Study on genotype and phenotype of novel CYP2D6 variants using pharmacokinetic and pharmacodynamic models with metoprolol as a substrate drug

To investigate the pharmacokinetic and pharmacodynamic profiles of volunteers carrying CYP2D6 genotypes with unknow metabolic phenotypes, a total of 22 volunteers were recruited based on the sequencing results. Peripheral blood and urine samples were collected at specific time points after oral administration of metoprolol. A validated high-performance liquid chromatography (HPLC) method was used to determine the concentrations of metoprolol and α-hydroxymetoprolol. Blood pressure and electrocardiogram were also monitored. The results showed that the main pharmacokinetic parameters of metoprolol in CYP2D6*1/*34 carriers are similar to those in CYP2D6*1/*1 carriers. However, in individuals carrying the CYP2D6*10/*87, CYP2D6*10/*95, and CYP2D6*97/*97 genotypes, the area under the curve (AUC) and half-life (t1/2) of metoprolol increased by 2-3 times compared to wild type. The urinary metabolic ratio of metoprolol in these genotypes is consistent with the trends observed in plasma samples. Therefore, CYP2D6*1/*34 can be considered as normal metabolizers, while CYP2D6*10/*87, CYP2D6*10/*95, and CYP2D6*97/*97 are intermediate metabolizers. Although the blood concentration of metoprolol has been found to correlate with CYP2D6 genotype, its blood pressure-lowering effect reaches maximum effectiveness at a reduction of 25 mmHg. Furthermore, P-Q interval prolongation and heart rate reduction are not positively correlated with metoprolol blood exposure. Based on the pharmacokinetic-pharmacodynamic model, this study clarified the properties of metoprolol in subjects with novel CYP2D6 genotypes and provided important fundamental data for the translational medicine of this substrate drug.

Determination of metoprolol tartrate in tablets and human urine using flow-injection chemiluminescence method

In this paper, a simple, rapid and sensitive flow-injection chemiluminescence method has been developed for the determination of metoprolol tartrate, which acts as a kind of sensitizer in the chemiluminescence emission from the redox of SO(3)(2-) with Ce(IV) in acidic medium. Under the optimized conditions, the proposed method allows the measurement of metoprolol tartrate over the range of 1.5 x 10(-8) to 7.3 x 10(-6)mol/L with a detection limit of 4.7 x 10(-9)mol/L (3sigma), and the relative standard deviation for 7.3 x 10(-7)mol/L metoprolol tartrate (n=11) is 2.20%. The utility of this method was demonstrated by determining metoprolol tartrate in tablets and human urine sample.

In vivo biotransformation of metoprolol in the horse and on-column esterification of the aminocarboxylic acid metabolite by alcohols during solid phase extraction using mixed mode columns

The in vivo biotransformation of metoprolol tartrate in the thoroughbred racehorse was studied after administration of a single oral dose. Metoprolol and its basic and bifunctional phase I metabolites were isolated from urine and plasma using mixed mode solid phase extraction (SPE) cartridges. The isolates were derivatised as trimethylsilyl ethers and analysed by capillary column gas chromatography--positive ion electron ionisation and ammonia chemical ionisation mass spectrometry. Metabolism was primarily confined to the oxidative transformations of the p-(2-methoxy)ethyl substituent. Metoprolol and five phase I metabolites were detected in horse urine. In common with man, rat and dog, the zwitterionic compound (+/-)-4-(2-hydroxy-3-isopropylaminopropoxy)-phenylacetic acid (H117/04), was the principle metabolite in the horse. This compound was readily isolated from both plasma and urine samples by SPE and, in addition, an unusual on-column esterification of the carboxylic acid moiety by alcohols was observed. Metoprolol and the major aliphatic acid metabolite were detected for about 10 and 40 h, respectively in unhydrolysed urine. After enzymatic hydrolysis, the detection period increased to 15 and 60 h, respectively indicating some phase II metabolism of metoprolol and its metabolites in the horse.

Endogenous Uremic Substances are not Involved in the Reduced Hepatic Extraction of Metoprolol in Bilateral Ureter-Ligated Rats†

The hepatic extraction of metoprolol is reduced in rats with bilateral ureter ligation (BUL)-induced renal failure. The aim of the present study was to evaluate the effect of uremic substances on the hepatic metabolism of metoprolol in rats with BUL. The metabolic rate in the liver microsomes of BUL rats was similar to that in sham rats, and there was no significant difference between sham and BUL rats in the effect of the supernatant of liver homogenates on the metabolism. The rate of metabolism in the liver microsomes in the presence of the plasma of BUL rats was also similar to that in the presence of the plasma of sham rats. These findings indicated that uremic substances which accumulate in BUL rats do not directly inhibit the activity of CYP2D2, which is responsible for the metabolism of metoprolol in the rat liver.

Gas Chromatographic-Mass Spectrometric Differentiation of Atenolol, Metoprolol, Propranolol, and an Interfering Metabolite Product of Metoprolol

Over a 10-year period, 1993-2002, Federal Aviation Administration identified 50 pilot fatalities involving atenolol, metoprolol, and propranolol, which is consistent with the fact that these drugs have been in the lists of the top 200 drugs prescribed in the U.S. In a few of the 50 pilot fatality cases, initial analysis suggested the presence of atenolol and metoprolol. However, there was no medical history with these cases supporting the use of both drugs. Therefore, atenolol, metoprolol, and/or propranolol, with their possible metabolite(s), were re-extracted from the selected case specimens, derivatized with pentafluoropropionic anhydride (PFPA), and analyzed by gas chromatography-mass spectrometry (GC-MS). The MS spectra of these three antihypertensives and a metoprolol metabolite are nearly identical. All of the PFPA derivatives had baseline GC separation, with the exception of a metoprolol metabolite product, which co-eluted with atenolol. There were four primary mass fragments (m/z 408, 366, 202, and 176) found with all of the PFPA-beta-blockers and with the interfering metabolite product. However, atenolol has three unique fragments (m/z 244, 172, and 132), metoprolol has two unique fragments (m/z 559 and 107), propranolol has four unique fragments (m/z 551, 183, 144, and 127), and the metoprolol metabolite product has two unique fragments (m/z 557 and 149). These distinctive fragments were further validated by using a computer program that predicts logical mass fragments and performing GC-MS of deuterated PFPA-atenolol and PFPA-propranolol and of the PFPA-alpha-hydroxy metabolite of metoprolol. By using the unique mass fragments, none of the pilot fatality cases were found to contain more than one beta-blocker. Therefore, these mass ions can be used for differentiating and simultaneously analyzing these structurally similar beta-blockers in biological samples.

