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

Physiologically-based pharmacokinetic modeling to predict drug-drug interactions of dabigatran etexilate and rivaroxaban in the Chinese older adults

INTRODUCTION

Drug-drug interaction (DDI) is one of the major concerns for the clinical use of NOACs in the older adults considering that coexistence of multiple diseases and comorbidity were common. Current guidelines on the DDI management were established based on clinical studies conducted in healthy adults and mainly focus on the Caucasians, whereas systemic and ethnic differences may lead to distinct management in the Chinese older adults.

OBJECTIVES

To investigate the impact of aging on the DDI magnitude between P-gp and/or CYP3A4 inhibitors with dabigatran etexilate and rivaroxaban in older adults, providing additional information for the use in clinical practice.

RESULTS

Compared with the simulated adult, the AUC of the simulated older adults increased by 42-88% (DABE) and 21-60% (rivaroxaban), respectively, during NOACs monotherapy. Simulation on DDIs predicted that verapamil and clarithromycin further increase the exposure of dabigatran by 29-72% and 40-47%, whereas clarithromycin, fluconazole, and ketoconazole increase the exposure of rivaroxaban by 21-30%, 16-24%, and 194-247% in the older adults. Overall, our simulation result demonstrated that aging and DDIs both increased the exposure of NOACs. However, aging does not have a drastic impact on the extent of DDIs. The DDI ratios of young and old older adults were similar to the adults and were also similar between Caucasians and Chinese.

DISCUSSION

We further simulated the interactions under steady-state based on the EHRA guideline (2021). Our simulation results revealed that recommended reduced dosing regimen of dabigatran etexilate during comedication with verapamil and clarithromycin (110 and 75 mg BID for Chinese young and old older adults) will result in exposure (trough concentration) that was either slightly higher or similar to the trough concentration of patients with any bleeding events. Routine monitoring of bleeding risk is encouraged. Further studies on the use of rivaroxaban in Chinese older adults are warranted.

CONCLUSION

Aging and DDI increases exposure of drug in Chinese older adults. However, aging does not have a drastic impact on the extent of DDIs. Clinical management of DDIs in Chinese older adults in the absence of complex polypharmacy can a priori be similar to the EHRA guideline but routine monitoring of bleeding risk is encouraged when dabigatran etexilate given with verapamil and clarithromycin.

Mass loading and removal of pharmaceuticals and personal care products including psychoactives, antihypertensives, and antibiotics in two sewage treatment plants in southern India

Environmental contamination by pharmaceuticals and personal care products (PPCPs) is barely studied in India despite being one of the largest global producers and consumers of pharmaceuticals. In this study, 29 pharmaceuticals and six metabolites were determined in sewage treatment plants (STPs) in Udupi (STP_U: population served ∼150,000) and Mangalore (STP_M: population served ∼450,000); the measured mean concentrations ranged from 12 to 61,000 ng/L and 5.0 to 31,000 ng/L, respectively. Atorvastatin (the most prescribed antihypercholesterolemic in India), mefenamic acid, and paraxanthine were found for the first time in wastewater in India at the mean concentrations of 395 ng/L, 1100 ng/L, and 13,000 ng/L, respectively. Select pharmaceutical metabolites (norverapamil and clopidogrel carboxylic acid) were found at concentrations of upto 7 times higher than their parent drugs in wastewater influent and effluent. This is the first study in India to report mass loading and emission of PPCPs and their select metabolites in STPs. The total mass load of all PPCPs analyzed in this study at STP_U (4.97 g/d/1000 inhabitants) was 3.6 times higher than calculated for STP_M. Select recalcitrant PPCPs (carbamazepine, diazepam, and clopidogrel) were found to have negative or no removal from STP_U while additional treatment with upflow anaerobic sludge blanket reactor at STP_M removed (up to 95%) these PPCPs from STP_M. Overall, 5.1 kg of caffeine, 4.1 kg of atenolol, 2.7 kg of ibuprofen, and 1.9 kg of triclocarban were discharged annually from STP_U. The PPCP contamination profile in the Indian STP was compared with a similar study in the USA.

