Studies of the Metabolism of Digoxin and Digitoxin Using Double Isotope Dilution Derivative Methods
The double isotope dilution derivative assays for digoxin, digoxigenin, and 3-epidigoxigenin were used to study the disposition of digoxin in three subjects maintained on the drug in standard oral tablet form. Only 21%–55% of the daily dose was excreted in the urine and feces as the chemically intact glycoside; the rest was biotransformed. Since the only acid-hydrolyzable derivative of digoxigenin that was recovered from urine and feces was digoxin itself, the genin portion of the molecule was chemically modified in the process of biotransformation. Only 0.3%–1.0% of the drug was excreted as 3-epidigoxigenin. Incubation of urine with glucuronidase and aryl sulfatase did not release additional digoxin or acid-labile derivatives of digoxigenin.
After digoxin was discontinued in two subjects, plasma concentration and daily urinary and fecal excretion of digoxin decreased exponentially during the subsequent 9 days with a half-life of 1.4 days in one subject and 1.6 days in the other; 23%–56% of the calculated body pool of the drug was cumulatively excreted in the urine and stool, and digoxin was the only acid-hydrolyzable derivative of digoxigenin recovered.
Biotransformation of orally administered digoxin was more extensive than previously reported and equalled or exceeded urinary excretion of the drug even in one subject with hepatic disease. Since only 1%–16% of the daily dose appeared in the feces, the tablet preparation was well-absorbed. The bioavailability of soluble oral preparations of digoxin may be primarily limited by a “first pass effect” manifested by enhanced hepatic uptake and metabolic conversion of the drug.
In the first application of the double isotope dilution derivative principle to analysis of cardiac glycosides in submicrogram quantities, digitoxin was converted to a readily measureable 14C-triacetate derivative, and 3H-digitoxin was first added to the sample to correct for subsequent analytic losses of the glycoside (Lukas and Peterson, 1964). This assay was subsequently used to study the disposition and pharmacokinetics of digitoxin in man (Lukas and Peterson, 1966; Lukas, 1971,1973 a, b), and in the course of those studies, in order to investigate the biotransformation of digitoxin, additional double isotope dilution derivative methods were developed for digitoxigenin, 3-epidigitoxigenin, digoxigenin, and 3-epidigoxigenin (Lukas, 1971, 1973 a, b, c).
This basic analytic method also proved to be applicable to digoxin. Since the method was published (Lukas, 1973 c), it has undergone a few modifications and preliminary trials in studies of the metabolism of digoxin. Thusfar, only a few patients have been studied, but the data are the first to be obtained by an organic chemical method specific for digoxin as well as an independent assay for digoxigenin and are the subject of this report.
KeywordsCardiac Glycoside Fecal Excretion Aryl Sulfatase Body Store Daily Maintenance
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