Stereoselective metabolism of metoprolol: enantioselectivity of alpha-hydroxymetoprolol in plasma and urine

Direct stereoselective separation on chiral stationary phase was developed for HPLC analysis of the four stereoisomers of alpha-hydroxymetoprolol in human plasma and urine. Plasma samples were prepared using solid-phase extraction columns and urine samples were prepared by liquid-liquid extraction. The stereoisomers were separated on a Chiralpak AD column at 24 degrees C with fluorescence detection and a mobile phase consisting of a mixture of hexane:ethanol:isopropanol:diethylamine (88:10.2:1.8:0.2) for plasma samples and hexane:ethanol:diethylamine (88:12:0.2) for urine samples. Calibration curves for the individual stereoisomers were linear within the concentration range of 2.0-200 ng/ml plasma or 0.125-25 microg/ml urine. The methods were validated with intra- and interday variations less than 15%. The absolute configuration of the pure stereoisomers were assigned by circular dichroism spectra. The methods were employed to determine the concentrations of alpha-hydroxymetoprolol stereoisomers in a metabolism study of multiple-dose administration of racemic metoprolol to hypertensive patients phenotyped as extensive metabolizers of debrisoquine. We observed stereo-selectivity in the alpha-hydroxymetoprolol formation favoring the new 1'R chiral center from both metoprolol enantiomers (AUC(0-24) (1'R1'S) = 3.02). The similar renal clearances (Cl(R)) of the four stereoisomers demonstrated absence of stereoselectivity in their renal excretion. (-)-(S)-metoprolol was slightly more alpha-hydroxylated than its antipode (AUC(0-24) (2S/2R) = 1.19), suggesting that this pathway is not responsible for plasma accumulation of this enantiomer in humans.

Isozyme-specific induction of low-dose aspirin on cytochrome P450 in healthy subjects

OBJECTIVE

This study was designed to define the effect of low-dose aspirin administration on the activity of cytochrome P450 (CYP) in normal human subjects.

METHODS

Aspirin, 50 mg daily, was given for 14 days to 18 nonsmoking healthy male volunteers. A modified 5-drug cocktail procedure consisting of caffeine, mephenytoin, metoprolol, chlorzoxazone, and midazolam was performed to simultaneously assess in vivo activity of CYP1A2, CYP2C19, CYP2D6, CYP2E1, and CYP3A, respectively. The activities were assessed on 4 occasions including at baseline, after 7 and 14 daily doses of aspirin, and at 7 days after discontinuation of aspirin. Concentrations of parent drugs and corresponding metabolites in biologic samples were assayed by reversed-phase HPLC.

RESULTS

Both 7-day and 14-day aspirin intake increased the activity of CYP2C19 significantly, as indicated by 4-hydroxymephenytoin urinary recovery (P <.001). Induction of low-dose aspirin on CYP2C19 was time-dependent. CYP3A activity indices increased moderately but significantly by both 7-day and 14-day aspirin treatment (P <.05), but the percentage changes in CYP3A activity indices were not significant. Low-dose aspirin had no effect on CYP1A2, CYP2D6, and CYP2E1 in vivo activity by either 7-day or 14-day treatment.

CONCLUSIONS

The effect of low-dose aspirin on CYPs was enzyme-specific. Both 7-day and 14-day low-dose aspirin induced the in vivo activities of CYP2C19 but did not affect the activities of CYP1A2, CYP2D6, and CYP2E1. The effect of low-dose aspirin on CYP3A activity awaits further confirmation. When low-dose aspirin is used in combination with drugs that are substrates of CYP2C19, doses of the latter should be adjusted to ensure their efficacy.

Enantioselective determination of metoprolol acidic metabolite in plasma and urine using liquid chromatography chiral columns: applications to pharmacokinetics

Enantioselective separations on chiral stationary phases with or without derivatization were developed and compared for the HPLC analysis of (+)-(R)- and (-)-(S)-metoprolol acidic metabolite in human plasma and urine. The enantiomers were analysed in plasma and urine without derivatization on a Chiralcel OD-R column, and in urine after derivatization using methanol in acidic medium on a Chiralcel OD-H column. The quantitation limits were 17 ng of each enantiomer/ml plasma and 0.5 microgram of each enantiomer/ml urine using both methods. The confident limits show that the methods are compatible with pharmacokinetic investigations of the enantioselective metabolism of metoprolol. The methods were employed in a metabolism study of racemic metoprolol administered to a patient phenotyped as an extensive metabolizer of debrisoquine. The enantiomeric ratio (+)-(R)/(-)-(S)-acid metabolite was 1.1 for plasma and 1.2 for urine. Clearances were 0.41 and 0.25 l/h/kg, respectively, for the (+)-(R)- and (-)-(S)-enantiomers. The correlation coefficients between the urine concentrations of the acid metabolite enantiomers obtained by the two methods were >0.99. The two methods demonstrated interchangeable application to pharmacokinetics.

Enantioselective determination of metoprolol and major metabolites in human urine by capillary electrophoresis

The enantiomeric separation of metoprolol and its metabolites in human urine was undertaken using capillary electrophoresis (CE). Resolution of the enantiomers was achieved using carboxymethyl-beta-cyclodextrin (CM-beta-CD) as the chiral selector. A 100-mM acetate buffer (pH 4.0) containing 5% 2-propanol and 10 mM CM-beta-CD resulted in the optimum separation of the metoprolol enantiomers and its acidic metabolite in human urine. Following a single metoprolol oral administration of 100 mg racemic metoprolol tartrate, stereoselective pharmacokinetic analysis showed that urinary acidic metabolite 3 of metoprolol accounted for 62.3% of the dose with an R/S ratio of 1.23 and urinary unchanged metoprolol 1 accounted for 6.3% of the dose with an R/S ratio of 0.72.

