The disposition kinetics of digitoxin have not been as thoroughly examined as those of digoxin. Digitoxin appears to be rapidly and completely absorbed after oral or intramuscular administration although there have been no estimates of absolute bioavailabilily. There is only one study that has examined the relative bioavailability of two commercial digitoxin tablets (USA) and no differences were found other than in rates of absorption. The near identical responses produced by equal oral and intravenous doses of digitoxin support the suggestion of completeness of absorption. The digitoxin plasma concentration-lime curve seems to be adequately described by a two compartment open model, although such data have not been rigorously analysed. Distribution is complete within 4 to 6 hours and response is not associated with plasma concentrations during the distributive phase, suggesting that the site of action resides in a tissue compartment of a multi-compartment pharmacokinetic model. Digitoxin is strongly bound (>90%) to plasma protein (albumin) with an association constant of 9.62 × 104 litre/mole. The volume of distribution of digitoxin is approximately 0.6 L/kg, although estimates vary considerably. This volume is much smaller than digoxin, consistent with the great plasma protein binding of digitoxin. As with the estimation of the other pharmacokinetic parameters of digitoxin, estimates of elimination vary greatly, primarily as a result of differences in assay methods. Digitoxin is eliminated by hepatic metabolism and as unchanged drug in the urine and faeces. Metabolism is considered to be the major route of elimination, accounting for about 70% of a dose. While most reports indicate that only 30% of a digitoxin dose is eliminated intact (in urine and faeces) this value may be as high as 48%. Digitoxin elimination appears to be independent of dose and route of administration, although there is substantial inter-patient variant in elimination half-life. Half-lives range from 2.4 to 16.4 days with a mean value of approximately 7.6 days. The shortest mean half-life (4.8 days) was observed with the most specific assay and the longest mean half-life (9.8 days) with the least specific method. Renal insufficiency has been reported to result in either decreased or increased digitoxin elimination. While elimination may be expected to increase if plasma protein binding is reduced, there are conflicting reports on the influence of renal insufficiency on digitoxin binding. Part of this conflict may be resolved with the observation that patients undergoing haemodialysis and who receive heparin have a greater fraction of free digitoxin in plasma soon after heparin administration. It appears that plasma free fatty acid concentrations increase in response to heparin which in turn competes with digitoxin for albumin binding sites. Digitoxin half-life is reported to be shortened and volume of distribution increased in nephrotic patients. The influence of hepatic impairment on digitoxin elimination has not been thoroughly examined and there is only one report suggesting reduced elimination in this situation. Cholestyramine has been shown to reduce digiloxin half-life by interfering with the en-terohepatic recycling of the drug. There have been no reports to indicate that other drugs may displace digitoxin from plasma protein binding sites. Concurrent administration of phenylbulazone, phenobarbitone, and phenytoin decrease digitoxin plasma concentrations; presumably by inducing digitoxin metabolism. Phenobarbitone, rifampicin and spironolactone have been reported to decrease digiloxin half-life. Numerous studies have attempted to better define the therapeutic plasma concentration range of digitoxin. Plasma concentrations greater than 35 to 40ng/ml are generally considered to be associated with potential toxicity while concentrations from 15 to 25ng/ml are considered to be within the therapeutic range. As with digoxin there is considerable variation and overlap in plasma concentrations associated with toxicity and therapeutic response.
ASJC Scopus subject areas
- Pharmacology (medical)