Renal Tubular Secretion Of Digoxin Research Articles

Verapamil metabolites: potential P-glycoprotein-mediated multidrug resistance reversal agents.

Multidrug resistance in cancer chemotherapy frequently correlates with overexpression of the P-glycoprotein drug transporter. Attempts to reverse P-glycoprotein-mediated multidrug resistance with racemic verapamil or its less toxic (R)-enantiomer have been complicated by cardiotoxicity. The objective of this study was to investigate the effects of the major verapamil metabolite, norverapamil, as well as the PR-22 and D-620 metabolites, on P-glycoprotein-mediated drug transport. We measured the basolateral-to-apical fluxes of the P-glycoprotein substrates digoxin and vinblastine in the presence and absence of verapamil, (R)-norverapamil, (S)-norverapamil, racemic norverapamil, PR-22, or D-620 across confluent monolayers of Madin-Darby canine kidney (MDCK) cells that express P-glycoprotein on their apical membranes. Verapamil and norverapamil nonstereospecifically inhibited the renal tubular secretion of digoxin and vinblastine similarly in a dose-dependent manner. However, there was no decrease in the cellular accumulation of digoxin and vinblastine, suggesting that neither verapamil nor norverapamil prevent the substrates from entering the MDCK cells. Furthermore, the norverapamil metabolite P-22 also inhibited the secretion of these P-glycoprotein substrates. Our results suggest that the verapamil metabolites norverapamil and PR-22, which are less cardiotoxic than the parent compound, have comparable inhibitory abilities to verapamil (norverapamil greater than PR-22) and may be useful in reversing resistance to P-glycoprotein substrates.

From Bench to Bedside: Utilization of an In Vitro Model to Predict Potential Drug-Drug Interactions in the Kidney: The Digoxin-Mifepristone Example

Drug interactions are a common source of drug-induced toxicity. For drugs with narrow therapeutic windows, such as digoxin, an understanding of the potential mechanisms by which drugs might interact is essential to clinical practice. This article describes the utility of a renal tubular cell culture model in the prediction of drug interactions involving P-glycoprotein. Digoxin is a cardiac glycoside that undergoes active secretion in the renal tubules by the MDR1 (P-glycoprotein) drug efflux pump. Mifepristone (RU486) is a recently introduced abortifacient that is largely unstudied in terms of drug-drug interactions. The authors used an in vitro model to study the effects of mifepristone on the renal tubular secretion and cellular uptake of digoxin by Madin-Darby canine kidney (MDCK) cells. Mifepristone significantly inhibited the renal tubular secretion of digoxin (p = 0.0005), without interfering with its ability to enter the renal tubular cell. Similar results were found with the P-glycoprotein substrate vinblastine. The findings suggest that drug interactions may result if mifepristone is administered with P-glycoprotein substrates, highlighting the usefulness of this model in the study of not only common but also rare combinations of drugs.

From Bench to Bedside: Utilization of an In Vitro Model to Predict Potential Drug‐Drug Interactions in the Kidney: The Digoxin‐Mifepristone Example

Drug interactions are a common source of drug-induced toxicity. For drugs with narrow therapeutic windows, such as digoxin, an understanding of the potential mechanisms by which drugs might interact is essential to clinical practice. This article describes the utility of a renal tubular cell culture model in the prediction of drug interactions involving P-glycoprotein. Digoxin is a cardiac glycoside that undergoes active secretion in the renal tubules by the MDR1 (P-glycoprotein) drug efflux pump. Mifepristone (RU486) is a recently introduced abortifacient that is largely unstudied in terms of drug-drug interactions. The authors used an in vitro model to study the effects of mifepristone on the renal tubular secretion and cellular uptake of digoxin by Madin-Darby canine kidney (MDCK) cells. Mifepristone significantly inhibited the renal tubular secretion of digoxin (p = 0.0005), without interfering with its ability to enter the renal tubular cell. Similar results were found with the P-glycoprotein substrate vinblastine. The findings suggest that drug interactions may result if mifepristone is administered with P-glycoprotein substrates, highlighting the usefulness of this model in the study of not only common but also rare combinations of drugs.