Effect of gender, sex hormones, time variables and physiological urinary pH on apparent CYP2D6 activity as assessed by metabolic ratios of marker substrates

The effects of gender, time variables, menstrual cycle phases, plasma sex hormone concentrations and physiologic urinary pH on CYP2D6 phenotyping were studied using two widely employed CYP2D6 probe drugs, namely dextromethorphan and metoprolol. Phenotyping on a single occasion of 150 young, healthy, drug-free women and men revealed that the dextromethorphan: dextrorphan metabolic ratio (MR) was significantly lower (P < 0.0001) in 56 female extensive metabolizers (0.008+/-0.021) compared to 86 male extensive metabolizers (0.020 +/-0.040). Urinary pH was a significant predictor of dextromethorphan: dextrorphan MRs in men and women (P < 0.001). Once-a-month phenotyping with dextromethorphan of 12 healthy young men (eight extensive metabolizers and four poor metabolizers) over a 1-year period, as well as every-other-day phenotyping with dextromethorphan of healthy, pre-menopausal women (10 extensive metabolizers and 2 poor metabolizers) during a complete menstrual cycle, did not follow a particular pattern and showed similar intrasubject variability ranging from 24.1% to 74.5% (mean 50.9%) in men and from 20.5% to 96.2% (mean 52.0%) in women, independent of the CYP2D6 phenotype (P = 0.342). Using metoprolol as a probe drug, considerable intrasubject variability (38.6+/- 12.0%) but no correlation between metoprolol: alpha-hydroxymetoprolol MRs and pre-ovulatory, ovulatory and luteal phases (mean +/- SD metoprolol: a-hydroxymetoprolol MRs: 1.086+/- 1.137 pre-ovulatory; 1.159+/-1.158 ovulatory and 1.002+/-1.405 luteal phase; P> 0.9) or 17beta-oestradiol, progesterone or testosterone plasma concentrations was observed. There was a significant inverse relationship between physiologic urinary pH and sequential dextromethorphan: dextrorphan MRs as well as metoprolol: alpha-hydroxymetoprolol MRs in men and women, with metabolic ratios varying up to six-fold with metoprolol and up to 20-fold with dextromethorphan (ANCOVA P < 0.001). We conclude that apparent CYP2D6 activity is highly variable, independent of menstrual cycle phases, sex hormones, time variables or phenotype. Up to 80% of the observed variability can be explained by variations of urinary pH within the physiological range. An apparent phenotype shift as a result of variations in urinary pH may be observed in individuals who have metabolic ratios close to the population antimode.

Significant interaction between the nonprescription antihistamine diphenhydramine and the CYP2D6 substrate metoprolol in healthy men with high or low CYP2D6 activity

The prototype "classic" over-the-counter antihistamine diphenhydramine was shown to interact with the polymorphic P450 enzyme CYP2D6. This project was undertaken to investigate (1) whether diphenhydramine inhibits the biotransformation of the clinically relevant CYP2D6 substrate metoprolol in vitro and (2) whether this in vitro interaction results in a clinically significant pharmacokinetic and pharmacodynamic drug interaction in vivo. In vitro incubations were carried out with microsomes obtained from lymphoblastic cells transfected with CYP2D6 complementary deoxyribonucleic acid to determine the type and extent of inhibition. We then randomized 16 subjects with genetically determined high (extensive metabolizers) or low (poor metabolizers) CYP2D6 activity to receive metoprolol (100 mg) in the presence of steady-state concentrations of diphenhydramine or placebo. In vitro, diphenhydramine was a potent competitive inhibitor of metoprolol alpha-hydroxylation, exhibiting an inhibitory constant of 2 micromol/L and increasing the Michaelis-Menten constant of metoprolol sixfold. In vivo, diphenhydramine decreased metoprolol oral and nonrenal clearances twofold and metoprolol-->alpha-hydroxymetoprolol partial metabolic clearance 2.5-fold in extensive metabolizers (all P < .05) but not in poor metabolizers (P > .2). Although the hemodynamic response to metoprolol was unaltered by diphenhydramine in poor metabolizers (P > .05), metoprolol-related effects on heart rate, systolic blood pressure, and Doppler-derived aortic blood flow peak velocity were more pronounced and lasted significantly longer in extensive metabolizers receiving diphenhydramine compared with poor metabolizers and extensive metabolizers receiving placebo. We conclude that diphenhydramine inhibits the metabolism of metoprolol in extensive metabolizers, thereby prolonging the negative chronotropic and inotropic effects of the drug. Clinically relevant drug interactions may occur between diphenhydramine and many CYP2D6 substrates, particularly those with a narrow therapeutic index.

Inhibition of CYP2D6 by quinidine and its effects on the metabolism of cilostazol

OBJECTIVE

In vitro results are inconclusive as to whether cilostazol is metabolised by cytochrome P450 isoenzyme 2D6 (CYP2D6). The goals of this study were (1) to assure the dose of quinidine and timing relative to cilostazol used in this study were adequate to cause inhibition of CYP2D6, (2) to evaluate carryover effects of quinidine administration, and (3) to evaluate the effect of CYP2D6 deficiency and administration of quinidine (a CYP2D6 inhibitor) on the pharmacokinetics of a single 100 mg oral dose of cilostazol.

DESIGN

This study was conducted as a single-centre, open-label, randomised sequence, 2-period, crossover pharmacokinetic trial. Water alone (treatment without quinidine) or two 200 mg oral doses of quinidine sulfate with water were administered 25 hours and 1 hour prior to a single 100 mg dose of cilostazol in period 1. Study participants were crossed over to opposite treatment in period 2. Metoprolol 25 mg, used as a positive control, was administered 1 hour after quinidine sulfate with water or using water alone to assess the magnitude of CYP2D6 inhibition by quinidine.

STUDY PARTICIPANTS

22 healthy nonsmoking Caucasian (14 male and 8 female) volunteers participated in the study.

MAIN OUTCOME MEASURES

Serial blood and urine samples were collected at predose and after cilostazol administration to characterise cilostazol and its metabolite pharmacokinetics. Additional plasma samples were taken to assess the pharmacokinetics of quinidine. Urine samples were collected to measure metoprolol and hydroxymetoprolol.