Pharmacokinetics of verapamil and its major metabolite, norverapamil from oral administration of verapamil in rabbits with hepatic failure induced by carbon tetrachloride

The aim of this study was to investigate the pharmacokinetic changes of verapamil and its major metabolite, norverapamil, after oral administration of verapamil (10 mg/kg) in rabbits with slight, moderate and severe hepatic failure induced by carbon tetrachloride. The plasma verapamil concentrations in all groups of hepatic failure were significantly higher (p < 0.01) than the control. However, the plasma norverapamil concentrations in severe hepatic failure were significantly higher (p < 0.05) than the control. The peak concentrations (Cmax) and the areas under the plasma concentration-time curve (AUC) of verapamil in the rabbits were significantly (p<0.01) higher than the control. The absolute bioavailability (F(A.B)) and the relative bioavailability (F(R.B)) of verapamil in the rabbits with hepatic failure were significantly higher (13.6-22.2% and 150-244%, respectively) than the control (9.1% and 100%, respectively). Although the AUC and Cmax of its major metabolite, norverapamil, in slight, moderate hepatic failure were not significantly lower than the control, the metabolite-parent AUC ratio in all groups of hepatic failure was decreased significantly (p < 0.05, in slight group; p < 0.01, in moderate and severe group) than the control. This could be due to decrease in metabolism of verapamil in the liver because of suppressed hepatic function in the hepatic failure groups because verapamil is mainly metabolized in the liver. From our data, it would seem appropriate that in patients with liver disease, doses of verapamil should be decreased by degree of hepatic failure.

PREDICTION OF CYTOCHROME P450 3A INHIBITION BY VERAPAMIL ENANTIOMERS AND THEIR METABOLITES

Verapamil inhibition of CYP3A activity results in many drug-drug interactions with CYP3A substrates, but the mechanism of inhibition is unclear. The present study showed that verapamil enantiomers and their major metabolites [norverapamil and N-desalkylverapamil (D617)] inhibited CYP3A in a time- and concentration-dependent manner by using pooled human liver microsomes and the cDNA-expressed CYP3A4 (+b5). The values of the inactivation kinetic parameters kinact and KI obtained with the cDNA-expressed CYP3A4 (+b5) were 0.39 min(-1) and 6.46 microM for R-verapamil, 0.64 min(-1) and 2.97 microM for S-verapamil, 1.12 min(-1) and 5.89 microM for (+/-)-norverapamil, and 0.07 min(-1) and 7.93 microM for D617. Based on the ratio of kinact and KI, the inactivation potency of verapamil enantiomers and their metabolites was in the following order: S-norverapamil>S-verapamil>R-norverapamil>R-verapamil>D617. Using dual beam spectrophotometry, we confirmed that metabolic intermediate complex formation with CYP3A was the mechanism of inactivation for all compounds. The in vitro unbound fraction was 0.84 for S-verapamil, 0.68 for R-verapamil, and 0.84 for (+/-)-norverapamil. A mechanism-based pharmacokinetic model predicted that the oral area under the curve (AUC) of a CYP3A substrate that is eliminated completely (fm=1) by the hepatic CYP3A increased 1.6- to 2.2-fold after repeated oral administration of verapamil. For midazolam (fm=0.9), a drug that undergoes extensive intestinal wall metabolism, the predicted increase in oral AUC was 3.2- to 4.5-fold. The predicted results correlate well with the in vivo drug interaction data, suggesting that the model is suitable for predicting drug interactions by mechanism-based inhibitors.

