Analytical methodologies for the enantiodetermination of citalopram and its metabolites

Citalopram (CIT) is a highly selective serotonin reuptake inhibitor (SSRI) frequently used in the treatment of major depressive disorders. It has a chiral centre in its structure and is used in therapy both as a racemic mixture (R,S-CIT) and a pure enantiomer (S-CIT). The differences between the pharmacokinetic and pharmacological profiles of the two enantiomers are well established. Consequently, the development of new efficient chiral analysis methods for their enantiomeric separation is a topic of great actuality. CIT metabolism is stereoselective as it is metabolized in chiral active metabolites, which retain considerable SSRI activity and contribute to the pharmacological effect. Chiral analytical methods are employed for the determination of enantiomeric ratio in pharmaceutical preparations and for monitoring the enantiomer levels in biological samples for therapeutic and toxicologic purposes. The current study reviews the published literature for the chiral analysis of CIT and its metabolites based on chromatographic and electrophoretic methods coupled with UV, fluorescence and mass spectrometry detectors.     

Stereospecific effects of benzo[d]isothiazolyloxypropanolamine derivatives at beta-adrenoceptors: synthesis, chiral resolution, and biological activity in vitro

We performed the asymmetric synthesis of four enantiopure benzo[d] isothiazo-3-or 5-yloxypropanolamine derivatives, previously described as competitive antagonists at beta-adrenoceptors. The chemical characterization of each enantiomer was accomplished by (1)H NMR and HPLC/DAD/CD. The direct chromatographic separation of the enantiomers via chiral HPLC was investigated. The best resolutions were achieved using cellulose tris (3,5-dimethylphenyl carbamate) (Chiralcel OD-H) and amylose tris (3,5-dimethylphenyl carbamate) (Chiralpak AD). The enantiomers obtained had enantiomeric purities suitable for biological assays. Tested in isolated rat cardiac and intestinal tissues to evaluate their effects at beta(1)- and beta(3)-adrenoceptors, the (S)-enantiomers revealed a higher degree of antagonism than (R)-enantiomers at both subtypes, even though their activity was greater at the cardiac beta(1)-subtype. The potent and cardiospecific antagonistic effect exerted by the compounds tested suggests that the benzisothiazole moiety could be an interesting scaffold for discovering new chiral beta-blocking drugs.     

Chiral investigation of midodrine, a long-acting alpha-adrenergic stimulating agent

Midodrine hydrochloride is a peripheral alpha(1)-adrenoreceptor agonist that induces venous and arterial vasoconstriction. Midodrine, after oral or intravenous administration, undergoes enzymatic hydrolysis and releases deglymidodrine, a pharmacologically active metabolite. Midodrine and deglymidodrine have a chiral carbon in the 2-position. To investigate the bioactivity of racemates and enantiomers of the drug and metabolite, three chromatographic chiral stationary phases, Chiralcel OD-H, Chiralcel OD-R, and alpha(1)-AGP, were evaluated for enantiomeric resolution. Good enantioseparation of midodrine racemate was obtained using the Chiralcel OD-H column. This stationary phase was then used to collect separately the midodrine enantiomers. By alkaline hydrolysis of rac-midodrine and each separated enantiomer, rac-deglymidodrine and its enantiomers were prepared. The control of the enantiomeric purity was carried out by alpha(1)-AGP stationary phase, while the hydrolysis of rac-midodrine and its enantiomers was controlled by capillary electrophoresis using trimethyl-beta-cyclodextrin as chiral selector. The pharmacological activity of the two racemates and the two enantiomeric pairs was tested in vitro on a strip of rabbit descending thoracic aorta. The tests continued that the activity of the drug and metabolite is due only to the (-)-enantiomer because neither of the (+)-enantiomers is active.     

Synthesis and biochemical evaluation of highly enantiomerically pure (R,R)- and (S,S)-alexidine

Alexidine is in everyday human use as oral disinfectant and contact lens disinfectant. It is used as a mixture of stereoisomers. Since all of alexidine’s known biological targets are chiral, the biological activity of any of its chiral stereoisomers could be significantly higher than that of the mixture of stereoisomers. This makes a synthetic methodology for obtaining the individual enantiomers of the chiral diastereoisomer highly desirable. Here, we describe the first synthesis of both enantiomers of alexidine in high enantiomeric purity, and demonstrate their activity against the protein–protein interaction between the anti-apoptotic protein Bcl-x_L and the pro-apoptotic protein Bak.     