RESULTS

Administration of metoprolol with quinidine caused a significant (p < 0.001) decrease in the urinary 4-hydroxymetoprolol/metoprolol ratio compared with administration of metoprolol alone (42-fold decrease, 0.065 vs 2.707). Hence, quinidine effectively converted extensive metabolisers of CYP2D6 to poor metabolisers of CYP2D6. The 21-day washout period was adequate to have complete recovery from quinidine inhibition of CYP2D6. The analysis of variance demonstrated that the mean maximum plasma concentration (Cmax) for cilostazol, both adjusted and unadjusted for the free fraction, was higher in the control group than in the quinidine group (p = 0.023). However, the time to Cmax (p = 0.669), the area under the plasma concentration-time curve from time zero to infinity (AUC infinity; p = 0.133), and the apparent oral clearance (p = 0.135) were unchanged. The geometric mean ratios (90% confidence interval) comparing with quinidine (test) and without quinidine (reference) coadministration for Cmax and AUC infinity are 0.86 (0.77, 0.95) and 0.92 (0.84, 1.00), respectively. Similar patterns were observed for OPC-13015 and OPC-13213 with regard to Cmax, area under the plasma concentration-time curve from time zero to the last measurable concentration at time t, and AUC infinity (where determinable). The slight decrease in the systemic availability of cilostazol and its metabolites was thought to be a result of the increased gastrointestinal motility secondary to quinidine.

CONCLUSIONS

Administration of quinidine sulfate 200 mg profoundly inhibited CYP2D6-mediated metabolism. The effects of quinidine inhibition of CYP2D6 metabolism were completely reversible during the 21-day washout period. Coadministration of quinidine with cilostazol had no substantial effect on cilostazol or its metabolites (OPC-13015 and OPC-13213). Hence, CYP2D6 does not have a significant contribution in the metabolic elimination of cilostazol.

Determination of metoprolol enantiomers in human urine by coupled achiral-chiral chromatography

Achiral chiral column switching HPLC assay was developed to allow the separation and quantitation of the enantiomers of metoprolol in human urine by means of fluorescence detection. Urine samples were prepared by liquid liquid extraction, followed by HPLC. The racemic metoprolol and internal standard were separated from the interfering components in urine and quantified on the silica column, and the enantiomers were determined on a Chiralcel OD chiral stationary phase. The two columns were connected by a switching valve equipped with a silica trap column. Detection limit was 25 ng/ml for each enantiomer. The intra-day variation ranged between 0.38 and 4.94% in relation to the measured concentration and the inter-day variation was 0.15-3.13%. It has been applied to the determination of (R)-(+)-metoprolol and (S)-(-)-metoprolol in urine from healthy volunteers dosed with racemic metoprolol tartrate.

Micellar liquid chromatography: a worthy technique for the determination of beta-antagonists in urine samples

Several beta-antagonists (acebutolol, atenolol, celiprolol, labetalol, metoprolol, nadolol, propranolol) were determined in urine samples with fluorometric detection after direct injection, in less than 15 min, with a micellar mobile phase of 0.1 M sodium dodecyl sulfate (SDS), 15% propanol, and 1% triethylamine at pH 3. The limits of detection (38 criterion) were usually between 3 and 30 ng/mL. The addition of propanol and triethylamine and the reduction of the pH of the mobile phase improved the efficiency of the chromatographic peaks that was rather low in pure micellar eluents. The selection of the composition of the mobile phase was easily performed through the use of an interpretive procedure which considered the retention times and peak shapes of the beta-antagonists in six chromatograms, obtained at varying concentrations of SDS (0.05-0.15 M) and propanol (5-15% v/v). The chromatograms of urine samples from healthy volunteers, which were administered atenolol, metoprolol, and propranolol, showed only one peak for the former drug and several peaks for the other two. These peaks corresponded to the parent drug and metabolites, which indicated the partial and the extensive degradation of metoprolol and propranolol, respectively.

Efficient high-performance liquid chromatographic assay for the simultaneous determination of metoprolol and two main metabolites in human urine by solid-phase extraction and fluorescence detection

An improved, more efficient method for the determination of metoprolol and its two metabolites in human urine is reported. The simultaneous analysis of the zwitterionic metoprolol acidic metabolite (III, H117/04) with the basic metabolites alpha-hydroxymetoprolol (II, H119/66), metoprolol (I) and guanoxan (IV, internal standard) was achieved employing solid-phase extraction and isocratic reversed-phase HPLC. The analytes were extracted from urine (100 microliters) using C18 solid-phase extraction cartridges (100 mg), and eluted with aqueous acetic acid (0.1%, v/v)-methanol mixture (40:60, v/v, 1.2 ml). The eluents were concentrated (250 microliters) under vacuum, and aliquots (100 microliters) were analysed by HPLC with fluorescence detection at 229 nm (excitation) and 309 nm (emission) using simple isocratic reversed-phase HPLC (Novapak C18 radial compression cartridge, 4 microns, 100 x 5 mm I.D.). Acetonitrile-methanol-TEA/phosphate buffer pH 3.0 (9:1:90, v/v) was employed as the eluent (1.4 ml/min). All components were fully resolved within 18 min, and the calibration curves for the individual analytes were linear (r2 > or = 0.996) within the concentration range of 0.25-40.0 mg/ml. Recoveries for all four analytes were greater than 76% (n = 4). The assay method was validated with intra-day and inter-day variations less than 2.5%.

Age might influence the frequency distribution of metoprolol hydroxylation polymorphism in a Chinese population

The frequency distribution of the 8-hour urinary ratio of metoprolol/alpha-hydroxymetoprolol (MR) was investigated in 206 healthy rural Chinese volunteers. The frequency of the poor metabolizer phenotype of metoprolol alpha-hydroxylation was 0% in this Chinese population. When the 206 subjects were subgrouped into four groups by age, the excretion of alpha-hydroxymetoprolol was significantly lower in the subjects with age older than 41 years compared to two younger groups (16-30 years), and the metoprolol hydroxylation metabolic ratio(metoprolol/a-hydroxymetoprolol, MR) was higher in these middle-aged subjects, indicating that age may affect the frequency distribution of the 8-hour urinary MR or log MR, although multiple regression analysis did not show a significant correlation between age, urine volume and MR. Also, a significant correlation was found between age and the excretion (percentage of dose) of alpha-hydroxymetoprolol in 8-hour urine (r = 0.259, P < 0.001). On the other hand, both multiple regression analysis and nonparametric tests showed that the larger the amount of urine excreted, the more metoprolol was recovered, demonstrating that the urinary excretion of unchanged metoprolol is renal flow-limited.

