Acute and chronic pharmacodynamic interaction of verapamil and digoxin in atrial fibrillation

Validation of a Drug Transporter Probe Cocktail Using the Prototypical Inhibitors Rifampin, Probenecid, Verapamil, and Cimetidine

Background and Objective

A novel cocktail containing four substrates of key drug transporters was previously optimized to eliminate mutual drug–drug interactions between the probes digoxin (P-glycoprotein substrate), furosemide (organic anion transporter 1/3), metformin (organic cation transporter 2, multidrug and toxin extrusion protein 1/2-K), and rosuvastatin (organic anion transporting polypeptide 1B1/3, breast cancer resistance protein). This clinical trial investigated the effects of four commonly employed drug transporter inhibitors on cocktail drug pharmacokinetics.

Methods

In a randomized open-label crossover trial in 45 healthy male subjects, treatment groups received the cocktail with or without single oral doses of rifampin, verapamil, cimetidine or probenecid. Concentrations of the probe drugs in serial plasma samples and urine fractions were measured by validated liquid chromatography-tandem mass spectrometry assays to assess systemic exposure.

Results

The results were generally in accordance with known in vitro and/or clinical drug–drug interaction data. Single-dose rifampin increased rosuvastatin area under the plasma concentration–time curve up to the last quantifiable concentration (AUC_0–tz) by 248% and maximum plasma concentration (C_max) by 1025%. Probenecid increased furosemide AUC_0–tz by 172% and C_max by 23%. Cimetidine reduced metformin renal clearance by 26%. The effect of single-dose verapamil on digoxin systemic exposure was less than expected from multiple-dose studies (AUC_0–tz unaltered, C_max + 22%).

Conclusions

Taking all the interaction results together, the transporter cocktail is considered to be validated as a sensitive and specific tool for evaluating transporter-mediated drug–drug interactions in drug development.

Clinical Trial Registration

EudraCT number 2017-001549-29.

Electronic supplementary material

The online version of this article (10.1007/s40262-020-00907-w) contains supplementary material, which is available to authorized users.

Rate control in permanent atrial fibrillation

Guidelines on the use of digoxin are inconsistent with evidence from randomised trials

Identification of severe potential drug-drug interactions using an Italian general-practitioner database

OBJECTIVE

To analyze prescriptions in a general-practitioner database over 1 year to determine the frequency, the characteristics, and the monitoring of the severe potential drug-drug interactions (DDIs).

METHODS

We retrospectively analyzed the clinical records from 16 general practitioners in the Veneto region, an area in northern Italy. The study covered the period from January 1 to December 31, 2004. We selected all severe and well-documented interactions according to the book Drug Interaction Facts by David S. Tatro (Facts and Comparisons, St. Louis, MO, 2006). We grouped severe potential DDIs according to their specific potential risk, and for the most frequently interacting drug pairs, we investigated whether some specific tests had been prescribed by physicians for safety monitoring.

RESULTS

During the study period, 16,037 patients (55% female) with at least one drug prescription were recorded, and a total of 185,704 prescriptions relating to 1,020 different drugs were analyzed. Ramipril was the most frequently prescribed drug followed by acetylsalicylic acid and atorvastatin. The final number of different types of severe potential DDIs was 119, which occurred 1,037 times in 758 patients (4.7% of the total number of patients). More than 80% of drugs involved in severe potential DDIs were cardiovascular drugs. Digoxin was the most frequently involved drug. Electrolyte disturbances, increase in serum digoxin levels, risk of hemorrhage, severe myopathy or rhabdomyolysis, and cardiac arrhythmias were the most commonly implicated potential risks. When considering patients using digoxin with loop or thiazide diuretics for more than 5 months, 72% had at least one test to monitor potential digoxin toxicity, whereas 28% had no tests. Sixty-four percent of patients using digoxin with amiodarone, verapamil, or propafenone had an ECG and/or digoxin monitoring, and 36% of them did not have any tests.

CONCLUSIONS

The present study revealed that, in a group of Italian general practitioners, the risks of severe potential drug interactions are relatively low and the drugs concerned are few. Analyses of specific tests showed that physicians are generally aware of the potential risks caused by digoxin drug associations. However not all patients were closely monitored and this should be improved.

P-glycoprotein system as a determinant of drug interactions: the case of digoxin-verapamil

Digoxin, which has a very narrow therapeutic window, is one of the most commonly prescribed drugs in the treatment of congestive heart failure. In some cases of atrial fibrillation digoxin is used in combination with verapamil. Verapamil can increase the plasma concentration of digoxin up to 60-90%. So far the precise mechanism of this pharmacokinetic drug-drug interaction is not known. Many studies suggest that verapamil reduces the renal clearance of digoxin. The energy-dependent membrane-bound transport enzyme, P-glycoprotein, may also be involved. Reports from oncology research show that verapamil can interact with P-glycoprotein as a modulator. Also taking into account that digoxin, like many anticancer drugs, is a substrate for P-glycoprotein, it is likely that P-glycoprotein modulation accounts for the digoxin-verapamil interaction. Current knowledge suggest that the non-competitive digoxin-verapamil interaction is due to inhibition of P-glycoprotein activity by verapamil resulting in a decreased renal tubular elimination of digoxin.

