Simultaneous determination of verapamil and its seven metabolites by high-performance liquid chromatography

Direct Monitoring of Verapamil Level in Exhaled Breath Condensate Samples

Background: In this research, an enhanced fluorimetric assay was developed for the direct monitoring of verapamil in exhaled breath condensate (EBC). The method is based on a binding–induced rigidity inside the sodium dodecyl sulfate (SDS) micelle which eliminate collisional quenching and vibrational modes responsible for non-radiative decay. This process produces an enhancement in the emission intensity of verapamil. Methods: Fluorescence intensity measurements were made at 15 ˚C on a FP-750 spectrofluorometer with maximum excitation and emission wavelengths of 280 nm and 310 nm, respectively. The important parameters influencing the analytical signal in experimental steps were investigated and optimized. The method was validated with considering of the linearity, recovery and limit of detection. Results: Under the optimized experimental conditions, the calibration graph was linear in the range of 0.02 − 12.0 µg.mL−1 of verapamil with a detection limit of 0.008 µg.mL–1. Conclusion: The proposed method was found to be suitable and accurate for the determination of verapamil and the validated method was successfully used for analysis of verapamil in EBC of patients receiving verapamil with the satisfactory results.

Straightforward solid-phase extraction method for the determination of verapamil and its metabolite in plasma in a 96-well extraction plate

Straightforward solid-phase extraction (SPE) methods were developed for the determination of verapamil and its metabolite in a plasma matrix. The spiked plasma sample was pretreated with 2% phosphoric acid followed by two different SPE methods using a Waters Oasis HLB 96-well extraction plate. Recoveries greater than 90% were obtained using both a generic and a selective SPE methods. The generic method is a good starting protocol, and it is applicable to a wide range of compounds. This generic method consists of using 5% methanol as the wash solvent, and 100% methanol for the elution. The limitation of the non-specific method is that it does not remove all plasma constituents that interfere with the quantitation of the metabolite, norverapamil. A second, more specific method was developed using the same Oasis HLB sorbent which removes more plasma interferences and provides cleaner extracts for the HPLC-UV analysis. This selective method uses both the methanol concentration and the pH advantageously to preferentially isolate the analytes of interest from a complex sample matrix. Recoveries of greater than 90% with R.S.D.s less than 3.8% were obtained with this selective method.

Simplified reversed-phase HPLC method with spectrophotometric detection for the assay of verapamil in rat plasma

A high-performance liquid chromatographic (HPLC) method was developed for the assay of verapamil in rat plasma. After deproteinization of the plasma sample with an acetonitrile-perchloric acid (8:2) mixture containing dextromethorphan, the internal standard, an aliquot of the supernatant was directly analyzed on a cyanopropylsilane column with methanol-acetonitrile-triethylamine acetate buffer (10:30:60) as the mobile phase and detection at 235 mm. At a flow rate of 1.5 ml min-1, a complete analysis was completed in less than 6 min. The method was linear for verapamil concentrations in the range 0.5-10 micrograms ml-1 (r = 0.9999). Recoveries for the same drug concentrations from spiked rat plasma ranged from 85.6-93.0% (n = 8). The mean RSD values for intraday and interday assay reproducibility (n = 3) were, in both cases, less than 0.9%. The limit of detectability was about 0.1 microgram ml-1. The method was found useful to monitor the plasma levels of verapamil in rats that had received this drug by the nasal, oral and intravenous routes of administration.

Automated determination of verapamil and norverapamil in human plasma with on-line coupling of dialysis to high-performance liquid chromatography and fluorometric detection

