Serum glycoside concentrations after single or repeated intravenous doses of beta-methyl-digoxin and digoxin.

The aim of the present investigation was to estimate the ratio of the intravenous doses of beta-methyl-digoxin and digoxin required to produce identical serum glycoside concentrations in man. 20 patients on intravenous maintenance therapy were changed from beta-methyl-digoxin to the identical dose of digoxin or vice versa. Each drug was given for 7 days. Serum concentrations 13% higher were found during administraton of beta-methyl-digoxin. Assuming a half life of 60 h after withdrawal, the dose of digoxin producing the same minimum serum concentration was estimated to be 1.16 times higher than that of beta-methyl-digoxin. 18 healthy volunteers received 0.4 mg beta-methyl- digoxin, and 23 the same dose of digoxin, as an intravenous infusion over 2 h. The serum concentrations and urinary glycoside excretion were measured over a period of 32 hrs. During the first hour after the infusion the serum concentration of digoxin declined more rapidly than that of beeta-methyl-digoxin. Thereafter, the ratio of the serum concentrtions did not change appreciably up to the end of the investigation. The area under the serum concentration/time curve was about 13% greater for beta-methyl-digoxin than for digoxin; this difference was not significant. The average renal clearance was 96 +- 9 ml for beta-methyl-digoxin, 151 +- 13 ml for digoxin. Since the total body clearance of digoxin is only about 1.16 times higher than that of beta-methyl-digoxin, the lower renal clearance of beta-methyl-digoxin must partly be compensated by higher extrarenal clearance. From the ratios of the areas under the serum concentration/time curves after single doses of beta-methyl-digoxin and digoxin, and the minimum serum concentrations during maintenance therapy, it was concluded that the dose of digoxin to produce the same average serum concentrations would be about 1.15 times higher than that of beta-methyl-dogoxin. In comparison wtih the large variations in individual dosage of digoxin and beta-methyl-digoxin, this difference is too small to be of practical importance.