Pharmacologic management of atrial fibrillation

In an era when many electrophysiologic problems are routinely treated with invasive procedures or implantable devices, drugs remain the cornerstones of treatment for atrial fibrillation. Atrial fibrillation may present as an episodic rhythm in patients who are primarily in sinus rhythm or it may be manifested as rhythm disorder that is permanent. Patients who appear to have an episodic rhythm disorder may be found to be in atrial fibrillation permanently when followed for long periods of time, and prognosis in the two forms is similar. It is, therefore, useful to consider them different manifestations in the same spectrum of disease. This review will address pharmacologic approaches designed to: (1) slow ventricular response; (2) restore sinus rhythm; (3) reduce occurrences of atrial fibrillation; and (4) prevent thromboembolic complications. Nonpharmacologic approaches to treating atrial fibrillation will be briefly reviewed.

Control of heart rate during transition from intravenous to oral diltiazem in atrial fibrillation or flutter

We tested whether patients presenting with atrial fibrillation (AF) or flutter (AFl) with a rapid ventricular response could maintain control of heart rate while transferring from a bolus and continuous infusion of intravenous diltiazem to oral diltiazem. Forty patients with AF or AFI and sustained ventricular rate > or = 120 beats/min received intravenous diltiazem "bolus" (20 to 25 mg for 2 minutes) and "infusion" (5 to 15 mg/hour for 6 to 20 hours). Oral long-acting diltiazem (diltiazem CD 180, 300, or 360 mg/24 hours) was administered in patients in whom stable heart rate control was attained during constant infusion. Intravenous diltiazem infusion was discontinued 4 hours after the first oral dose, and patients were monitored during 48 subsequent hours of "transition" to oral therapy. Response to diltiazem was defined as heart rate <100 beats/min, > or = 20% decrease in heart rate from baseline, or conversion to sinus rhythm. Other rate control or antiarrhythmic medications were not allowed during the study period. Thirty-seven of 40 patients maintained heart rate control during the bolus, and 35 of the remaining 37 maintained control during the infusion of intravenous diltiazem. Of the 35 patients achieving heart rate control with intravenous diltiazem who entered the transition to oral therapy, 27 maintained heart rate control (response rate of 77%/, 95% confidence interval 63% to 91%). The median infusion rate of intravenous diltiazem was 10 mg/hour, and the median dose of oral diltiazem CD was 300 mg/day. Oral long-acting diltiazem was 77% effective in controlling ventricular response over 48 hours in patients with AF or AFl in whom ventricular response was initially controlled with intravenous diltiazem.

Substantial excretion of digoxin via the intestinal mucosa and prevention of long-term digoxin accumulation in the brain by the mdr 1a P-glycoprotein

1. We have used mice with a disrupted mdr 1a P-glycoprotein gene (mdr 1a (-/-) mice) to study the role of P-glycoprotein in the pharmacokinetics of digoxin, a model P-glycoprotein substrate. 2. [3H]-digoxin at a dose of 0.2 mg kg-1 was administered as a single i.v. or oral bolus injection. We focussed on intestinal mucosa and brain endothelial cells, two major pharmacological barriers, as the mdr 1a P-glycoprotein is the only P-glycoprotein normally present in these tissues. 3. Predominant faecal excretion of [3H]-digoxin in wild-type mice shifted towards predominantly urinary excretion in mdr 1a (-/-) mice. 4. After interruption of the biliary excretion into the intestine, we found a substantial excretion of [3H]-digoxin via the gut mucosa in wild-type mice (16% of administered dose over 90 min). This was only 2% in mdr 1a (-/-) mice. Biliary excretion of [3H]-digoxin was not dramatically decreased (24% in wild-type mice versus 16% in mdr 1a (-/-) mice). 5. After a single bolus injection, brain levels of [3H]-digoxin in wild-type mice remained very low, whereas in mdr 1a (-/-) mice these levels continuously increased over a period of 3 days, resulting in a approximately 200 fold higher concentration than in wild-type mice. 6. These data demonstrate the in vivo contribution of intestinal P-glycoprotein to direct elimination of [3H]-digoxin from the systemic circulation and to the pattern of [3H]-digoxin disposition, and they underline the importance of P-glycoprotein for the blood-brain barrier.

Calcium antagonists in the elderly. A risk-benefit analysis

Calcium antagonists are effective in lowering blood pressure, relieving anginal symptoms and improving exercise tolerance in older and younger patients with coronary artery disease. Verapamil and diltiazem are effective in slowing ventricular response rates to supraventricular arrhythmias in both older and younger patients. Although they belong to at least 3 distinct chemical classes, a moderate decrease in the clearance of all calcium antagonists occurs with aging. Most clinical trials of these drugs have used the same dosages in older and younger patients, confounding analyses of sensitivity in older compared with younger patients. Greater reductions in blood pressure usually occur in older compared with younger patients receiving the same dosages of calcium antagonists; similarly, the dosage required to reduce blood pressure to a certain level is usually lower in older compared with younger patients. Drug acquisition costs are generally higher for calcium antagonists than for beta-blockers or diuretics. Compared with younger patients, greater heart rate suppression may be seen in older patients treated with verapamil and diltiazem; conversely, heart rate increases are usually seen with dihydropyridines. Calcium antagonists have not been shown to provide long-term benefits or decreased morbidity or mortality in elderly patients with hypertension. Verapamil, but not dihydropyridines, decreases mortality after myocardial infarction in patients without congestive heart failure. Calcium antagonists have not been shown to be beneficial in the treatment of acute stroke. Adverse effects, such as a postural hypotension, may be more frequent in elderly compared with younger patients. In addition, the elderly are at greater risk for drug interactions with calcium antagonists due to the higher likelihood that they are receiving other drugs.

