended for studies on the degradation kinetics and environmental behaviors and providing additional information for reliable risk assessment of these chiral antibiotics.

Chemoenzymatic synthesis of (2S)-2-arylpropanols through a dynamic kinetic resolution of 2-arylpropanals with alcohol dehydrogenases

We applied Horse Liver Alcohol Dehydrogenase (HLADH) to the enantioselective synthesis of six (2S)-2-arylpropanols, useful intermediates in the synthesis of Profens. The influence of substrate structure and reaction conditions on yields and enantioselectivity were investigated. The high yields and high enantioselectivity towards the (S)-enantiomer obtained in the bioreduction of 2-arylpropionic aldehydes, clearly indicate the achievement of a DKR process through a combination of an enzyme-catalyzed kinetic reduction with a chemical base-catalyzed racemization of the unreacted aldehydes. The racemization step is represented by the keto-enol equilibrium of the aldehyde and can be controlled by modulating pH and reaction conditions.

Investigation of a dual CD chiral CE system for separation of glitazone compounds

A dual CD-CE method for chiral separation of enantiomers of pioglitazone, rosiglitazone and balaglitazone was investigated for the purpose of optimizing the chiral separation. In a previous work a dual CD chiral CE method was used for investigation of glitazone compounds in drug substance and pharmaceutical formulation and the studies showed that all studied glitazones were racemic mixtures. This CE method could separate the enantiomers with a resolution (R(S)) of about 3. However, another study on single glitazone enantiomers pointed out that a higher R(S) is needed to achieve more accurate results for separation of a small amount of one enantiomer in the presence of a high amount of the other enantiomers. The focus of this investigation was thus directed toward the effect of CDs and the pH of the running buffer to achieve a better enantioseparation. Initially CE systems with each of heptakis(2,6-di-O-methyl)-beta-CD (DM-beta-CD) and heptakis(6-sulfobutylether)-beta-CD (SB-beta-CD) as single CD added were investigated at three different pH values (2.5, 5.0 and 9.3). After having chosen the best of these three pH values a dual CD system was further investigated and optimized. The optimization work was then focused on the concentration of the two CDs and the pH of the running buffer and was performed using factorial design experiments. A mixture of a DM-beta-CD and SB-beta-CD was found to be optimal and necessary to achieve enantioseparation with sufficiently high R(S). In order to further verify the importance of the SB-beta-CD, a CE system with the DM-beta-CD added and substitution or partial substitution of the SB-beta-CD by SDS was studied for comparison. (1)H-NMR studies were performed to get a more detailed understanding of the interactions between the glitazones and the CDs used.The optimized dual CD-CE method for chiral separation of the enantiomers of pioglitazone, rosiglitazone and balaglitazone using a running buffer containing 50 mM borate buffer pH 9.7, 12 mM of SB-beta-CD and 3 mM of DM-beta-CD provided a high R(S) (R(S) between 5.5 and 8.8).

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%.

Enantioselective 2-hydroxylation of RS-8359, a selective and reversible MAO-A inhibitor, by cytochrome P450 in mouse and rat liver microsomes

RS-8359, (+/-)-4-(4-cyanoanilino)-5,6-dihydro-7-hydroxy-7H-cyclopenta[d]-pyrimidine is a racemic compound with a selective and reversible monoamine oxidase A (MAO-A) inhibition activity. The substrate and product enantioselectivity with respect to 2-hydroxylation of RS-8359 enantiomers was studied using mouse and rat liver microsomes. In mice, the (S)-enantiomer was transformed to the cis-diol metabolite, whereas the (R)-enantiomer to the trans-diol metabolite. The Vmax/Km value for the formation of the cis-diol metabolite from the (S)-enantiomer was sevenfold greater than that for the formation of the trans-diol metabolite from the (R)-enantiomer. The greater Vmax/Km value for the (S)-enantiomer was due to the tenfold smaller Km value compared to that for the (R)-enantiomer. The results were in fair agreement with the previously reported low plasma concentrations of the (S)-enantiomer and the high recovery of the cis-diol metabolite derived from the (S)-enantiomer in urine after oral administration of RS-8359 to mice. Similarly to mice, in rats the (R)-enantiomer was transformed to the trans-diol metabolite, whereas the (S)-enantiomer yielded the cis-diol and trans-diol metabolites. The Vmax/Km value for the (R)-enantiomer was larger than that for the (S)-enantiomer in rats, indicating that the low plasma concentration of the (S)-enantiomer in rats might be caused by a metabolic reaction other than P450-dependent hydroxylation. CYP3A was shown to be responsible for the trans-diol formation from the (R)-enantiomer.

Pharmacodynamics and pharmacokinetics of nonsteroidal anti-inflammatory drugs in species of veterinary interest

