Amiodarone-digoxin interaction. Clinical and experimental observations

Effects of neferine on the pharmacokinetics of amiodarone in rats

Amiodarone, an iodinated benzofuran derivative with predominantly class III anti-arrhythmic effects, is used to treat supraventricular and ventricular arrhythmias. The purpose of this study was to assess the potential of neferine, an effective anti-pulmonary fibrosis drug isolated from the embryo of Nelumbo nucifera Gaertner's seeds, to alter the pharmacokinetic profile of amiodarone. Experimental Sprague-Dawley rats were randomly divided into two groups. In groups 1 and 2, amiodarone was given to rats by intragastric and intravenous administration, respectively, while neferine was co-administratered by intragastric administration. Blood samples were collected from the orbital venous plexus at indicated time points and were analyzed for amiodarone concentration using RP-HPLC. The geometric mean ratio for C(max) and AUC(0-96) was calculated. There were no significant differences between the pharmacokinetics parameters of amiodarone administered intravenously or intragastrically and the control (without neferine) group (with ratios of 0.7-1.4 in all experimental groups), suggesting that neferine had no effect on amiodarone plasma pharmacokinetics. The dosage regimen of amiodarone does not need to be taken into consideration when combined with neferine.

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.

Plasma digoxin concentration fluctuations associated with timing of plasma sampling and amiodarone administration

A 31-year-old man with dilated cardiomyopathy was hospitalized for new-onset atrial fibrillation. Oral amiodarone 600 mg/day was started to control his arrhythmia, and the patient continued to receive digoxin 0.125 mg/day, which was prescribed 4 days earlier at a heart failure clinic. The patient's digoxin plasma concentration peaked early on hospital day 3 at 2.93 ng/ml; digoxin was withheld. Over the next 3 days, the patient's digoxin plasma concentrations rose and fell daily. These fluctuations correlated with the timing of blood sampling in relation to oral amiodarone administration. The patient's renal function remained stable, and he developed no signs or symptoms of digoxin toxicity. To our knowledge, no case reports have associated significant fluctuations of digoxin plasma concentrations that correspond to the timing of oral amiodarone administration. Tissue-to-plasma redistribution appears to be a possible mechanism for this interaction, with the most significant effect occurring 8-10 hours after amiodarone administration. Clinicians should be aware that digoxin plasma concentrations may not correlate with digoxin tissue concentrations in this setting. When a loading dose of oral amiodarone is required in a patient receiving digoxin, the digoxin dosage should first be reduced, and digoxin therapy should be adjusted based on signs and symptoms of digoxin toxicity.

Role of Organic Anion Transporting Polypeptide 2 in Pharmacokinetics of Digoxin and β-Methyldigoxin in Rats

Recently, we found that potent P-glycoprotein (P-gp) inhibitors, such as verapamil and cyclosporin A, markedly modulated the pharmacokinetics of digoxin in rats, whereas they did not affect beta-methyldigoxin pharmacokinetics significantly. Digoxin is also a substrate of rat organic anion transporting polypeptide 2 (Oatp2). Here, we compared the magnitude of Oatp2-mediated drug interaction of digoxin and beta-methyldigoxin using amiodarone as an Oatp2 inhibitor in rats. Amiodarone (20 mg/kg) given intravenously significantly increased plasma levels and decreased biliary excretion, liver distribution, and intestinal distribution of digoxin administered intravenously at a dose of 10 mug/kg. Amiodarone also significantly decreased biliary excretion and liver distribution of beta-methyldigoxin, but the change in plasma levels of beta-methyldigoxin was quite small. These findings may give a clue in selecting these cardiac glycosides in clinical pharmacotherapy for patients receiving multiple drugs towards escape from Oatp2-mediated drug interactions.

Amiodarone Interaction Time Differences with Warfarin and Digoxin

Background:

Amiodarone has pharmacokinetic and pharmacodynamic interactions with various therapeutic agents. The mechanism of interaction between warfarin and amiodarone is the inhibition of warfarin metabolism by amiodarone, and that between digoxin and amiodarone is the inhibition of digoxin transport by amiodarone.