Gender-specific effects on verapamil pharmacokinetics and pharmacodynamics in humans

Pharmacokinetic studies of i.v. and oral racemic verapamil and 14C-erythromycin breath tests (ERBT) were performed in 84 healthy men (n = 42) and women (n = 42). Verapamil was measured by HPLC, concentration versus time data were analyzed by noncompartmental models, protein binding was measured by equilibrium dialysis, and statistical analyses were performed by ANOVA. Clearance of i.v. and p.o. verapamil was 13.7 +/- 4.3 and 58.4 +/- 35 ml/min/kg (mean +/- SD) in women compared to 12.6 +/- 3.4 and 82.6 +/- 70 ml/min/kg in men (p = 0.076). Bioavailability was higher in women (0.25 +/- 0.09) compared to men (0.20 +/- 0.09, p = 0.019) with a significant Gender x Formulation interaction (p = 0.04). ERBT were higher in women (p < 0.0001). Verapamil (i.v. and p.o.) decreased blood pressure in all subjects with greater heart rate increases after p.o. verapamil in women compared to men (p = 0.041). The findings suggest that sex-specific differences in drug metabolism may occur in both the gut and the liver and involve multiple metabolic pathways and that pharmacokinetic differences will alter pharmacodynamic responses.

Effect of verapamil on renal haemodynamics in a portal hypertensive rat model

In order to investigate the effects of verapamil on renal haemodynamics in rats with portal hypertension, verapamil was given at either a low (0.2 mg/kg) or high (2 mg/kg) dose to rats after portal vein ligation. An approximate 12% decrease in mean arterial pressure followed administration of low dose verapamil, with a significant decrease in cardiac output and renal blood flow, as well as reduced portal pressure, observed; these signs were all indicative of a rise in renal vascular resistance. In contrast, the marked fall in both mean arterial pressure and cardiac output with high dose verapamil, accompanied by a significant decrease in portal pressure and no change in renal blood flow, suggests a reduction in renal vascular resistance. This study shows that the acute effects of verapamil on renal haemodynamics may vary with the dose used. Also, acute verapamil administration tends to decrease renal blood flow to alter the autoregulation of the kidney; thus, caution should be taken in the clinical use of verapamil in the treatment of cirrhosis with portal hypertension.

Acute verapamil toxicity in a patient with chronic toxicity: possible interaction with ceftriaxone and clindamycin

OBJECTIVE

To report a case of acute toxicity in a patient with chronic verapamil toxicity, possibly precipitated by intravenous administration of the highly protein-bound drugs ceftriaxone and clindamycin.

DATA SOURCES

Case reports, review articles, and relevant laboratory and clinical studies identified by MEDLINE (1984-forward), and relevant cross references from those articles.

DATA EXTRACTION

Data were abstracted from pertinent sources by one author and reviewed by the remaining authors.

CASE SUMMARY

A 59-year-old man who had been receiving sustained-release verapamil 240 mg q12h for more than two years for hypertension, and phenytoin 300 mg/d for many years for prophylaxis against seizures, was noted to be in junctional rhythm when he presented to the emergency room with bilateral pneumonia. Administration of intravenous ceftriaxone 1 g and clindamycin 900 mg precipitated symptoms of acute verapamil toxicity in this patient. The toxicity led to complete heart block requiring cardiopulmonary resuscitation and insertion of a temporary pacemaker. He spontaneously reverted to normal sinus rhythm after 16 hours. Subsequent cardiac evaluation, including echocardiogram, 48-hour dynamic electrocardiographic recording (Holter), and exercise stress test were normal. The patient has remained in sinus rhythm for more than one year after this episode.

CONCLUSIONS

We believe that junctional rhythm on admission was a result of chronic verapamil toxicity. This may have been because of increased bioavailability of the drug or increased sensitivity of the receptors. Administration of ceftriaxone, clindamycin, or both agents might have precipitated acute verapamil toxicity by displacing verapamil from its protein-binding sites. Extreme caution is necessary when a highly protein-bound drug is given to a patient already receiving verapamil.

Requirements for drug monitoring of verapamil: experience from an unselected group of patients with cardiovascular disease