This review summarises selected aspects of the pharmacokinetics (PK) and pharmacodynamics (PD) of nonsteroidal anti-inflammatory drugs (NSAIDs). It is not intended to be comprehensive, in that it covers neither minor species nor several important aspects of NSAID PD. The limited objective of the review is to summarise those aspects of NSAID PK and PD, which are important to an understanding of PK-PD integration and PK-PD modelling (the subject of the next review in this issue). The general features of NSAID PK are: usually good bioavailability from oral, intramuscular and subcutaneous administration routes (but with delayed absorption in horses and ruminants after oral dosing), a high degree of binding to plasma protein, low volumes of distribution, limited excretion of administered dose as parent drug in urine, marked inter-species differences in clearance and elimination half-life and ready penetration into and slow clearance from acute inflammatory exudate. The therapeutic effects of NSAIDs are exerted both locally (at peripheral inflammatory sites) and centrally. There is widespread acceptance that the principal mechanism of action (both PD and toxicodynamics) of NSAIDs at the molecular level comprises inhibition of cyclooxygenase (COX), an enzyme in the arachidonic acid cascade, which generates inflammatory mediators of the prostaglandin group. However, NSAIDs possess also many other actions at the molecular level. Two isoforms of COX have been identified. Inhibition of COX-1 is likely to account for most of the side-effects of NSAIDs (gastrointestinal irritation, renotoxicity and inhibition of blood clotting) but a minor contribution also to some of the therapeutic effects (analgesic and anti-inflammatory actions) cannot be excluded. Inhibition of COX-2 accounts for most and possibly all of the therapeutic effects of NSAIDs. Consequently, there has been an intensive search to identify and develop drugs with selectivity for inhibition of COX-2. Whole blood in vitro assays are used to investigate quantitatively the three key PD parameters (efficacy, potency and sensitivity) for NSAID inhibition of COX isoforms, providing data on COX-1:COX-2 inhibition ratios. Limited published data point to species differences in NSAID-induced COX inhibition, for both potency and potency ratios. Members of the 2-arylpropionate sub-groups of NSAIDs exist in two enantiomeric forms [R-(-) and S-(+)] and are licensed as racemic mixtures. For these drugs there are marked enantiomeric differences in PK and PD properties of individual drugs in a given species, as well as important species differences in both PK and PD properties.

PK-PD integration and PK-PD modelling of nonsteroidal anti-inflammatory drugs: principles and applications in veterinary pharmacology

Much useful information relevant to elucidation of mechanism of action of nonsteroidal anti-inflammatory drugs (NSAIDs) at the molecular level can be obtained from integrating pharmacokinetic (PK) and pharmacodynamic (PD) data, such data being obtained usually, although not necessarily, in separate studies. Integrating PK and PD data can also provide a basis for selecting clinically relevant dosing schedules for subsequent evaluation in disease models and clinical trials. The principles underlying and uses of PK-PD integration are illustrated in this review for phenylbutazone in the horse and cow, carprofen and meloxicam in the horse, carprofen and meloxicam in the cat and nimesulide in the dog. In the PK-PD modelling approach for NSAIDs, the PK and PD data are generated (usually though not necessarily) in vivo in the same investigation and then modelled in silico, usually using the integrated effect compartment or indirect response models. Drug effect is classically modelled with the sigmoidal E(max) (Hill) equation to derive PD parameters which define efficacy, potency and sensitivity. The PK-PD modelling approach for NSAIDs can be undertaken at the molecular level using surrogates of inhibition of cyclooxygenase (COX) isoforms (or indeed other enzymes e.g. 5-lipoxygenase). Examples are provided of the generation of PD parameters for several NSAIDs (carprofen, ketoprofen, vedaprofen, flunixin and tolfenamic acid) in species of veterinary interest (horse, calf, sheep and goat), which indicate that all drugs investigated except vedaprofen were non-selective for COX-1 and COX-2 in the four species investigated under the experimental conditions used, vedaprofen being a COX-1 selective NSAID. In these studies, plasma concentration was linked to COX inhibitory action in the biophase using an effect compartment model. Data for S-(+)-ketoprofen have been additionally subjected to inter-species modelling and allometric scaling of both PK and PD parameters. For several species values of four PK parameters were highly correlated with body weight, whilst values for PD parameters based on COX inhibition lacked allometric relationship with body weight. PK-PD modelling of NSAIDs has also been undertaken using clinical end-points and surrogates for clinical end-points in disease models. By measurement of clinically relevant indices in clinically relevant models, data generated for PD parameters have been used to set dosages and dose intervals for evaluation and confirmation in clinical trials. PK-PD modelling of NSAIDs is likely to prove superior to conventional dose titration studies for dosage schedule determination, as it sweeps the whole of the concentration-effect relationship for all animals and therefore permits determination of genuine PD parameters. It also introduces time as a second independent variable thus allowing prediction of dosage interval. Using indirect response models and clinically relevant indices, PD data have been determined for flunixin, phenylbutazone and meloxicam in the horse, nimesulide in the dog and meloxicam in the cat.

Role of biological matrices during the analysis of chiral drugs by liquid chromatography

The review article covers advances of chiral drugs analysis by high-performance liquid chromatography (HPLC) methods achieved during last 10 years. Emphasis is given to various aspects of influence of biological matrix in pharmacodynamics, pharmacokinetics, HPLC analysis. Discussed is composition of main biological matrices from the point of view of potential interferences to above-mentioned fields of study. Beside typical analytical approaches to chiral recognition in HPLC, sample pretreatment and/or clean-up by conventional extraction procedures, column switching (CSW) techniques using restricted access materials (RAMs), microdialysis (MCD) is discussed. Measurement of unbound drug concentration and discussion of column maintenance and remedy is an additional source of information and field where knowledge on complex properties and interactions of biological matrix is usefully applicable.

Albendazole sulphoxide enantiomers in pregnant rats' embryo concentrations and developmental toxicity

Three single oral doses (8.5, 10, and 14 mg/kg) of a racemic formulation of albendazole sulphoxide (ABZSO) were administered to pregnant rats on day 10 of gestation. Mother plasma and embryo concentrations of ABZSO enantiomers and albendazole sulphone (ABZSO(2)) were determined 9 h after administration. The (-)-ABZSO enantiomer showed higher peak concentrations in both maternal plasma and embryo than the (+) enantiomer. An increase in embryo concentrations of ABZSO enantiomers and ABZSO(2) was only observed when dose rose to 14 mg/kg. There was an increase in resorption when the dose increased, but significant differences were only found in the higher dose group when compared with the other groups. The incidence of external and skeletal malformations (mostly of the tail, vertebrae and ribs) rose significantly in the 10 mg/kg group, producing almost 20% and 90% of malformed fetuses, respectively, and gross external and skeletal abnormalities in the thoracic region and limbs were also found.

