β-Blocking Agents: Determination of Biological Levels Using High Performance Liquid Chromatography

Analysis of oxprenolol in formulations by high-performance liquid chromatography

A simple, accurate, and rapid high-performance liquid chromatographic method for the analysis of oxprenolol in commercial formulations is described. The analysis was performed on a cyano radial compression cartridge, with 0.0539 M, pH 3 phosphate buffer-acetonitrile-methanol (76: 15.6:8.4) as the mobile phase. The flow rate was 5 mL/min, with detection at 272 nm; the mobile phase was employed for extraction. The assay was applied to the content uniformity test of three oxprenolol hydrochloride tablet formulations of different strengths and the contents of a 2-mg dry ampule for intravenous/intramuscular injection. The percent of label claim for each formulation tested was within 91.7-110%. The applicability of this assay to the analysis of some other beta-blocking drugs was investigated. It was found that under the above conditions, atenolol, metoprolol, oxprenolol, and propranolol can be fully resolved in less than 3 min.

Determination of metoprolol and its alpha-hydroxylated metabolite in human plasma by high-performance liquid chromatography

A high-performance liquid chromatographic method has been developed for the simultaneous determination of metoprolol and its alpha-hydroxylated metabolite in plasma, Metoprolol, alpha-hydroxymetoprolol and alprenolol (internal standard) are extracted from plasma at alkaline pH with diethyl ether-dichloromethane (4:1, v/v) and back-extracted with 0.01 N sulfuric acid. A 100-microliter volume of the acidic extract is injected into the chromatographic system. The compounds are eluted in about 12 min with acetonitrile-acetate buffer (75:25, v/v) on a LiChrosorb RP-8 (5 micron) column. The quantitative determinations are made fluorometrically. Concentrations down to 35 nmol/1 (10 ng/ml) of metoprolol base and 30 nmol/1 (8 ng/ml) of alpha-hydroxymetoprolol base in plasma can be determined with good precision and accuracy.

Study of the influence of nifedipine on the pharmacokinetics and pharmacodynamics of propranolol, metoprolol and atenolol

The influence of chronic therapy with nifedipine on the pharmacokinetics of propranolol 80 mg twice daily, metoprolol 100 mg twice daily and atenolol 100 mg once daily was investigated in eight healthy volunteers. Nifedipine 10 mg three times daily did not affect the pharmacokinetics of metoprolol and atenolol whereas nifedipine shortened the time to peak plasma concentration for propranolol by about 1 h. Propranolol, metoprolol and atenolol provoked comparable decreases in heart rate measured at rest and during exercise. The beta-adrenoceptor blocking properties of propranolol, metoprolol and atenolol were not affected by concomitant therapy with nifedipine. The present study did not show significant pharmacokinetic and pharmacodynamic interactions between nifedipine and lipophilic beta-adrenoceptor blockers.

Multiple inverse isotope dilution assay for oxprenolol and nine metabolites in biological fluids

An isotope dilution assay for the specific determination of 14C-labelled oxprenolol and nine of its metabolites in the same biological sample is described. After addition of unlabelled carriers to the sample, oxprenolol and the metabolites were isolated by base- and acid-specific extraction and separated by normal-phase high-performance liquid chromatography using two different mobile phases. Quantitation of the various peaks was performed by on-line ultraviolet detection at 275 nm and off-line radiometry by liquid scintillation counting. Endogenous compounds and unknown metabolites did not interfere in the assay. The analysis of rat and dog blood, plasma and urine samples spiked with [14C]oxprenolol hydrochloride, showed mean recoveries between 98.7 and 99.8%. The assay was used to investigate the metabolic fate of [14C]oxprenolol in the dog. Analyses of blood and urine demonstrated the quantitative significance of the various metabolites in the biotransformation of oxprenolol.

Gas chromatographic determination of metoprolol in human plasma

A method for the determination of metoprolol at concentrations down to 10 ng/ml in human plasma is described. After addition of oxprenolol as internal standard, both compounds are extracted into diethyl ether--dichloromethane (4:1, v/v) at basic pH, transferred into an acidic aqueous solution and back-extracted at basic pH into diethyl ether--dichloromethane. They are then derivatized with heptafluorobutyric anhydride. The derivatives are quantitatively determined by gas chromatography using a 63Ni electron-capture detector. The linearity was demonstrated, and the technique was formally validated in the concentration range 10-500 ng/ml. The technique was applied in a study of the bioavailability of metoprolol after oral administration to man; mean plasma concentrations are reported.