Serum verapamil and metabolite concentrations were determined by HPLC in 29 patients in routine treatment with verapamil, and 23 were in steady state. Dosage levels and corresponding mean trough levels (+/- S.D.) were as follows: 120 mg daily: 79.1 (+/- 77) nmol/l, 240 mg daily: 173.3 (+/- 200.1) nmol/l, 360 mg daily: 204 (+/- 110.2) nmol/l and 480 mg daily: 361.0 (+/- 231.4) nmol/l. The variation coefficients were 97.3, 115.4, 54.0, and 62.1, respectively, thus showing considerable interpatient variation. Repeated determination of trough levels showed, in contrast, only small intrapatient variation (variation coefficient 35.8, 1.9, and 7.4, at the dosage levels 120, 240 and 340 mg per day). No significant correlation was found between serum verapamil levels age, sex, or weight. No significant effect of digoxin on the concentration of serum verapamil was found. No relation was observed between serum verapamil concentrations and desired effect or side-effects. Two patients showed no measurable serum verapamil, but one of these had detectable levels of metabolites. Such patients may represent subgroups of fast metabolizers or non-absorbers. Measurements of the metabolites nor-verapamil, D 620 and D 617 indicated saturation of the first-pass metabolism. In conclusion, therapeutic drug monitoring is not indicated during routine verapamil treatment, whereas single measurements of verapamil may be warranted in patients not responding to treatment in order to identify fast metabolizers or non-absorbers.

Effect of verapamil on splanchnic haemodynamics in a portal hypertensive rat model

To elucidate the effects of verapamil on splanchnic haemodynamics in rats with portal hypertension, verapamil was given at a low dose (0.2 mg/kg) and a high dose (2 mg/kg) to the rat model after portal vein ligation. Approximately 10% decrease in arterial pressure was caused by the low dose of verapamil, with significant decreases in cardiac output and portal venous inflow as well as reduced portal pressure; these were all indicative of a rise in portal vascular resistance. In contrast, the marked fall in both arterial pressure and cardiac output in the high dose, accompanied by a significant decrease in the portal pressure and the unchanged portal venous inflow, suggested a reduction in portal vascular resistance. This study shows that the acute effects of verapamil on portal hypertension may vary with the dosage used. These results also demonstrate that, since the therapeutic efficacy and safety of verapamil is only in a very limited range of dose, caution should be taken in its clinical use in the treatment of cirrhosis with portal hypertension.

Influence of food on the bioavailability of a sustained-release verapamil preparation

The effects of food on the bioavailability of a sustained-release (SR) formulation of verapamil (SR-verapamil; Isoptin SR) were determined in an open, three-way single-dose study involving 12 volunteers receiving (in randomized order) the SR preparation (1 X 240 mg) either fasting or with food and a conventional formulation of verapamil (3 X 80 mg) fasting. Compared with the conventional formulation, SR-verapamil had a reduced Cmax, prolonged tmax, and unchanged t1/2, consistent with its SR formulation. The AUC was 80% of the conventional preparation. Concomitant food administration significantly prolonged the tmax of SR-verapamil from 7.3 +/- 3.4 to 11.7 +/- 6.3 h, but had little effect on Cmax, t1/2, or AUC. Similar results were obtained with the metabolite, norverapamil. Food administration also had little effect on the blood pressure and ECG effects of SR-verapamil. Cautions regarding taking this preparation with food therefore appear to be unnecessary.

The effects of encainide versus diltiazem on the oxidative metabolic pathways of antipyrine

The effects of diltiazem and encainide on the pharmacokinetics and metabolism of antipyrine were compared in nine healthy male volunteers. Diltiazem 90 mg every 8 hours for 5 days decreased the oral clearance of antipyrine from 2.34 to 1.86 L/hour (p less than 0.05) and increased half-life from 12.7 to 15.9 hours (p less than 0.05). Diltiazem reduced the formation rate constants for 3-hydroxymethylantipyrine by 27% (p less than 0.05) and 4-hydroxyantipyrine by 37% (p less than 0.05). There was also a 21% reduction in the formation rate constant for norantipyrine (0.05 less than p less than 0.10). Encainide 25 mg every 8 hours for 5 days had no apparent effect on the oral clearance or half-life of antipyrine, or on the formation rate constants for metabolites of antipyrine. In contrast to a previously published report in rats, encainide, unlike diltiazem, does not inhibit the oxidative metabolism of antipyrine in humans.