Pharmacodynamics, chiral pharmacokinetics and PK-PD modelling of ketoprofen in the goat

There have been few studies of the pharmacodynamics of nonsteroidal antiinflammatory drugs (NSAIDs) using PK-PD modelling, yet this approach offers the advantage of defining the whole concentration-effect relationship, as well as its time course and sensitivity. In this study, ketoprofen (KTP) was administered intravenously to goats as the racemate (3.0 mg/kg total dose) and as the single enantiomers, S(+) KTP and R(-) KTP (1.5 mg/kg of each). The pharmacokinetics and pharmacodynamics of KTP were investigated using a tissue cage model of acute inflammation. The pharmacokinetics of both KTP enantiomers was characterized by rapid clearance, short mean residence time (MRT) and low volume of distribution. The penetration of R(-) KTP into inflamed (exudate) and noninflamed (transudate) tissue cage fluids was delayed but area under the curve values were only slightly less than those in plasma, whereas MRT was much longer. The S(+) enantiomer of KTP penetrated less readily into exudate and transudate. Unidirectional inversion of R(-) to S(+) KTP occurred. Both rac-KTP and the separate enantiomers produced marked inhibition of serum thromboxane B2 (TxB2) synthesis (ex vivo) and moderate inhibition of exudate prostaglandin E2 (PGE2) synthesis (in vivo); pharmacodynamic variables for S(+) KTP were Emax (%) = 94 and 100; IC50 (microg/mL) = 0.0033 and 0.0030; N = 0.45 and 0.58, respectively, where Emax is the maximal effect, IC50 the plasma drug concentration producing 50% of Emax and N the slope of log concentration/effect relationship. The IC50 ratio, serum TxB2:exudate PGE2 was 1.10. Neither rac-KTP nor the individual enantiomers suppressed skin temperature rise at, or leucocyte infiltration into, the site of acute inflammation. These data illustrate for KTP shallow concentration-response relationships, probable nonselectivity of KTP for cyclooxygenase (COX)-1 and COX-2 inhibition and lack of measurable effect on components of inflammation.

Ketoprofen in the cat: pharmacodynamics and chiral pharmacokinetics

The non-steroidal anti-inflammatory drug ketoprofen (KTP) was administered as the racemate to cats intravenously (IV) and orally at clinically recommended dose rates of 2 and 1 mg/kg, respectively, to establish its chiral pharmacokinetic and pharmacodynamic properties. After IV dosing, clearance was more than five times greater and elimination half-life and mean residence time were approximately three times shorter for R(-) KTP than for S(+) KTP. Absorption of both S(+) and R(-) enantiomers was rapid after oral dosing and enantioselective pharmacokinetics was demonstrated by the predominance of S(+) KTP, as indicated by plasma AUC of 20.25 (S(+)KTP) and 4.09 (R(-)KTP) microg h/mL after IV and 6.36 (S(+)KTP) and 1.83 (R(-)KTP) microg h/mL after oral dosing. Bioavailability after oral dosing was virtually complete. Reduction in ex vivo serum thromboxane (TX)B(2) concentrations indicated marked inhibition of platelet cyclo-oxygenase (COX)-1 for 24 h after both oral and IV dosing and inhibition was statistically significant for 72 h after IV dosing. Both oral and IV rac-KTP failed to affect wheal volume produced by intradermal injection of the mild irritant carrageenan but wheal skin temperature was significantly inhibited by IV rac-KTP at some recording times. Possible reasons for the disparity between marked COX-1 inhibition and the limited effect on the cardinal signs of inflammation are considered. In a second experiment, the separate enantiomers of KTP were administered IV, each at the dose rate of 1mg/kg. S(+)KTP again predominated in plasma and there was unidirectional chiral inversion of R(-) to S(+)KTP. Administration of both enantiomers again produced marked and prolonged inhibition of platelet COX-1 and, in the case of R(-)KTP, this was probably attributable to S(+)KTP formed by chiral inversion.

Enantioselective analytical methods in pharmacokinetics with specific reference to genetic polymorphic metabolism

The new trend towards developing enantiospecific drugs has increased the interest in enantiospecific pharmacokinetics of chiral drugs, mainly in the case where only one of the two enantiomers is responsible for the pharmacological activity. Enantiospecific bioassays are also useful in investigating the pharmacokinetic behaviour of the two enantiomers when a given drug is marketed as racemate. The stability of the stereogenic centre in vitro and in vivo, as far as unidirectional and bidirectional inversions are concerned, is another reason for requiring stereospecific assay and bioassay. These assays are often complicated in order to achieve quantification, mainly for in vivo measurements, which are often in the low pg/ml range. This paper considers the enantiospecific bioassays, the methods and approaches used, the need for chemical derivatization, and the difficulties involved. It includes a specific discussion for the genetic polymorphic metabolism involving stereogenic centres.

