Excretion of digoxin and its metabolites in urine after a single oral dose in healthy subjects

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 x 10(-7) cm/s versus 2.11 x 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 x 10(-6) cm/s versus 1.46 x 10(-6) cm/s, p<0.01 and 2.11 x 10(-6) cm/s versus 1.60 x 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 microM) 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 microM), 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.

Digoxin disposition in elderly humans with hypochlorhydria

Digoxin (D3) metabolism is partially mediated by the gastrointestinal tract via acid hydrolysis of digitoxose sugar moieties and bacterial reduction of the lactone. The hypothesis that hypochlorhydria influences digoxin disposition was tested in six normochlorhydric (NC) and four hypochlorhydric (HC) subjects. D3 tablets were administered daily for 19 to 28 days, and quantitative urine and fecal samples were collected over the last 3 days (steady state). Samples were analyzed for D3 and its extractable metabolites by fluorescence-derivatization HPLC. Excretion of D3 in urine increased from 37% of the dose in NC to 46% in HC, whereas excretion of D3 in feces decreased from 29 to 14%. These changes were statistically significant (P < .05) and consistent with decreased hydrolysis of D3 by stomach acid and increased intestinal metabolism in HC. In each subject, D3 was added to anaerobic cultures of both feces and jejunal fluid. Digoxin was reduced in all but two of the fecal incubates, and was not reduced in any jejunal fluid incubates. Because dihydrodigoxin (DHD3) was found in only two hypochlorhydric subjects, in vitro measures of bacterial reduction of D3 were not predictive of in vivo excretion of reduced metabolites. Sugar-hydrolyzed, reduced metabolites were not found in any subjects. It is concluded that D3 disposition is altered by hypochlorhydria, and that an understanding of the metabolic mechanisms requires further study.

The bioavailability of digoxin from three oral formulations measured by a specific h.p.l.c. assay

1. We have studied the absolute bioavailability of three oral formulations of digoxin, 1.0 mg, in 12 young healthy volunteers in a four way randomised cross-over study using an intravenous control. 2. Digoxin tablets (250 micrograms), liquid filled digoxin capsules (100 micrograms) and an experimental enteric-coated capsule (100 micrograms) were evaluated. In vitro dissolution at pH 1 demonstrated extensive hydrolytic breakdown of digoxin from the tablets and capsules but not from the enteric-coated capsules. 3. Serum 'digoxin' concentrations were measured by fluorescence polarization immunoassay (FPI). The systemic availability (+/- s.d.) of the capsules was 70.5 +/- 11.3%, and that of the tablets 71.5 +/- 8.6%. Drug was less available from the enteric-coated capsules (62.1 +/- 10.3%) measured with FPI. These results were reflected in the urinary drug recoveries measured by FPI. 4. By contrast, there were no differences in urinary recovery of unchanged digoxin between any of the oral treatments, when this was measured by h.p.l.c. The cross-reactivity of immunoassays for metabolites of digoxin may produce artefactual results and the optimal pharmaceutical formulation for digoxin remains to be determined.

Inhibition by basic drugs of digoxin secretion into human bile

Bile samples from previous interaction studies in man were re-analysed by a combined HPLC/radioimmunoassay method. Quinidine, quinine and verapamil but not probenecid or spironolactone were found to inhibit the biliary secretion of unchanged digoxin.

Quinidine reduces biliary clearance of digoxin in man

Quinidine is known to reduce the renal clearance of digoxin, but this effect does not completely explain the influence of quinidine on the total clearance of digoxin. We therefore studied the effect of quinidine administration on biliary clearance of digoxin in five patients with atrial fibrillation. Biliary clearance of digoxin under steady state conditions before and during treatment with quinidine was investigated using a duodenal-marker-perfusion technique. Quinidine caused an average 42% (range 21-65%, P less than 0.02) reduction of the measured biliary clearance of digoxin. We conclude that the biliary effect adds to the previously demonstrated inhibitory effect of quinidine on the renal clearance of digoxin and helps to explain the decrease in total clearance of the drug. This is the first demonstration in man of a pharmacokinetic drug interaction at the level of biliary excretion.