Objective:

To investigate the pharmacokinetic magnitude of the time differences between amiodarone–warfarin and amiodarone–digoxin interactions.

Methods:

Amiodarone was administered concomitantly to 79 inpatients who had been receiving fixed-maintenance doses of warfarin or digoxin. Seventy-seven inpatients were prescribed warfarin therapy, and 54 inpatients were prescribed digoxin therapy. To determine serum concentrations of the warfarin enantiomers digoxin, amiodarone, and desethylamiodarone blood samples were obtained with coadministration of amiodarone. Serum S- and R-warfarin, amiodarone, and desethylamiodarone concentrations were measured by HPLC methods, and serum digoxin concentrations were measured by a fluorescence polarization immunoassay.

Results:

A remarkable decrease of S-warfarin clearance was observed within approximately the first 2 weeks after coadministration of amiodarone. Only a small decrease in R-warfarin clearance was observed. Digoxin clearance was gradually decreased with time, and a good reverse correlation was obtained between amiodarone or desethylamiodarone concentrations and digoxin clearance.

Conclusions:

Relatively short-term monitoring of warfarin clearance is required when amiodarone is coadministered. Long-term monitoring of digoxin serum amiodarone and desethylamiodarone concentrations is necessary to detect the amiodarone–digoxin interaction.

Organic anion transporter oatp2-mediated interaction between digoxin and amiodarone in the rat liver

PURPOSE

The interaction between amiodarone and digoxin has been known to increase serum concentrations of digoxin in humans and rats. In this study, we assessed the molecular mechanism(s) of that drug interaction, focusing on digoxin transport mediated by P-glycoprotein (Pgp) and by rat liver organic anion transporter (oatp2).

METHODS

Digoxin transport by Pgp and oatp2 was assessed using Pgp-overexpressing transfectant LLC-GA5-COL150 monolayers and oatp2-expressing Xenopus oocytes, respectively. The digoxin uptake into the isolated rat hepatocytes was also examined.

RESULTS

Amiodarone (10 microM) inhibited slightly the transcellular transport of digoxin in LLC-GA5-COL150 monolayers, whereas itraconazole (10 microM), a potent Pgp inhibitor, markedly blocked the transport. The digoxin uptake by the isolated rat hepatocytes and by the oatp2-expressing Xenopus oocytes was decreased markedly in the presence of amiodarone but not in the presence of itraconazole. In addition, amiodarone inhibited the oatp2-mediated digoxin uptake in a competitive manner with an apparent inhibition constant value of 1.8 microM.

CONCLUSION

These findings suggest that rat oatp2 rather than Pgp may be one of the interaction sites for digoxin and amiodarone in the liver.

A review of class III antiarrhythmic agents for atrial fibrillation: maintenance of normal sinus rhythm

A noteworthy shift from class I to class III antiarrhythmic agents for suppression of atrial fibrillation has occurred. Sotalol, amiodarone, and dofetilide have been evaluated for their ability to maintain sinus rhythm in patients with chronic atrial fibrillation. All of these agents are moderately effective; however, amiodarone appears to be most efficacious. Aside from their common class III actions, these agents have profoundly different pharmacologic, pharmacokinetic, safety, and drug interaction profiles that help guide drug selection. Amiodarone and dofetilide are safe in patients who have had a myocardial infarction and those with heart failure. The safety of commercially available d,l-sotalol in these patients is poorly understood. Torsades de pointes is the most serious adverse effect of sotalol and dofetilide, and risk increases with renal dysfunction. Amiodarone has minimal proarrhythmic risk but has numerous noncardiac toxicities that require frequent monitoring. Overall, an ideal antiarrhythmic agent does not exist, and drug selection should be highly individualized.