A fully automated method for the simultaneous determination of verapamil and its main metabolite norverapamil in human plasma is described. This method is based on on-line sample preparation using dialysis followed by clean-up and enrichment of the dialysate on a precolumn and subsequent HPLC analysis with fluorometric detection. All sample handling operations were performed automatically by a sample processor equipped with a robotic arm (ASTED system). The plasma samples were dialysed on a cellulose acetate membrane (cut-off: 15 kD) and the dialysate was purified and enriched on a short pre-column filled with cyanopropyl silica. Before starting dialysis, this trace enrichment column (TEC) was first conditioned with the HPLC mobile phase and then with pH 3.0 acetate buffer. 370 microliters of plasma sample spiked with the internal standard (gallopamil) were dialysed in the static-pulsed mode. The solution at the donor side was pH 3.0 acetate buffer containing Triton X-100 while the acceptor solution was made of the same acetate buffer. When dialysis was discontinued, the analytes were desorbed from the TEC by the HPLC mobile phase and transferred to the C18 analytical column by means of a switching valve. This mobile phase consisted of a mixture of acetonitrile, pH 3.0 acetate buffer and 2-aminoheptane. The influence of different parameters of the dialysis process on the recovery of verapamil and norverapamil has been studied. The effect of the volume, the aspirating and dispensing flow-rates of the dialysis solution has been investigated. The recoveries of verapamil and norverapamil in plasma were close to 75% and the limits of quantification were 5 ng/ml for both analytes. The method was found to be linear in the concentration range from 5 to 500 ng/ml (r2: 0.9996 for both analytes). The intra-day and inter-day reproducibilities at a concentration of 100 ng/ml were 2.3% and 5.6% for verapamil and 1.7% and 5.1% for norverapamil, respectively.

Pharmacokinetics of verapamil and norverapamil enantiomers after administration of immediate and controlled-release formulations to humans:evidence suggesting input-rate determined stereoselectivity

Verapamil is a racemic calcium channel-blocking drug that undergoes extensive hepatic first-pass metabolism to an active metabolite, norverapamil. The enantiomers of verapamil and norverapamil have differing negative inotropic, chronotropic, and dromotropic activities and differing effects on vascular smooth muscles; the S-enantiomers having greater activity. It is hypothesized that the R/S concentration ratio of verapamil enantiomers may be input-rate dependent. The pharmacokinetics of verapamil and norverapamil enantiomers were studied in 11 young, healthy male and female volunteers after oral administration of 80 mg immediate-release (IR) verapamil every 8 hours, and a 240 mg dose once daily of controlled-release (CR) formulation on two separate occasions. Both dosage regimens were continued for 1 week with a minimum 1-week period between the two drug treatments. After the last dose of each regimen, plasma samples were collected over the period corresponding to the dosing interval. Enantiomer concentrations were determined using a microwave-facilitated precolumn derivatization with high performance liquid chromatographic quantification. Stereospecific assay revealed that: (1) stereoselective R- and S-enantiomer disposition occurred regardless of formulation administered; (2) a trend of R:S concentration ratios of verapamil differed between the two formulations; and (3) fluctuations between Cmax and Cmin values of the two formulations were statistically different over respective dosing intervals (greater fluctuation after CR administration). Using nonstereospecific data analyses, however, the pharmacokinetic parameters for verapamil and norverapamil were similar for both formulations over a 24-hour period. We suggest that kinetic differences can be attributed to differences in release rates of drug from the tablet matrices. The relative bioavailabilities of verapamil and norverapamil from the two products may, therefore, be subject to input rate-dependent processes.

Enhancement of cytotoxicity of doxorubicin by verapamil in the hepatic artery infusion for liver tumors in rats

BACKGROUND

The calcium channel blocker has been demonstrated to be effective in the accumulation and retention of chemotherapeutic agents in tumor cells.

METHODS

The effect of verapamil on cytotoxicity of doxorubicin was investigated in a hepatic artery infusion (HAI) for liver tumors of Walker 256 carcinosarcoma in rats. Doxorubicin was infused by way of a hepatic artery by a bolus injection intra-arterially (IA) (1 mg/kg) and a continuous infusion intra-arterially (CIA) (6 mg/kg/day for 6 days).

RESULTS

Doxorubicin increased 90% and 66% in tumor tissue following HAI of verapamil by a bolus and continuous infusion (P < 0.05), respectively. However, no enhancement of the accumulation of doxorubicin in the tumor tissue was found in an intravenous administration of verapamil. The CIA infusion of verapamil with doxorubicin inhibited the tumor growth by 73% in comparison with doxorubicin only (P < 0.05). Verapamil administered intravenously (IV) could not induce this inhibitory effect. The CIA administration of verapamil reduced the serum concentration by 45% (P < 0.001) in comparison with the CIV route. Furthermore, the administration of verapamil did not increase the accumulation of doxorubicin in the normal liver and heart tissues. No enhancement of bone marrow suppression and hepatic biochemical influence by doxorubicin was revealed by the concomitant use of verapamil.

