Digoxin urinary excretion

The co-administration of drugs which inhibit the transporters involved in biliary excretion can reduce the biliary excretion of drugs which are substrates of the transporter, leading to elevated plasma concentrations of the drugs. For example, biliary and urinary excretion of digoxin, both mediated by P-gp, are inhibited by quinidine which is an inhibitor of P-gp. 

Figure 7 Change in the biliary and renal clearance of digoxin caused by quinidine or quinine treatment. After a steady state concentration of quinine or quinidine was achieved by multiple oral administrations, the plasma concentration and biliary and urinary excretion of digoxin after oral administration were measured in healthy volunteers. The steady state concentrations of quinine and quinidine were 7.0 2.5 and 4.5 0.5 pM, respectively. Source From Ref. 291.

Disposition in the Body. Incompletely absorbed after oral administration and metabolised to digoxin. After intravenous administration, about 80% of a single dose is excreted in the urine in 72 hours, mostly as unchanged drug, and about 10% of a dose is eliminated in the faeces. During maintenance treatment, about 30% of a daily dose is excreted in the 24-hour urine. After oral administration digoxin accounts for about 50% of the total urinary excretion material. 

Lindenbaum J, Tse-Eng D, Butler VP, Rund DG. Urinary excretion of reduced metabolites of digoxin. AmJMed( 9S ) 71, 67-74. 

In one study in 10 healthy subjects, the bioavailability of a single 750-microgram dose of digoxin, using urinary excretion as a measure, was reduced by 20% by aminosalicylic acid 2 g four times daily for 2 weeks. This seems to be just another aspect of the general malabsorption caused by aminosalicylic acid. The importance of this interaction in patients is not known (it is probably small) but it would be prudent to monitor concurrent use. 

Yo ida A, Fujita hL Kurosawa N, Nioka M, ShichinoheT, Arakawa M, Fukuda R, Owada E, Ito K. Effects of diltiazem on plaana level and urinary excretion of digoxin inhealll subjects. Clin Pharmacol Ther ( 9ZA) 35,681-5. 

---