The drug treatment of atrial fibrillation

1. Atrial fibrillation is an inefficient cardiac rhythm associated with impaired exercise tolerance, exertional dyspnoea, palpitation and a substantial risk of thromboembolism. 2. The first decision in management is to consider cardioversion which can be achieved in suitable cases electrically, or pharmacologically with a class Ic antiarrhythmic drug like flecainide or propafenone. 3. Prophylaxis in paroxysmal atrial fibrillation is best achieved with a class Ic drug or a class III drug such as sotalol or amiodarone. 4. Control of ventricular rate in chronic atrial fibrillation can be achieved by pharmacological manipulation of the atrioventricular node by digoxin alone, or in combination with the calcium channel blockers verapamil or diltiazem, or beta-adrenoceptor blockers with intrinsic sympathomimetic activity like pindolol or xamoterol. 5. In view of the considerable risk of thromboembolism in patients with chronic atrial fibrillation anticoagulation or at least treatment with aspirin should be considered.

Neuropsychiatric considerations in the treatment of hypertension

Algorithms have been developed to guide the treatment of simple hypertension. The basic algorithms are modified in the face of concurrent medical conditions, taking into account the various pharmacological effects of antihypertensive agents. This article reviews the neuropsychiatric effects of the major classes of antihypertensive agents (ganglionic blockers, centrally acting agents, diuretics, vasodilators, beta-blockers, calcium channel blockers and angiotensin converting enzyme inhibitors). The purported efficacy of some antihypertensive agents in the treatment of psychiatric conditions is also discussed. Beneficial as well as adverse neuropsychiatric effects are reviewed. In this way, guidelines for the treatment of hypertension are suggested which take into account a broad spectrum of neuropsychiatric considerations.

Acute myocardial uptake of digoxin in humans: correlation with hemodynamic and electrocardiographic effects

Acute myocardial uptake of digoxin was measured at a constant paced heart rate (75 beats/min) for 30 min after an intravenous bolus injection of 500 micrograms of digoxin in 14 patients with ischemic heart disease. Myocardial digoxin content, determined by serial measurement of aortocoronary sinus digoxin concentration gradients and coronary sinus blood flow, was expressed relative to coronary sinus blood flow at rest and correlated with simultaneous hemodynamic and electrocardiographic changes. Myocardial digoxin uptake was extensive (4.1 +/- 0.7% of total injected dose at 30 min) and prolonged, with rapid initial uptake (75.3 +/- 6.6% of maximum at 3 min), followed by a variable phase of slower accumulation. Peak left ventricular positive first derivative of left ventricular pressure (dP/dt) increased progressively (p less than 0.01), with a similar time course to that of myocardial digoxin accumulation; maximal change was 18.5 +/- 4.7% at 27 min. The ratio of inotropic effect to myocardial digoxin content did not vary significantly over the period of the experiment. However, peak inotropic effects in individual patients were not significantly related to peak myocardial digoxin content. The spontaneous PR interval increased transiently, with a peak increase of 5.9 +/- 1.8% (p less than 0.05) 12 min after digoxin administration. It is concluded that after intravenous bolus administration, 1) peak effects of digoxin on atrioventricular (AV) conduction occur early, whereas positive inotropic effects increase progressively for greater than or equal to 27 min; and 2) digoxin accumulation in the human myocardium is prolonged and is a determinant of inotropic effects, but not of prolongation of AV node conduction.

Lack of effect of enoximone on the pharmacokinetics of digoxin in patients with congestive heart failure

The aim of this study was to investigate if the concomitant administration of the positive inotropic drug enoximone (100 mg tid) has any effect on the morning through levels of the cardiac glycoside digoxin in 17 patients with congestive heart failure (NYHA II-IV). Plasma concentrations of digoxin were 1.05 +/- 0.37 ng/mL before enoximone, 0.95 +/- 0.31 ng/mL at the end of the enoximone treatment period of 1 week and 0.95 +/- 0.36 ng/mL 1 week after cessation of enoximone treatment. Thus, concomitant administration of enoximone had no effect on plasma concentrations of digoxin while on the other hand the hemodynamics as assessed by NYHA-classification and determination of the heart volume improved significantly.

The electrophysiological effects of alpha-chloralose anesthesia in the intact dog: (1) alone and (2) in combination with verapamil

The electrophysiological effects of alpha-chloralose anesthesia were determined in 13 chronically instrumented dogs and compared to baseline electrophysiological parameters in the conscious state. Alpha-chloralose anesthesia (75 mg/kg of a 4% solution in polyethylene glycol (PEG) delayed conduction and prolonged refractoriness of the AV node: (1) the P-R interval increased from 108 +/- 14 msec (mean +/- SD) in the conscious state to 125 +/- 23 msec (P less than 0.02); (2) the A-H from 98 +/- 12 msec to 108 +/- 16 msec (P less than 0.04); (3) the AV nodal effective refractory period from 136 +/- 16 to 153 +/- 29 msec (P = .05) and the AV nodal functional refractory period from 232 +/- 58 to 247 +/- 46 msec (P = 0.07); and (4) the AV block cycle length from 228 +/- 54 msec to 248 +/- 43 msec (P less than 0.04). Chloralose anesthesia also increased the ventricular refractory period from 139 +/- 13 msec to 161 +/- 22 msec (P less than .03) and the QTc interval from 273 +/- 22 to 306 +/- 32 msec (P less than 0.0002). To determine whether these effects on AV nodal conduction would influence experimental results, responses to verapamil were studied in the conscious state and during chloralose anesthesia. During chloralose anesthesia, (1) no relationship was detected between the sinus cycle length and verapamil concentrations; (2) a greater increment in AV conduction time was seen for a given verapamil concentration; and (3) AV block occurred at verapamil concentrations associated with 1:1 conduction in the conscious state. We conclude that chloralose anesthesia has significant electrophysiological effects and that these effects must be taken into consideration during the interpretation of experiments performed in animals during chloralose anesthesia.

