Chiral Amplification of Oligopeptides via Polymerization in Two-dimensional Crystallites on Water

A possible role that might have been played by ordered clusters at the air/water interface for the generation of homochiral oligopeptides under prebiotic conditions has been probed by a catalyzed polymerization of amphiphilic activated alpha-amino acids that assembled as two-dimensional (2-D) crystallites at this interface. Three type of processes are described: (i) polymerization of racemates of activated alpha-amino acids that undergo spontaneous resolution into enantiomorphous 2-D crystallites to yield racemic mixtures of oligopeptides enriched with the oligomers of homochiral sequence, (ii) enhanced formation of racemic mixtures of homochiral oligopeptides via lattice-controlled polymerization within 2-D racemic compounds and (iii) generation of homochiral oligopeptides of a single handedness from chiral non-racemic mixtures of monomers that self-assemble into two different phases, racemic crystallites composed from both enantiomers and enantiomorphous crystallites of the enantiomer in excess. The structures of the 2-D crystallites have been determined by grazing incidence X-ray diffraction and the diastereoisomeric composition of the oligopeptides by matrix-assisted laser-desorption time-of-flight mass spectrometry with enantio-labeling.     

Anticancer Effect of Chlorambucil Enhanced by Chiral Phthalidyl Promoiety

Phthalidyl promoiety has been used in several drugs, but they were all marketed in racemic form. The pharmaceutical effects of each enantiomer have not been clearly demonstrated. In this project, an anticancer chemotherapy drug, chlorambucil, was modified as enantiopure phthalidyl prodrugs. The enantiomers, together with phthalidyl unit and their racemic mixture, were then subject to the in vivo bioactivity tests against B16F10 melanoma cells. It was found that proper chirality within the promoiety had noticeably better in vivo pharmacological effects than the parent drug, the enantiomer and racemic mixture. This merit perhaps could be extended from the phthalidyl prodrugs to other chirality containing prodrugs.     

Stereoselective pharmacokinetics of ornidazole after intravenous administration of individual enantiomers and the racemate

The pharmacokinetics of ornidazole (ONZ) were investigated following i.v. administration of racemic mixture and individual enantiomers in beagle dogs. Plasma concentrations of ONZ enantiomers were analyzed by chiral high-performance liquid chromatography (HPLC) on a Chiralcel OB-H column with quantification by UV at 310 nm. Notably, the mean plasma levels of (-)-ONZ were higher in the elimination phase than those of (+)-ONZ. (-)-ONZ also exhibited greater t1/2, MRT, AUC(0-t) and smaller CL, than those of its antipode. The area under the plasma concentration-time curve (AUC(0-t)) of (-)-ONZ was about 1.2 times as high as that of (+)-ONZ. (+)-ONZ total body clearance (CL) was 1.4 times than its optical antipode. When given separately, there were significant differences in the values of AUC(0-infinity) and CL between ONZ enantiomers (P < 0.05), indicating that elimination of (+)-ONZ was more rapid than that of (-)-ONZ. No significant differences were found between the estimates of the pharmacokinetic parameters of (+)-ONZ or (-)-ONZ, obtained following administration as the individual and as a racemic mixture. This study demonstrates that the elimination of ONZ enantiomers is stereoselective and chiral inversion and enantiomer/enantiomer interaction do not occur when the enantiomers are given separately and as racemic mixture.     

Experimental studies of competition between enantiomers in chiral liquid chromatography through their system peaks

Competition between the (+)- and (?) enantiomers of 2,2,2-trifluoro-1-(9-anthryl) ethanol as mobile phase additives was indicated by the chromatographic behavior of their system peaks. Two types of chiral stationary phases were used, one based on dinitrobenzoylphenylglycine and the other on dinitrobenzylphenylethylamine plus tartaric acid. The racemic mixture was used as the mobile phase additive and k′ of their system peaks was studied as a function of the mixture concentration in the mobile phase in both cases. A shift in k′ of the two system peaks was observed and considered as an indication that competition occurred. The areas of the two system peaks were also studied as a function of the concentration of the enantiomers in the samples, using two different compositions of the mobile phase. The dependency of system peaks' area on the sample composition indicated whether competition between the enantiomers occurred. One mobile phase contained 0.1 mM of the racemic mixture, where the area of the two retained system peaks behaved independently, i.e., only the peak corresponding to the enantiomer was affected by its presence in the sample. The other mobile phase contained 0.75 mM of the racemic mixture, and both peaks were affected by the injection of any one of the enantiomers. The interdependency of the system peaks' area on both the enantiomers indicated that their distribution in the chiral system was interrelated due to mutual interactions. A quantitative treatment of the interdependency and competition was excluded, due to the irreversible adsorption of the two enantiomers on the chiral stationary phase after using overloading concentrations. This irreversible adsorption was visualized by the appearance of two retained system peaks of the two residual enantiomers. These system peaks were detected only when the sample contained pure enantiomers due to competition between the enantiomer in the sample with the residual enantiomers in the stationary phase. © 1994 Wiley-Liss, Inc.     

