Comparison of the pharmacokinetics of digoxin and dihydrodigoxin in cats in single-dose studies

Pharmacokinetics of 3H-dihydrodigoxin and 3H-digoxin after single intravenous and intraduodenal administration in cats are compared. Data could be described by an open two compartment body model. The beta half-life in plasma of dihydrodigoxin after initial rapid distribution is 4.6h compared to 10.4 h after digoxin administration. The volume of tissue distribution of dihydrodigoxin is 7 times smaller than that of digoxin (0.311 versus 2.051). The "specific uptake" of dihydrodigoxin into myocardium and some other tissues is very low. Over 5.5h the cumulative biliary and urinary elimination of dihydrodigoxin is definitely higher (45.7% versus 14.4%). An unexpected peak in TLC plates after dihydrodigoxin administration in blood, bile and urine was identified to be the sodium salt of the opened lactone structure: dihydrodigoxin acid.