Low- and medium-dose diltiazem in chronic atrial fibrillation: comparison with digoxin and correlation with drug plasma levels

The safety and efficacy of diltiazem were compared with digoxin maintenance therapy for control of ventricular response in 19 patients with chronic atrial fibrillation. The relationship between drug plasma levels and cardiovascular effects was also investigated. After 7 days of combined therapy with diltiazem (60 mg three times a day in 10 patients and four times a day in nine patients) and digoxin (0.125 mg/day in two patients and 0.250 mg/day in 17 patients), the 24-hour mean heart rate derived from ambulatory ECG recording was reduced by 16.3% in comparison with digoxin therapy alone; the serum digoxin level was not significantly changed (1.06 +/- 0.43 vs 1.05 +/- 0.61 ng/ml). After a standardized bicycle exercise test (50 watts for 3 minutes), maximal heart rate was reduced by 19.9%, diastolic blood pressure was decreased by 8.9%, and systolic pressure-rate product was decreased by 12.5%. Diltiazem plasma levels (mean 120.9 +/- 63.3 ng/ml) were linearly correlated with percentage variations in maximal heart rate, diastolic blood pressure, systolic blood pressure, and pressure-rate product during exercise. Eighteen patients in succession discontinued digoxin therapy; after 14 days of diltiazem alone, the 24-hour mean heart rate returned to control values of digoxin therapy, whereas maximal heart rate and pressure-rate product during exercise were significantly reduced (-17.2% and -14.1%, respectively), with no changes in blood pressure. Diltiazem plasma levels (135.0 +/- 83.2 ng/ml) showed a linear correlation with the percentage of reduction in maximal heart rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Digoxin-felodipine interaction in patients with congestive heart failure

A possible interaction between felodipine and digoxin was studied in 23 patients with congestive heart failure before and after 8 weeks treatment with both drugs. A modest, non-significant increase in serum digoxin level 2 h postdose (+15%) was found in the felodipine group (n = 11) compared to placebo (n = 12), with no change in the trough and 6 h postdose levels. There was a bimodal distribution of the observed changes in serum digoxin level 2 h postdose: a significant increase (p less than 0.001) was observed only in patients with a high plasma felodipine level, which may have been caused by changes in the absorption rate in those patients. Changes in the elimination of digoxin after felodipine therapy appeared unlikely, since the trough and 6 h post-dose levels were unchanged. Analysis of the clinical characteristics, haemodynamics and laboratory values revealed no significant differences between the subgroups. The observed increase in serum digoxin warrants monitoring the trough and peak levels digoxin in patients with congestive heart failure who are also being treated with felodipine.

Diltiazem and exercise performance in patients with chronic atrial fibrillation

To evaluate the influence of calcium entry blockade (diltiazem 60 mg qid) on exercise capacity in patients with chronic atrial fibrillation, nine men (mean age 65 years) with atrial fibrillation underwent maximal treadmill exercise on and off diltiazem therapy. Heart rate, blood pressure, and measured ventilatory parameters were assessed at a standard submaximal workload (3.0 mph/0% grade), the gas exchange anaerobic threshold (ATge), and maximal exercise. Significant reductions in heart rate at all stages of exercise were demonstrated: maximum heart rate decreased from 171 +/- 30 beats/min to 142 +/- 27 beats/min (17 percent, p less than .01) and submaximal exercise heart rate decreased from 123 +/- 22 beats/min to 96 +/- 16 beats/min (22 percent, p less than .01). However, there were no significant changes in blood pressure or gas exchange data, ie, oxygen uptake, minute ventilation, or respiratory exchange ratio at any of the exercise workloads. These data demonstrate that in patients with chronic atrial fibrillation, diltiazem controls the ventricular rate response throughout exercise without attenuating blood pressure or exercise capacity.

Comparative effects of nadolol-digoxin combination therapy and digoxin monotherapy for chronic atrial fibrillation

In some patients with chronic atrial fibrillation, treatment with digitalis alone may fail to produce a satisfactory decrease in heart rate at rest or during exercise or emotional stress. Findings of a few clinical studies suggest that beta blockade in combination with digitalis therapy may be of benefit in these patients. In a randomized, double-blind, placebo-controlled, parallel-group, 8-week study of 32 patients with chronic atrial fibrillation, the effects of digoxin therapy alone were compared with a combination of digoxin and nadolol. Criteria for entry into the study included ventricular rate at rest greater than or equal to 80/min or greater than or equal to 120/min with exercise, and serum digoxin levels within the therapeutic range. After digoxin dose titration to produce therapeutic levels, digoxin dosage remained constant throughout the balance of the study. After a 2-week, single-blind placebo lead-in period, patients were randomized to receive either digoxin plus placebo or a combination of digoxin and nadolol. The dose of nadolol/placebo was titrated from 20 to 120 mg daily as tolerated. Twenty-four hour ambulatory electrocardiographic (Holter) recordings, symptom-limited exercise treadmill tests and serum digoxin and nadolol levels were obtained at the end of the single and double-blind treatment periods. Comparing endpoint with baseline, results from Holter recordings showed that patients treated with a combination of digoxin and nadolol had significant (p less than 0.001) decreases in 24 hour average (78 +/- 4 to 63 +/- 3).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of beta-adrenergic blockade on exercise performance in patients with chronic atrial fibrillation