Crystal structures of peptide enantiomers and racemates: probing conformational diversity in heterochiral Pro-Pro sequences

Multiple conformational states in heterochiral diproline sequences have been characterized in the solid state by the determination of the crystal structures of seven tripeptides in enantiomeric and racemic forms. The sequences of the type Piv-DPro-LPro-DXxx-NHMe (D-L-D) [DXxx=DVal 1, DLeu 3, and DPhe 5] and their corresponding enatiomeric L-D-L sequences [LXxx=LVal 2, LLeu 4, and LPhe 6] have been investigated. Single crystals have been obtained for the pure enantiomers 1, 2, 3, 4 and for the racemates 1/2, 3/4, and 5/6. For Xxx=Leu, mirror image conformations (type II/II' beta-turns) at Pro-Leu segment are obtained. For Xxx=Val, a LPro-DPro type II beta-turn in 2 and an open/extended structure is obtained in the solvated form of the enantiomer 1. For Xxx=Phe, suitable crystals could not be obtained for enatiomeric peptides. The racemate 5/6 revealed a cis peptide bond between the diproline segment with the absence of any intramolecular hydrogen bonds. Crystal structures of enantiomers and racemates prove useful in characterizing the multiple conformational states that are accessible to Pro-Pro segments.     

Field responses of the Asian larch bark beetle,Ips subelongatus,to potential aggregation pheromone components: disparity between two populations in northeastern China

Abstract Behavioral responses of the Asian larch bark beetle, Ips subelongatus Motsch. to three potential aggregation pheromone components, ipsenol (racemic or [?]‐enantiomer), ipsdienol (racemic or [+]‐enantiomer) and 3‐methyl‐3‐buten‐1‐ol, were tested using partial or full factorial experimental designs in two provinces (Inner Mongolia and Jilin) of northeastern China. Our field bioassays in Inner Mongolia (Larix principis‐rupprechtii Mayr. plantation) clearly showed that ipsenol, either racemic or 97%‐(?)‐enantiomer, was the only compound that significantly attracted both sexes of I. subelongatus, while all other compounds (singly or in combinations) were unattractive. There were no two‐ or three‐way synergistic interactions. However, in Jilin Province (L. gmelini[Rupr.] Rupr. Plantation), all the individual compounds tested were inactive, except a very weak activity by 97%‐(?)‐ipsenol in 2004 when the beetle population was very high. While a combination of ipsenol and ipsdienol (racemates or enantiomerically pure natural enantiomers) showed a significant attraction for both sexes of I. subelongatus, indicating a two‐way synergistic interaction between these two major components, addition of 3‐methyl‐3‐buten‐1‐ol to these active binary blend(s) did not have any effects on trap catches, suggesting that ipsenol and ipsdienol are the synergistic aggregation pheromone components of I. subelongatus in Jilin Province. It seems that 97%‐(?)‐ipsenol in Inner Mongolia or the binary blend of 97%‐(?)‐ipsenol and 97%‐(+)‐ipsdienol in Jilin Province are superior to their corresponding racemates, which might be due either to weak inhibitory effects of the antipode enantiomers or to reduced release rates of the active natural enantiomer(s) in the racemate(s). Our current bioassay results suggest that there is a strong geographical variation in aggregation pheromone response of I. subelongatus in northeastern China. Future research on the pheromone production and response of I. subelongatus from different regions in Northeast Asia will surely improve our understanding of the dynamic aggregation pheromone system of this economically important forest pest insect.     

