Intestinal Absorption Of Digoxin Research Articles

Fructose-induced metabolic syndrome decreases protein expression and activity of intestinal P-glycoprotein

ObjectvesMetabolic syndrome (MetS) is a health disorder that increases the risk for cardiovascular complications such as heart disease and type 2 diabetes. Some drugs used in patients with MetS are substrates of intestinal P-glycoprotein (P-gp), one of the most important efflux pumps that limit the absorption of xenobiotics. Thus, their bioavailability could be affected by changes in this transporter. Because one of the major causes of MetS in humans is excessive sugar intake, the aim of this study was to evaluate the effect of a fructose-rich diet on intestinal P-gp activity and protein expression in male Sprague-Dawley rats. MethodsFructose-drinking animals received standard chow and 15% (w/v) fructose in the drinking water over 8 wk; control rats were fed on standard chow and tap water. ResultsIleal protein expression of P-gp was 50% lower in fructose-drinking rats than in control animals. This reduction was confirmed by immunofluorescence microscopy. These results correlated well with the decrease of about 50% in the transport rate of the substrate rhodamine 123 in everted intestinal sacs. Finally, an increase of 62% in the intestinal absorption of digoxin, a P-gp substrate used as therapeutic drug, was observed in vivo, in fructose-drinking animals. ConclusionThe present study demonstrated that MetS-like conditions generated by enhanced fructose intake in rats decreased the protein expression and activity of ileal P-gp, thus increasing the bioavailability of P-gp substrates.

Acetaminophen (AP) inhibits intestinal P‐glycoprotein (P‐gp) activity in vitro and in vivo.

AP induces long‐term regulation of expression of intestinal P‐gp. Whether it can acutely modulate P‐gp activity is unknown.AimsTo study the effect of AP on rat intestinal P‐gp activity in vivo and in vitro.M&MIn vitro. Intestinal everted sacs were filled with rhodamine 123 (serosal), a tipical P‐gp substrate, (R123; 4.5; 9; 12; 18 or 36 μM) and incubated in KH buffer (mucosal) alone or with AP (100 μM), and secretion of R123 was assessed along a 40‐min period. Serosal‐mucosal transport of AP was studied in a similar fashion, in the presence/absence of Psc 833 (10 μM), a specific inhibitor of P‐gp. Transport of AP (10 μM) was also evaluated in P‐gp knock down HepG2 cells. In vivo. Intestinal absorption of digoxin (Dig, 25.6 nmol/kg) was studied by portal vein sampling (30–min period) in the presence or absence of AP (100 μM).ResultsR123 transport adjusted well to a sigmoideal curve. Its Vmax was decreased by AP (P<0.05), with no modification in EC50 or slope. AP transport was not affected by PSC 833 or by knocking down P‐gp. In vivo intestinal absorption of Dig was increased (+214%) by AP (P<0.05).ConclusionAP inhibited P‐gp activity in vitro and in vivo. AP‐induced changes in Vmax of R123 transport, together with lack of AP transport by P‐gp, suggest that inhibition is unlikely competitive. Bioavailability and therapeutic efficacy of drugs, substrates of P‐gp, can be altered when they are co‐administered with AP.

Effect of polyoxyl 35 castor oil and Polysorbate 80 on the intestinal absorption of digoxin in vitro.

Surfactants are classically used to improve the solubilization of lipophilic drugs such as digoxin. Polysorbate 80 and Cremophor EL (polyoxyl 35 castor oil) are such surfactants but they may also modulate the action of P-glycoprotein, an energy-dependent "counter-transport" system implicated in the phenomenon of multidrug resistance in cancer cells. P-glycoprotein is also present in the intestine on the apical membrane of mature enterocytes and can potentially reduce the absorption of a wide range of drugs. In this study, using the improved everted gut sac method, the effects of Polysorbate 80, Cremophor EL and cyclosporin on the absorption of digoxin were studied. An increase in the uptake of digoxin in the presence of these three products could be shown with our in vitro model. Cremophor EL and Polysorbate 80 had no toxic effects at the concentrations used. These results suggest that surfactants such as Cremophor EL and Polysorbate 80 should not only support solubilization but can also modulate the P-glycoprotein system to improve the bioavailability of poorly absorbed drugs.

