Metoprolol Enantiomers Research Articles

Enantioselective and diastereoselective aspects of the oxidative metabolism of metoprolol

Enantio- and diastereoselective aspects of oxidative metabolism of metoprolol ( 1) were examined in the presence of rat liver and human liver microsomes using a pseudoracemate of 1, made up of equal molar (2R)- 1- d 0 and (2S)- 1- d 2 , as substrate. Both O-demethylation and α-hydroxylation showed only slight enantioselectivity, 2R 2S ratios being 1.18 and 0.93 for these pathways in rat liver microsomes and 1.09 and 0.92 in human liver microsomes. In the presence of the rat liver microsomal fraction, α-hydroxylation yielded predominantly the 1′ R-hydroxy product, 1′ R 1′S ratio > 12 , regardless of the stereochemistry of the side chain. In humans (extensive metabolizers) administered a single 50 mg oral dose of pseudoracemic metoprolol tartrate, urinary α-hydroxymetoprolol ( 2) accounted for 9.3 ± 2.4% of the dose, 2R 2S ratio of 0.85 ± 0.14 , and the carboxylic acid metabolite 4, accounted for 52.7 ± 6.8% of the dose, 2R 2S ratio 1.15 ± 0.09. The data suggested that preferential O-demethylation of the (2 R)-enantiomer of 1 could contribute to the 2 S > 2 R plasma ratio of metoprolol enantiomers observed in this population.

Chiral chloroformates as transparent reagents for the resolution of metoprolol enantiomers by reversed-phase liquid chromatography

Ten chiral chloroformates were prepared and tested as derivatizing reagents, using metoprolol and tyrosine enantiomers as model compounds. Chloroformates derived from isosorbide and isomannide monoesters gave separation factors of 1.07–2.16 for tyrosine and 1.06–1.08 for metoprolol in reversed-phase liquid chromatography. With tert.-butyl 3-(chloroformoxy)butyrate as derivatizing reagent a separation factor of 1.10 and a resolution of 2.0 were obtained in 12 min for ( R)_- and ( S)-metoprolol. The reagent and its by-products did not interfere with the UV or fluorescence detection of metoprolol.

Enantioselective determination of metoprolol in plasma by liquid chromatography on a silica-bonded α 1-acid glycoprotein column

The enantiomers of metoprolol were determined in plasma samples after direct resolution on a silica bonded α 1-acid glycoprotein column. Metoprolol was extracted from plasma into a diethyl ether-dichloromethane mixture and after back extraction to dilute phosphoric acid and adjustment of pH the sample was injected on a Chiral- AGP column for separation of R- and S-metoprolol. It was possible to measure down to 2 nmol per litre plasma with a relative standard deviation of less than 15% by use of gradient elution and fluorescence detection. The analytical method was employed to study the pharmacokinetics of the metoprolol enantiomers after administration of the racemate to humans.

Analysis of metoprolol enantiomers in human serum by liquid chromatography on a cellulose-based chiral stationary phase

Metoprolol is a lipophilic, cardioselective β-adrenergic blocking agent commercially available as a racemic compound. A normal phase high-performance liquid chromatographic method was developed to directly determine individual enantiomeric concentrations of metoprolol in human serum. Separation of the enantiomers was accomplished by a cellulose-tris(3,5-dimethylphenylcarbamate) chiral stationary phase. Metoprolol enantiomers were detected by means of fluorescence with excitation and emission wavelengths of 275 and 315 nm, respectively. Standard curves were linear over the concentration range 12.5–400 ng/ml for each enantiomer. Within-day coeffient of variation was <15% at all concentrations and the between-day coefficient of variation ranged from 4.1 to 11.2%. The limit of detection was determined to be 5 ng/ml for each enantiomer and the stereoselective resolution (α) of R- and S-metoprolol was 3.08. The assay was employed to determine enantiomeric serum concentrations of metoprolol in healthy male volunteers.

Stereospecific synthesis of specifically deuterated metoprolol enantiomers from chiral starting materials

AbstractEnantiomers of metoprolol (1) containing six deuterium atoms in the isopropyl methyl groups [(2R‐2], two deuterium atoms at C‐2 and C‐6 of the aromatic ring [(2S)‐3], and two deuterium atoms at C‐3 of the propanolamine side chain [(2S)‐4] were prepared. Chiral 2,2‐dimethyl‐1,3‐dioxolane‐4‐methanols [(4R)‐5 and (4S)‐5] were key synthons. Sources of deuterium were [2H6]‐isopropylamine, 4‐(2‐methoxyethyl)‐2,6‐[2H2]‐phenol (12), prepared by 2HCl/2H2O exchange, and (4S)‐2,2‐dimethyl‐1,3‐dioxolane‐4‐[2H2]‐4‐methanol (19), prepared by LiAl2H4 reduction of (4S)‐methyl 2,2‐dimethyl‐1,3‐dioxolane‐4‐carboxylate. Enantiomeric excess was greater than 94% for each of the prepared enantiomers, as determined independently by 1H NMR spectroscopy on diastereomeric derivatives and by chiral column HPLC.