Nonlinear stereoselective pharmacokinetics of ketoconazole in rat after administration of racemate

The stereoselective pharmacokinetics of ketoconazole (KTZ) enantiomers were studied in rat after i.v. and oral administration of (+/-)-KTZ. Sprague-Dawley rats were administered racemic KTZ as 10 mg/kg i.v. or orally over the range 10-80 mg/kg as single doses. Serial blood samples were collected over a 24-h period via surgically placed jugular vein cannulae. Plasma was assayed for KTZ enantiomer concentrations using stereospecific HPLC. Enantiomeric plasma protein binding was determined using an erythrocyte partitioning method at racemic concentrations of 10 and 40 mg/L. Stereoselective metabolism was tested by incubating the racemate (0.5-250 microM) with rat liver microsomes. In all rats, (+)-KTZ plasma concentrations were higher (up to 2.5-fold) than (-)-KTZ. The clearance and volume of distribution of the (-) enantiomer were approximately twofold higher than antipode. Half-life did not differ between the enantiomers. After oral doses the t(max) was not stereoselective. For both enantiomers with higher doses the respective half-life were found to increase. The mean unbound fraction of the (-) enantiomer was found to be up to threefold higher than that of the (+) enantiomer. At higher concentrations nonlinearity in plasma protein binding was observed for both enantiomers. There was no evidence of stereoselective metabolism by liver microsomes. Stereoselectivity in KTZ pharmacokinetics is attributable to plasma protein binding, although other processes such as transport or intestinal metabolism may also contribute.     

Enantiomerization of an atropisomeric drug

The antipsoriatic 10-(3-chlorophenyl)-6,8,9,10-tetrahydrobenzo[b][1,8] naphthyridin-5(7H)-one, Sch 40120 , is chiral only because it lacks planarity and possesses a stereogenic axis. It comprises short-lived, interconverting atropisomeric enantiomers distinguished by the chlorine substitutent. The atropisomers form diastereomeric complexes with the shift reagent (R)-(−)-2,2,2-trifluoro-1-(9-anthryl)ethanol, which were detected by ¹H NMR spectroscopy. Liquid chromatography on an ovomucoid chiral column isolated each enantiomer from the racemic mixture. Re-injections of the separated enantiomers onto the same column held constant at 10°C established that each enantiomer formed the other. Under identical chromatographic conditions, both stereoisomers independently recreated the racemic mixture. The calculated enantiomer half-life lasted 1.6 min at the physiological temperature of 37°C. Simulations of dynamic liquid chromatograms acquired with a chiral stationary phase indirectly yielded values of the half-lives. The chromatograms were modeled with the computer program SIMUL. Also determined were the rate constants for enantiomerization and the corresponding Gibbs free energies of activation, all at varying temperatures. At 37°C, the rate constant and activation energy respectively equaled 0.213 min⁻¹ and 21.6 kcal mole⁻¹. An Arrhenius plot was linear. The intractably brief life spans necessitated development of the racemic drug, rather than advancement of one enantiomer only. The pharmacological, biological, and chemical consequences of molecular asymmetry inherent to the drug were therefore nil. © 1996 Wiley-Liss, Inc.     

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.     

Single enantiomeric β-blockers—The existing technologies

A number of β-blocking drugs are available in the world market, only few compounds are found as single enantiomers. The need to use the single enantiomeric β-blockers affects development of drugs and technology. Many processes have been exploited to replace the existing racemates. Two main routes are established: (1) asymmetric syntheses and (2) racemic resolutions. The syntheses give medium-high yields and excellent enantiomeric excess, but the resolutions are limited by 50% yield. Both technologies involve new techniques such as dynamic kinetic resolution (DKR) and membrane-based extraction. The synthetic ways utilise various substrates and catalysts. A simultaneous formation is also afforded by these processes. They offer oriented alternatives to the single enantiomeric β-blockers. Resolutions of the racemates appear with many attractive separation methods. Direct or indirect resolutions show excellent characteristics and produce high enantiomeric excess. The existing processes operate continuously at mild operating temperatures compared to the asymmetric synthesis. In situ separation is also exploited. Development of the single enantiomeric β-blockers using the DKR based on enzymatic membrane(s) is encouraged. Integration of acetylation, racemisation and hydrolysis followed by separation of the enantiomers in the enzymatic membrane reactors could be a better option in resolution and separation of the β-blocker racemates.     

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-(diethylamino)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 enantioners, 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, enantiomeric 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. © 1993 Wiley-Liss, Inc.     

