Interaction between digoxin and propafenone in children

Characterisation of (R/S)-propafenone and its metabolites as substrates and inhibitors of P-glycoprotein

Digoxin is a drug with a narrow therapeutic index, which is a substrate of the ATP-dependent efflux pump P-glycoprotein. Increased or decreased digoxin plasma concentrations occur in humans due to the inhibition or induction of this drug transporter in organs with excretory function such as small intestine, liver and kidney. It is well known that serum concentrations of digoxin increase considerably in humans if propafenone is given simultaneously. However, it has not been investigated in detail whether propafenone and its metabolites are substrates and/or inhibitors of human P-glycoprotein. The aim of this study, therefore, was to investigate the P-glycoprotein-mediated transport and inhibition properties of propafenone and its major metabolites 5-hydroxypropafenone and N-desalkylpropafenone in Caco-2 cell monolayers. Inhibition of P-glycoprotein-mediated transport by propafenone and its metabolites was determined using digoxin as a P-glycoprotein substrate. No polarised transport was observed for propafenone and N-desalkylpropafenone in Caco-2 cell monolayers. However, 5-hydroxypropafenone translocation was significantly greater from basal-to-apical compared with apical-to-basal (P(app) basal-apical vs. P(app) apical-basal, 10.21+/-2.63 x 10(-6) vs. 4.34+/-1.84 x 10(-6) cm/s; P<0.01). Moreover, propafenone, 5-hydroxypropafenone and N-desalkylpropafenone inhibited P-glycoprotein-mediated digoxin transport with IC(50) values of 6.8, 19.9, and 21.3 microM, respectively. In summary, whereas propafenone and N-desalkylpropafenone are not substrates of P-glycoprotein, 5-hydroxypropafenone is translocated by human P-glycoprotein across cell monolayers. In addition, propafenone and its two major metabolites 5-hydroxypropafenone and N-desalkylpropafenone are inhibitors of human P-glycoprotein and therefore contribute to the digoxin-propafenone interaction observed in humans.

Digoxin-carvedilol interactions in children

Digoxin is often coadministered with carvedilol in children with severe ventricular failure. In eight children (age 2 weeks to 8 years), the oral clearance of digoxin decreased by half with carvedilol, and two of them had digoxin toxicity. Carvedilol increases serum concentrations of digoxin in children, and its dose may need to be reduced to avoid toxicity.

Clinically significant drug interactions with cyclosporin. An update

Since its approval in 1983 for immunosuppressive therapy in patients undergoing organ and bone marrow transplants, cyclosporin has had a major impact on organ transplantation. It has significantly improved 1-year and 2-year graft survival rates, and decreased morbidity in kidney, liver, heart, heart-lung and pancreas transplantation. Several studies have supported the efficacy of cyclosporin in preventing graft-versus-host disease in bone marrow transplantation. Cyclosporin is also possibly effective in treating diseases of autoimmune origin and as an antineoplastic agent. The introduction of therapeutic drug monitoring of cyclosporin was extremely useful because of the wide inter- and intraindividual variability in the pharmacokinetics of cyclosporin after oral or intravenous administration. Optimal long term use of cyclosporin requires careful monitoring of the blood (or plasma) concentrations. Sustained and clinically significant drug-drug interactions can occur during long term therapy with cyclosporin. The coadministration of multiple drugs with cyclosporin could result in graft rejection, renal dysfunction or other undesirable effects. Any interaction that leads to modified cyclosporin concentrations is of potential clinical importance. Cyclosporin itself may have significant effects on the pharmacokinetics and/or pharmacodynamics of coadministered drugs, such as digoxin, HMG-CoA reductase inhibitors and antineoplastic drugs affected by multidrug resistance. Many drugs have been shown to affect the pharmacokinetics and/or pharmacodynamics of cyclosporin. Interactions between cyclosporin and danazol, diltiazem, erythromycin, fluconazole, itraconazole, ketoconazole, metoclopramide, nicardipine, verapamil, carbamazepine, phenobarbital (phenobarbitone), phenytoin, rifampicin (rifampin) and cotrimoxazole (trimethoprim/sulfamethoxazole) are well documented in a large number of patients. Other interactions (such as those with aciclovir, estradiol and imipenem) are documented only in isolated case studies.