Beta-adrenergic blocking agents are commonly used in combination with digitalis to control excessive heart rate during exercise in patients with chronic atrial fibrillation. However, little is known about the effect of beta-adrenergic blockade on exercise capacity in these patients. Accordingly, a randomized, double-blind, cross-over placebo-controlled study was performed to assess the efficacy of celiprolol, a new cardioselective beta-blocker with partial intrinsic sympathomimetic activity, on exercise performance in nine men with chronic atrial fibrillation. All but one patient was receiving maintenance digitalis during the study. Heart rate, blood pressure and gas exchange variables were measured at rest and during treadmill exercise testing while the patients were receiving maintenance celiprolol or placebo. Significant reductions in heart rate and systolic blood pressure compared with control values were observed at submaximal exercise, at the gas exchange anaerobic threshold and at maximal exertion while the patients were taking celiprolol. However, oxygen uptake at the gas exchange anaerobic threshold during celiprolol therapy was 12.3 versus 14.0 ml oxygen/kg per min during placebo administration (a 12% difference, p less than 0.01). Similarly, oxygen uptake at maximal exertion during celiprolol therapy was 17.6 versus 21.0 ml/kg per min during placebo administration (a 16% difference, p less than 0.01). Treadmill time was also reduced during the celiprolol phase compared with placebo (11.3 versus 10.3 minutes; a 19% difference, p less than 0.01). These results indicate that in patients with atrial fibrillation the major beneficial effects of beta-adrenergic blockade--reduced submaximal and maximal exercise heart rate and blood pressure--must be weighed against the decrease in exercise capacity.

Pharmacokinetic drug interactions between digoxin and antiarrhythmic agents and calcium channel blocking agents: an appraisal of study methodology

While preliminary screening for interactions between new cardiovascular pharmacotherapeutic agents and digoxin can be efficiently and safely conducted in normal healthy volunteers, it is particularly important to detect and quantify drug interactions in patients with varying degrees of cardiac, hepatic and/or renal dysfunction. Much of the previously published literature provides only minimal data to guide clinical practice because of limitations of study design including sample size and measurement techniques. Important factors that determine the ability of a particular study design to detect a drug interaction with digoxin include the accuracy and precision of the assay method for serum digoxin concentrations, intrasubject and intersubject variability in serum digoxin concentration, and sample size. The format of the trial (chronic versus single digoxin dosing in cardiac patients; chronic versus single digoxin dosing in normal subjects) and the method of assessment of alterations in digoxin handling (formal determination of digoxin clearance, comparison of multiple or single digoxin measurements during various phases of trial) also impact greatly on the clinical relevance of such investigations. Guidelines for future studies of drug interactions with digoxin in cardiac patients are proposed with particular emphasis on laboratory methods; measurement techniques during baseline, placebo, and active drug phases; calculation of the statistical power of the study; time course of the trial; and assessment of the clinical significance of the findings.

Efficacy of oral diltiazem to control ventricular response in chronic atrial fibrillation at rest and during exercise

Although digoxin is often the first choice for control of ventricular response in chronic atrial fibrillation, it fails to slow exercise rates. Diltiazem, a calcium channel antagonist that slows atrioventricular conduction, was administered to 16 patients who failed to achieve adequate rate control on low level exercise testing despite digoxin therapy. Therapeutic response to diltiazem was assessed with submaximal and maximal exercise tests and 24 hour ambulatory electrocardiographic monitoring. During the diltiazem treatment phase, ventricular response at rest diminished (96 +/- 17 versus 69 +/- 10 beats/min, p less than 0.001) as did rate during submaximal exercise (155 +/- 28 versus 116 +/- 26, p less than 0.001), maximal exercise (163 +/- 14 versus 133 +/- 26, p less than 0.001) and average ventricular response during 24 hour monitoring (87 +/- 13 versus 69 +/- 10, p less than 0.001). Rate at rest decreased 26 +/- 15% and submaximal exercise rate diminished 24 +/- 12%. Thirteen (81%) of the 16 patients exhibited at least 15% slowing of rate at rest and during submaximal exercise. Eleven patients (69%) reported alleviation of symptoms. There was no change in serum digoxin levels during diltiazem treatment (1.3 +/- 0.5 versus 1.3 +/- 0.6 ng/ml, p = NS). On withdrawal of diltiazem, ventricular response returned to baseline values. Diltiazem is an effective agent for control of ventricular response, both at rest and during exercise, in digoxin-treated patients with chronic atrial fibrillation.

Verapamil-induced changes in digoxin kinetics in cirrhosis

The influence of a single low dose of verapamil (80 mg) on the serum levels of digoxin (single dose of 0.5 mg) was studied in 6 patients with hepatic cirrhosis and in 6 healthy volunteer controls. In the cirrhotic patients verapamil increased the peak serum level and the total AUC of digoxin by 98% and 32%, respectively. There was an associated 23% decrease in the renal digoxin clearance. In normal subjects only marginal alterations in digoxin kinetics were observed following verapamil administration. The results indicate that cirrhosis magnifies the influence of verapamil on digoxin kinetics.