Bi-directional chiral inversion of ketoprofen in CD-1 mice

The R enantiomers of some of the 2-arylpropionic acid non-steroidal antiinflammatory drugs (NSAIDs) are known to undergo metabolic chiral inversion to their more pharmacologically active antipodes. This process is drug and species dependent and usually unidirectional. The S to R chiral inversion, on the other hand, is rare and has been observed, in substantial extents, only for ibuprofen in guinea pigs and 2-phenylpropionic acid in dogs. After i.p. administration of single doses of racemic ketoprofen or its optically pure enantiomers to male CD-1 mice and subsequent study of the concentration time-course of the enantiomers, we noticed substantial chiral inversion in both directions. Following racemic doses, no stereoselectivity in the plasma-concentration time courses was observed. After dosing with optically pure enantiomer, the concentration of the administered enantiomer predominated during the absorption phase. During the terminal elimination phase, however, the enantiomers had the same concentrations. Our observation is suggestive of a rapid and reversible chiral inversion for ketoprofen enantiomers in mice. Chirality 9:29–31, 1997. © 1997 Wiley-Liss, Inc.     

Model-based mutagenesis to improve the enantioselective fractionation properties of an antibody

The binding affinity and specificity of recombinant antibodies can be modified by site-directed mutagenesis. Here we have used molecular modelling of the variable domains of an enantiospecific antibody fragment to fine-tune its affinity so it is more suitable for the fractionation of the drug enantiomers. We have shown earlier that the Fab fragment of this antibody specifically recognizes one enantiomer from the racemic mixture of a medical drug and that it can be used for the fractionation of these enantiomers by affinity chromatography. However, the affinity was unnecessarily high, requiring harsh elution conditions to release the bound enantiomer. Thus, the continuous use of the antibody affinity columns was impossible. We made a homology model of the antibody and designed mutations to the antigen-binding site to decrease the affinity. Four out of five point mutations showed decreased affinity for the hapten. Two of the mutations were also combined to construct a double mutant. The affinity columns made using one of the single mutants with lowered affinity and the double mutant were capable of multiple rounds of enantioseparation.     

Stereoselective inhibition of dopaminergic activity by gamma vinyl-GABA following a nicotine or cocaine challenge: a PET/microdialysis study

Elevation of endogenous GABA by the racemic mixture of gamma vinyl-GABA (GVG, Vigabatrin) decreases extracellular nucleus accumbens (NAc) dopamine (DA) levels and diminishes the response to many drugs of abuse known to elevate DA in the mesocorticolimbic system. We investigated the effects of the individual enantiomers (S(+)-GVG, R(-)-GVG) on cocaine-induced NAc DA in rodents as well as the effects of nicotine-induced increases in primates. In a series of microdialysis experiments in freely moving animals, S(+)-GVG (150 mg/kg), R(-)-GVG (150 mg/kg) or racemic (R, S) GVG (300 mg/kg) was administered 2.5 hours prior to cocaine (20 mg/kg) administration. When compared with cocaine alone, the R(-) enantiomer did not significantly inhibit cocaine induced NAc DA release. S(+)-GVG, at half the dose of the racemic mixture (150 mg/kg), inhibited cocaine-induced DA elevation by 40%, while the racemic mixture (300 mg/kg) inhibited cocaine-induced DA release by 31%. In addition, our PET studies in primates demonstrated that S(+)-GVG completely inhibits nicotine-induced increases in the corpus striatum, again at half the dose of the racemic mixture. The R(-) enantiomer was ineffective. Although the S(+) enantiomer has been well established as the active compound in the treatment of epilepsy, the efficacy of this enantiomer with regard to mesolimbic DA inhibition generates a complex series of clinical and neurochemical issues. Further investigations will determine the locus of action and physiologic properties of each enantiomer.     

Pharmacokinetics of the active antifungal enantiomer, SCH 42427 (RR), and evaluation of its chiral inversion in animals following its oral administration and the oral administration of its racemate genaconazole (RR/SS)

Genaconazole (SCH 39304) is a potent triazole antifungal agent that is active both orally and topically. Genaconazole is a racemic mixture which contains 50% of the RR (SCH 42427) and 50% of the SS (SCH 42426) enantiomers. The RR isomer accounts for most of the antifungal activity of genaconazole. Serum concentrations of the RR and SS enantiomers were analyzed by a chiral HPLC method which involved extraction of serum with organic solvent followed by separation on a Cyclobond I column and quantification by UV absorbance at 205 nm. The bioavailability and pharmacokinetic profiles of the two enantiomers after oral administration of the racemate (genaconazole) were very similar in cynomolgus monkeys. In rats following dosing with genaconazole, the RR enantiomer had a lower C(max) and a longer t(1/2) than the SS enantiomer, while the AUC(I) values of the two enantiomers were similar. Based on chiral HPLC analysis, there was no evidence for the inversion of the RR to the SR isomer, or of the SS to the SR isomer, indicating that there was no chiral inversion of the RR or SS enantiomers in either species. Genaconazole at 20 mg/kg and the RR (SCH 42427) enantiomer at 10 mg/kg had very similar serum concentration-time profiles and C(max), AUC(I), and t(1/2) values for the RR enantiomer in both rats and monkeys, indicating that the two treatments were equivalent with respect to the bioavailability of the RR enantiomer.     