Phenotype and genotype analysis of debrisoquine hydroxylase (CYP2D6) in a black Zimbabwean population. Reduced enzyme activity and evaluation of metabolic correlation of CYP2D6 probe drugs

OBJECTIVE

Debrisoquine hydroxylase (CYP2D6) is responsble for the oxidative metabolism of many clinically used drugs. Since this enzyme has been poorly studied in the southern part of Africa, we examined the CYP2D6 phenotypes and genotypes in 103 unrelated black Zimbabweans.

METHODS

Phenotyping for CYP2D6 activity was done using debrisoquine and metoprolol as probe drugs by measuring the urinary metabolic ratio (MR) of parent drug to metabolite concentration ratios. Genotyping was done using polymerase chain reaction (PCR), restriction fragment length polymorphism (RFLP), single-strand conformation polymorphism (SSCP) and sequencing analyses with respect to CYP2D6 variants of interest.

RESULTS AND CONCLUSION

Phenotyping with debrisoquine revealed two poor metabolisers (PMs), whereas 5 subjects out of 94 were PMs using metoprolol as probe drug. Genotypes predictive of the poor metaboliser status were observed for the two subjects who were PMs with both probe drugs, whereas no mutations could explain the PM phenotype for metoprolol among the three remaining subjects, a fact possibly explained by lack of compliance in metoprolol intake. There was a moderate correlation of 0.67 between the debrisoquine and metoprolol metabolic ratios in the 89 subjects who were extensive metabolisers for both probe drugs. The median values for the metabolic ratios for debrisoquine and metoprolol as probe drugs were 1.00 and 1.35, respectively, which are higher than those observed in Caucasian populations. This is indicative of a decreased capacity for metabolism of CYP2D6 substrates by Zimbabweans compared to Caucasians. Evaluation of the DNA samples for the known allelic variants CYP2D6A, CYP2D6B, CYP2D6C, CYP2D6D or CYP2D6Ch1 yielded no explanation for these results.

Metabolic ratios of four probes of CYP2D6 in Turkish subjects: a cross-over study

The relationships among the metabolic ratios for the standard probe drugs of CYP2D6 activity, such as debrisoquine, sparteine, metoprolol and dextromethorphan, were studied in 32 Turkish subjects. All subjects were randomly selected according to their phenotypes from a group of 111 Turkish subjects whose oxidation status had been tested for debrisoquine previously. All subjects were given a 10 mg debrisoquine tablet, a 100 mg sparteine tablet, a 100 mg. metoprolol tablet and a 20 mg dextromethorphan capsule orally with a wash-out period of at least 1 week between each probe administration. Metabolic ratios were calculated as percentage of dose excreted as parent drug/percentage of dose excreted as its hydroxymetabolite of parent drug in 0-8 h urine. Three poor metabolisers (PM) of debrisoquine were identified. They were also PMs of the other test probes and no misclassification by the 4 phenotyping methods was observed. All six correlations among the metabolic ratios of the 4 probe drugs assessed by Spearman's rank test were highly significant (P < 0.001). The present findings indicate that the oxidative metabolism of debrisoquine, sparteine, metoprolol and dextromethorphan is catalysed by the same cytochrome P450 in the Turkish subjects.

Determination of some cardiovascular drugs in serum and urine by capillary isotachophoresis

The separation and determination of amiloride, metoprolol, deacetylmetipranolol, labetalol and furosemide in human serum and urine by capillary isotachophoresis were investigated. Amiloride and beta-blockers were separated by cationic isotachophoresis in the electrolyte system sodium morpholinoethanesulfonate buffer (pH 5.5) (cL = 10 mM)-glutamic acid. Furosemide was separated using the anionic electrolyte system histidine hydrochloride buffer (pH 6.2) (cL = 10 mM)-morpholinopropanesulfonic acid. Endogenous and the possible exogenous compounds were almost totally removed from serum and urine by solid-phase extraction using a Separon SGX C18 cartridge. The recovery of compounds varied from 98.2 to 103.2%. The linearity range for the compounds was 50-1000 ng/ml. The relative standard deviations varied from 0.1 to 5.6%. The overall limits of determination ranged from 32 to 46 ng/ml of urine and from 39 to 46 ng/ml of serum, depending of the type of drugs.

Detection of beta-blockers in urine by solid-phase extraction-supercritical fluid extraction and gas chromatography-mass spectrometry

The most convenient way to perform supercritical fluid extraction (SFE) of liquid sample matrices is to combine it with solid-phase extraction (SPE). beta-Blockers from urine were collected on an Empore disc, which was then placed into an extraction cell for derivatization and SFE. SPE recovery was best at pH 10. Effects of temperature, pressure and volume of pyridine on the acetylation and SFE processes were studied. Without acetylation the beta-blockers were not significantly soluble in CO2. SFE temperatures of 70 degree C and 150 degree C together with 200 microliters of acetic anhydride and 400 microliters pyridine gave the best results. With the SPE-SFE-GC-MS method developed here, beta-blockers like oxprenolol, metoprolol and propranolol could easily be detected in urine samples, and the limit of detection (LOD) for these compounds was found to be 20 ng/ml, 30 ng/ml and 40 ng/ml, respectively.

Determination of the enantiomers of metoprolol and its major acidic metabolite in human urine by high-performance liquid chromatography with fluorescence detection

Enantiomers of metoprolol and its acidic metabolite H 117/04 were determined in human urine by high-performance liquid chromatography (HPLC) with fluorometric detection after chiral derivatization. The carboxyl functional group of the major metabolite was blocked by esterification after solid-phase extraction, which helped to quantitate this compound from interfering substances. The assay method was validated. The recovery of (-)- and (+)-metoprolol from urine was 86.3-90.5%; and the recovery of the (-)- and (+)-acidic metabolite H 117/04 from urine was 74.4-83.9% at different concentrations.