Serum binding of nifedipine and verapamil in patients with ischaemic heart disease on monotherapy

Serum free fractions of nifedipine, verapamil and some of their metabolites were measured in patients with ischaemic heart disease receiving single oral dose and chronic monotherapy and were compared with those obtained in vitro. The percentages of unbound nifedipine and verapamil in vitro (concentration range 50-200 and 150-400 ng ml-1, respectively) were 2.51 and 7.23%, respectively, and did not differ from those found on monotherapy with these drugs (2.05 and 8.08%, respectively), and after single dosing. It is suggested that, during treatment with nifedipine or verapamil, their serum metabolites do not affect binding of the parent drugs to serum proteins.

Calcium channel antagonists: Part VI: Clinical pharmacokinetics of first and second-generation agents

A survey of the pharmacokinetic properties of the three prototypical calcium antagonist agents shows that they have in common a very high rate of hepatic first-pass metabolism with, in the case of verapamil and diltiazem, the formation of an active metabolite that affects the dose during chronic therapy. Therefore, the major factor altering the pharmacokinetic properties and the dose of the drug required is the capacity of the liver to metabolize the drug, which in turn depends on the hepatic blood flow and the activity of the hepatic metabolizing systems. Hence liver disease, a low cardiac output, and coadministration of certain drugs inducing or inhibiting the hepatic enzymes, all indirectly affect the pharmacokinetic properties of the calcium antagonists. There are also other potential drug interactions of a kinetic or dynamic nature that may arise. In general, renal disease has little effect on the pharmacokinetics of calcium antagonists.

Verapamil. An updated review of its pharmacodynamic and pharmacokinetic properties, and therapeutic use in hypertension

Although verapamil is a well-established treatment for angina, cardiac arrhythmias and cardiomyopathies, this review reflects current interest in calcium antagonists as anti-hypertensive agents by focusing on the role of verapamil in hypertension. Verapamil is a phenylalkylamine derivative which antagonises calcium influx through the slow channels of vascular smooth muscle and cardiac cell membranes. By reducing intracellular free calcium concentrations, verapamil causes coronary and peripheral vasodilation and depresses myocardial contractility and electrical activity in the atrioventricular and sinoatrial nodes. Verapamil is well suited for the management of essential hypertension since it produces generalised systemic vasodilation resulting in a marked reduction in systemic vascular resistance and, consequently, blood pressure. Evidence from clinical studies supports the role of oral verapamil as an effective and well-tolerated first-line treatment for the management of patients with mild to moderate essential hypertension. Clinical studies have shown that verapamil is more effective the higher the pretreatment blood pressure and some authors have found a more pronounced antihypertensive effect in older patients or in patients with low plasma renin activity. Sustained release verapamil formulations are available for oral administration which, as a single daily dose, are as effective in lowering blood pressure over 24 hours as equivalent doses of conventional verapamil formulations given 3 times daily. As a first-line antihypertensive agent, oral verapamil is equivalent to several other calcium antagonists, beta-blockers, diuretics, angiotensin-converting enzyme (ACE) inhibitors and other vasodilators, and is not associated with many of the common adverse effects of these treatments. Verapamil may be preferred as an alternative first-line antihypertensive treatment to diuretics in elderly patients because it has similar efficacy in these patients without causing the adverse effects commonly linked with diuretic treatment. Furthermore, because verapamil does not cause bronchoconstriction, it may be used in preference to beta-blockers in patients with asthma or chronic obstructive airway disease. Reflex tachycardia, orthostatic hypotension or development of tolerance is not evident following verapamil administration. As a second- or third-line treatment for patients refractory to established antihypertensive regimens, verapamil produces marked blood pressure reductions when combined with diuretics and/or ACE inhibitors, beta-blockers and vasodilators such as prazosin.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacokinetic characterization of the antiarrhythmic drug diprafenone in man

The pharmacokinetics of the antiarrhythmic drug diprafenone have been investigated in 6 healthy volunteers following single intravenous (50 mg) and oral doses (50 and 150 mg). Diprafenone was mainly eliminated by metabolism in the liver. Following i.v. infusion of 50 mg diprafenone, the terminal half-life of elimination was 1.50 h, the volume of distribution at steady-state was 1.23 l.kg-1, and the free fraction in plasma was 1.68%. Mean total plasma clearance was 741 ml.min-1.70 kg-1, which approaches normal liver blood flow after correction for the blood/plasma concentration ratio. Thus, diprafenone can be classified as a high extraction drug. Following oral administration, a dose-dependent increase in bioavailability from 10.9 (50 mg dose) to 32.5% (150 mg dose) was observed. The data suggest that diprafenone is subject to saturable hepatic first-pass metabolism.