Enantiomeric selectivity in behavioural and electrophysiological responses of Aedes aegypti and Culex quinquefasciatus mosquitoes

1-Octen-3-ol is a kairomone for many haematophagous insects including mosquitoes. Numerous studies have examined the effects of racemic 1-octen-3-ol; however, few studies have investigated the role of individual enantiomers in relation to mosquito attraction. In the present study, we investigated the behavioural and electrophysiological responses of two mosquito species, Aedes aegypti and Culex quinquefasciatus, to individual enantiomers and mixtures of 1-octen-3-ol, employing a laboratory Y-tube olfactometer and single sensillum recordings. The olfactory receptor neurons of both Ae. aegypti and Cx. quinquefasciatus had a significantly higher response to the (R)-1-octen-3-ol enantiomer compared to the (S)-1-octen-3-ol enantiomer at 10-9 g μl-1 to 10-6 g μl-1. Behaviourally, Ae. aegypti was more responsive to the (R)-1-octen-3-ol enantiomer, showing an increase in flight activity and relative attraction compared to Cx. quinquefasciatus. The (R)-1-octen-3-ol enantiomer caused an increase in activation for Cx. quinquefasciatus. However, the most notable effect was from an (R:S)-1-octen-3-ol mixture (84:16) that caused significantly more mosquitoes to sustain their flight and reach the capture chambers (demonstrated by a reduced non-sustained flight activity), suggesting that it may have a behaviourally excitatory effect. For Cx. quinquefasciatus, a reduced relative attraction response was also observed for all treatments containing the (R)-1-octen-3-ol enantiomer, either on its own or as part of a mixture, but not with the (S)-1-octen-3-ol enantiomer. This is the first time enantiomeric selectivity has been shown for Ae. aegypti using electrophysiology in vivo. The implications of these results for exploitation in mosquito traps are discussed.     

Enantioselective analysis of citalopram and demethylcitalopram in human and rat plasma by chiral LC-MS/MS: application to pharmacokinetics

Citalopram (CITA) is available as a racemic mixture and as a pure enantiomer. Its antidepressive action is related to the (+)-(S)-CITA and to the metabolite (+)-(S)-demethylcitalopram (DCITA). In the present investigation, a method for the analysis of CITA and DCITA enantiomers in human and rat plasma was developed and applied to the study of pharmacokinetics. Plasma samples (1 ml) were extracted at pH 9.0 with toluene:isoamyl alcohol (9:1, v/v). The CITA and DCITA enantiomers were analyzed by LC-MS/MS on a Chiralcel OD-R column. Recovery was higher than 70% for both enantiomers. The quantification limit was 0.1 ng/ml, and linearity was observed up to 500 ng/ml plasma for each CITA and DCITA enantiomer. The method was applied to the study of the kinetic disposition of CITA administered in a single oral dose of 20 mg to a healthy volunteer and in a single dose of 20 mg/kg (by gavage) to Wistar rats (n = 6 for each time). The results showed a higher proportion of the (-)-(R)-CITA in human and rat plasma, with S/R AUC ratios for CITA of 0.28 and 0.44, respectively. S/R AUC ratios of DCITA were 0.48 for rats and 1.04 for the healthy volunteer.     