Effect of dofetilide on the pharmacokinetics of digoxin

The effect of dofetilide on the steady-state pharmacokinetics of digoxin was evaluated in a randomized, double-blind study. Five days of dofetilide treatment did not significantly affect steady-state pharmacokinetic variables of digoxin compared with placebo; therefore, the use of dofetilide does not necessitate an adjustment in digoxin dose to maintain therapeutic digoxin levels.

Antiarrhythmic agents: drug interactions of clinical significance

The management of cardiac arrhythmias has grown more complex in recent years. Despite the recent focus on nonpharmacological therapy, most clinical arrhythmias are treated with existing antiarrhythmics. Because of the narrow therapeutic index of antiarrhythmic agents, potential drug interactions with other medications are of major clinical importance. As most antiarrhythmics are metabolised via the cytochrome P450 enzyme system, pharmacokinetic interactions constitute the majority of clinically significant interactions seen with these agents. Antiarrhythmics may be substrates, inducers or inhibitors of cytochrome P450 enzymes, and many of these metabolic interactions have been characterised. However, many potential interactions have not, and knowledge of how antiarrhythmic agents are metabolised by the cytochrome P450 enzyme system may allow clinicians to predict potential interactions. Drug interactions with Vaughn-Williams Class II (beta-blockers) and Class IV (calcium antagonists) agents have previously been reviewed and are not discussed here. Class I agents, which primarily block fast sodium channels and slow conduction velocity, include quinidine, procainamide, disopyramide, lidocaine (lignocaine), mexiletine, flecainide and propafenone. All of these agents except procainamide are metabolised via the cytochrome P450 system and are involved in a number of drug-drug interactions, including over 20 different interactions with quinidine. Quinidine has been observed to inhibit the metabolism of digoxin, tricyclic antidepressants and codeine. Furthermore, cimetidine, azole antifungals and calcium antagonists can significantly inhibit the metabolism of quinidine. Procainamide is excreted via active tubular secretion, which may be inhibited by cimetidine and trimethoprim. Other Class I agents may affect the disposition of warfarin, theophylline and tricyclic antidepressants. Many of these interactions can significantly affect efficacy and/or toxicity. Of the Class III antiarrhythmics, amiodarone is involved in a significant number of interactions since it is a potent inhibitor of several cytochrome P450 enzymes. It can significantly impair the metabolism of digoxin, theophylline and warfarin. Dosages of digoxin and warfarin should empirically be decreased by one-half when amiodarone therapy is added. In addition to pharmacokinetic interactions, many reports describe the use of antiarrhythmic drug combinations for the treatment of arrhythmias. By combining antiarrhythmic drugs and utilising additive electrophysiological/pharmacodynamic effects, antiarrhythmic efficacy may be improved and toxicity reduced. As medication regimens grow more complex with the aging population, knowledge of existing and potential drug-drug interactions becomes vital for clinicians to optimise drug therapy for every patient.

Pharmacokinetics and pharmacodynamics in the critically ill patient

1. Until recently, when drugs were used in critically ill patients they were expected to behave in the same way as in less seriously ill patients. Now the unpredictability of even the most reliable drugs has been recognized. With this there is an awareness of the adverse effects drugs may have on organs other than the ones the drug was intended to act on. In patients with multiorgan dysfunction, poly-pharmacy is usually needed. The drugs may not only interfere with the action of each other at the receptor and enzyme level, but may also change protein binding and elimination. All these effects may be unimportant in less seriously ill patients, but may affect outcome in the critically ill. A high degree of awareness and suspicion of unknown drug-induced adverse reaction is needed by clinicians and pharmacologists alike.

Treatment of digitalis intoxication with emphasis on the clinical use of digoxin immune Fab

Many studies and cases of digitalis intoxication have been reported since the time of William Withering's first publication in 1785. Recognition and management of digitalis toxicity is challenging. Before digoxin immune Fab was commercially available, treatment consisted of managing the signs and symptoms of toxicity until the digitalis was eliminated. Digoxin immune Fab offers a safe, effective, and specific method of quickly reversing digitalis toxicity. Factors that must be considered with the clinical use of this agent include the dosage calculation, administration technique, postdose monitoring, pharmacokinetics, mechanism of action, interference with commercially available digoxin assays, partial neutralizing dosing, rebound of free digoxin, and indications for use. For severe, life-threatening toxicity, digoxin immune Fab is the treatment of choice.