A model for the prediction of digoxin-drug interactions at the renal tubular cell level.

Digoxin-drug interactions are relatively common causes of digitalis toxicity. Recently, the clinical importance of the renal tubular secretion of digoxin has been proven by documenting drug interactions at this level. The authors describe a model using cultured renal tubular cell monolayers that can be used to predict drug interactions with the cardiac glycoside. This model accurately documents known clinical digoxin interactions such as those with verapamil and propafenone. The common feature of these interactions is that they involve P-glycoprotein substrates (e.g., digoxin, vincristine, vinblastine) or inhibitors (e.g., quinidine, cyclosporine). In the case of the newly described interaction of digoxin with itraconazole, the model preceded the emergence of clinical cases.

P-glycoprotein-mediated renal tubular secretion of digoxin: The toxicological significance of the urine-blood barrier model

We provide direct evidence that verapamil inhibits active digoxin secretion in renal tubular cells (LLC-PK1), and that verapamil increases cellular accumulation of digoxin. These findings suggest that verapamil inhibits the digoxin active secretory transport at the apical membranes, supporting the theory that P-glycoprotein mediates digoxin secretion in the renal tubular cells. Based on existing data on digoxin transport, we present a hypothetical model for the renal handling of digoxin, implying that P-glycoprotein functions as a driving mechanism of a unidirectional “urine-blood” barrier.

Energy-dependent transport of digoxin across renal tubular cell monolayers (LLC-PK1).

Digoxin secretory transport across renal tubular cell monolayers (LLC-PK1) grown on permeable filters was characterized. Metabolic inhibitors reduced total and specific basolateral to apical (B-A) flux of digoxin and conversely increased the apical to basolateral (A-B) flux. The specific transport of digoxin from the basolateral to the apical compartment was saturable, with a maximum velocity of transport of 184.5 +/- 38.0 pmol.cm-2.h-1 and a Michaelis-Menten constant (Km) of 14.1 +/- 1.6 microM. In addition, B-A flux of digoxin resulted in accumulation of digoxin in the apical compartment against the concentration gradient. P-Glycoprotein inhibitors such as quinidine, verapamil, vincristine, and cyclosporine increased the net A-B flux and inhibited the total B-A flux without affecting the nonspecific flux significantly. Tetraethylammonium, a prototype substrate for an organic cation transport system, had no such effect. Our results suggest that digoxin undergoes transepithelial secretion by an energy-dependent, carrier-mediated process in renal tubules, a process that seems to be distinct from the tetraethylammonium transport system.

Clinical pharmacokinetic significance of the renal tubular secretion of digoxin.

Tubular secretion appears to be a major route of the renal elimination of digoxin. Secretion of the drug by the tubules is modulated by renal blood flow, by a number of commonly coadministered drugs (e.g. quinidine, spironolactone, verapamil, amiodarone), and by age. The maximal transport capacity does not appear to be achieved with clinically relevant concentrations. The tubular transport of digoxin does not appear to be associated with the anionic or cationic transport systems, nor the Na+/K+-ATPase receptor. Further studies are needed to elucidate the exact mechanisms involved in the transtubular movement of the glycoside.

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.

Pubertal changes in net renal tubular secretion of digoxin.

To evaluate the effect of puberty on the net renal tubular secretion of digoxin, we measured the ratio of digoxin clearance to creatinine clearance in 23 patients aged 4 to 21 yr and correlated this ratio with both sexual maturity (Tanner stage) and chronologic age. All subjects were at steady-state levels for digoxin treatment; all had normal serum creatinine values for age as well as normal serum potassium levels. Mean ratio for immature children (n = 14, Tanner 1 through 3.5) was 1.45 +/- 0.66. Mean ratio for mature adolescents (n = 9, Tanner 4 through 5) was 0.95 +/- 0.28. The difference between the two groups was significant (P less than 0.05). When patients were regrouped by age using either 13 or 15 yr as a cutoff, the difference in ratios was no longer statistically significant. Based on 45 subjects (new and from our previous study) aged 2 mo to 80 yr, there was a significant decrease in the clearance ratio with increasing age, but when the 23 subjects aged 4 to 21 yr were analyzed separately, the correlation between ratio and age was not significant. It appears that the decrease in net renal tubular secretion of digoxin from childhood to adulthood correlates better with full sexual maturation at puberty (Tanner 4 through 5) than with chronologic age. This observation may represent a developmental change in pharmacokinetics with broader significance than for digoxin disposition alone.