Induction of Rat Intestinal P-glycoprotein by Spironolactone and Its Effect on Absorption of Orally Administered Digoxin

The effect of the diuretic spironolactone (SL) on expression and function of intestinal P-glycoprotein (P-gp), as well as its impact on intestinal absorption of digoxin, was explored. Rats were treated with daily doses of 200 micromol/kg b.wt. of SL intraperitoneally for 3 consecutive days. The small intestine was divided into four equal segments of approximately 25 cm, with segment I being the most proximal. Brush-border membranes were isolated and used in analysis of P-gp expression by Western blot analysis. P-gp content increased in the SL group by 526, 292, 210, and 622% over controls for segments I, II, III, and IV, respectively. Up-regulation of apical P-gp was confirmed by immunofluorescence microscopy. P-gp transport activity was explored in intestinal sacs prepared from segment IV using two different model substrates. Serosal to mucosal transport (efflux) of rhodamine 123 was 140% higher, and mucosal to serosal transport (absorption) of digoxin was 40% lower in the SL group, both indicating increased P-gp function. In vivo experiments showed that intestinal absorption of a single dose of digoxin administered p.o. was attenuated by SL pretreatment. Thus, concentration of digoxin in portal and peripheral blood was lower in SL versus control groups, as well as its accumulation in kidney and liver. Urinary excretion of digoxin was significantly decreased in the SL group, probably reflecting decreased systemic availability of digoxin for subsequent urinary elimination. We conclude that SL induces P-gp expression with potential impact on intestinal absorption of substrates with therapeutic application.

The complexity of intestinal absorption and exsorption of digoxin in rats

The potential multiple carrier-mediated mechanisms involved in the transport of digoxin in rat intestine were investigated by the rapid filtration method in rat intestinal brush-border vesicles (BBMV) and in vitro Ussing chambers. The uptake of digoxin showed a typical overshoot phenomenon in the presence of an inward proton gradient and an outward bicarbonate gradient, or an outward glutathione gradient in BBMV. Good fitting to an equation consisting of both saturable and linear terms was obtained using non-linear regression analysis. GF120918, a specific P-gp inhibitor, significantly increased the absorptive permeability of digoxin in rat ileum (7.02 × 10 −7 cm/s versus 2.11 × 10 −6 cm/s with GF120918) but the addition of DIDS (0.5 mM), an anionic transporter inhibitor, or bromosulfophthalein (0.1 mM), an Oatp inhibitor, in the presence of GF120918 decreased the absorptive permeability compared with GF120918 alone (2.11 × 10 −6 cm/s versus 1.46 × 10 −6 cm/s, p < 0.01 and 2.11 × 10 −6 cm/s versus 1.60 × 10 −6 cm/s, p < 0.05, respectively). The above results suggest the involvement of carrier-mediated uptake mechanism, possibly Oatp, in digoxin absorption. Interestingly, GF120918 (1 μM) did not abolish the polarized transport of digoxin in rat jejunum and ileum, and DIDS (0.5 mM), not a P-gp inhibitor, and MK571 (50 μM), an MRP-selective inhibitor, can also significantly decrease the exsorptive permeability of digoxin. This result indicates the involvement of non-P-gp efflux transporter in digoxin secretion and this transporter is DIDS and MK571-sensitive. Contrary to conventional concept, our studies show that intestinal absorption of digoxin may involve both active uptake and efflux transporters. Our study may have clinical implications in drug–drug or drug–food interactions involving transporters.

MDR1 genotype-related pharmacokinetics and pharmacodynamics.