No increase in serum digoxin concentration with high-dose diltiazem

The effect of incremental diltiazem dosing during concomitant digoxin administration over a four-week period in eight healthy adult volunteers (mean age, 28 +/- 4 years) was studied. The study group received 0.25 mg of digoxin twice daily for two days, after which they received 0.25 mg daily for the duration of the study. Following baseline electrocardiographic evaluation and measurement of trough digoxin levels, all subjects received 120 mg of diltiazem daily for one week, then 240 mg daily for one week, followed by 360 mg daily for one week. Resting electrocardiographic parameters (heart rate, P-R interval), renal function, electrolyte values, and digoxin and diltiazem concentrations were measured weekly. Daily administration of 360 mg of diltiazem plus 0.25 mg of digoxin resulted in a significant decrease in heart rate (from 68 +/- 9 beats per minute to 61 +/- 10 beats per minute; p less than 0.05), a marginal increase in P-R interval (from 169 +/- 22 msec to 179 +/- 21 msec; p = 0.08), and no significant change in trough serum digoxin concentration (from 0.85 +/- 0.08 ng/ml to 0.90 +/- 0.08 ng/ml; p = NS). The administration of up to 360 mg of diltiazem per day with 0.25 mg of digoxin per day was not associated with significant increases in serum digoxin concentrations in healthy subjects.

Clinical digoxin toxicity in the aged in association with co-administered verapamil. A report of two cases and review of the literature

Digoxin toxicity occurs more commonly in aged than younger individuals. Cardioactive drugs such as quinidine effect digoxin pharmacokinetics so as to increase the potential for digoxin toxicity. The calcium-channel antagonists have become extensively used for cardiac disorders and are often co-administered with digoxin. Despite documented calcium-channel antagonist interactions with digoxin, clinically significant digoxin toxicity associated with their concurrent use is apparently unusual. Two elderly patients receiving digoxin and verapamil simultaneously are presented to demonstrate the clinical importance and potential danger of the concomitant use of these drugs.

The influence of diltiazem hydrochloride on trough serum digoxin concentrations

A significant drug interaction between verapamil and digoxin, resulting in elevated serum digoxin concentrations, has been well documented in the medical literature. However, a similar interaction between digoxin and the calcium channel blockers nifedipine and diltiazem has not been conclusively established. This study investigated the influence of diltiazem hydrochloride on trough serum concentrations of concurrently administered digoxin in eight healthy volunteers. During the control phase of the study, volunteers were administered digoxin 0.25 mg/d for 13 days, and subsequently judged to be at steady state by serial determinations of digoxin serum concentrations. Twenty-four hour urine collections were done for creatinine clearance and urinary digoxin clearance determinations. Phase II of the study involved the addition of diltiazem hydrochloride 30 mg qid to the on-going, daily regimen of digoxin. After 14 days of concomitant therapy, steady-state trough digoxin concentrations were again determined, as well as creatinine clearances and urinary digoxin clearances. This investigation demonstrates that concomitant administration of diltiazem hydrochloride with digoxin results in significantly elevated steady-state trough digoxin concentrations (0.32 +/- 0.07 ng/ml increasing to 0.48 +/- 0.06 ng/ml, p less than 0.01). Urinary digoxin clearance decreased from 223.5 +/- 35.7 ml/min to 153.4 +/- 17.5 ml/min (p less than 0.05). Creatinine clearances were unaltered. A review of the current literature on this topic is included.

Evaluation of the pharmacodynamic and pharmacokinetic interaction between calcium antagonists and digoxin

Therapeutic uses of calcium antagonists have expanded to include not only ischemic heart disease but arrhythmias, systemic hypertension, congestive heart failure, and various pulmonary and gastrointestinal diseases. Many patients receiving a calcium antagonist concomitantly receive digoxin. Although the potential interaction between these agents has frequently been investigated, literature reports are confusing and inconsistent. We summarized the pharmacodynamics, pharmacokinetics, and mechanisms of interaction to help clinicians evaluate the potential calcium antagonist-digoxin interaction.

Digitalis: its place in therapy

Since the initial introduction of digitalis 200 years ago by Withering, its low therapeutic ratio has limited the use of this agent. The utility of digitalis in patients with congestive heart failure and a recent myocardial infarction has been questioned recently. Findings of rigorously controlled clinical studies suggest a small but definite hemodynamic and clinical improvement in patients administered digitalis. Congestive heart failure can be effectively treated without cardiac glycosides. However, when used judiciously, digitalis provides an additional agent in our therapeutic armamentarium. The inotropic, dormotropic, and vagomimetic properties are uniquely suited for the patient with supraventricular arrhythmias and compromised left ventricular function.

Digoxin and nifedipine

Many investigators have studied the potential interactions between calcium-channel antagonists and digoxin. Digoxin is usually well absorbed, and its excretion is dependent on renal mechanisms, primarily glomerular filtration. Several studies have reported a decrease in digoxin clearance and an increase of approximately 50% in digoxin levels when verapamil was added to digoxin therapy. Because renal digoxin clearance was decreased but no concomitant change in creatinine clearance was shown, the presumed major mechanism for decreased renal digoxin clearance is an alteration in renal tubular secretion of digoxin. Although an early report described a digoxin-nifedipine interaction, several subsequent studies have shown no significant changes in digoxin kinetics during nifedipine administration. Four studies found no significant decrease in creatinine clearance of digoxin during nifedipine administration. Thus significant changes in glomerular filtration are unlikely. Physiologic endpoints were measured by 2 groups describing a digoxin-nifedipine interaction and, although there was an increase in serum digoxin concentration, no changes were found in electrophysiologic correlates. Thus, if a digoxin-nifedipine interaction does exist, steady-state digoxin levels might increase from 24 to 45% when nifedipine therapy is added. Studies to date have involved small numbers of subjects with and without cardiac disease and have used different study protocols. Nonetheless, little evidence exists for any clinically significant increase in physiologic effects and no adverse effects have been found in patients receiving combined nifedipine and digoxin.

