A steady-state evaluation of the effects of propantheline bromide and cholestyramine on the bioavailability of digoxin when administered as tablets or capsules

Drug interactions can profoundly alter the absorption of digoxin in tablet form. This study evaluated whether digoxin solution in capsules, a new dosage form with 90% to 100% bioavailability, would reduce such alterations, specifically those caused by cholestyramine and propantheline bromide. The investigation used a six-treatment, steady-state, balanced, incomplete block design with 18 healthy adults studied for four continuous two-week treatment periods. Treatments were either two 0.25 mg digoxin tablets or two 0.20 mg digoxin capsules administered alone, with propantheline, 15 mg qid, or with cholestyramine, 8 g qd. Bioavailability was determined from steady-state, 24-hour area under the serum concentration-time curve (AUC, ng X h/mL) and from 0- and 24-hour trough serum digoxin concentrations (ng/mL). The AUCs for tablets alone, with cholestyramine, and with propantheline were 32.8 +/- 13.3 (+/- SD), 22.4 +/- 12.1, and 40.6 +/- 13.9, respectively, while corresponding values for capsules were 31.7 +/- 9.3, 24.7 +/- 7.9, and 35.9 +/- 12.8. The trough concentrations for tablets alone, with cholestyramine, and with propantheline were 0.88 +/- 0.47, 0.61 +/- 0.38, and 1.09 +/- 0.35, respectively; trough concentrations for capsules were 0.77 +/- 0.28, 0.74 +/- 0.28, and 0.96 +/- 0.48, respectively. The only significant differences in AUC were seen when comparing tablets alone versus tablets with cholestyramine (P less than .0005) and tablets with propantheline (P less than .01). A significant finding was also observed when comparing trough concentrations for tablets alone versus tablets with cholestyramine (P less than .005).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of particle size on the bioavailability of digoxin

The bioavailability of digoxin in three tablets prepared from materials with different particle sizes was measured in healthy volunteers in a cross-over study using an alcoholic solution of digoxin as a reference standard. Its bioavailability in tablets with particle sizes of 7 mu or 13 mu was 78-97% of that of digoxin in solution. The tablet with largest particle size (102 mu) showed markedly lower bioavailability than the reference solution, namely 39%. It is obvious that particle size is an important determinant of the dissolution rate and bioavailability of digoxin.

Bioavailability studies with Digoxin-Sandoz and Lanoxin

Various brands of digoxin tablets, and even different batches of one brand, may differ greatly in bioavailability. Digoxin-Sandoz tablets have been compared with Lanoxin manufactured between 1969 and 1972 and after May 1972. Comparisons were also made between and within batches of Digoxin-Sandoz tablets. Three separate cross-over studies were conducted involving a total of 20 volunteers. Digoxin-Sandoz tablets were shown to have a constant bioavailability and to produce plasma concentrations very similar to ""new'' Lanoxin. Storage for 2 years of one batch of Digoxin-Sandoz did not alter the bioavailability. Particle size was shown to influence bioavailability. Care should be exercised when plasma data alone are interpreted as an index of bioavailability. Measures of bioavailability based on plasma data obtained up to 6 h after administration differed from those based on cumulative urinary excretion data (in this study by a factor of about 2), which can lead to the belief that a difference in bioavailability is much greater than is actually the case. Data from cumulative urinary excretion, collected over a sufficiently long period of time, are likely to be the most reliable method for determining the bioavailability of a substance such as digoxin.

[Comparison of digitoxin bioavailability from tablets and elixir during maintenance therapy (author's transl)]

The bioavalability of digoxin tablets and solution has been studied during maintenance therapy in a cross over study. Each preparation was given over a period of at least 7 days to patients with compensated congestive heart failure. Urine concentrations and plasma levels were analysed for digitoxin. There was no significant difference between the two preparations. Determination of steady state serum concentrations and urinary excretion during maintenance therapy as an index of bioavalability are more cumbersome than a single dose study. From a pharmacokinetic point of view however, analyses of steady-state conditions are preferable to a single dose study. In addition, steady state of drug input and output resembles the usual digitalis therapy.