Chiral purity determination of tobacco alkaloids and nicotine-like compounds by 1H NMR spectroscopy in the presence of 1,1′-binaphthyl-2,2′-diylphosphoric acid

The enantiomeric purity of several tobacco alkaloids and nicotine-like compounds was determined using ¹H NMR (300 MHz) spectroscopy in the presence of (-)-(R)-1,1′binaphthyl-2,2′-diylphosphoric acid (BNPPA) as a chiral complexing agent. The most significant signal splitting resulting from diastereoisomeric complexation are seen for chemical shifts in the proximity of the pyridinyl nitrogen. Chemical shift data exclude any contribution of the pyrrolidinyl protons to chiral recognition, but when the pyrrolidine ring is replaced by a piperidine ring, i.e., for compounds such as rac-anabasine and rac-anatabine, non-equivalence between enantiomers was observed for protons close to the piperidine ring. A new approach for the preparation of the pure (-)-(S)-and (+)-(R)-enantiomers of nornicotine, a tobacco alkaloid and metabolite of nicotine, was developed. The optically pure enantiomers thus obtained were used to establish the minimum sensitivity of the NMR spectroscopic method of chiral analysis. These findings provide a new, general, and facile method for the determination of enantiomeric purity of tobacco alkaloids and nicotine-like compounds. © 1996 Wiley-Liss, Inc.     

2-Amino-3-(3-hydroxy-1,2,5-thiadiazol-4-yl)propionic acid: resolution, absolute stereochemistry and enantiopharmacology at glutamate receptors

In order to identify new subtype-selective (S)-glutamate (Glu) receptor ligands we have synthesized (RS)-2-amino-3-(3-hydroxy-1,2,5-thiadiazol-4-yl)propionic acid [(RS)-TDPA]. Resolution of (RS)-TDPA by chiral chromatography was performed using a Crownpac CR(+) column affording (R)- and (S)-TDPA of high enantiomeric purity (enantiomeric excess=99.9%). An X-ray crystallographic analysis revealed that the early eluting enantiomer has R-configuration. Both enantiomers showed high affinity as well as high agonist activity at (RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) receptors, determined using a [(3)H]AMPA binding assay and an electrophysiological model, respectively. The affinities and agonist activities obtained for (R)-TDPA (IC(50)=0.265 microM and EC(50)=6.6 microM, respectively) and (S)-TDPA (IC(50)=0.065 microM and EC(50)=20 microM, respectively) revealed a remarkably low AMPA receptor stereoselectivity, (S)-TDPA showing the highest affinity and (R)-TDPA the most potent agonist activity. In addition, (S)-TDPA was shown to interact with synaptosomal Glu uptake sites displacing [(3)H](R)-aspartic acid (IC(50 ) approximately 390 microM). An enantiospecific and subtype-selective agonist activity was observed for (S)-TDPA at group I metabotropic Glu (mGlu) receptors (EC(50)=13 microM at mGlu(5) and EC(50)=95 microM at mGlu(1)).     

Chiral discrimination of multiple profens as diastereomeric (R)-(+)-1-phenylethylamides by achiral dual-column gas chromatography

Profens were converted into diastereomeric (R)-(+)-1-phenylethylamides using ethyl chloroformate and triethylamine in dichloromethane. Gas chromatographic analysis on dual-columns with different polarities provided complete enantioresolution of eight profens, facilitating chiral discrimination based on matching with retention index sets characteristic of each enantiomer. The present method was linear (r >/= 0.9992) with good precision (0.8-6.0%) and accuracy (-9.3 to 0.003%), allowing detection of trace (R)-profens in optical purity test on four (S)-profen mixture in a single run. And the method allowed simultaneous enantiomeric screening for ibuprofen enantiomers and their chiral metabolites excreted in urine following administration of racemic ibuprofen.     

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.     