Digitalis and the autonomic nervous system

Digitalis produces many of its effects in intact animals and human beings by modifying the properties of the autonomic nervous system. The parasympathetic limb of the autonomic nervous system is most sensitive to these effects of digitalis, and its properties are significantly altered with therapeutic concentrations of the drug. These actions are particularly important in mediating the electrophysiologic effects of digitalis. With toxic concentrations of digitalis, stimulation of sympathetic nerve activity may also occur. This latter action may be involved in the arrhythmogenic effects of digitalis. These effects of digitalis on the autonomic nervous system play a major role in determining the pharmacodynamic actions of the drug in patients. The effects of digitalis on the autonomic nervous system also provide a setting for important interactions with other drugs that modify the properties of the sympathetic and parasympathetic nervous systems.

Effect of diltiazem on renal clearance and serum concentration of digoxin in patients with cardiac disease

The effect of diltiazem on digoxin serum concentration was evaluated in 9 patients who had been treated chronically for heart disease with digoxin, 0.25 mg/day. The indications for digoxin therapy were arrhythmias in 5 patients and mild heart failure in the other 4. Renal digoxin clearance was also evaluated in 8 of these patients. Serum digoxin concentration was measured at control, 7 +/- 2 days after initiation of 120 mg/day of diltiazem and 11 +/- 5 days after increasing the dose of diltiazem to 240 mg/day. Serum digoxin concentration was 0.9 +/- 0.4 ng/ml at control, 0.8 +/- 0.4 ng/ml with 120 mg/day of diltiazem, and 0.8 +/- 0.3 ng/ml during therapy with 240 mg/day. The differences between these values were not significant. Renal digoxin clearance also did not show a significant change after diltiazem therapy (44 +/- 15 ml/min before diltiazem and 46 +/- 13 ml/min with 240 mg/day of diltiazem). This study shows no effect of diltiazem in doses of 120 to 240 mg/day on serum digoxin concentration or renal digoxin clearance in patients who are treated chronically for heart disease with digoxin. In this dose range, diltiazem has advantages over verapamil, which markedly elevates digoxin levels.

Diltiazem, verapamil, and quinidine in patients with chronic atrial fibrillation

The comparative effects of diltiazem and verapamil in 30 patients with long-standing atrial fibrillation were evaluated in a prospective clinical trial. After a one- to two-day washout period during which drugs other than digoxin were withdrawn, patients were randomly assigned to diltiazem or verapamil treatment groups. All therapy was double blind. Both drugs were given in ascending doses as follows: days 1-6 (part I): diltiazem, 180 mg/d, or verapamil, 240 mg/d; days 7-12 (part II): diltiazem, 360 mg/d, or verapamil, 480 mg/d. Patients failing to convert to sinus rhythm after 12 days had dosage reduced to 180 mg/d of diltiazem or 240 mg/d of verapamil, and quinidine, 750 mg/d, was coadministered for another six days (part III). Medication compliance was verified by frequent measurement of serum drug concentrations. Three verapamil patients dropped out during part I due to adverse reactions (dyspnea, pulmonary congestion, skin rash, or hepatotoxicity). The higher dosage of either verapamil or diltiazem in part II was not well tolerated, and in eight patients part III had to be initiated early due to symptomatic bradycardia. Only one patient in the diltiazem group converted to sinus rhythm, whereas five converted with verapamil (two with verapamil alone, three when combined with quinidine). Thus, diltiazem and verapamil alone are unlikely to convert atrial fibrillation to sinus rhythm. The combination of verapamil and quinidine, however, is a potentially useful pharmacologic approach, having converted atrial fibrillation to sinus rhythm in nearly 50% of patients.

Calcium channel blockers that prolong the QT interval

The development of a group of calcium channel blockers that prolong the QT interval has raised interest in their possible proarrhythmic potential. In animal experiments lidoflazine has no observed arrhythmogenic potential and no antiarrhythmic potential in a model sensitive to the antiarrhythmic properties of both conventional and investigational agents. This lack of antiarrhythmic potential reduces the possible proarrhythmic potential even further, since it has been my observation that many of the more potent antiarrhythmic agents may facilitate arrhythmias in a considerable percentage of the most severely ill patients. Bepridil has both slow channel blocking properties and fast channel blocking effects and thus possesses an antiarrhythmic effect beyond the prolongation of repolarization. Even though early studies with humans showed the drug to be no more effective than the conventional reference agent procainamide, its potential cannot be fully assessed by these preliminary studies. The assessment of arrhythmogenicity is determined by use of the technique of PES. This technique has been shown effective in identifying patients with electrical instability and in identifying the effectiveness of antiarrhythmic agents in preventing VT and sudden death. Thus it seems probable that PES studies may be effective in predicting which drug or agent would facilitate arrhythmias. Alcohol, caffeine, cigarette smoke, and catecholamines facilitate PES induction of VT, lending support to the hypothesis that PES studies can be used to identify proarrhythmic drugs. In fact, patients who experience cardiac arrest on a class I agent can have VT provoked on that agent but not on antiarrhythmic drug therapy. PES thus identifies the proarrhythmic effect of the drug in that particular patient.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics of calcium-entry blockers

Effective use of drugs in therapy depends not only on clinical acumen but also on the availability of relevant pharmacokinetic and pharmacodynamic data. Such information assists in development of safe dosing regimens, prediction of abnormal handling of drugs in states of disease and disorder and anticipation of drug interactions. For the calcium-entry blocking agents now available in the United States (verapamil, nifedipine and diltiazem), these data appeared well after clinical patterns of use evolved. Nonetheless, their relevance continues to be demonstrated by the dependence of each agent on intact liver blood flow and function for normal rates of elimination; by the nonlinear kinetic characteristics for verapamil and diltiazem (and probably for nifedipine, as well) and the derivative implications for decreased dosing frequency requirements; and by observations now appearing on the relation between plasma drug levels and drug effects, both therapeutic and toxic. Such data are discussed herein, with emphasis on those aspects that impact on the clinical use of the calcium-entry antagonists.