A general overview of amiodarone toxicity: its prevention, detection, and management

Although amiodarone is a highly effective antiarrhythmic agent, it has a high incidence of side effects, some of which can be serious or even lethal. With close monitoring, side effects can be found in essentially all patients, but fortunately most of these are mild and well tolerated. Furthermore, many will respond to dosage reduction in a relatively short period of time, ie, days to weeks, which is remarkable considering the long period of time amiodarone has been shown to persist in tissues. There is reasonable evidence that toxicity, particularly the early toxic manifestations with large loading dosages, can be favorably modified by reducing the dosage. Similarly, reducing the maintenance dosage will, in most instances, reduce or eliminate most toxic manifestations. The mechanisms of toxic effects are uncertain, but suggestive evidence exists for and against both an immunologic reaction and an intracellular lysosomal lipoidosis. Principles of use of amiodarone should include individualizing administration of dosages for each patient due to the unusual pharmacokinetic properties of this drug and continuous long-term attempts at using the lowest effective dosage. There are no definite tests that predict amiodarone efficacy or toxicity, but the serum level can be used as a rough guide of absorption and distribution in the attempt to minimize the maintenance dosage. No guidelines regarding screening tests for toxicity can be made at this time since great variability in these tests has been reported, and no evidence exists for their benefit in preventing adverse effects to amiodarone. However, follow-up testing at the intervals noted in the package insert are reasonable and important. The possibility of interactions with drugs already reported and with others not yet reported should always be kept in mind, and appropriate monitoring for clinical evidence of toxicity due to the concomitantly used drugs should be undertaken. Amiodarone can have a tremendous beneficial effect in the proper circumstances, but it is a drug that should command utmost respect because of its side effects and requires constant vigilance from any physician wishing to use it.

Pharmacokinetic interactions with digoxin

Numerous pharmacological agents have been shown to produce clinically significant pharmacokinetic interactions with digoxin. Drugs which reduce digoxin absorption include the antacids aluminium hydroxide, magnesium hydroxide and magnesium trisilicate, the antidiarrhoeals kaolin and pectin, the hypocholesterolaemic agent cholestyramine and the chemotoxins cyclophosphamide, vincristine and bleomycin. Certain antibiotics including sulphasalazine, neomycin and aminosalicylic acid reduce digoxin absorption while others, including erythromycin and tetracycline, increase the bioavailability of digoxin in some patients. Capsule preparations of digoxin in solution are less subject to several of the interactions which affect the absorption and bioavailability of digoxin tablets. Various drugs induce alterations in the volume of distribution and clearance of digoxin. Cardiac patients receiving digoxin therapy are particularly prone to interactions with commonly co-administered medications such as the antiarrhythmics quinidine and amiodarone, the calcium channel blockers verapamil and nifedipine, and possibly some vasodilating agents. Studies of digoxin interactions have yielded discrepant results, indicating the need for careful analysis of investigational design before arriving at clinical conclusions.

Concentration response relationships of amiodarone and desethylamiodarone

Twelve patients with frequent ventricular premature depolarizations (VPDs) received amiodarone, 600 mg/day, for up to 8 weeks. On days 0, 1, 4, 8, 15, 22, 36, and 57 of treatment, 24-hour ambulatory ECGs were obtained, and multiple blood samples were taken for determination of amiodarone and desethylamiodarone plasma concentrations. All patients had at least 75% suppression of VPDs. The mean duration of therapy before the onset of antiarrhythmic effect was 13.2 days (range 1 to 36 days). Trough amiodarone and desethylamiodarone plasma concentrations at the time of onset of antiarrhythmic effect were 0.86 +/- 0.48 mg/L and 0.23 +/- 0.15 mg/L, respectively. Sixty-seven percent of patients responded at amiodarone concentrations below 1.0 mg/L. For each patient there was a progressive decrease in frequency of VPDs as both amiodarone and desethylamiodarone concentrations increased. Regression modeling indicated that both amiodarone and desethylamiodarone plasma concentrations explained significant variability in the frequency of VPDs, and amiodarone and desethylamiodarone plasma concentrations were highly correlated with each other. There was a trend for desethylamiodarone to explain more variability in frequency of VPDs than amiodarone.