The importance of stereoselective determination of drugs in the clinical laboratory

About 56% of the drugs currently in use are chiral compounds, and 88% of these chiral synthetic drugs are used therapeutically as racemates. Only a few of these drugs qualify for a stereospecific determination in a clinical laboratory for therapeutic drug monitoring of patients. If the qualitative and quantitative pharmacokinetic and pharmacodynamic effects are similar, the enantiomers do not need to be separated. However, if the metabolism of the different stereoisomers is handled by different enzymes which are either polymorphic or can be induced or inhibited, and if their pharmacodynamic effects have differences either in strength or in quality, enantiospecific analysis is urgently needed. Unfortunately, there are many racemic drugs where the stereospecificity of the metabolism and/or the pharmacodynamic effects of the enantiomers is not known today. For these drugs, there is a great need for studies concentrating on these differences to improve treatment of the patients.

A sensitive gas chromatographic/mass spectrometric method for the resolution and quantification of ethosuximide enantiomers in biological fluids

A modified specific, sensitive and reproducible chiral gas chromatographic (GC) method for the resolution and quantification of ethosuximide enantiomers in urine and plasma was developed. The samples were extracted by liquid-liquid extraction, using diethylether and the enantiomers were separated and quantified on a chiral gas chromatographic column (25QC2 / CYDEX- beta 0.25). The method involved the use of GC/MS instrumentation for the acquisition of data in the electron impact selective-ion monitoring mode, collecting ions characteristic of both ethosuximide and alpha, alpha - dimethyl - beta - methylsuccinimide, the internal standard and of mass-to-charge ratio (m/z) exactly equal to 55 and 70 units. The limit of quantitation of the method was 2.5 microg/ml for both urine and plasma with both enantiomers. The method proved to be linear, precise and reproducible in the 5-300 microg/ml concentration range for urine samples and in the 10-250 microg/ml concentration range for plasma samples. Future research work envisaged the application of this method in pharmacokinetic and pharmacodynamic studies.

Direct determination of pindolol enantiomers in human serum by column-switching LC-MS/MS using a phenylcarbamate-beta-cyclodextrin chiral column

A direct analytical method of pindolol enantiomers in body fluids was developed by means of column-switching semi-microcolumn liquid chromatography/tandem mass spectrometry (LC-MS/MS). A pre-column packed with a silica-based cation-exchanger was used for on-line sample cleanup. Subsequent enantioseparation was conducted with a phenylcarbamate-beta-cyclodextrin (ph-beta-CD) bonded semi-micro chiral column (2.0 mm inner diameter (i.d.)). A 25-microl aliquot of serum/urine samples was directly injected into the system after simple filtration with a membrane filter. Separated enantiomers were monitored with positive electrospray ionization (ESI) and selected reaction monitoring (SRM). R(+)- and S(-)-pindolol in serum and urine were determined separately within 16 min at a resolution factor of 1.9. The detection limits at a signal-to-noise (S/N) ratio of 5 were 0.13 ng ml(-1) for both enantiomers. The linearity of the method was in the range of 0.25-100 ng ml(-1) with regression coefficient greater than 0.997. Recoveries from spiked serum and urine samples, estimated by the external standard method, were between 94.8 and 117.6% with a relative standard deviation (RSD) ranging from 2.1 to 18%.

Stereochemistry and drug efficacy and development: relevance of chirality to antidepressant and antipsychotic drugs

Many drugs contain a chiral center or a center of unsaturation, or such centers result during metabolism of these drugs. Often such drugs are marketed as a mixture of the resultant enantiomers (racemates) or of geometric isomers, respectively. These enantiomers (molecules that are not superimposible on their mirror image) or geometric isomers may differ markedly from each other with regard to their pharmacodynamic and/or pharmacokinetic properties. This review deals primarily with drugs with chiral centers, and possible complications arising from the use of racemates are discussed. Recent developments in resolution of enantiomers, increased knowledge of the molecular structure of specific drug targets and a heightened awareness of several possible advantages of using single enantiomers rather than racemic mixtures of drugs have led to an increased emphasis on understanding the role of chirality in drug development. This has resulted in increased investigation of individual enantiomers early on in the development of drugs and in 'chiral switching', i.e. the replacement of a racemate of a drug which has already been approved or marketed by a single enantiomer. Although stereochemistry is an important matter to consider in drugs of virtually all classes, this review focuses on the relevance of chirality to antidepressant and antipsychotic drugs. Examples of the effects of chiral centers on the properties of antidepressants (tricyclics, selective serotonin reuptake inhibitors, monoamine oxidase inhibitors, viloxazine, bupropion, mianserin, venlafaxine, mirtazapine and reboxetine), antipsychotics and/or some of their metabolites are discussed.

Enantiomeric separation of alanyl and leucyl dipeptides by capillary electrophoresis with cyclodextrins as chiral selectors

Eight neutral cyclodextrins were tested for the enantiomeric separation of alanyl and leucyl dipeptides by capillary electrophoresis at pH 3, and seven out of the eight cyclodextrins proved suitable for the separation of one or more of the dipeptide enantiomer pairs. The best results were obtained with heptakis(2,6-di-O-methyl)-beta-cyclodextrin. The dipeptides that were separated were mainly the aromatic and the more lipophilic aliphatic dipeptides. Mobility difference plots at pH 3.0 with malonic acid-triethanolamine as background electrolyte showed that the aromatic dipeptides had higher affinities for the cyclodextrin than the nonpolar, aliphatic dipeptides. The results suggested that, under the conditions applied, the C-terminal amino acid rather than the N-terminal one is involved in the chiral discrimination.