CONCLUSIONS

The continuous HAI of verapamil remarkably enhanced the cytotoxicity of HAI with doxorubicin for the treatment of hepatic tumor without aggravating the side effects induced by doxorubicin.

Verapamil attenuates postischemic oxidative injury in the rat liver

We assessed the effects of the calcium channel blocker verapamil on postischemic oxidative injury in the rat liver. In the untreated rats, the values of tissue lipid peroxidation products (thiobarbituric acid-reactive substances) remained unchanged during 90 min of warm ischemia. However, the values increased significantly after the next 60 min of reperfusion compared with those in the sham-operated rats (P less than 0.01). Intravenous infusion of verapamil (5 micrograms.kg-1.min-1) significantly reduced the extent of lipid peroxidation during reperfusion compared with that in the untreated rats (P less than 0.02). The percentages of tissue water content and the serum lactate dehydrogenase activities after 60 min of reperfusion were significantly lower in the treated rats than in the untreated rats (P less than 0.02 and P less than 0.01, respectively). We also investigated the influence of verapamil on superoxide-generating activity determined by the superoxide-dependent cytochrome c reduction of peritoneal polymorphonuclear leukocytes (PMNs) harvested from normal, non-ischemic, and non-treated rats in vitro. This demonstrated that there was no apparent effect with the highest verapamil concentration level (8 microM) observed in the rat plasma during our experiment. These findings suggest that verapamil might reduce the postischemic oxidative injury in the rat liver by mechanisms perhaps not related to the suppression of rat PMNs superoxide-generating activity.

Impact of food on the pharmacokinetics and electrocardiographic effects of sustained release verapamil in normal subjects

To evaluate the impact of food on the pharmacokinetics and electrocardiographic effects of sustained release (SR) verapamil tablets, 9 healthy men each received 3 single doses of verapamil in a randomized, crossover manner: 10 mg of intravenous verapamil, 240 mg SR verapamil on an empty stomach, and 240 mg SR verapamil with a standardized meal. PR intervals and racemic verapamil serum concentrations were measured serially over 30 hours after administration. The time to peak concentration was longer (7.5 +/- 3.0 vs 4.4 +/- 2.3 hours), resulting in a lower peak verapamil serum concentration (118 +/- 43 vs 175 +/- 50 ng/ml) when SR verapamil was administered with food (p < 0.05). Food tended to decrease the bioavailability of SR verapamil (34 +/- 12 vs 49 +/- 14%), although this difference did not reach statistical significance (p = 0.065). Precipitous or exaggerated release of verapamil from the SR tablet was not observed in any subject during the fasting state. Prolongation of the PR interval paralleled these alterations in serum concentration. The maximal change in the PR interval was greater (21 +/- 8 vs 14 +/- 5%; p < 0.05) when SR verapamil was given in the fasting state. Although an exaggerated verapamil release or effect was not observed, food significantly altered the absorption and electrocardiographic effects of a single dose of SR verapamil. Manipulation of the administration condition may be helpful in achieving desired outcomes.

Determination of verapamil and norverapamil in human plasma by liquid chromatography: comparison between a liquid-liquid extraction procedure and an automated liquid-solid extraction method for sample preparation

A conventional liquid-liquid extraction (LLE) procedure with high-performance liquid chromatography (HPLC) has been developed for the determination of verapamil and its main metabolite, norverapamil, in plasma. After addition of the internal standard, plasma samples were basified with phosphate buffer (pH 9.0) and extracted with a mixture of cyclohexane-dichloromethane. After centrifugation, the organic layer was separated and the analytes were extracted back into a 0.1 N sulphuric acid solution containing 2-aminoheptane. An aliquot of this aqueous phase was then injected directly onto the HPLC column. This LLE procedure has been compared with an automated liquid-solid extraction (LSE) method that has been developed in parallel. Good linearity was obtained using both extraction methods. The absolute recoveries for the two analytes were ca 95% with the automated LSE procedure and slightly lower (ca 84%) for the LLE method. The automated method gives better results with respect to detectability and precision, but the LLE procedure is simpler to develop, requires much less expensive equipment, and remains a useful alternative when the number of samples to be analysed is limited.