Optimization of detection sensitivity for enantiomers of metoprolol on silica-bonded α1-acid glycoprotein

On the chiral phases CHIRAL-AGP and EnantioPac, organic modifiers, pH and temperature have been varied with isocratic and gradient techniques to obtain minimum retention and maximum peak height by baseline resolution of R-and S-metoprolol. The decrease in retention obtained by gradient elution gives, by limiting baseline resolution, 20–60% greater peak heights than those obtained by the isocratic technique. A reduction of the retention by a change in the pH is more favourable than addition of modifiers such as acetonitrile or 2-propanol. It gives a smaller decrease in resolution by isocratic elution and a pH gradient can give considerably stronger peak compression than an acetonitrile gradient as indicated by an estimation of the detection limits. For metoprolol enantiomers, CHIRAL-AGP gives about three times lower reduced plate heights than does EnantioPac.

Coupled column chromatography—mass spectrometry : Thermospray liquid chromatography—Mass spectrometric and liquid chromatography—tandem mass spectrometric analysis of metoprolol enantiomers in plasma using phase-system switching

The applicability of phase-system switching using thermospray tandem mass spectrometry is demonstrated for the bioanalysis of the enantiomers of the betablocker metoprolol. Independent optimization of the chromatography, using an α 1-acid glycoprotein chiral stationar phase, and the mass spectrometric detection system is realized. By utilizing peak compression, 10 ng (37 pmol) of each enantiomer is easily detected in standard solutions. In plasma samples, with use of tandem mass spectrometry for additional selectivity, levels of 61 ng/ml (230 nmol/l) can be measured by using [ 2H 6]metoprolol as internal standard.

Coupled-column chromatography on immobilized protein phases for direct separation and determination of drug qenantiomers in plasma

Columns packed with immobilized α 1-acid glycoprotein and albumin were used in coupled-column chromatography to increase their utility for determining low concentrations of enantiomers in biological samples. The two enantiomers eluted from the protein columns were trapped and compressed on two separate columns, packed with hydrophobic stationary phase, and subsequently transferred to a fourth column for final separation. The overall effect was an increase in efficiency and selectivity. Examples are given of separations of the enantiomers of terbutaline, metoprolol, oxazepam and bupivacaine in plasma. For quantitative determination a single calibration can be used for both enantiomers.

Rapid high-performance liquid chromatographic method for the measurement of the enantiomers of metoprolol in serum using a chiral stationary phase

Metoprolol, a β-adrenergic blocker, is only available as a racemic mixture for clinical use. A high-performance liquid chromatographic method for the separation of the optical isomers (enantiomers) in human serum is described utilizing a commercially available column with a cellulose tris(3,5-dimethylcarbamate) chiral stationary phase. Separation of the enantiomers is accomplished without precolumn derivatization using a mobile phase of hexane-2-propranol-diethylamine (91:8:1, v/v). The moethod can be successfully applied to pharmacokinetic studies in man.

Measurement of underivatised metoprolol enantiomers in human plasma by high-performance liquid chromatography with a chiral stationary phase

We report the direct assay of underivatised metoprolol enantiomers in human plasma using a combination of solvent extraction and solid-phase extraction followed by HPLC with a new stationary phase:cellulose tris (3,5 dimethylphenyl-carbamate) polymer bonded to macroporous silica commercially prepached

Reversed-phase high-performance liquid chromatographic assay to quantitate diastereomeric derivatives of metoprolol enantiomers in plasma

Reversed-phase high-performance liquid chromatographic assay to quantitate diastereomeric derivatives of metoprolol enantiomers in plasma

Determination of metoprolol enantiomers in plasma and urine using ( S)-(−)-phenylethyl isocyanate as a chiral reagent

Determination of metoprolol enantiomers in plasma and urine using ( S)-(−)-phenylethyl isocyanate as a chiral reagent