Pharmacokinetics of reboxetine enantiomers in the dog

Reboxetine, (RS)-2-[(RS)-α-(2-ethoxyphenoxy)benzyl]morpholine methanesulphonate, is a racemic compound and consists of a mixture of the (R,R)- and (S,S)-enantiomers. The pharmacokinetics of reboxetine enantiomers were determined in a crossover study in three male beagle dogs. Each animal received the following oral treatments, separated by 1-week washout period: 10 mg/kg reboxetine, 5 mg/kg (R,R)- and 5 mg/kg (S,S)-. Plasma and urinary levels of the reboxetine enantiomers were monitored up to 48 h post-dosing using an enantiospecific HPLC method with fluorimetric detection (LOQ: 1.1 ng/ml in plasma and 5 ng/ml in urine for each enantiomer). After reboxetine administration mean t_max was about 1 h for both enantiomers. C_max and AUC were about 1.5 times higher for the (R,R)- than for the (S,S)-enantiomer, mean values ± SD being 704 ± 330 and 427 ± 175 ng/ml for C_max and 2,876 ± 1,354 and 1,998 ± 848 ng.h/ml for AUC, respectively. No differences between the (R,R)- and (S,S)-enantiomers were observed in t½ (3.9 h). Total recovery of the two enantiomers in urine was similar, the Ae (0–48 h) being 1.3 ± 0.7 and 1.1 ± 0.7% of the enantiomer dose for the (R,R)- and the (S,S)-enantiomers, respectively. No marked differences in the main plasma pharmacokinetic parameters were found for either enantiomer on administration of the single enantiomers or reboxetine. No chiral inversion was observed after administration of the separate enantiomers, as already observed in humans. Chirality 9:303–306, 1997. © 1997 Wiley-Liss, Inc.     

Pharmacokinetic behaviour of R-(+)- and S-(−)-amlodipine after single enantiomer administration

Amlodipine, 3-ethyl 5-methyl-2-[(2-aminoethoxymethyl]-4-(2-chlorophenyl)-1,4-dihydro-6-methyl-3,5-pyridinedicarboxylate, is a chiral calcium antagonist, currently on the market and in therapeutic use as a racemate. The pharmacokinetic behaviour of R-(+)- and S-(−)-amlodipine after single enantiomer administration to healthy male human volunteers together with comparative administration of the racemic mixture of both enantiomers were studied. Plasma levels were studied as a function of time and assayed using an enantioselective chromatographic method (coupled chiral and achiral HPLC) with on-line solid-phase extraction and UV absorbance detection. The method was validated separately for the R-(+)- and S-(−)-enantiomer, respectively. Results of the study indicate that the pharmacokinetic behaviour of R-(+)- and S-(−)-amlodipine after single enantiomer administration is comparable to that of each enantiomer after administration of the racemate. No racemization occurs in vivo in human plasma after single enantiomer administration.     

Protein binding of indobufen enantiomers: pharmacokinetics of free fraction-studies after single or multiple doses of rac-indobufen

The binding of the enantiomers of indobufen (INDB) to human serum proteins was investigated using the racemic mixture or the pure (+)-S-enantiomer in a concentration range of 2.5-100.0 mg/L. In addition, the pharmacokinetics of free (unbound) and total INDB enantiomers were studied 1) following administration of a single 200 mg rac-INDB tablet to healthy volunteers, and 2) in obliterative atherosclerosis patients at steady state. The free fraction of INDB was obtained by ultrafiltration. Using the racemic mixture, the binding parameters of the two enantiomers were different, showing enantioselectivity in protein binding. The (-)-R-enantiomer was bound more strongly to human serum albumin, with association constant K = 11.95 +/- 0.98 x 10(5) M(-1) and n = 0.72 +/- 0.02 binding sites. The comparable data for the (+)-S-enantiomer were K = 4.65 +/- 0.02 x 10(5) M(-1), n = 0.92 +/- 0.01. When the binding of (+)-S-enantiomer was studied alone, the association constant K (2.10 +/- 0.18 x 10(5) M(-1)) was lower and the number of binding sites was increased, to n = 1.87 +/- 0.17. Competition occurred between the enantiomers, with the (-)-R-enantiomer displacing its antipode. The fraction of both enantiomers bound to serum proteins was 99.0%, which increased with decreasing initial concentration of the enantiomers. In healthy volunteers the (+)-S-enantiomer was eliminated faster than its (-)-R antipode, resulting in a lower AUC for the (+)-S-enantiomer. Significant differences were observed in the total INDB enantiomer concentrations. The mean unbound fraction of (-)-R- and (+)-S-INDB was 0.45% and 0.43%, respectively. Levels of the free (+)-S-enantiomer were higher than its (-)-R-antipode at steady state in patients with obliterative atherosclerosis who also took other drugs. The free enantiomer fraction increased to around 1% upon repeated administration. We conclude that the more rapid elimination of the (+)-S enantiomer is associated with its weaker binding to serum proteins.