Direct determination of the enantiomeric ratio of verapamil, its major metabolite norverapamil and gallopamil in plasma by chiral high-performance liquid chromatography

Two methods for the determination of the enantiomeric ratio of verapamil in plasma by high-performance liquid chromatography have been developed. On an alpha 1-acid glycoprotein chiral stationary phase (Chiral-AGP) verapamil was separated after the acetylation of the main metabolite norverapamil, which interferes with the resolution of verapamil. On an amylose tris-3,5-dimethylphenylcarbamate column (Chiralpak AD) verapamil and norverapamil were determined simultaneously without prior derivatization. Gallopamil was separated on both columns under similar conditions.

Plasma level monitoring of D,L-verapamil and three of its metabolites by reversed-phase high-performance liquid chromatography

A high-performance liquid chromatographic assay was developed for determination of verapamil, norverapamil (M1) and its N-dealkylated metabolites (M2 and M3) in plasma. Plasma samples were vortex-mixed, deproteinized and centrifuged. The analysis was performed on a C18 reversed-phase column with fluorimetric detection. Since the polarity of verapamil and norverapamil differs considerably from that of M2 and M3, two different eluents were used for rapid high-performance liquid chromatographic separation. The eluent for the separation of verapamil and norverapamil was acetonitrile-0.07% orthophosphoric acid (33:67, v/v), and for M2 and M3 acetonitrile-0.07% orthophosphoric acid (25:75, v/v). The high-performance liquid chromatographic assay allowed rapid, sensitive and reliable quantitation of verapamil and three of its metabolites in plasma without an extraction procedure. The limit of detection was less than 5 ng/ml (plasma) for all compounds. No interferences with other commonly co-administered drugs was observed. Plasma concentrations of verapamil and its metabolites were determined in 21 patients receiving a continuous infusion of verapamil for tachyarrhythmia of acute onset. The steady-state plasma concentration data of verapamil and its three main metabolites in these patients gave evidence that the plasma concentration of verapamil and its active metabolite norverapamil was primarily determined by the extent of the formation of M2.

Evaluation of the pharmacokinetics and electrocardiographic effects of intravenous verapamil with intravenous calcium chloride pretreatment in normal subjects

To evaluate the effects of calcium pretreatment on the disposition and electrocardiographic effects of verapamil, 8 healthy male volunteers received treatment in each of 3 phases in a randomized, double-blind, crossover manner. Phase I denoted 10 ml of 0.9% intravenous sodium chloride followed by 10 mg of intravenous verapamil; phase II denoted 10 ml of 10% intravenous calcium chloride followed by 4 ml of 0.9% intravenous sodium chloride; and phase III denoted 10 ml of 10% intravenous calcium chloride followed by 10 mg of intravenous verapamil. Blood samples for the determination of verapamil concentrations were drawn at 5, 10, 15, 20, 30, 45, 60 and 90 minutes, and at 2, 4, 6, 10 and 24 hours. Blood pressure, heart rate and PR intervals were also measured at these times. Pretreatment of verapamil with intravenous calcium did not alter the disposition of intravenous verapamil. Blood pressure was not significantly altered in any treatment phase, although calcium tended to increase mean arterial pressure and verapamil abolished this effect. Calcium had no significant affect on verapamil-induced PR prolongation (maximum percent change in PR interval: phase I = 19 +/- 11%, phase III = 18 +/- 7%; time to maximal prolongation: phase I = 0.38 +/- 0.21 hours, phase III = 0.37 +/- 0.26 hours; and area under the percent change in PR vs time curve: phase I = 15.5 +/- 10, phase III = 21 +/- 9). Verapamil caused a reflex increase in heart rate of similar magnitude in both phases I and III (24 +/- 10% and 21 +/- 7%, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Does verapamil limit myocardial infarct size in a heart deficient in xanthine oxidase?