Syntheses and β‐adrenergic binding affinities of (R)‐ and (S)‐fluoronaphthyloxypropanolamines

The β‐adrenergic receptors mediate several physiological processes including heart rate (β₁), bronchodilation (β₂), and lipolysis (β₃). Therefore, selectivity is important for a possible therapeutic agent acting via these receptors. Aryloxypropanolamines are β‐receptor agonists or antagonists, depending on the aryl group and its substituents. We therefore hypothesized that fluorine substitution on the aromatic ring in this class could lead to significant biological effects because of the unique chemical characteristics of fluorine. Because the target compound has a chiral center, we set out to synthesize the two enantiomers so that effects of stereochemistry on biological activity could be evaluated. Syntheses of the enantiomers were performed starting with commercially available fluoronaphthalene and subsequent use of the chiral synthon (2R)‐ or (2S)‐glycidyl 3‐nitrobenzenesulfonate, depending on the desired enantiomer. High‐pressure liquid chromatography (HPLC) methods were used to characterize %ee. Each enantiomer was synthesized. They exhibited nanomolar binding activities on β‐adrenergic receptors. The (S)‐enantiomer was found to be up to 310 times more potent than the (R). It was also found to be about five‐fold more selective for β₂‐ than for β₁‐receptors. The current report demonstrates the importance of stereochemistry for the fluoroaromatic β‐receptor ligands. Chirality 11:144–148, 1999. © 1999 Wiley‐Liss, Inc.     

Simultaneous LC/ESI‐MS Separation Method for the Enantioseparation of Some New Anticonvulsant Drugs

A sensitive and specific method for the simultaneous determination of the enantiomeric purity of 2,6‐dimethylphenoxyacetyl derivatives as trans or cis racemic and enantiomeric forms with 2‐ or 4‐aminocyclohexanol moiety ( 1 , 2 , 3 , 6 ) and their amine analogs ( 8 , 9 ) was developed. The compounds studied are known for their anticonvulsant activity and the most interesting pharmacological results were those for (±)‐trans‐2‐(2,6‐dimethylphenoxy)‐N‐(2‐hydroxycyclohexyl)acetamide ( 1 ) as well as (±)‐trans‐2‐[(2,6‐dimethylphenoxy)ethyl]aminocyclohexanol ( 8 ). The analytical method for determining the enantiomeric purity of the compounds studied is based on direct separation of the analytes using a chiral stationary phase (Chiralpak AS column). The mass spectrometric analysis was done on a coupled liquid chromatograph–mass spectrometer system with an electrospray ionization source (LC/ESI‐MS). For the compounds 1 , 8 , and 9 , the method allows an excellent separation of enantiomers, with a resolution higher than 3.2, and a tailing factor of less than 1.67 with a final enantiomer purity better than 97.5%. Chirality 26:144–149, 2014. © 2014 Wiley Periodicals, Inc.     

Studies on the Enantiomers of RC-33 as Neuroprotective Agents: Isolation,Configurational Assignment,and Preliminary Biological Profile

In this study we addressed the role of chirality in the biological activity of RC-33 , recently studied by us in its racemic form. An asymmetric synthesis procedure was the first experiment, leading to the desired enantioenriched RC-33 but with an enantiomeric excess (ee) not good enough for supporting the in vitro investigation. An enantioselective high‐performance liquid chromatography (HPLC) procedure was then successfully carried out, yielding both RC-33 enantiomers in amounts and optical purity suitable for the pharmacological study. The absolute configuration of pure enantiomers was easily assigned exploiting the asymmetric synthesis previously devised. As emerged in the preliminary in vitro biological investigation, (S)‐ and (R)-RC-33 possess a comparable affinity towards the σ₁ receptor and a very a similar behavior in the calcium influx assay, resulting in an equally effective σ₁ receptor agonist. Overall, the results obtained so far suggest that the interaction with the biological target is nonstereoselective and leads us to hypothesize that there is a lack of stereoselectivity in the biological activity of RC-33 . Chirality 25:814–822, 2013. © 2013 Wiley Periodicals, Inc.     