Enantiospecific pharmacokinetics and pharmacodynamics of ketoprofen in sheep

Pharmacokinetic and pharmacodynamic parameters were established for the enantiomers of the 2-arylpropionic acid (APA) nonsteroidal anti-inflammatory drug (NSAID), ketoprofen (KTP). Each enantiomer was administered separately (1.5 mg/kg) and in a racemic mixture (3 mg/kg) intravenously (i.v.) to a group of eight sheep in a four-way, four-period cross-over study using a tissue cage model of inflammation. Plasma disposition of each KTP enantiomer was similar following separate administration of the pure compounds compared to administration of the racemic mixture. S(+)KTP volume of distribution (Vd(area)) was higher and clearance (ClB) faster than those of R(-)KTP. S(+) and R(-)KTP achieved relatively low concentrations in exudate and transudate. Unidirectional limited chiral inversion of R(-) to S(+)KTP was demonstrated. After R(-)KTP administration S(+)KTP was detected in plasma, but not in either exudate or transudate. Pharmacokinetic/pharmacodynamic (PK/PD) modelling of the data could not be undertaken following R(-)KTP administration because of chiral inversion to S(+)KTP, but the pharmacodynamic parameters, calculated maximum effect (Emax), concentration producing 50% effect (EC50), Hill's coefficient (N), rate constant of elimination of drug effect from the compartment (KeO) and mean equilibration half-life (t1/2KeO) were determined for S(+)KTP after administration of the racemic mixture as well as the pure compound.

Chirality and drugs used in psychiatry: nice to know or need to know?

1. Many drugs used to treat psychiatric disorders contain a chiral center or a center of unsaturation and are marketed as a mixture of the resultant enantiomers or geometric isomers, respectively. These enantiomers or geometric isomers may differ markedly with regard to their pharmacodynamic and/or pharmacokinetic properties. 2. Examples of the effects of chiral centers or geometric centers on such properties are given for drugs from the following classes: antidepressants (tricyclics, selective serotonin reuptake inhibitors, monoamine oxidase inhibitors, viloxazine, bupropion, trazodone, mianserin, venlaflaxine); benzodiazepines, zoplicone, and antipsychotics. 3. As described in this review, there are several notable examples of psychiatric drugs currently available where the individual enantiomers or geometric isomers differ considerably with regard to factors such as effects on amine transport systems, interactions with receptors and metabolizing enzymes, and clearance rates from the body. Indeed, relatively recent developments in analytical and preparative resolution of racemic and geometric drug mixtures and increased interest in developing new drugs which interact with specific targets, which have been described in detail at the molecular level, have resulted in increased emphasis on stereochemistry in drug development.

Passage of S-(+)- and R-(-)-ketoprofen across the human isolated perfused placenta

Ketoprofen is a chiral non-steroidal anti-inflammatory drug (NSAID) available as a racemic (rac) mixture of S-(+)- and R-(-)-isomers. Its inhibitory effect on prostaglandin biosynthesis resides virtually in the S-form. Interestingly, R-ketoprofen does not undergo substantial metabolic inversion in humans. Though contraindicated during the last trimester of pregnancy, NSAIDs, including ketoprofen, are used as tocolytic agents in some cases. The S/R plasma concentration ratio was reported to average 2.3 in premature neonates whose mothers were given rac-ketoprofen and to be close to 1 in the maternal plasma. Thus, we investigated the placental transfer of rac-ketoprofen in vitro using Schneider's perfused human cotyledon model. Glucosed Earle solutions with and without human serum albumin (HSA) were used. Several maternal perfusates were tested with different rac-ketoprofen concentrations together with 20 mg L-1 of antipyrine as a reference substance. Ketoprofen enantiomers were assayed by a specific HPLC method with derivatization procedure. HSA concentrations in maternal perfusate influenced the placental transfer of ketoprofen enantiomers. In the absence of HSA in the maternal perfusate, the S-(+)/R-(-) concentration ratio was close to 1 in the fetal perfusate. By contrast, this ratio averaged 1.44 after addition of HSA 10 g L-1 on the maternal side. Similar results were found for dialysis experiments using an inert Spectrapor 2 membrane suggesting that the S-(+)-free concentration is superior to the R-(-)-free concentration in the presence of HSA. Direct measurements of the free concentrations by centrifugal ultrafiltration confirmed this hypothesis. Accordingly, the data observed in vivo may result, at least in part, from the stereoselective protein binding of ketoprofen.

Production and characterization of polyclonal anti-S 20499 antibodies: influence of the hapten structure on stereospecificity

Immunoassays were studied as an alternative to HPLC methods for the stereoselective determination of a chiral drug, S 20499, a new anxiolytic compound that is chemically related to buspirone. The production of highly stereospecific polyclonal antibodies was sought following the construction of appropriately optimized hapten-protein conjugates. This process involved the selection of the structure and the length of the spacer arm used to couple S 20499 to the carrier protein as well as deciding on the location of the coupling site with respect to the chiral center. Two haptens were prepared: one a derivative resembling the original structure of S 20499, with the effective addition of a carboxylic acid group, and a second with the effective addition of a butanoic acid moiety that is supposed to favor stereorecognition. Six stereospecific polyclonal antisera were obtained in rabbits with two groups of antibody families defined in terms of specificity. Both approaches gave high levels of stereospecificity (cross-reactivity towards the optical antipode of S 20499 ranged from 4.1% to < 0.1%). Although it did not decrease the mean apparent affinity constant, the longer spacer improved antibody specificity by decreasing cross-reactions towards dealkylated S 20499 derivatives. Hence, the addition of a four carbon atom bridge should be a valuable tool for increasing antibody stereospecificity with no drawbacks in terms of specificity and affinity. It was also shown that long immunization periods appear to have no effect on the stereospecificity of the antibodies obtained.

