Assessment of Methodology in Single-Dose Studies of Digoxin Bioavailability

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Abstract
Eight healthy males received 0.75 mg of digoxin by 10 modes of administration in a single-dose multicrossover bioavailability study. Digoxin concentrations in multiple blood samples drawn after each dose and in 6 consecutive 24-h urine collections were used to calculate the areas under the 4-, 8- and 24-h serum concentration curve (A-4, A-8, A-24), and the excretion of digoxin during 1 day (U-1) and 6 days (U-6) following each dose. All 5 methods of assessment gave very similar information on bioavailability. Individual values of A-4 and A-8 were highly correlated (r = 0.973) and had similar variability. A-24 was more variable than A-4 and A-8, and was not as well correlated with either. U-1 and U-6 were highly correlated (r = 0.944), and had nearly identical variability which was less than that of any of the area measures. Urinary excretion data provides more reliable and reproducible information about completeness of absorption of digoxin than data based upon serum concentrations. Extending the period of urine collection beyond 1 day or the blood sampling period beyond 4 or 8 h does not enhance the reliability or usefulness of digoxin bioavailability studies.

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