Assay of metoprolol and alpha-hydroxymetoprolol in human urine by reversed-phase liquid chromatography with direct-injection

AIM

To develop an HPLC method with direct injection for the simultaneous determination of metoprolol (M) and alpha-hydroxymetoprolol (HM) in human urine.

METHODS

Urine (200 microliters) was diluted with eluate and injected into the chromatograph. Samples were separated on an ODS column by isocratic binary elution and monitored by fluorescence detection.

RESULTS

No potential interfering peaks were identified. M and HM gave rapid elution and baseline resolution. The linear curves of both analytes ranged between 0.2 and 100 mg.L-1. The response sensitivity was approximately 0.1 mg.L-1 and the coefficients of variation in the assay were within 8% for both compounds. A typical application in oxidation phenotyping was presented for one healthy volunteer who received 100 mg of oral metoprolol.

CONCLUSION

The method can be used for the investigation of genetic polymorphism of metoprolol oxidation in the large populations.

Renal selective N-acetyl-L-gamma-glutamyl prodrugs: studies on the selectivity of some model prodrugs

1. In this study, a number of structurally different N-acetyl-L-gamma-glutamyl prodrugs were investigated with respect to selective uptake by the kidney in male Wistar rats. 2. All prodrugs were tested in vitro in rat kidney slices and kidney homogenate to study their uptake and conversion. It was found that the prodrugs of para-nitroaniline (agPNA), aminophenyl acetic acid (agAFA), sulphamethoxazole (agSM), sulphadimethoxine (agSDM), propranolol (agPP) and metoprolol (agMP) were accumulated by a probenecid-sensitive carrier. The prodrug of 4'-aminoantipyrine (agAAP) was not accumulated by a probenecid- or buthionine sulphoximine-sensitive carrier. Unlike all other prodrugs, agAAP and agMP were not, or only a very limited extent converted to the parent compound in vitro. 3. agPNA, agAFA and agPP were also investigated in vivo. The tissue distribution of the prodrugs and the parent drugs was established, as was their urinary excretion and pharmacokinetic behaviour. agPNA and agAFA showed selective uptake by the kidney, in contrast to agPP which accumulated in the liver. The distribution of the parent compounds following prodrug administration was as follows: agPNA was found in kidney and plasma: agAFA in kidney only; agPP in liver only. 4. The factors which determine the selectivity of N-acetyl-L-gamma-glutamyl prodrugs are discussed. The main factors are: the transport into the kidney, the conversion rate, the residence time of the prodrug in the kidney and the presence or absence of competition for uptake and conversation by other tissues, e.g. the liver. It is concluded that this prodrug approach offers the possibility of delivering drugs selectively to the kidney, but also that it is not universally applicable.

Variation of pharmacokinetics after oral administration of slow-release metoprolol tablets and pharmacogenetic considerations

The maximum plasma concentrations (Cmax) after oral administration of 120 mg tablets of slow-release metoprolol (CAS 37350-58-6) to 75 Japanese healthy male volunteers and 15 arrhythmic patients were measured. Extensive and poor metabolizers after oral administrations of slow-release metoprolol tablets were classified by means of the frequency distribution of Cmax values. In addition, the frequency distribution of Cmax values after oral administration of slow-release metoprolol 120 mg tablets was compared with that of conventional metoprolol 40 mg tablets. 1. Mean +/- S.E. values of Cmax, tmax and AUC0-24 after oral administration of slow-release metoprolol tablets to 75 healthy male volunteers and 15 arrhythmic patients were 95.3 +/- 6.6 ng/ml, 4.4 +/- 0.2 h and 1000.4 +/- 70.9 ng.h/ml, respectively. 2. The number of poor metoprolol metabolizers after the oral administration of slow-release tablets in 75 healthy volunteers and 15 patients was estimated to be 2 subjects (2.2%). 3. The frequency distribution of Cmax values after oral administration of conventional metoprolol tablets was similar to that of slow-release metoprolol tablets. In addition, from the result of this study and that obtained in another study, in which the frequency of poor metoprolol metabolizers in British people has been examined, it is concluded that the frequency of poor metoprolol metabolizers varies between ethnic groups (2.2% in Japanese population and 11.3% in British population.

Utility of a one-point (3-hour postdose) plasma metabolic ratio as a phenotyping test using metoprolol in two east Asian populations

We examined the utility of the postdose 3-h plasma metabolic ratio (MR) as a phenotyping method for assessing genetically determined debrisoquine-type oxidation polymorphism after an oral dose of 100 mg of metoprolol tartrate administered to 402 unrelated, healthy, and native East Asian (218 Korean and 184 Japanese) subjects. All of them were phenotyped simultaneously with the reported MR employing urine samples collected during an 8-h postdose period. In the two populations, the distribution histograms and probit plots of log10plasma MRs derived from the metoprolol/alpha-hydroxymetoprolol concentration values indicated a large gap between the extensive and poor metabolizers who were phenotyped by the reported criteria of the 8-h urinary MR. There were statistically significant (p less than 0.001) correlations (rs = 0.688 and 0.810, respectively) between the postdose urinary and plasma MRs in the Korean and Japanese populations. Two poor metabolizers (one each included in the two racial groups) identified according to the 8-h urinary MR gave the greatest plasma MRs (i.e., 549.7 among the Koreans and 150.0 among the Japanese). The results suggest that the one-point, postdose 3-h plasma MR is also useful for the phenotyping purpose of oxidation pharmacogenetic polymorphism of metoprolol, a widely prescribed beta-adrenoceptor blocking drug.

Liquid chromatographic separation of the enantiomers of metoprolol and its alpha-hydroxy metabolite on Chiralcel OD for determination in plasma and urine

The two enantiomers of metoprolol and the four enantiomeric forms of alpha-hydroxymetoprolol were separated by liquid chromatography on a Chiralcel OD column containing a cellulose tris(3,5-dimethyl-phenylcarbamate) chiral stationary phase. The column efficiency was strongly dependent on the flow-rate and the enantioselectivity was influenced by temperature. Of utmost importance for the chiral separation was the water content of the mobile organic phase. The separation system was used for the separation and determination of the enantiomers in plasma and urine samples. The metoprolol enantiomers could be determined by fluorescence down to 10 nmol/l of each in plasma with a relative standard deviation of less than 15%.

