Recent data point to the contribution of P-glycoprotein (P-gp) to digoxin elimination. On the basis of clinical observations of patients in whom digoxin levels decreased considerably when treated with rifampin, we hypothesized that concomitant rifampin therapy may affect digoxin disposition in humans by induction of P-gp. We compared single-dose (1 mg oral and 1 mg intravenous) pharmacokinetics of digoxin before and after coadministration of rifampin (600 mg/d for 10 days) in 8 healthy volunteers. Duodenal biopsies were obtained from each volunteer before and after administration of rifampin. The area under the plasma concentration time curve (AUC) of oral digoxin was significantly lower during rifampin treatment; the effect was less pronounced after intravenous administration of digoxin. Renal clearance and half-life of digoxin were not altered by rifampin. Rifampin treatment increased intestinal P-gp content 3.5 +/- 2.1-fold, which correlated with the AUC after oral digoxin but not after intravenous digoxin. P-gp is a determinant of the disposition of digoxin. Concomitant administration of rifampin reduced digoxin plasma concentrations substantially after oral administration but to a lesser extent after intravenous administration. The rifampin-digoxin interaction appears to occur largely at the level of the intestine. Therefore, induction of intestinal P-gp could explain this new type of drug-drug interaction.

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The role of intestinal P-glycoprotein in the interaction of digoxin and rifampin

Drug transport in the central nervous system is highly regulated not only by the blood-brain and the blood-cerebrospinal fluid barriers but also in brain parenchyma. The novel localization of drug transporters in brain parenchyma cells, such as microglia and astrocytes, suggest a reconsideration of the present conceptualization of brain barriers as it relates to drug transport. That is, the cellular membranes of parenchyma cells act as a second "barrier" to drug permeability and express transporters whose properties appear similar to those localized at the conventional brain barriers. This review will focus on the molecular characteristics, localization, and substrate specificities of several classes of well known membrane drug transporters (i.e., the organic cation, organic anion, nucleoside, P-glycoprotein, and multidrug resistance proteins) in the brain. Comparisons to similar transporters localized within the peripheral system and clinical implications of the functional expression of specific drug transport families will be discussed when appropriate. Nutrient and neurotransmitter transporters, whose characteristics have been reviewed extensively elsewhere, will not be considered in this review.

Drug Transporters in the Central Nervous System: Brain Barriers and Brain Parenchyma Considerations

1. The molecular and functional characterization of transport proteins is emerging rapidly and significant numbers of drugs have been shown to be substrates or inhibitors. The purpose of this review is to highlight the in vivo preclinical and clinical evidence that supports a role for transport proteins in attenuating the absorption, distribution and excretion (ADE) of drugs. 2. For absorption, a clear role has emerged for P-glycoprotein in limiting permeability across the gastrointestinal tract. As a result, a wide variety of drugs suffer from incomplete, variable and non-linear absorption. Similarly, at the blood-brain barrier a range of drugs has limited brain penetration due to P-glycoprotein-mediated efflux, which can limit therapeutic effectiveness of CNS agents. In the liver, transport proteins are present on the sinusoidal membrane that can be the rate-limiting step in hepatic clearance for some drugs. Mechanistic studies clearly suggest a key role and broad substrate specificity for the OATP family of sinusoidal transporters. Mainly ATP-dependent transport proteins such as P-glycoprotein and MRP2 govern active biliary excretion. 3. Drug-drug interactions have been demonstrated involving inhibition or induction of transport proteins. Clinically significant interactions in the gastrointestinal tract and kidney have been observed with inhibitors such as ketoconazole, erythromycin, verapamil, quinidine, probenecid and cimetidine. Clinically significant inhibition at the blood-brain barrier is more difficult to demonstrate, relying on pharmacodynamic and toxicodynamic changes, but an example is quinidine increasing loperamide-induced central effects in humans. 4. This review highlights the emerging role of transport proteins in ADE of drugs and suggests these need to be considered, in drug discovery and development, with respect to variability in drug disposition and response.

Role of transport proteins in drug absorption, distribution and excretion.

Hepatobiliary secretion of organic compounds; molecular mechanisms of membrane transport.

Objective
Our objective was to quantitate the contribution of the genetic polymorphism of the human MDR1 gene to the bioavailability and interaction profiles of digoxin, a substrate of P-glycoprotein.

Methods
The pharmacokinetics of digoxin was studied in 15 healthy volunteers, who were divided into 3 groups (n = 5 each) on the basis of genotyping for the MDR1 gene, in a 4-dose study after single doses of digoxin alone (0.5 mg orally and intravenously) and coadministered with clarithromycin (400 mg orally for 8 days). The dose of digoxin was reduced during the clarithromycin phase (0.25 mg orally and intravenously).