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

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

The use of toxicokinetics for the safety assessment of drugs acting in the brain

Pharmacological and toxicological studies undertaken on drugs that affect the brain are frequently performed in disparate species under various experimental conditions, at doses often greatly in excess of those expected to be administered to humans, and the findings are extrapolated implicitly or explicitly with scant regard to differences in the biodisposition of the drugs. Such considerations are necessary since: 1. Species; 2. Strain; 3. Gender; 4. Route; 5. Dose; 6. Frequency and time of administration; 7. Temperature; 8. Coadministration of drugs; and 9. Surgical manipulation are but some of the factors that have been shown to influence the kinetics and metabolism of drugs. This article, using MDMA and other phenylethylamines as examples, provides evidence for the need to measure the exposure of the drugs and their active metabolites in blood and brain (toxicokinetics) in order that conclusions based only on dynamic, biochemical, or histological evidence are more pertinent. Further, the combined use of toxicokinetic-dynamic modeling can lead to a better appreciation of the mechanisms involved and a more useful approach to the calculation of safety margins.

Effect of the aluminium ion on hepatic elimination of quinine and quinidine

A study of the influence of the aluminum ion on the blood and hepatic kinetics of two alkaloid stereoisomers--quinine and quinidine--after their p.o. and s.c. administration (80 mg/kg) to rats was carried out. It appeared that the mode of application of the stereoisomers had different effects on their resorbtion in control animals. In the case of s.c. application, blood concentrations of quinine in some time intervals reached significantly higher levels than those found for quinidine. Hepatic elimination of quinine appeared to be independent of the mode of its application, whereas the elimination of quinidine was significantly increased after its s.c. application. Pretreatment of rats with aluminium chloride (600 mg/kg, i.p.) 2 h before injecting one of the stereoisomers had a different effect on their concentrations in blood, and significantly higher effect on their bile elimination. The quinine concentrations in blood after its p.o. administration were not changed significantly, but after s.c. application these concentrations were increased in some time intervals. The presence of the aluminum ion caused a significant increase in the rate of hepatic elimination of quinine, whereas it had no significant effect on quinidine elimination.

Enantioselective metabolism and pharmacokinetics of Casodex in the male rat

1. Casodex, a non-steroidal antiandrogen, is a racemic mixture of R-Casodex, the pharmacologically active (-)-enantiomer, and S-Casodex, the inactive (+)-enantiomer. Single oral doses of pseudo-racemic 14C-Casodex (10 mg/kg), prepared from mixtures of either 14C-labelled R-Casodex and unlabelled S-Casodex, or 14C-S-Casodex and unlabelled R-Casodex, were administered to the intact and bile duct-cannulated male rat. 2. Neither enantiomer underwent stereochemical inversion, but the pharmacokinetics of Casodex showed marked enantioselectivity. 3. After dosing R-labelled Casodex, plasma concentrations of R-Casodex increased slowly to reach a peak of 3.50 +/- 0.05 micrograms/ml (mean +/- SEM) at 12 h and, thereafter, declined monoexponentially with an elimination half-life of 24 h. Plasma concentrations of S-Casodex rose rapidly to reach a much lower peak of 0.97 +/- 0.06 microgram/ml at 3 h and, thereafter, declined rapidly, although there were insufficient data to determine the half-life. R-Casodex had a much higher AUC0-24 (66 micrograms.h/ml) than S-Casodex (12 micrograms.h/ml). Plasma drug concentrations measured using an achiral assay were in very good agreement with the sum of the enantiomer concentrations throughout the profile. R-Casodex comprised 94% of the total plasma radioactivity at 12 h, decreasing to 75% at 120 h. 4. Plasma concentration data generated after administration of S-Casodex were similar to those observed after dosing R-labelled Casodex. S-Casodex comprised about 74% of the total plasma radioactivity at 6 h and only 41% at 24 h. 5. The urine of intact animals contained 36 +/- 2 and 48 +/- 3% of the dose respectively up to 48 and 120 h after dosing with R-labelled Casodex, and 33 +/- 4 and 34 +/- 4% respectively after dosing with S-labelled Casodex. The urine and bile of the cannulated rat contained 43 +/- 2 and 21 +/- 2% of the dose respectively up to 48 h after dosing with R-labelled Casodex and 37 (n = 2) and 50% respectively after dosing with S-labelled Casodex. 6. After dosing with R- or S-labelled Casodex, the urinary radioactivity consisted of the carboxylic acid metabolite formed by hydrolytic cleavage at the amide, whereas biliary radioactivity consisted of hydroxy-Casodex and Casodex, mainly conjugated with glucuronic acid. The clearance of R-Casodex by each of these pathways of metabolism was less than that of S-Casodex, with direct glucuronidation and hydroxylation showing greater enantioselectivity than hydrolysis.

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.

The cost benefit ratio of enantiomeric drugs

Several drugs possess a chiral structure, i.e. they contain one or more stereogenic centres in their molecule. While naturally occurring active principles usually contain a single enantiomer, most chiral drugs produced by chemical synthesis are used in the form of racemic mixtures of two or more diastereoisomers. These stereoisomers (including enantiomers) may interact in different ways with biological structures and, therefore, may exhibit widely different pharmacokinetic properties. In the pharmaceutical industry, partly in response to increasing demands raised by regulatory authorities, these considerations justify the current trend to develop the single enantiomer characterized by the most favourable profile of activity (eutomer). The availability of new chemical and analytical technologies facilitates stereoselective synthetic processes and separation of individual enantiomers from racemic mixtures. Any decision to develop a drug as a single enantiomer, however, should be made only after careful evaluation of the cost-benefit ratio, i.e. when the advantages of the eutomer in terms of efficacy and tolerability outweigh the associated increase in production and development costs with respect to the racemic drug. This article takes into consideration synthetic procedures and pharmacological profiles for a number of chiral drugs in therapeutic use (naproxen, labetalol, and warfarin) or selected for clinical development, such as the beta-blocker dilevalol or the mucokinetic agent 3'-hydroxyfarrerol. These examples demonstrate that the kinetic, pharmacological and toxicological properties of individual enantiomers need to be clearly characterized before any decision can be made concerning the development of a chiral drug. The choice of preferentially developing a single enantiomer should be based on careful consideration of production and development costs and actual therapeutic advantages especially in terms of improved safety.