The multidrug resistant transporter MDR1/P-glycoprotein, the gene product of MDR1, is a glycosylated membrane protein of 170 kDa, belonging to the ATP-binding cassette superfamily of membrane transporters. MDR1 acts as an energy-dependent efflux pump that exports its substrates out of cells. MDR1 was originally isolated from resistant tumor cells as part of the mechanism of multidrug resistance, but over the last decade, it has been elucidated that human MDR1 is also expressed throughout the body to confer intrinsic resistance to the tissues by exporting unnecessary or toxic exogeneous substances or metabolites. A number of structurally unrelated drugs are substrates for MDR1, and MDR1 and other transporters are recognized as an important class of proteins for regulating pharmacokinetics and pharmacodynamics. In 2000, Hoffmeyer et al. performed a systemic screening for MDR1 polymorphisms and detected 15 single nucleotide polymorphisms (SNPs). They also indicated that a polymorphism in exon 26 at position 3435 (C3435T), a silent mutation, affected the expression level of MDR1 protein in duodenum, and thereby the intestinal absorption of digoxin. To date, the genotype frequencies of C3435T have been investigated extensively using a larger population and interethnic difference has been elucidated, and a total of 28 SNPs have been found at 27 positions on the MDR1 gene. Clinical studies on MDR1 genotype-related MDR1 expression and pharmacokinetics have also been performed around the world; however, results were not always consistent with Hoffmeyer's report. In this review, published reports are summarized for the future individualization of pharmacotherapy based on MDR1 genotyping. In addition, recent investigations have raised the possibility that MDR1 and related transporters play a fundamental role in regulating apoptosis and immunology, and in fact, there are reports of MDR1-related susceptibility to inflammatory bowel disease, HIV infection and renal cell carcinoma. Herein, these issues are also summarized, and the current status of the knowledge in the area of pharmacogenomics of other transporters is briefly introduced.

Digitalis. An update of clinical pharmacokinetics, therapeutic monitoring techniques and treatment recommendations.

The intestinal absorption of digoxin is essentially a passive non-saturable diffusion process, although a saturable carrier-mediated component also plays an important role. The bioavailability varies between 40 and 100%: the presence of food may reduce the peak serum concentration, but does not reduce the amount of digoxin absorbed. Recent development of a capsule containing a hydroalcoholic vehicle may reduce interindividual variations in absorption. Pharmacokinetic analysis of the distribution of digoxin suggests 3 compartments, the slow distribution phase accounting for the lag time between the inotropic effects and the plasma concentration profile. Digoxin is extensively bound to tissues such as myocardium, renal, skeletal muscle as well as red blood cells, but not to adipose tissue. Plasma protein binding varies between 20 and 30%: displacement of digoxin from protein binding sites does not cause significant clinical effects. As expected, haemodialysis or exchange transfusions do not significantly alter the body load of digoxin. The apparent volume of distribution of digoxin varies between 5 and 7.3 L/kg; this may be reduced by, for example, electrolyte abnormalities which reduce digoxin binding to the myocardium. The elimination half-life of digoxin is 36 hours, with 60 to 80% being excreted unchanged, by passive glomerular filtration and active tubular secretion. The remainder is excreted non-renally. Clearance is therefore dependent on renal function and declines in renal disease and in elderly patients. Digoxin interacts with other drugs at any stage of absorption (e.g. cholestyramine), distribution (e.g. quinidine), metabolism (e.g. phenytoin) or elimination (e.g. diltiazem). Patients should, therefore, be carefully monitored when changing a therapeutic regimen which includes any drugs known to interact with digoxin. Clinical monitoring is more important than therapeutic drug monitoring which should be reserved for suspected toxicity, doubts about efficacy, or in cases of poor compliance. With the advent of newer treatment modalities, digoxin is no longer the treatment of first choice in supraventricular arrhythmias and congestive heart failure. However, with careful monitoring, digoxin remains an important therapeutic option.

BIOAVAILABILITY OF DIGOXIN FROM RAPIDLY DISSOLVING PREPARATIONS

1 Intestinal absorption of digoxin was assessed by determination of peak plasma concentrations, areas under plasma concentration curves over 80 h, and 10 day urinary excretion. Absorption was equal after ingestion of single doses of standard Lanoxin (Wellcome) tablets, tablets and capsules of ultra-rapid dissolution rate material, or an oral solution of digoxin in water. 2 Mean plasma concentrations and dosage-interval urinary excretion were highly similar during 14 day courses of either Lanoxin or ultra-rapid dissolution tablets. Increased bioavailability does not result from encapsulation of solid dosage presentations, nor from increasing tablet dissolution rate beyond 75% in 15 minutes. 3 Fourteen day courses of tablets of slow dissolution rate produced lower and less consistent mean plasma concentrations and urinary excretion. Slow dissolution rates are associated with greater individual variability in absorption.

Relationship of in vitro binding of digoxin to its intestinal absorption in rats.

Digoxin is bound in aqueous solution to cholestyramine and various commercially available antacids. However, chronic ingestion of cholestyramine and an antacid in rats does not interfere with the intestinal absorption of digoxin.