Pharmacokinetics of calcium antagonists under development

Calcium antagonist drugs under clinical development are of the Type I (verapamil, diltiazem-like) and Type II (nifedipine-like) classes. Tiapamil, the only Type I drug currently available, is a high clearance, widely distributed drug which undergoes extensive presystemic elimination. Pharmacokinetically it is quite similar to verapamil; however, it does have increased biliary excretion and decreased binding to plasma proteins. Eight Type II (dihydropyridine) drugs are reviewed. Seven of these drugs (felodipine, isradipine, nicardipine, nilvadipine, nimodipine, nisoldipine and nitrendipine) are pharmacokinetically similar to nifedipine, with high clearance, extensive distribution, and significant presystemic elimination. Amlodipine has lower clearance, even greater peripheral distribution, and greatly decreased presystemic elimination. Three of the 8 dihydropyridines have been reported to have plasma protein binding greater than 90%. Unlike nifedipine, each dihydropyridine drug under development has an asymmetric centre; therefore each in fact is a racemic mixture. Human pharmacokinetic and pharmacodynamic data have not been reported for any of the racemates. Each of the drugs has been studied in patients with hepatic and renal disease. Predictably, patients with severe hepatic disease have decreased presystemic clearance and, in some cases decreased clearance after intravenous administration of the dihydropyridines, although renal failure has little influence on their pharmacokinetics. Unfortunately, disease-drug interaction studies of this class of drugs do not generally report plasma protein binding. The effect of age on the disposition of 2 of the dihydropyridines has been reported; however, only for nicardipine can a conclusion be drawn, namely that volume of distribution may increase with age and clearance may remain unchanged. A variety of potential drug-drug interactions have been evaluated, most commonly the effect of these drugs on cardiac glycoside disposition and effect, and the effect of cimetidine on the disposition of dihydropyridines. Tiapamil, like verapamil, impairs digoxin clearance significantly. Among the dihydropyridines, although minor pharmacokinetic effects have in some cases been reported, the magnitude of the interactions suggest they have limited clinical importance. From drugs currently under development, it is clear that a large number of calcium antagonists will soon be introduced into clinical use. Only 1 of the newer drugs, amlodipine, has significant pharmacokinetic differences from the agents currently in use, although possible pharmacodynamic differences among the drugs have been suggested.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacokinetics and pharmacodynamics of two formulations of verapamil

The pharmacokinetics and pharmacodynamics of sustained release verapamil were compared with the conventional formulation in 10 healthy adult volunteers after single and multiple dosing. The mean time of maximum plasma concentrations of verapamil were significantly prolonged and the absorption rate constants significantly reduced after sustained release verapamil on both day 1 and day 10. On day 10 there was no significant difference between formulations in the relative bioavailability of verapamil. However, the area under the plasma concentration-time curve and maximum concentration (Cmax) for both formulations increased significantly with repeat dosing. On day 10, the difference in Cmax between formulations was significant. The day 10 mean peak/trough plasma verapamil concentration ratio was significantly less following the sustained release dose form. The mean PR interval was significantly prolonged by both formulations on day 1 and day 10. There were no differences between formulations other than a significantly longer PR interval following the conventional formulation 2 h after dosing on day 10.