Increased metabolism to dihydrodigoxin after intake of a microencapsulated formulation of digoxin

A capsule preparation containing small, enteric-coated granules of digoxin was developed to prevent acid hydrolysis of the drug in the stomach and to diminish the variation in plasma glycoside concentration during the intervals between doses. The absorption and metabolism of tritiated digoxin after a single oral loading dose of this formulation (Formulation C) were compared to those after ingestion of a digoxin solution (Formulation S) by 8 healthy men. Drug concentrations were measured by radioimmunoassay (RIA) and liquid chromatography (LC). The percentage of the digoxin dose excreted in the urine during 72 h, as measured by RIA, was significantly lower after the capsule (20.5 +/- 2.0% vs 36.2 +/- 3.0% after S, mean +/- SEM) but total urinary radioactivity after the two treatments was similar (C 35.3 +/- 5.2 and S 41.2 +/- 2.6%; p greater than 0.05). The discrepancy was mainly due to significantly greater excretion of dihydrodigoxin after the capsule (m 12.8%, range 0-28.6% of the dose) than after the digoxin solution (m 5.4%, range 0-14.5%). Dihydrodigoxin was not measured by the RIA. The recovery of hydrolysis metabolites (LC) was greater during the first 24 h after S (2.3 +/- 0.6% vs 0.9 +/- 0.3% after C; p less than 0.05). The peak plasma concentration of digoxin (RIA) was significantly reduced and delayed after intake of C (2.5 +/- 0.4 nmol/l at 3.8 +/- 0.3 h vs. 8.3 +/- 0.8 nmol/l at 0.9 +/- 0.1 h after S), and so was the shortening of electromechanical systole at 1.5 h, 2.5 h, and 3 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolism of digoxin after oral and intrajejunal administration

To study the influence of administration site on metabolism of digoxin in the gut, urinary excretion of digoxin and its metabolites was compared after oral and intrajejunal administration of [3H]-digoxin solution to eight healthy volunteers, using liquid chromatography for the drug analysis. Dihydrodigoxin was not excreted in higher amounts after the more distal administration. The excretions of hydrolytic and non-extractable metabolites were significantly greater after the oral administration.

Fasting and postprandial absorption of digoxin from a microencapsulated formulation

The absorption of digoxin from a capsule preparation containing a large number of small, enteric-coated granules of the glycoside (Preparation CR) was compared in 10 volunteers with that from a rapidly dissolving tablet (Preparation L). Plasma and urine digoxin concentrations were measured by radioimmunoassay. In the fasting state, after a loading dose of digoxin (0.76 mg), peak plasma concentrations were significantly (p less than 0.001) lower after CR (2.0 +/- 0.5 nmol/l, mean +/- SD) than L (4.7 +/- 1.1 nmol/l). Peak concentrations after CR were significantly (p less than 0.001) delayed compared to L (3.3 +/- 0.6 h vs 1.1 +/- 0.4 h). Also, postprandial peak plasma concentrations at steady state, were significantly (p less than 0.01) lower after CR (1.0 +/- 0.3 nmol/l) than L (2.7 +/- 0.5 nmol/l), and the peak concentrations occurred later (3.9 +/- 1.7 h vs 1.4 +/- 0.9 h). The area under the plasma concentration-time curves was smaller (p less than 0.01) for CR (17.7 +/- 5.9 nmol X 1(-1) X h) than for L (22.4 +/- 4.1 nmol X 1(-1) X h), and so was the amount of drug excreted in urine (174 +/- 25 micrograms vs 190 +/- 31 micrograms; p less than 0.005). Thus, the absorption rate of digoxin from the enteric-coated formulation was markedly reduced but at the cost of a variable reduction in the amount absorbed.