Calcium antagonists--drug interactions

Evaluations of drug interactions should be done with caution. One needs to be aware of the reported interactions and apply the information on an individual basis. This review may therefore serve as a guide to the more common drug interactions and when drug therapy should be monitored closely in clinical practice. Major drug interactions with calcium antagonists are summarized in Table 2.

Effects of nadolol on the spontaneous and exercise-provoked heart rate of patients with chronic atrial fibrillation receiving stable dosages of digoxin

Nadolol, a long-acting beta-adrenergic-blocking agent, was evaluated in 20 patients with chronic atrial fibrillation by means of a randomized, double-blind, crossover study. Patients were required either to demonstrate resting heart rates in excess of 80 bpm or to show a rate of 120 bpm or an increment of greater than 50 bpm during mild treadmill exercise provocation (3 minutes, 1.75 mph, 10% grade). With placebo the group averaged a heart rate of 92 +/- 19 bpm, determined by 24 hours of ambulatory ECG recordings; this rate was significantly reduced to 73 +/- 16 bpm (p less than 0.001) with nadolol (mean dosage, 87 +/- 43 mg/day). During standardized exercise testing, heart rates increased to 153 +/- 26 bpm with placebo and to 111 +/- 24 bpm with nadolol (p less than 0.001), representing 65% and 52% increments, respectively. Digoxin blood levels averaged 0.8 +/- 0.5 ng/ml with placebo and were similar with nadolol (0.9 +/- 0.4; p = NS). Total exercise time on a modified Bruce treadmill protocol was 466 +/- 143 seconds with placebo and was significantly decreased by nadolol (380 +/- 143; p less than 0.01). During initial dose titration with nadolol, one patient was dropped from study for intolerable fatigue and one for worsened claudication. No patients were dropped from the double-blind treatment periods, although two patients receiving nadolol and one patient receiving placebo complained of moderate fatigue. We conclude that nadolol is a safe and effective agent for the control of spontaneous and exercise-provoked heart rates in patients with chronic atrial fibrillation who were already receiving digoxin treatment.

Studies on the possible mechanisms of lidoflazine arrhythmogenicity

Lidoflazine is a calcium channel blocking agent that is effective and safe in the treatment of angina pectoris, but has been reported to be associated with sudden death when administered for the treatment of supraventricular arrhythmias. Studies were performed in dogs to determine if lidoflazine caused a rise in serum digoxin concentration that could cause arrhythmias or if it was directly arrhythmogenic. Dogs received chronic injections of digoxin and then digoxin in combination with lidoflazine. No increase in digoxin concentration was found. Dogs also underwent programmed electrical stimulation while not receiving medications and then after incremental doses of lidoflazine administered intravenously. Lidoflazine did not cause spontaneous ventricular tachycardia and did not lower the threshold of ventricular tachycardia induction. Combined administration of lidoflazine and digoxin did not facilitate arrhythmia induction. These studies do not support a digoxin-lidoflazine interaction or a direct arrhythmogenic action of lidoflazine.

Calcium channel blockers in emergency medicine

Diltiazem, nifedipine, and verapamil inhibit calcium entry into cells via different mechanisms with different pharmacologies. They display different relative effects on different cardiovascular functions, a complex interplay of direct actions and adrenergic reflexes. Peripheral arterial vasorelaxation causes adrenergic reflex activity which opposes their direct negative chronotropic, dromotropic, inotropic, and hypotensive actions. Verapamil's most potent activity is electrophysiologic, and nifedipine's effects are hemodynamic; diltiazem acts like a less-potent combination of verapamil and nifedipine. All three drugs are efficacious in angina. These three drugs may not be interchangeable in all patients, but individualization of therapy is possible. Future indications for calcium channel blocker therapy may include hypertrophic cardiomyopathy, cerebral vasospasm, migraine headaches, pulmonary hypertension, asthma, esophageal spasm, intestinal ischemia, Raynaud's phenomenon, dysmenorrhea, and premature labor.

Amiodarone-digoxin interaction. Clinical and experimental observations

Twenty-two patients were given amiodarone for refractory cardiac arrhythmias, and pre- and post-amiodarone serum digoxin levels were studied. The interval between pre- and post-amiodarone serum digoxin levels ranged from five days to nine months (mean interval, seven weeks). The mean (+/- SD) pre-amiodarone serum digoxin level was 1.0 +/- 0.4 ng/ml, and the post-amiodarone serum digoxin level was 1.9 +/- 0.8 ng/ml (p less than .001). To develop an animal model for study of the digoxin-amiodarone interaction, 18 pigs were given digoxin for a four-week period. Half of the animals were given amiodarone as well as digoxin for the last two weeks of the study. At the end of the initial two-week period, there was no difference in serum digoxin levels between the two groups. At the end of the second two-week period, the serum digoxin level in the group receiving digoxin alone was 0.6 +/- 0.2 ng/ml, and the serum digoxin level in the group receiving the digoxin and amiodarone was 1.2 +/- 0.6 ng/ml (p less than .01). These data confirm the presence of an amiodarone-digoxin interaction in man and show that the pig is an appropriate model for study of this clinical phenomenon in the animal laboratory.