1. The delay of ischaemic myocardial necrosis by verapamil has been reported in the dog heart, which contains a high level of xanthine oxidase, a potential source of cytotoxic free radicals. To test whether the retardation of ischaemic myocyte death by verapamil is not an isolated phenomenon in the xanthine oxidase rich heart, we assessed the effect of verapamil in the rabbit heart, which lacks xanthine oxidase. 2. Verapamil (200 micrograms/kg, i.v. bolus plus 40 micrograms/kg per min) was administered in a group of rabbits (n = 5) to test the haemodynamic response to this agent. The heart rate, blood pressure and left ventricular dp/dt max were reduced by 11, 25 and 57%, respectively, and the plasma concentration of verapamil was maintained at 300-400 ng/mL during the infusion. 3. In other groups of rabbits, the effect of the same dosage of verapamil on the size of myocardial infarct after 20 or 30 min ischaemia and 72 h reperfusion was examined. The verapamil was administered for 45 min, starting 15 min prior to ischaemia. The percentage of area at risk infarcted (%I/AAR) was 15.2 +/- 3.9% in the 20 min ischaemia control group and 15.4 +/- 4.5% in the 20 min ischaemia verapamil group, 49.1 +/- 3.4% in the 30 min ischaemia control group and 41.2 +/- 3.3% in the 30 min ischaemia verapamil group. The %I/AAR was significantly smaller in the 20 min ischaemia control groups and 15.4 +/- 4.5% in the 20 min ischaemia there was no difference in %I/AAR between the control and verapamil treated animals in either the 20 or the 30 min ischaemia groups. 4. These results suggest that verapamil does not delay the transition from reversible to irreversible myocardial injury during coronary occlusion in the rabbit, which like the human, lacks myocardial xanthine oxidase.

Chromatography of calcium channel blockers

Numerous publications during the past ten years have described the determination of various calcium channel blockers in biological fluids, using gas and liquid chromatographic techniques. Diltiazem, verapamil, flunarizine and a growing number of dihydropyridines belong to this group of drugs, which in most instances are active at low plasma concentrations. From a bioanalytical point of view these compounds have many features in common, such as high lipophilicity and favourable detection properties.

Stereoselective protein binding of verapamil enantiomers

The binding of the (+)- and (-)-enantiomers of verapamil (V) to purified albumin (40 g/L), alpha 1-acid glycoprotein (0.55 g/L) and fresh serum has been studied over a wide range of verapamil concentrations (0.055 to 22 microM). The free fraction of the pharmacologically more potent (-)-V was always greater than that of (+)-V. Similar free fractions were observed in solutions of alpha 1-acid glycoprotein ((+)-V 0.079 +/- 0.016; (-)-V 0.142 +/- 0.020) and fresh serum ((+)-V 0.096 +/- 0.009; (-)-V 0.136 +/- 0.006), however the free fraction was higher in a solution of albumin ((+)-V 0.400 +/- 0.030; (-)-V 0.572 +/- 0.029). Saturation of verapamil binding sites was observed for alpha 1-acid glycoprotein only. Enantioselective verapamil serum binding was also noted in samples collected from five healthy volunteers following oral and intravenous verapamil administration. The free fraction of the individual isomers in vitro when added to predose serum as the pseudoracemic drug ((+)-V 0.06 +/- 0.01, (-)-V 0.12 +/- 0.02) was similar to that observed for the enantiomers when studied separately in vitro, indicating that the binding of each enantiomer is independent of the other optical isomer. The free fraction ex vivo after intravenous therapy ((+)-V 0.06 +/- 0.01, (-)-V 0.12 +/- 0.02) was similar to that observed in vitro in that subjects pre-dose serum. The free fraction of both enantiomers, however, was higher after oral drug therapy ((+)-V 0.13 +/- 0.02, (-)-V 0.23 +/- 0.03). The lower binding noted may be a result of competition for serum binding sites by verapamil metabolites, which attain higher concentrations following oral dosing.