Effects of oral prazosin on total plasma digoxin levels

Prazosin and digoxin are frequently coadministered in clinical practice. To determine the effects of oral prazosin treatment on steady-state digoxin levels, 20 patients receiving a constant maintenance dose of digoxin, who had normal renal and liver functions and were not receiving any other treatment, were given 5 mg of prazosin for 3 days. Plasma digoxin levels were measured before, on days 1 and 3 of prazosin treatment, and after prazosin had been discontinued. It was found that prazosin significantly increased plasma digoxin levels. On discontinuation of prazosin digoxin levels returned to their previous values.

Interactions of amiodarone with digoxin in rats

1. The influence of oral amiodarone treatment on the blood and tissue concentrations of digoxin was investigated in the anaesthetized rat by use of unlabelled and [3H]-digoxin. 2. Amiodarone diminished the total body clearance and the apparent volume of digoxin distribution by 60%. This reduction was due to a 50% reduction of the hepatobiliary clearance, whereas the renal clearance did not change. 3. Amiodarone treatment increased blood, myocardial and skeletal muscle [3H]-digoxin concentrations by 200% indicating passive equilibration between blood and these tissues, and resulting in unaltered tissue to blood ratios. In contrast, the liver concentration increased by 70% only and the liver to blood ratio therefore decreased under amiodarone treatment. 4. It is concluded that the hepatobiliary elimination of digoxin is decreased in amiodarone-treated rats compared to controls and is responsible for the increased levels of blood and tissue glycoside.

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.

Treatment of adverse digitalis effects by hemoperfusion through columns with antidigoxin antibodies bound to agarose polyacrolein microsphere beads

Ten patients with an array of moderate to severe adverse effects resulting from digitalis were effectively treated by hemoperfusion through small columns which contained antidigoxin antibodies bound to polyacrolein microspheres in agarose macrospheres (APAMB). The procedure was well tolerated. There was no detectable damage to formed blood elements and no changes in electrolytes, liver enzymes, or other related biochemical parameters. Despite some theoretic considerations to the contrary, the removal of a relatively small load of digoxin resulted in amelioration of the clinical symptoms and ECG abnormalities associated with digitalis. No rebound phenomena of intoxication or posthemoperfusion increase in digoxin serum levels were noted over the subsequent 5 to 6 days. A further increase in the capacity of the columns may render this method a safe and convenient emergency procedure for patients with digitalis toxicity.

Pharmacokinetic interactions between digoxin and other drugs

Drug interactions with digoxin are important because of this agent's narrow therapeutic index. Among the drugs that can decrease digoxin bioavailability are cholestyramine, antacid gels, kaolin-pectate, certain antimicrobial drugs and cancer chemotherapeutic agents. In selected patients, antibiotics may enhance digoxin bioavailability by eliminating intestinal flora that metabolize digoxin. Antiarrhythmic drugs, such as quinidine and amiodarone, can markedly increase steady state serum digoxin levels. Certain calcium channel blocking drugs, particularly verapamil, have a similar effect. Potassium-sparing diuretic drugs, such as spironolactone, can alter digoxin pharmacokinetics. Indomethacin may decrease renal excretion of digoxin in preterm infants. Finally, rifampin, an antibiotic used in the treatment of tuberculosis, may lower steady state serum digoxin levels in patients with severe renal disease. Physicians must maintain constant vigilance whenever medications are added to or withdrawn from a therapeutic regimen that includes digoxin.