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.

Stereoselective pharmacokinetics of mefloquine in healthy Caucasians after multiple doses

Mefloquine (MQ) is a chiral antimalarial agent effective against chloroquine-resistant Plasmodium falciparum. It is commercially available as a racemic mixture of the (+) and (-) enantiomers for oral administration. The pharmacokinetics of the (+) and (-) enantiomers of MQ were studied in eight healthy volunteers after administration of a first oral dose of 250 mg of racemic MQ and at steady state after 13 repeated doses of 250 mg given at 1-week intervals. Plasma samples were collected, and concentrations of each enantiomer were determined using a previously described achiral-chiral double column-switching liquid chromatographic method. At each time point, higher plasma concentrations values were found for the (-) enantiomer (p < 0.001). At steady state, Cmax values of (-)-MQ were higher than those of (+)-MQ (1.42 +/- 0.19 versus 0.26 +/- 0.05 mg/L; p < 0.001). Similarly, the plasma concentrations 7 days after the final dose were higher for (-)-MQ (1.01 +/- 0.26 versus 0.11 +/- 0.04 mg/L; p < 0.001). AUC values at steady state were also higher for (-)-MQ (197.3 +/- 36.7 versus 30.1 +/- 8.9 mg/L x h; p < 0.001). The terminal half-life values (T1/2beta) were longer for (-)-MQ (430.4 +/- 225.2 versus 172.8 +/- 56.5 h; p < 0.001). This study shows that the pharmacokinetics of MQ is highly stereoselective.

Stereoselective pharmacokinetics and interconversions of flosequinan enantiomers containing chiral sulphoxide in rat

1. In order to study the pharmacokinetics of flosequinan enantiomers ((+-)-7-fluoro-1-methyl-3-methylsulphinyl-4-quinolone) containing chiral sulphur, plasma levels of (+)-(R)- and (-)-(S)-flosequinan (R-FSO and S-FSO) and two metabolites (flosequinan sulphide (FS) and flosequinan sulphone (FSO2)) were measured after oral and i.v. administration of racemic flosequinan (rac-FSO), R-FSO and S-FSO in male rat. 2. The pharmacokinetic parameters of the enantiomers were different after oral and i.v. administration of R-FSO and S-FSO. The plasma clearance of R-FSO was higher than S-FSO. 3. The major metabolites of boh R-FSO and S-FSO was FSO2. A minor metabolite, FS, was also detected in plasma. 4. Interconversions occurred after the oral and i.v. administration of R-FSO and S-FSO. The amount of interconversion from S-FSO and R-FSO was greater than that from R-FSO to S-FSO. The rate of interconversion after oral administration was higher than that after i.v. administration. 5. After i.v. administration of FS, R-FSO and S-FSO were detected in plasma, suggesting that the interconversion occurred via formation of FS. 6. The pharmacokinetic parameters of R-FSO after administration of rac-FSO differed from that after administration of R-FSO, indicating the interaction between each enantiomer.

Semipreparative enantiomeric separation of a series of putative melatonin receptor agents using tri-acetylcellulose as chiral stationary phase

In order to obtain milligram amounts of the enantiomers of a series of compounds to be tested for binding to the melatonin binding site, a system for semipreparative enantiomeric separation was set up using tri-acetylcellulose as the chiral stationary phase. Interactions of this class of compounds with tri-acetylcellulose were examined on an analytical scale with a series of 20 compounds. Apparently, both steric and electrostatic interactions determine retention behavior on tri-acetylcellulose. Semipreparative separations were carried out for a subset of seven compounds. The purity of the first eluting enantiomer usually was around 99%, whereas the purity of the second eluting enantiomer was slightly less. The system described is easy to use and has the major advantage that a series of compounds can be separated with one technique. The purities obtained are sufficient for a first screen of their affinity.

Stereochemical aspects of the molecular pharmaceutics of ibuprofen

Thermal analysis, thermodynamics of solution and molecular modelling of (+)-ibuprofen and (+/-)-ibuprofen gave information on how heterochiral or homochiral interactions would affect the processing of ibuprofen. The study confirmed that (+/-)-ibuprofen exists as a true racemate with a 10% eutectic pure enantiomer composition. Both the racemate and the (+)-isomer crystal unit cells include four molecules and crystallize in the P2(1/c) and P2(1) space groups, respectively. Thus the intermolecular forces were different in each crystal. As a consequence the (+)-enantiomer lattice was more fragile but only slightly more soluble than the racemate in aqueous media. The solid-state structure contributions to solubility were different for the two crystals (delta H (+) = 51.1 and delta H(+/-) = 32.2 kJ mol-1) but the standard free energies of the solutions were comparable for both compounds.