Screening and confirmatory analysis of beta-agonists, beta-antagonists and their metabolites in horse urine by capillary gas chromatography-mass spectrometry

A method for the screening and confirmatory analysis of beta-agonists and -antagonists in equine urine is described. Following initial enzymic hydrolysis, the basic drugs and metabolites are extracted using Clean Screen DAU or Bond Elut Certify cartridges, and analysed as their trimethylsilyl ether or 2-(dimethyl) silamorpholine derivatives by capillary gas chromatography-mass spectrometry. The method proved to be very sensitive and selective for basic drugs. After administration of therapeutic doses of propranolol, metoprolol, timolol, isoxsuprine and clenbuterol to thoroughbred horses, the parent compound/metabolites could be detected in urine for upto 14-120 h depending on the drug.

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-d0 and (2S)-1-d2, as substrate. Both O-demethylation and alpha-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, alpha-hydroxylation yielded predominantly the 1'R-hydroxy product, 1'R/1'S ratio greater than 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 alpha-hydroxymetoprolol (2) accounted for 9.3 +/- 2.4% of the dose, 2R/2S ratio 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 (2R)-enantiomer of 1 could contribute to the 2S greater than 2R plasma ratio of metoprolol enantiomers observed in this population.

A comparison of a short half-life marker (low-dose isoniazid), a long half-life pharmacological indicator (low-dose phenobarbitone) and measurements of a controlled release 'therapeutic drug' (metoprolol, Metoros) in reflecting incomplete compliance by volunteers

1. Although, long half-life compounds appear to be more appropriate pharmacological indicators of compliance with treatment, short half-life markers or measurements of short half-life therapeutic drugs are frequently used. 2. We have compared the usefulness of low-dose phenobarbitone (a long half-life indicator), low dose isoniazid (a short half-life marker) and controlled release metoprolol (Metros) (a controlled release formulation of a short half-life 'therapeutic' drug) in seven volunteers with simulated partial (two thirds) compliance. 3. Detection of isoniazid metabolites in urine had an 83% sensitivity and 94% specificity for detecting ingestion within the previous 24 h and 100% sensitivity and 82% specificity for detecting ingestion within the past 6 h but gave no indication of the longer term pattern of compliance. 4. At 28 days (a time when steady-state would be obtained for all three drugs) phenobarbitone plasma levels were 70% (66-76%)--median and interquartile range--of the expected steady-state level if compliance had been complete. Corresponding figures for metoprolol were 82% (37-100%). 5. Measurement of phenobarbitone was much superior to isoniazid or metoprolol measurements in reflecting partial compliance over the previous 1 to 4 weeks.

Metoprolol alpha-hydroxylation polymorphism in the San Bushmen of southern Africa

1. The metabolic oxidation of metoprolol has been studied in a group of 98 San Bushmen. 2. The amounts of metoprolol and alpha-hydroxy metoprolol excreted in 0-8 h urine collection, after dosing with 100 mg metoprolol, were measured and the metabolic ratio (% dose excreted as metoprolol/% dose excreted as alpha-hydroxy metoprolol) calculated. 3. Frequency distribution and probit plots of the metabolic rate data showed a bimodal distribution with 4.1% of the population exhibiting slow metabolism with an MR greater than 10. 4. These results are much less than found in Caucasians (8.4%) but very different from the unimodal distribution found for Nigerians. 5. A previous study in the same group of Bushmen had revealed that 18 of 96 subjects were poor or non-metabolizers of debrisoquine to 4-hydroxy debrisoquine, but only one of the poor metoprolol metabolizers was a poor metabolizer of debrisoquine. 6. On the basis of these results, the claim of debrisoquine type of polymorphism for beta-adrenoceptor antagonists found in Caucasians cannot be extrapolated to the San Bushmen, and one must query the use of debrisoquine as measure of oxidative status in any group other than Caucasians.

Simultaneous determination of metoprolol and alpha-hydroxymetoprolol in human plasma and urine by liquid chromatography with a preliminary observation on metoprolol oxidation in Japanese subjects

A simple, sensitive, and highly reliable liquid chromatographic method using fluorescence detection is described for the simultaneous determination of metoprolol and alpha-hydroxymetoprolol in plasma and urine. This method involves a single extraction of the compounds with the internal standard pindolol from alkalinized plasma or urine into dichloromethane. A reconstituted aliquot with a mobile phase is injected onto a reversed-phase, Zorbax ODS column, and the detection is achieved by the excitation and emission wavelengths at 230 and 300 nm, respectively. The assay is reproducible and precise for metoprolol and alpha-hydroxymetoprolol in both plasma and urine samples, as judged by a coefficient of variation of less than 9.2% at all concentrations examined. The standard curves for metoprolol and alpha-hydroxymetoprolol are linear over 10-200 ng/ml in plasma and over 0.5-10 micrograms/ml in urine. The lower detection limit is 2 ng/ml for each of the compounds in plasma using a 0.5-ml sample. Preliminary data on the oxidation polymorphism of metoprolol in Japanese subjects are reported using the current assay method. In 183 Japanese subjects no poor metabolizer of metoprolol has been identified so far.

Analysis of alpha-hydroxy metabolites of metoprolol in human urine after phosgene/trimethylsilyl derivatization

Three metoprolol metabolites containing an alpha-hydroxy group were identified in human urine by capillary column gas chromatography/mass spectrometry. After aqueous phase cyclization with phosgene the neutral or acidic derivatives formed were isolated by solvent extraction at pH 10 or 3, respectively. Following silylation the electron impact mass spectra of the metabolites exhibited a characteristic ion at m/z 336 of high abundance which originated from cleavage of the bond adjacent to the alpha-OTMS group. Most probably the identified compounds were formed by further biotransformations of alpha-hydroxy metoprolol, which is a primary metabolite. The analytical method is applicable to detect the metoprolol metabolites reported so far. A quantitative assay for one of the metabolites (H 119/72) with nitrogen selective detection is described. The total amount of this metabolite excreted by one subject within 24 h after dosing was about 0.25% of the given dose.