Effects of verapamil on hepatic and systemic hemodynamics and liver function in patients with cirrhosis and portal hypertension

The effects of verapamil on hepatic and systemic hemodynamics and on liver function were investigated in 10 patients with portal hypertension due to advanced micronodular cirrhosis to verify whether, as it has been suggested, this calcium channel blocker may improve liver function and reduce portal pressure in these patients. The oral administration of 100 mg of verapamil caused systemic vasodilation, evidenced by a significant reduction in mean arterial pressure (-8.1 +/- 7.6%, p less than 0.025) and systemic vascular resistance (-12.5 +/- 9.5%, p less than 0.001), and increased heart rate (+13.9 +/- 10.4%, p less than 0.01). However, no beneficial effect was noted on portal pressure evaluated by hepatic vein catheterization (baseline 19.8 +/- 4.0, verapamil 20.2 +/- 3.6 mmHg, NS), hepatic blood flow (1.45 +/- 0.64 vs. 1.47 +/- 0.62 liters per min, NS) and hepatic vascular resistance (1.314 +/- 611 vs. 1,266 +/- 513 dyn per sec per cm-5, NS). Similarly, no change was observed in portal blood flow, measured in six patients by pulsed Doppler flowmeter (0.94 +/- 0.30 vs. 0.89 +/- 0.35 liter per min, NS). In addition, verapamil did not increase the hepatic intrinsic clearance of these patients (0.20 +/- 0.07 vs. 0.19 +/- 0.06 liter per min, NS). This study suggests that verapamil is of no beneficial effect in patients with advanced cirrhosis of the liver.

Drug level monitoring: cardiovascular drugs

Methods for the determination of cardiovascular drugs in blood and plasma are critically reviewed with emphasis on gas and liquid chromatographic techniques. The importance of the various procedures is discussed, in particular sample work-up where the conditions for isolation and derivatization of the compounds are decisive for the accuracy and precision of the methods. Compared with other assay techniques chromatographic methods are generally to be preferred owing to their better selectivity. In the review the following groups are discussed: digitalis glycosides, antiarrhythmic agents, beta-adrenoceptor antagonists, vasodilating agents, antihypertensive compounds, and diuretics.

An investigation of the cause of accumulation of verapamil during regular dosing in patients

The accumulation of verapamil during regular dosing conditions was studied. Plasma concentrations of verapamil (V) and norverapamil (NV) were measured as were urinary concentrations of verapamil, norverapamil, and four other N- and O-dealkylated metabolites in nine patients after an initial single dose and after chronic oral verapamil administration to steady-state plasma concentrations. Indocyanine green (ICG) clearance was determined immediately prior to the initial verapamil dose and prior to the verapamil washout from regular dosing. An approximately two-fold accumulation of V had occurred during regular dosing. The area under the plasma concentration-time curve (AUC1) after the first dose was 417.4 +/- 276.7 ng ml-1 h (mean +/- s.d.) and increased to 786.5 +/- 54 ng ml-1 h (P less than 0.01) during one dosage interval at steady-state (AUCss). NV also tended to accumulate from an AUC1 of 552.6 +/- 411 to an AUCss 668.7 +/- 332 ng ml-1 h (P less than 0.09). The ratio of AUC-V to AUC-NV was unchanged. The verapamil elimination half-life (t 1/2) increased from 8.4 +/- 4.2 to 12.0 +/- 3.6 h (P less than 0.01) whereas the norverapamil t 1/2 was unchanged. ICG clearance was unchanged. Urinary excretion of NV increased slightly but the ratio of urinary V/NV concentrations was not significantly altered nor was the ratio of four other metabolites to verapamil or the ratio of the combined o-demethylated to the N-dealkylated metabolites.(ABSTRACT TRUNCATED AT 250 WORDS)