Disposition kinetics of ML-1035 sulfoxide enantiomers and the prochiral sulfide in rats

ML-1035, 4-amino-5-chloro-2-[2-(methylsulfinyl)ethoxy]-N-[2- (diethyl-amino)ethyl]benzamide, is a sulfoxide compound and a racemic gastroprokinetic agent with a chiral center at the sulfur atom. We have investigated the disposition kinetics of (R)-ML-1035 sulfoxide (R) and (S)-ML-1035 sulfoxide (S) after the single enantiomers and the racemic mixture were administered to rats in separate experiments. There was no noticeable chiral inversion after either enantiomer dose. Both enantiomers were rapidly absorbed. After dosing with enantiomers or with the racemate, the resulting plasma concentration-time curve of R was closely parallel to that of S in both intravenous and oral experiments, suggesting that the two enantiomers have approximately the same disposition kinetics. After intravenous enantiomer doses, only S underwent conversion to sulfide, suggesting that sulfidation in the liver is enantioselective. However, the enantioselective sulfidation after intravenous dosing did not introduce a difference in the global plasma disposition profiles between R and S, since the reduction reaction is a minor metabolic process. Other metabolic reactions such as sulfonation and mono-N-desethylations were not enantioselective. After oral administration, conversion to sulfide was observed for both enationers, implicating the existence of a nonhepatic pathway in sulfidation. Administration of a prochiral sulfide dose was associated with an enantioselective sulfoxidation, in which the R/S concentration ratios increased as a function of time. In addition, enatiomeric interaction causing changes in pharmacokinetic parameters was observed after the oral racemate dose, while the interaction is negligible after an intravenous racemate dose, indicating a route dependency in enantiomeric interaction.

Pharmacokinetic-pharmacodynamic relationships in phase I/phase II of drug development

Early investigation of pharmacokinetic-pharmacodynamic relationships in Phase I/II may facilitate the further clinical development of a new drug. Although some pharmacology assessments in Phase I are often only surrogates for the therapeutic effect, PK-PD modelling of those effects provides in general crucial information on the drug's potency in vivo. A mathematical PK-PD expression allows explorative simulations on the rate of onset of drug action, on the intensity and duration of the effects for doses in future clinical trials, or in situations of altered drug kinetics. Furthermore, understanding of the PK-PD relationship early on in drug development may anticipate unnecessary exposure of human subjects to inappropriate drug doses or trials.

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.

Chiral separation by capillary electrophoresis with oligosaccharides

Maltodextrins, i.e., mixtures of linear alpha-(1-4)-linked D-glucose polymers, were found to be effective as chiral electrolyte modifiers to perform direct, rapid separations by capillary electrophoresis of racemic mixtures of 2-arylpropionic acid non-steroidal anti-inflammatory compounds and coumarinic anticoagulant drugs, and also diastereomeric cephalosporin antibiotics. Enantioselectivity seemed to be dependent on an as yet unidentified combination of variables.

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.

Enantioselective pharmacokinetics of ethotoin in humans following single oral doses of the racemate

Racemic ethotoin (1000 mg) was administered orally as a single dose to six healthy adult volunteers. Blood samples were collected at appropriate times for 120 h following the dose. Ethotoin was quantified enantio-selectively in plasma using a novel chiral column HPLC procedure. One of the enantiomers of the chiral metabolite, 5-phenylhydantoin, was also quantified in the HPLC method. The Cmax and AUC0-infinity values for (+)-(S)-ethotoin were significantly greater than those for (-)-(R)-ethotoin (ratio of mean AUC0-infinity values 0.88), but the elimination half-lives of the isomers were virtually identical [12.35 +/- 5.15 h for (-)-(R)-ethotoin; 12.28 +/- 5.34 h for (+)-(S)-ethotoin]. Parameters derived from AUC0-infinity (Cl0/F and V(area)/F) also differed slightly between the isomers. The data were interpreted as indicating a small difference in the absorption of the two isomers; it seemed unlikely, in terms of the identical elimination rates, that their metabolic profiles would differ greatly. The 5-phenylhydantoin was eliminated with a significantly longer half-life (18.69 +/- 6.11 h) than that of ethotoin. Enantioselectivity in the pharmacokinetics of ethotoin is therefore a minor issue.

Rapid chiral separation of metoprolol in plasma--application to the pharmacokinetics/pharmacodynamics of metoprolol enantiomers in the conscious goat

The plasma concentrations of metoprolol enantiomers have been determined by means of a direct phenyl carbamate-cellulose-based chiral high performance liquid chromatography assay using fluorimetric detection. This assay has been used to investigate the pharmacokinetics and pharmacodynamics of metoprolol enantiomers in the conscious goat. There is evidence that the pharmacokinetics of metoprolol in the goat occurs stereoselectively and that enantiomer-enantiomer pharmacokinetic interactions occur. R-Metoprolol is less effective in reducing the mean arterial blood pressure than S- and R/S-metoprolol.

Enzymes in stereoselective pharmacokinetics of endogenous substances

The use of enzymes to assay individual components of the L-carnitine family in pharmaceuticals, foodstuffs, and biological fluids with various forms of detection is reviewed. The most useful enzyme in the assay of compounds of the L-carnitine family is carnitine acetyl transferase (CAT), which catalyses the reversible interconversion of L-carnitine and its short-chain acyl esters. CAT can be used in one or more coupled reactions combined with U.V., or radiolabelled detection, or combined with HPLC, allowing, enantioselective, structurally specific, and, in the case of radiolabelled tracing, highly sensitive assays to be carried out. When compared with chromatographic separation of enantiomers or diastereoisomers, enantioselective enzyme mediated assays may be cheaper, more sensitive, and simpler, but they do not allow the nonpreferred isomer to be assayed. Consequently, they are appropriate for the specific assay of endogenous enantiomeric substrates of the enzyme concerned, in biological samples. The analysis of the other enantiomer in raw materials or in pharmaceuticals must be more properly approached by enantioselective chromatographic methods.