CHARACTERISTICS OF DIGOXIN INTERACTION WITH QUINIDINE, VERAPAMIL AND AMIODARONE: IN VIVO AND IN VITRO STUDIES

Recently several drugs which are commonly coadministered with digoxin have been shown to cause toxic accumulation of the cardiac glycoside. The interactions with quinidine and verapamil have been attributed in part to reduced renal clearance of digoxin. Since GFR is not altered it appears that renal tubular secretion of digoxin must be inhibited. We studied the effect of amiodarone on serum digoxin concentrations (SDC) in 10 children aged 0.5-18 yrs. There was a 68-800% increase in SDC (P<.025) when both drugs were used in normally recommended doses. Digoxin renal clearance was reduced by 10-44% without corresponding alteration in creatinine clearance. A similar interaction was demonstrated in rats. The addition of amiodarone 30 mg/kg/day caused SDC to increase from 0.68±0.08 (mean ± SD) to 11.22 ± 2.16 ng/ml (P<0.005). Subsequent studies in vitro have examined the effect of quinidine, verapamil, and amiodarone on the uptake of 125I digoxin in renal cortical slices of rats. Therapeutic concentrations of quinidine (5 ug/ml), verapamil (500 ng/ml), and amiodarone(2 ug/ml) caused significant inhibition of digoxin uptake by renal slices (26%, 15%, and 16% respectively) (P<.01). Quinidine and verapamil were shown to cause further dose-dependent inhibition of digoxin uptake. These studies support the suggestion that elevated SDC is induced by reduction in the tubular secretion of digoxin caused by quinidine, verapamil, and amiodarone. Whenever these drugs are added to digoxin therapy, follow-up assessment of SDC is mandatory.

Digoxin toxicity associated with amiodarone therapy in children

The addition of amiodarone to digoxin therapy in nine children caused a sharp increase in digoxin serum concentrations (68% to 800%) in the presence of preserved serum creatinine and BUN concentrations. Digoxin half-life was prolonged. Digoxin accumulation could be attributed in part to the decrease in the renal clearance of digoxin resulting from inhibited tubular secretion of the drug and to the reduction in the distribution volume of digoxin caused by amiodarone. Creatinine clearance was not affected by amiodarone. This interaction appears to be more acute in children than in adults, presumably because of the more important role of the renal tubular secretion of digoxin in children. Whenever digoxin and amiodarone therapy are combined, the digoxin serum concentration should be monitored carefully, with appropriate reduction of the digoxin dose.

Interactions between digoxin and potassium-sparing diuretics.

A kinetic and hemodynamic study of digoxin was performed in six healthy subjects and similar studies were performed during digoxin with spironolactone and with triamterene. Spironolactone reduced renal tubular secretion of digoxin and attenuated its positive inotropic effect (evaluated by systolic time intervals and echocardiography) and triamterene reduced the extrarenal elimination of digoxin, but induced no changes in digoxin-elicited inotrophy. It is suggested that the renal handling of digoxin is influenced by the intracellular potassium concentration in the renal tubular cell. The results indicate a drug-receptor interaction between spironolactone metabolites and digoxin at the hypothetical inotropic digitalis receptor. Amiloride has been reported to suppress digoxin inotropism, whereas spironolactone induces minor inhibition and triamterene does not affect digoxin inotropism.