Results
The bioavailability of digoxin in G/G2677C/C3435, G/T2677C/T3435, and T/T2677T/T3435 subjects were 67.6% ± 4.3%, 80.9% ± 8.9%, and 87.1% ±8.4%, respectively, and the difference betweenG/G2677C/C3435 and T/T2677T/T3435 subjects was statistically significant (P <.05). The MDR1 variants were also associated with differences in disposition kinetics of digoxin, with the renal clearance being almost 32% lower in T/T2677T/T3435 subjects (1.9 ± 0.1 mL/min per kilogram) than G/G2677C/C3435 subjects (2.8 ± 0.3 mL/min per kilogram), and G/T2677C/T3435 subjects having an intermediate value (2.1 ± 0.6 mL/min per kilogram). Coadministration of clarithromycin did not consistently affect digoxin clearance or renal clearance. However, a significant increase in digoxin bioavailability was observed in G/G2677C/C3435subjects (67.6% ± 4.3% versus 85.4% ± 6.1%; P < .05) but not inthe other2 genotype groups.

Conclusion
The allelic variants in the human MDR1 gene are likely to be associated with altered absorption and/or disposition profiles of digoxin and P-glycoprotein-mediated drug interaction (Clin Pharmacol Ther 2002;72:209-19.)

Clinical Pharmacology & Therapeutics (2002) 72, 209–219; doi: 10.1067/mcp.2002.126177

Role of human MDR1 gene polymorphism in bioavailability and interaction of digoxin, a substrate of P-glycoprotein

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 μM. 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, ...

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

The effect of cationic drugs on the uptake of the prototypical organic cation N′-methylnicotinamide has been evaluated. Using purified brash border membrane vesicles prepared from dog kidney cortex and applying a rapid Millipore filtration technique, cationic drugs apparent inhibitory constants (Ki) were calculated from kinetic analysis of N′-methylnicotinamide uptake corrected for noncarrier-mediated transport (10 s uptake; outwardly directed H+ gradient; pH 7.4, 25 °C). All of the cationic drugs tested exhibited competitive inhibition of N′-methylnicotinamide uptake suggesting that they all share the organic base transport system at the renal proximal tubule. The Ki values were as follows, in order of decreasing apparent affinity: quinidine (0.7 μM), trimethoprim (1.3 μM), cimetidine (2.0 μM), famotidine (3.0 μM), quinine (7.0 μM), amiloride (5.8 μM), procainamide (21 μM), and nizatidine (30 μM). The different relative affinities of the drugs for the organic base transport system may explain the mutual ...

Inhibition kinetics of cationic drugs on N′-methylnicotinamide uptake by brush border membrane vesicles from the dog kidney cortex

Quinidine is known to inhibit the renal clearance of digoxin without affecting glomerular filtration rate. The renal interaction between these drugs was investigated by a combination of in vivo and in vitro methods. The uptake of digoxin by brush border membrane vesicles was not affected by quinidine. Similarly, digoxin did not inhibit the uptake of the cation N-methylnicotinamide by these vesicles and did not alter the binding kinetics of digoxin to the Na+, K+-adenosine triphosphatase by the antiluminal membrane vesicles. By using the in vivo multiple indicator dilution technique transtubular transport of digoxin was documented; renal-artery infusion of quinidine did not affect the recovery of digoxin in the renal vein or urine. Clearance studies documented that the decrease in the renal clearance of digoxin is paralleled by a significant fall in renal blood flow evidenced by a decrease in p-aminohippuric acid clearance. It is concluded that quinidine inhibits the renal excretion of digoxin not by competition at the tubular cell membrane level, but rather by decreasing renal blood flow. A parallel decrease in biliary clearance of digoxin is documented and may suggest a similar mechanism.

Effects of quinidine on the renal tubular and biliary transport of digoxin: in vivo and in vitro studies in the dog.

By performing single pass experiments in the dog, we could identify by thin layer chromatography a digoxin metabolite produced by the kidney. In vitro studies verify that this new species is produced by the brush border membrane of the renal tubular cell.

Melvin Silverman

Papers

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

Inhibition kinetics of cationic drugs on N′-methylnicotinamide uptake by brush border membrane vesicles from the dog kidney cortex

Effects of quinidine on the renal tubular and biliary transport of digoxin: in vivo and in vitro studies in the dog.

Evidence of digoxin metabolism by the kidney