The Polymorphic Oxidation of ??-Adrenoceptor Antagonists

Wide variability in response to some drugs such as debrisoquine can be attributed largely to genetic polymorphism of their oxidative metabolism. Most beta-blockers undergo extensive oxidation. Anecdotal reports of high plasma concentrations of certain beta-blockers in poor metabolisers (PMs) of debrisoquine have claimed that the oxidation of these drugs is under polymorphic control. Subsequently, controlled studies have shown that debrisoquine oxidation phenotype is a major determinant of the metabolism, pharmacokinetics and some of the pharmacological actions of metoprolol, bufuralol, timolol and bopindolol. The poor metaboliser phenotype is associated with increased plasma drug concentrations, a prolongation of elimination half-life and more intense and sustained beta-blockade. Phenotypic differences have also been observed in the pharmacokinetics of the enantiomers of metoprolol and bufuralol. In vivo and in vitro studies have identified some of the metabolic pathways which are subject to the defect, viz. alpha-hydroxylation and O-demethylation of metoprolol and 1'- and possibly 4- and 6-hydroxylation of bufuralol. In contrast, the overall pharmacokinetics and pharmacodynamics of propranolol, which is also extensively oxidised, are not related to debrisoquine polymorphism, although 4'-hydroxypropranolol formation is lower in poor metabolisers. As anticipated, the disposition of atenolol which is eliminated predominantly unchanged by the kidney and in the faeces, is unrelated to debrisoquine phenotype. The clinical significance of impaired elimination of beta-blockers is not clear. If standard doses of beta-blockers are used in poor metabolisers, these subjects may be susceptible to concentration-related adverse reactions and they may also require less frequent dosing for control of angina pectoris.

Oxidation phenotype and the metabolism and action of beta-blockers.

Variability in response to some drugs such as debrisoquine can be attributed to genetic polymorphism of their oxidative metabolism. Most beta-adrenoceptor antagonists (beta-blockers) are extensively metabolised via oxidative routes. Anecdotal reports of high plasma concentrations of certain beta-blockers in poor metabolisers of debrisoquine (PM) have claimed that their oxidation is under polymorphic control. Controlled studies have shown that debrisoquine oxidation phenotype is a major determinant of the metabolism, pharmacokinetics and some of the pharmacological actions of metoprolol, bufuralol and timolol. The PM phenotype is associated with an increased drug bioavailability, a prolongation of elimination half-life and more intense and sustained beta-blockade. Phenotypic differences were also noted in the pharmacokinetics of the enantiomers of metoprolol. In vivo and in vitro work has identified some of the metabolic pathways which are subject to the defect, namely, the alpha-hydroxylation and the O-dealkylation of metoprolol and the 1'-hydroxylation of bufuralol. In contrast, the pharmacokinetics and pharmacodynamics of propranolol which is also extensively oxidised, are not related to debrisoquine polymorphism, although 4'-hydroxypropranolol formation is lowered in PM subjects. The clinical significance of impaired elimination of beta-blockers is unclear. If standard doses of beta-blockers are used in PM subjects, they may be susceptible to concentration-related adverse reactions and they may also require lower and less frequent dosing for control of angina pectoris.

Differential stereoselective metabolism of metoprolol in extensive and poor debrisoquin metabolizers.

The hypothesis that variability in metoprolol metabolism stereoselectivity is related to debrisoquin oxidation phenotype was tested in six extensive (EM) and six poor (PM) debrisoquin metabolizers. In EM, plasma AUCs for (S)-metoprolol were 35% higher than for (R)-metoprolol, whereas in PM, AUCs for (S)-metoprolol were lower than for (R)-metoprolol. AUCs for total metoprolol correlated with the ratio of (S)- to (R)-metoprolol AUC. The renal clearance of metoprolol was also stereoselective but to the same extent in both EM and PM. Findings suggest that the enzyme system responsible for polymorphic oxidation of the debrisoquin-type is stereoselective. The relation between log total metoprolol plasma concentration and response (beta-blockade) was shifted to the right in PM relative to EM, which is compatible with a difference in pharmacologic activity of metoprolol enantiomers. Kinetic predictions based on total drug measurements will tend to overestimate dynamic differences between EM and PM, but the magnitude of the error is relatively small, and, in absolute terms, there is a large difference in pharmacologic activity between the phenotypes (beta-blockade at 24 hr: EM = 5.3 +/- 5.6%; PM = 18.9 +/- 3.8%).

Erythromycin

Rifampicin, a member of rifamycin sub-class of antibiotics which belongs to the naphthalenic ansamycin class of antibiotics, has a characteristic ansa structure, i.e., a ring structure or chromophore spanned by an aliphatic chain. The present work was designed to evaluate its potential as a chiral selector (CS) as its structure consisting of nine stereogenic centers, an aromatic moiety and several functional groups (i.e., one imine, one amide, one acetoxy residue, two aliphatic hydroxyl and three phenolic hydroxyl groups) was expected to instigate multiple enantioselective interactions, namely, hydrogen bonding and inclusion complexation with chiral analytes, and therefore resulting in efficient enantioseparations. Systematic experiments were performed to investigate the effects of concentration of CS, composition of background electrolyte (BGE) and applied voltage on chiral separation. Enantiomers of propranolol and metoprolol were baseline resolved using a BGE consisting of 20 mM CS and 50/50 (v/v) iso-propanol/phosphate buffer (100 mM, pH 7.0) whereas for enantiomers of sertraline, a BGE consisting of 23 mM CS and 40/60 (v/v) iso-propanol/phosphate buffer (100 mM, pH 7.0) resulted in baseline resolutions.