Antiarrhythmic actions of verapamil against ischaemic arrhythmias in the rat

The actions of intravenous verapamil against arrhythmias induced by occlusion of a coronary artery were investigated in conscious rats. Verapamil (2-20 mg kg-1, i.v. given pre-occlusion) dose-dependently reduced arrhythmias in rats with either large or small occluded zones at an ED50 of 6 mg kg-1. This dose was effective when given immediately post-occlusion. Severe arrhythmias, as opposed to PVC, were preferentially reduced. In conscious, and pentobarbitone-anaesthetized rats, verapamil (6 mg kg-1) had different effects on electrically-induced arrhythmias, and the ECG, from an equi-effective anti-arrhythmic dose of quinidine (20 mg kg-1, i.v.). Quinidine decreased following frequency, but increased threshold current and pulse width, whereas verapamil did not. Both drugs increased P-R interval, but only quinidine increased QRS and Q-T intervals. Thirty minutes post-occlusion, the verapamil content of tissue and blood was determined after a 6 mg kg-1 dose given pre- or post-occlusion. Measurable levels of verapamil were found in both normal and ischaemic myocardium. Plasma and plasma water concentrations were 3.6 +/- 0.8 mumol l-1 and 0.6 +/- 0.1 mumol l-1 (mean +/- s.e. mean), respectively following post-occlusion administration vs. 2.7 +/- 1.2 and 0.24 +/- 0.04 for pre-occlusion administration. Plasma water concentrations were close to IC50 values for inhibition of contractility in rat atria and ventricles. Similar concentrations depressed slow action potentials induced in rat ventricles by raised K+ We suggest that the ability of verapamil to prevent severe ventricular arrhythmias following myocardial ischaemia in the conscious rat is largely due to the calcium antagonist effects of the drug.

Determination of tiapamil and of its two main metabolites in plasma and in urine by high-performance liquid chromatography

Selective high-performance liquid chromatographic methods for the determination of tiapamil and its two main metabolites in plasma and urine are described. Tiapamil together with its metabolites is extracted at alkaline pH into dichloromethane. Separation is carried out using normal-phase high-performance liquid chromatography with ultraviolet detection (278 nm). The unchanged drug and the desmethyl metabolite are analysed simultaneously. The second metabolite is analysed separately under more polar conditions. The sensitivity limits are 50 ng/ml for tiapamil, 100 ng/ml for the desmethyl metabolite and 75 ng/ml for the second metabolite, using 0.5 ml of plasma. The sensitivity limits in urine are 100 ng/ml for all three compounds using a 0.5 ml specimen. The method has been applied to the analysis of human plasma and urine after intravenous (70 mg) and oral (400 mg) administration of tiapamil.

Inter- and intra-subject variation in the first-pass elimination of highly cleared drugs during chronic dosing. Studies with deuterated verapamil

The pharmacokinetics of verapamil in five healthy volunteers were investigated on 4 occasions during chronic administration of deuterated verapamil. There was no statistically significant difference in oral clearance, terminal half-life, bioavailability, morning trough level and peak concentration or in the time of their occurrence on the four occasions. The plasma clearance, however, exhibited considerable inter- and intra-individual variation, ranging between 26.3% and 85.4% and 12.0% and 48.0%, respectively. Comparison of these pharmacokinetic parameters with data from previous single dose studies in the same subjects revealed a significant (p less than 0.05) decrease in the clearance and an increase in the apparent bioavailability of verapamil during chronic administration, although no difference in the half-life was found. Due to the considerable variation in the oral clearance of verapamil during chronic dosing, steady-state conditions in a strict pharmacokinetic sense may never be attained, and pharmacokinetic data obtained in single dose studies will be of limited value in predicting steady-state plasma concentrations.

High-performance liquid chromatographic method for the determination of plasma and urine metapramine after dansylation

A high-performance liquid chromatographic method has been developed for the determination of metapramine in human plasma and urine. After selective extraction and derivatization with dansyl chloride, metapramine and the internal standard (maprotiline) are chromatographed on a reversed-phase LiChrosorb RP-18 column using a mixture of water--acetonitrile (35:65) as mobile phase. The eluted compounds are measured using a fluorescence detector. The detection limit of the assay for plasma and urine samples is about 1 ng/ml. The method has been successfully applied in a pharmacokinetic study following intravenous administration of 35 mg of metapramine.