Sensitivity improvement of circular dichroism detection in HPLC by using a low-pass electronic noise filter: Application to the enantiomeric determination purity of a basic drug

The quality control of chiral drugs requires the determination of their enantiomeric purity. Nowadays, circular dichroism (CD) spectroscopy is gaining increasing importance in pharmaceutical analysis because of the commercially available CD detector in liquid chromatography. The separation of the two enantiomers of a basic drug (efaroxan) was achieved by high performance liquid chromatography using an amylose-derivated column with both UV and CD detections. A baseline-resolved separation (resolution: 5) was obtained after optimization of the mobile phase composition with hexane-ethanol-diethylamine (90:10:0.05; v/v/v). The use of a commercial low-pass electronic noise filter of the CD signal has improved the signal-to-noise ratio by a factor twelve and allowed the quantitation of each enantiomer in the 1.25-300 microg ml(-1) concentration range. The CD linear calibration curve, expressed in terms of stereoisomer height ratio versus concentration ratio, was plotted over the 0.4-6% range. A correlation coefficient greater than 0.999 was obtained by least-squares regression and the limit of detection for the distomer/eutomer ratio was estimated at 0.14%. Although the method validation showed good repeatability on the retention times (RSD < 0.9%), on the peak height ratios (RSD < 8.7%) of each enantiomer only up to 99.2% enantiomeric purity was achieved.     

Enantioselective separation and degradation of the herbicide dichlorprop methyl in sediment

Chiral pesticides currently constitute about 50% of all pesticides dosage used in China, and this ratio is increasing as more complex structures are introduced. Dichlorprop methyl (DCPPM) is a chiral herbicide consisting of a pair of enantiomers. In this study, the enantiomeric separation of DCPPM was investigated by gas chromatography (GC) and high-performance liquid chromatography (HPLC) using chiral stationary phases (CSPs), and its enantiomeric degradation was characterized using a DCPPM-degrading bacterial strain isolated from an activated sludge from a textile-printing wastewater treatment plant. Baseline separation by both GC and HPLC was achieved. Incubation with DCPPM-degrading bacteria in different pH solutions showed that the R enantiomer was preferentially degraded over the S enantiomer of DCPPM. The degradation rate constant decreased with increasing pH in the order of k(pH5) approximately k(pH7) >k(pH9). In comparison, the enantioselectivity as indicated by EF followed the order of pH 7 > pH 9-pH 5.     

Preparation and configuration of racemic and optically active analgesic cycloaminoalkylnaphthalenes

Cycloaminoalkylnaphthalene 3 shows interesting opioid‐like analgesic properties. It possesses two chiral centers and can exist as two racemic pairs and four diastereomers. Since the binding of opioids with receptors is stereoselective, it was important to have the two racemic pairs as well as the four diastereomers. In this paper the synthesis of the (2R,3S/2S,3R) racemate and the (2R,3S) and (2S,3R) enantiomers of the 1,2‐dimethyl‐3‐[2‐(6‐hydroxynaphthyl)]‐3‐hydroxypyrrolidine 3 is considered and the determination of absolute configuration is described. The (2R,3S/2S,3R)‐ 3 racemate and the (2R,3S)‐ 3 and (2S,3R)‐ 3 enantiomers were prepared by reaction of the racemic and optically active 1,2‐dimethyl‐3‐pyrrolidone 2, respectively, with the lithiation product obtained from 2‐bromo‐6‐tetrahydropyranyloxy‐naphthalene 1 and acidic hydrolysis. The above‐mentioned enantiomers of 3 were also obtained by optical resolution via fractional crystallization of the salts with d ‐ and l ‐tartaric acids. The configuration of the optically active compounds was determined by X‐ray analysis of a crystal of (−)‐(2S,3R)‐ 3 · HCl · H₂O. The pharmacological test HPT showed that (−)‐(2S,3R)‐ 3 · HCl · H₂O enantiomer is able to induce opioid‐like analgesia with a relative potency 1.5 times that of (2R,3S/2S,3R)‐ 3 and ∼1.5 times that of morphine. Chirality 11:21–28, 1999. © 1999 Wiley‐Liss, Inc.     