New antiarrhythmic drugs in pediatric use: propafenone

Propafenone hydrochloride, a class 1C antiarrhythmic agent, combines sodium channel-blocking effects with beta-blocking capacities and a weak calcium antagonism. The drug exerts marked electrophysiologic effects on accessory atrioventricular pathways. In patients with atrioventricular nodal reentry tachycardia, propafenone is able to block conduction in the fast conducting pathway. In addition, propafenone is very effective in young patients with supraventricular tachycardia based on enhanced abnormal automaticity. In pediatric patients, left ventricular performance remains unimpaired. Proarrhythmic events have been noted in children only occasionally. In accordance with the electrophysiologic profile, intravenous and oral propafenone is an effective agent for treatment of supraventricular tachycardia based on a reentry mechanism and due to abnormal automaticity (i.e., supraventricular tachycardia based on an accessory atrioventricular pathway, atrioventricular nodal reentry tachycardia, junctional ectopic tachycardia, and atrial ectopic tachycardia). In children with ventricular dysrhythmias, efficacy seems to be related to the underlying cardiac diagnosis. Propafenone is well tolerated in the majority of young patients. Incidence of proarrhythmic events seems to be lower with propafenone than with other class 1C agents. However, the risk of these serious adverse events should be taken into account when therapy with propafenone is considered, particularly in patients with structural heart disease.

The effects of cardiac transplantation and cyclosporine therapy on digoxin pharmacokinetics

Most patients needing cardiac transplantation are treated with digoxin for heart failure. Because of its narrow therapeutic range, even recommended doses of digoxin may cause severe toxicity. Several drugs, including quinidine, amiodarone, verapamil, and propafenone can interact with digoxin, leading to toxic accumulation of the glycoside. The authors have recently reported two cases of severe digitalis toxicity after the initiation of cyclosporine treatment in patients awaiting cardiac transplantation. A preliminary study on two additional patients suggested that cyclosporine reduced the plasma clearance and volume of distribution of digoxin. To assess the mechanism of this interaction, the authors studied digoxin pharmacokinetics in patients awaiting cardiac transplantation and again after the surgery, during chronic cyclosporine therapy. To separate the effects of transplantation and cyclosporine on digoxin pharmacokinetics, pharmacokinetic studies were subsequently performed in dogs to allow controlled experimental conditions for evaluation of the digoxin-cyclosporine interaction.

Serum digoxin levels related to plasma propafenone levels during concomitant treatment

Nine patients with supraventricular rhythm disorders were treated during 5-day periods with different oral doses (300, 450, 600, and 900 mg daily) of propafenone concomitantly to long-term digoxin treatment. A poor correlation (r = .398; P less than .05) was obtained when the difference between the mean digoxin serum level (calculated with the Cmin data determined each of the 5 days) observed during a given propafenone dose and the mean digoxin serum level observed before propafenone treatment, was correlated with the dose of propafenone; but an evident correlation (r = .778; P less than .01) was found when the difference in digoxin level was correlated with the plasma propafenone concentration. The propafenone effect of increasing digoxin blood levels was thus concluded to be poorly dose dependent but strongly concentration dependent. The association of propafenone to a long-term digoxin treatment can be considered with a low risk of toxicity when plasma propafenone concentration does not exceed about 1000 ng/mL. Propafenone plasma levels are unpredictable in view of their wide interindividual variation for a given dose, so their measurement is advised to detect high levels and consequently to prevent a rise in digoxin serum concentrations with the possibility of toxicity. In clinical practice, when propafenone concentration determinations are not readily available, digoxin serum levels at least have to be carefully monitored.