Comparison of two long-acting preparations of metoprolol with conventional metoprolol and atenolol in healthy men during chronic dosing

1 Eight healthy men received two long-acting formulations of metoprolol 200 mg (SA Astra, SR Geigy), conventional metoprolol 200 mg and atenolol 100 mg once daily for 1 week each in balanced, crossover fashion. There was a washout period of at least a week between each phase. 2 On the last day of each phase, post-exercise heart rate was recorded at intervals and compared to pretreatment values. Plasma metoprolol concentrations were measured. 3 The mean AUC was similar after each of the three formulations of metoprolol (relative bioavailability of SA and SR v conventional was 97%) but with SA and SR metoprolol the time to peak was significantly delayed by about 2 h. 4 In comparison to conventional metoprolol only metoprolol SA was associated with significantly higher plasma metoprolol concentrations at the end of a dosing interval (mean values: conventional, 25 ng/ml, SR 37 ng/ml, SA 51 ng/ml). 5 Mean (+/- s.d.) reduction in exercise tachycardia at the end of a dosing interval was significantly greater with atenolol (14.8 +/- 4.5%) and metoprolol SA (13.7 +/- 10.3%) than with metoprolol SR (10 +/- 8.4%) and conventional metoprolol (8.2 +/- 7.1%). 6 The variability in beta-adrenoceptor blockade at 24 h was much greater with each of the three metoprolol formulations than that with atenolol. This was explained by the variability in metoprolol metabolism. 7 Oxidation phenotype testing with debrisoquine showed there were six extensive metabolisers and two poor metabolisers. The AUC, half-life and response to metoprolol at 24 h were much greater in poor metabolisers. Response to atenolol was not influenced by phenotype.

Defective metabolism of metoprolol in poor hydroxylators of debrisoquine

Eight healthy volunteers received oral metoprolol 200 mg once daily for a week. The AUC, half-life and duration of beta-adrenoceptor blockade on day 7 was much greater in two subjects than in the remaining six. This suggested that the metabolism of metoprolol was impaired in two and the effect was therefore prolonged. Subsequent testing of oxidation phenotype with oral debrisoquine showed that the subjects with high metoprolol availability were also poor hydroxylators of debrisoquine. The urinary debrisoquine/4-hydroxydebrisoquine ratio was highly correlated with metoprolol AUC, half-life and beta-adrenoceptor blockade at 24 h. Thus patients with a genetic defect in drug oxidation, when treated with metoprolol, are likely to have high plasma concentrations and a prolonged effect.

High-performance liquid chromatographic method for the quantitative analysis of the aryloxypropanolamines propranolol, metoprolol and atenolol in plasma and tissue

A simple and rapid high-performance liquid chromatographic method is described for the quantitative analysis of three beta-receptor blocking drugs of similar molecular structure (aryloxypropanolamines, AOPAs) but with different polarities. the method consists of extraction, reversed-phase ion-pair chromatography and fluorometric detection, whereby slight modifications in these parameters allow analysis of the different AOPAs in a similar way. The method was used to determine concentrations of propranolol, metoprolol and atenolol in plasma and various organs of the rat. In samples of 1 ml of plasma and 0.5--1.7 g of tissue, drug concentrations of at least 2 ng/ml and 5 ng/g, respectively, can be measured. Extension of the method to other AOPAs is possible.

Determination of metoprolol and alpha-hydroxymetoprolol in plasma by high-performance liquid chromatography

A sensitive, selective and reproducible high-performance liquid chromatographic assay for the simultaneous determination of metoprolol and alpha-hydroxymetoprolol in plasma was developed with (+/-)-ethyl-2-(4-(3-isopropyl-amino-2-hydroxy-propoxy)phenyl)-ethyl carbamate as the internal standard. Samples were acidified and subjected to an organic wash to remove interfering neutral and acidic components. Final extraction is made from the alkalinized aqueous phase with methylene chloride. The samples were chromatographed on a microparticulate silica gel column with fluorescent detection. Sensitivity was 3 ng/ml for metoprolol and 12.5 ng/ml for alpha-hydroxymetoprolol. Specificity was established by comparison of retention times of samples with standard materials. A typical absorption/disposition profile for metoprolol and alpha-hydroxymetoprolol is presented for one subject who received 50 mg of metoprolol tartrate (Lopressor).