Clinical pharmacokinetics of verapamil, nifedipine and diltiazem

The calcium antagonists diltiazem, nifedipine and verapamil are widely used in the treatment of coronary heart disease, arterial hypertension, certain supraventricular tachyarrhythmias and obstructive hypertrophic cardiomyopathy. During recent years their pharmacokinetic properties and metabolism have been studied in more detail. Although these 3 calcium antagonists exhibit great diversity in chemical structure, they exhibit common pharmacokinetic properties. These drugs are extensively metabolised and only traces of unchanged drugs are eliminated in urine. Their systemic plasma clearances are high and dependent on liver blood flow. Therefore, their bioavailabilities (diltiazem 40 to 50%; nifedipine 40 to 50%; verapamil 10 to 30%) are low despite almost complete absorption following oral administration. During long term treatment, oral clearance decreases and bioavailability increases due to saturation of hepatic first-pass metabolism. Pronounced intra- and inter-individual variations in clearance and bioavailability are observed. In patients with liver cirrhosis the various pharmacokinetic parameters are grossly altered. Clearance decreases, elimination half-life is substantially prolonged, and bioavailability more than doubles. In addition, the volume of distribution increases. Whereas renal disease has no impact on the pharmacokinetics of diltiazem and verapamil, elimination half-life of nifedipine increases in relation to the degree of renal impairment due to an increase in volume of distribution. Systemic clearance, however, remains unchanged. The data so far available indicate that the plasma concentrations of these drugs correlate with both their electrophysiological and haemodynamic effects. However, no effective therapeutic plasma concentration range has been firmly established. As reliable clinical end-points are available for dose titration of calcium antagonists, it is doubtful whether therapeutic drug monitoring will be of great value. Calcium antagonists are often administered in combination with a variety of other drugs. Thus, the potential for both pharmacodynamic and pharmacokinetic drug interaction exists. The interaction between digoxin and these drugs is of clinical importance. Verapamil and diltiazem cause a significant increase in plasma digoxin concentrations. In contrast, nifedipine does not lead to a significant increase in the plasma digoxin concentration. The mechanism responsible for this interaction is inhibition of both renal and non-renal digoxin clearance.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of oral verapamil therapy on antipyrine clearance

The influence of chronic verapamil treatment on antipyrine elimination was studied in eight angina patients. Antipyrine half-life (mean +/- s.d.) was 13.1 +/- 1.15 h at the start of therapy and 16.6 +/- 3.05 h (P less than 0.05) during chronic oral administration of verapamil (80-120 mg four or three times daily for 4 to 7 months). There was a significant decrease in antipyrine clearance (mean +/- s.d, 43.2 +/- 16.8 ml min-1 vs 28.7 +/- 16.6 ml min-1, P less than 0.01) while the change of distribution volume was insignificant. Verapamil elimination was also found to be impaired after chronic dosing as compared to single administration. Half-lives measured from the concentration vs time and urinary excretion rate vs time curves were both prolonged and oral clearance was decreased. Our results suggest that the inhibition of drug-metabolizing enzymes accounts for the impairment of verapamil elimination on chronic administration.

Comparative single dose and steady-state pharmacokinetics of bevantolol in young and elderly subjects

The pharmacokinetics of bevantolol were examined following single and repeated oral doses to young and elderly volunteers. Following administration of a single 200-mg bevantolol tablet mean maximum plasma bevantolol concentrations in young and elderly subjects were 1690 ng/ml and 1810 ng/ml, respectively. Maximum bevantolol concentrations occurred approximately 1.1 h postdose in both young and elderly subjects. There were no significant differences in mean steady state bevantolol concentrations on Day 14 between young and elderly subjects. However, disproportionate increases in Cmax, and in AUC, but not in tmax values were observed between Days 1 and 14. On Days 1 and 14, most young and elderly subjects exhibited monoexponential decline in bevantolol plasma concentrations after absorption phase. In those subjects Day 14 elimination half-lives in young and elderly were 1.9 and 2.2 h, respectively. In subjects who exhibited biexponential decline in bevantolol, an age effect in elimination became apparent, on Day 14 elimination half-lives were 5.7 and 11.2 h in young and elderly subjects, respectively. Bevantolol Metabolite III concentrations were observed in plasma of some subjects during the first 6 h after dosing. At steady-state AUC (0-ldc) values for the metabolite were less than 2% those of bevantolol. Bevantolol plasma levels accumulate to a small extent with repeated 200 mg daily doses. This is probably due to the contribution of a late and more persistent terminal elimination phase that was discernable in only certain individuals.