Influence of spironolactone pretreatment on pharmacokinetics and metabolism of digitoxin in rats

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Abstract
The influence of pretreatment with spironolactone (84 mg/kg at 3 days, twice per day) on the tritium levels in plasma, urine and feces of female SD-rats (n=9) was investigated at various time periods after oral administration of 25 μg/kg 3H-digitoxin. In plasma, the concentrations of total radioactivity are reduced in pretreated animals to about 20% of tritium levels in control rats, while the half-life of radioactivity in both groups is almost identical, 2.9 days in pretreated rats and 2.8 days in controls. The lower plasma levels of tritium in pretreated rats coincide with a six-fold decrease in the urinary 3H-elimination and a corresponding increase in the fecal excretion. This is due to a higher biliary clearance of tritiated products in the early phase of elimination. The separation of the excretion products by TLC shows that spironolactone pretreatment enhances the splitting of the glycosidic bonds of digitoxin. The amount of digitoxigenin-bis-digitoxoside and of digitoxigenin-mono-digitoxoside excreted in urine and feces within 96 hrs is four and ten times greater than that recovered in control animals, respectively. The formation of the hydroxylation products digoxin and digoxigenin-bis-digitoxoside is decreased from 50% of the total excreted radioactivity in control to 15% in pretreated rats. The conjugation reactions with glucuronic and sulfuric acid are increased after pretreatment with spironolactone. Thus, the effect of spironolactone on digitoxin kinetics is apparently related to an enhancement of the hepatic excretory mechanism as well as to an enhanced metabolism.

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