Drug Interactions with Digoxin

Multiple drug administration may affect the actions of digoxin either by altering its myocardial effects (dose-response relationship or pharmacodynamic effect) or by altering the absorption, storage or excretion of the drug (a pharmacokinetic effect). The major pharmacodynamic interactions include potentiation of digitalis toxicity by potassium-wasting diuretics and by succinlycholine. β-Blocking drugs also increase the potential for digoxin to produce adverse sinus or A-V node dysfunction. The pharmacokinetic interactions can be divided into drug interactions which alter digoxin absorption and those which change renal excretion. Estimates of the former are usually based on either steady-state or single dose studies of bioavailability. These studies have demonstrated significant reduction of absorption of oral digoxin by simultaneously administered liquid antacids, cholestyramine, kaolin-pectin, and meals of high fibre content. Reduced digoxin absorption also occurs secondary to therapy with sulphasalazine, para-aminosalicylic acid, and neomycin, probably due to their effect upon gut wall function. Propantheline and diphenoxylate with atropine, which decrease gut motility, increase digoxin absorption, whereas metoclopramide, which increases motility, depresses absorption. The renal excretion of digoxin is not changed importantly by diuretics such as frusemide but is decreased by spironolactone, possibly due to inhibition of renal tubular secretion of digoxin. Quinidine produces dramatic persistent increases in digoxin serum levels, due to decreased renal clearance of the glycoside, without changing the glomerular filtration. In addition to this renal effect, quinidine also seems to decrease the volume of distribution of digoxin, which results in acutely altered digoxin levels even within the first day after quinidine administration. Alteration of the timing of the administration of digoxin from that of a second drug (e.g. as with liquid antacids, kaolin-pectin, or meals of high fibre content), or alteration of the dose of digoxin based on the anticipated change in kinetics (as with spironolactone, quinidine or medications altering gut function or motility), can improve clinical effectiveness of digoxin therapy. However, because of individual variation in the impact of these interactions, routine monitoring of digoxin serum levels is strongly recommended under such circumstances to enhance clinical efficacy and patient safety.

MATURATION OF RENAL TUBULAR TRANSPORT OF DIGOXIN

Previous data have suggested an age related increase in renal tubular secretion of digoxin in infants and children receiving long term digoxin therapy. This phenomenon could be the result of a maturational process or secondary to chronic substrate stimulation. To investigate this question two groups of 2 week old paired litter-mate rats received intraperitoneal injections of either digoxin or an equal volume of normal seline (control) an alternate days until sacrificed at 4, 6, and 8 weeks of age. An additional group of 12 week old rats were studied as controls. I125 digoxin uptake was measured in renal cortical slices as the CPM/mg wet tissue slice to medium ratio (S/M). Both digoxin treated and control rats demonstrated significant age related increment in digoxin uptake. S/M ratios at 4, 6, 8, and 12 weeks in the control group were (mean ± S.D) 1.34 ± 0.06, 1.39 ± 0.14, 1.62 ± 0.18 and 1.93 ± 0.25 respectively (r = 0.81, p < 0.001) but did not differ significantly at each age from those in the control group. I125 uptake was significantly reduced by both Dinitrophenol (DNP) and sodium azide, as well as by a 100% nitrogen atmosphere. These results indicate that renal tubular transport of digoxin is an age related energy dependent process, which probably is not subject to substrate stimulation.

Renal Tubular Secretion of Digoxin

To study the mechanism of the renal handling of digoxin, simultaneous measurements of digoxin and inulin clearances were performed in 13 digitalized patients with congestive heart disease. The renal digoxin clearances exceeded inulin clearances, indicating an active tubular secretion of digoxin. No evidence of tubular backward diffusion was disclosed. Treatment with spironolactone (Aldactone) decreased the digoxin clearances, suggesting an inhibition of the tubular secretion of digoxin in the distal segment of the renal tubulus. After blocking the tubular secretion, the calculated renal clearances of nonproteinbound digoxin in plasma were similar to that of inulin, suggesting a glomerular filtration of free plasma digoxin. Plasma digoxin level rose during treatment with spironolactone.