Diethylstilbestrol metabolites and analogs. Stereochemical probes for the estrogen receptor binding site

Indenestrol A (IA) and indenestrol B (IB) are analogs and metabolites of diethylstilbestrol (DES). These compounds have high binding affinity with the estrogen receptor (ER) but possess weak uterotropic activity. Due to their chemical structures, IA and IB exist as mixtures of enantiomers. We investigated whether the poor biological activity of these compounds was due to differential activity of the enantiomers. We also utilized these compounds as probes to determine the extent of stereochemical sensitivity in the ER ligand binding site. The IA and IB enantiomers were separated to greater than 98% purity using a chiral high pressure liquid chromatography column. Their enantiomeric nature was confirmed by mass spectrometry and NMR. The purified IA enantiomer peak 1 was derivatized with 4-bromobenzoyl chloride. The resulting di(4-bronobenzoate) IA was analyzed by x-ray crystallography and the absolute enantiomeric conformation assigned is C(3)-R. The IA enantiomers designated IA-R and A-S were assayed by competitive binding to cytosolic ER. The competitive binding index was estradiol, 100; DES, 286; IA-Rac (racemic mixture of IA), 143; IA-R, 3; and IA-S, 285; the index showed that ER demonstrates a stereochemical chiral preference. The IB enantiomers did not show a binding preference: IB, 145; IB-1, 100; and IB-2, 143. The differences in the IA enantiomer binding were shown to be due to competitive interactions by Lineweaver-Burk analysis of saturation binding of estradiol to ER in the presence of 1-, 5-, and 10-fold molar excess of competitor. Differences in binding affinity of the enantiomers could be partially explained by differences in the association rate constant (k+1) determined by association rate inhibition studies in which IA-S was 15 times more active than IA-R. Nuclear estrogen receptor levels were measured 1 h after in vivo treatment with doses of 5-20 micrograms/kg. The IA-Rac produced only 60% of the levels is compared with DES. Nuclear ER levels were checked every 30 min up to 2 h with no apparent difference, indicating that the low early levels were not due to a delayed estrogen receptor retention. When the enantiomers were tested individually only a dose of 10 micrograms/kg IA-S translocated ER to a level comparable to DES, while IA-R showed low levels at several doses. These results suggest that the poor biological activity of IA may be related to the differential ER interaction of its enantiomers.(ABSTRACT TRUNCATED AT 400 WORDS)     

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.     

Chiral assay methods for lifibrol and metabolites in plasma and the observation of unidirectional chiral inversion following administration of the enantiomers to dogs

Lifibrol, a new drug for the treatment of hypercholesterolemia, contains a stereogenic center bearing a secondary alcohol group. A normal-phase achiral–chiral HPLC separation of the enantiomers of lifibrol and two of its metabolites was developed and validated for quantitation in dog plasma. A silica and a Chiralcel OD-H column were operated in series and all six enantiomeric components and internal standard were directly separated. An initial solid-phase extraction (phenyl) clean-up step and a column-switching step to eliminate late-eluting compounds were also utilized. The solid-phase extraction step was automated using a robotic system. Assay development, validation, and application of the method to a bioavailability study of the racemate and enantiomers of lifibrol in dogs are described. The lower limit of quantitation was 0.0125 μg/ml for each enantiomer of lifibrol using 200 μl of dog plasma with UV detection (255 nm). In dog plasma following oral or intravenous administration of the racemate, the (R)/(S) ratio of the enantiomers of lifibrol was greater than one and increased with time. Following administration of the individual enantiomers, chiral inversion of the (S)-enantiomer but not the (R)-enantiomer was observed. © 1994 Wiley-Liss, Inc.     

Evaluation of (R)-(-)-α-methoxy phenyl acetic acid as a chiral shift reagent for resolution and determination of R and S enantiomers of modafinil in bulk drugs and formulations by 1H NMR spectroscopy

(R)-(-)-α-Methoxy phenyl acetic acid, (S)-(-)-1,1'-(2-naphthol), and (R)-(+)-α-methoxy-α-trifluoromethyl phenyl acetic acid were evaluated as chiral shift reagents (CSRs) for (1)H NMR spectroscopic resolution and determination of R and S enantiomers of modafinil (MDL) in bulk drugs and formulations. Effects of the nature of CSR and the weight ratio of substrate to shift reagent on enantiomeric discrimination were investigated. Intramolecular and intermolecular hydrogen bonding interactions between the drug and the CSR seem to be the driving force for desired resolution. A mechanism was proposed to explain the interactions between (R, S)-enantiomers of MDL and (R)-(-)-α-methoxy phenyl acetic acid. The method was validated and applied successfully to determine the enantiomeric purity of MDL in tablet formulations.