Determination of metoprolol and two major metabolites in plasma and urine by column liquid chromatography and fluorometric detection

Metoprolol and its alpha-hydroxy metabolite were determined in plasma down to 2 nmol/l (S.D. 10-15%) after solvent extraction and bonded-phase liquid chromatography with fluorometric detection. The major metabolite with a carboxylic function was also measured in plasma when liquid-solid extraction on a column activated with dodecyl sulphate was applied. In urine the three components were assayed by direct injection of a diluted sample.

Rapid determination of the enantiomers of metoprolol, oxprenolol and propranolol in urine

A method is described that makes possible the rapid determination of the enantiomers of beta-blocking agents. After extraction from urine samples (at pH 9.9) using toluene, the enantiomers are derivatised with S-(+)-benoxaprofen chloride. The chromatographic separation can be performed on thin-layer plates with toluene-acetone as mobile phase. The derivatives can be detected by measuring the fluorescence (lambda ex = 313 nm,lambda em = 365 nm).

Pharmacokinetics of salicylates administered with metoprolol

The widely used acetylsalicylic acid (ASA, Colfarit) was administered in combination with metoprolol (Lopresor) during 7 days. Plasma concentrations of total salicylates (ASA and salicylic acid (SA) and metoprolol were monitored during the treatment period, on day 7 urinary excretion was also investigated. The kinetic data of the compounds were compared to those of the metoprolol and ASA control periods. Metoprolol kinetics remained uninfluenced whereas the maximal plasma concentrations of the salicylates were significantly higher than in the ASA control period.

Determination of the new beta-blocker bisoprolol and of metoprolol, atenolol and propranolol in plasma and urine by high-performance liquid chromatography

High-performance liquid chromatographic methods using fluorimetric detection have been developed for the determination in plasma and urine of bisoprolol, atenolol, metoprolol and propranolol. Bisoprolol, metoprolol and propranolol were extracted at alkaline pH with dichloromethane, atenolol with n-butanol-dichloromethane (25:75). After evaporation of the organic solvent the compounds were chromatographed on silica gel Si-60 columns (normal phase) using aqueous ammonium phosphate buffer (pH 4) containing 3-7% acetonitrile as eluent (method 1). Alternatively, the compounds were acetylated prior to chromatography on reversed-phase columns (RP-8), using acetonitrile-water mixtures as eluents (method 2). The detection limit was 1-2 ng/ml in plasma and 10 ng/ml in urine for bisoprolol and metoprolol with either method. For atenolol the detection limit was 5 ng/ml in plasma or 50 ng/ml in urine (method 1), for propranolol 1 ng/ml in plasma (method 2).

Investigation of drug absorption from the gastrointestinal tract of man. IV. Influence of food and digestive secretions on metoprolol jejunal absorption

The influence of nutrients and digestive secretions on the intestinal absorption and bioavailability of the beta-adrenoceptor antagonist, metoprolol, was investigated in an isolated segment of jejunum using an intestinal perfusion technique. Two solutions containing metoprolol, one with, and one without nutrients, were perfused into the jejunum with an occluding balloon inflated or deflated. Jejunal fluid, blood and urine samples were then collected for drug or metabolite estimation. In the segment studied, metoprolol absorption from the nutrient solution was four times that observed during perfusion of the saline solution. Bile salts did not enhance drug absorption. Both in the presence and absence of nutrients, a linear relationship was observed between the computed cumulative amount of drug absorbed from the gastrointestinal tract and the resulting plasma concentration at each sampling time, indicating that first-pass loss was not saturated. This result was also reflected in the similarity of the AUC:dose ratios, and in the lack of effect of nutrients on the metabolism of the drug.

Investigation of drug absorption from the gastrointestinal tract of man. III. Metoprolol in the colon

The colonic absorption of metoprolol was indirectly evaluated by measuring drug appearance in plasma following intravenous, jejunal or colonic infusion in six healthy volunteers. Plasma concentrations of alpha-hydroxymetoprolol and urinary excretion of the main metabolites were also measured. Plasma profiles of metoprolol after colonic and jejunal perfusion were similar, and the relative bioavailabilities of the drug from these two regions of the gut were not significantly different. The concentrations of alpha-hydroxymetoprolol, the major metabolite in plasma, were similar after jejunal and colonic perfusion, but higher than those observed after intravenous administration. The percentage of the dose recovered in urine over 24 h as two metabolites was not significantly influenced by the route of administration.

Simultaneous determination of metoprolol and metabolites in urine by capillary column gas chromatography as oxazolidineone and trimethylsilyl derivatives

A method for the determination of metoprolol and its main metabolites in urine is presented. The method comprises derivatization of the aminopropanol side-chain with phosgene at alkaline pH and isolation in an organic phase at acidic pH. After trimethylsilylation, separation and quantification are performed by capillary column gas chromatography with flame ionization detection. The reaction is performed at pH 12 with 60 microliters of 2 M phosgene in toluene added in three portions. Diethyl ether--dichloromethane is used as extraction medium and bis(trimethylsilyl) acetamide as silylating agent. With spiked samples linear standard curves were obtained for metoprolol and three of its main metabolites with a detection limit varying between 4 and 20 mumol/l of urine. The method was applied to urine samples from a normal individual who had taken 292 mumol of metoprolol as tartrate.

High-performance liquid chromatographic determination of metoprolol and alpha-hydroxymetoprolol concentrations in human serum, urine, and cerebrospinal fluid

A sensitive and simplified high-performance liquid chromatographic procedure was developed for the simultaneous quantification of metoprolol and alpha-hydroxymetoprolol in human serum, as well as cerebrospinal fluid and urine. Following protein precipitation with trichloroacetic acid, the sample was alkalinized with 1 M NaOH and extracted with dichloromethane. The mobile phase consisted of acetonitrile-water (50:50) containing 0.005 M 1-heptanesulfonic acid in 0.001% acetic acid. Using pronetalol as an internal standard, compounds were quantitated using fluorescence detection at 230 nm with a 300-nm emission filter and 0.02 AUFS. Extraction recovery is approximately 80% for both compounds. The lower limits of detection are 5 ng/mL and 4 ng/mL for metoprolol and alpha-hydroxymetoprolol, respectively.