Digoxin bioequivalence study: determination in human plasma by microparticle enzyme immunoassay

In this study a bioanalytical method for digoxin quantification was developed using Abbott AxSYM Digoxin II with fluorescence detection to assess the bioequivalence of two digoxin tablet formulations (Digox 0.25 mg tablet from Pharlab Ind. Ltd., Brazil as test formulation and Digoxin 0.25 mg tablet from Laboratório Glaxo SmithKline, Brazil as reference formulation).

30 healthy volunteers (both sexes) received a single oral dose of digoxin in an open, randomized, two-period crossover study with a seven half-life washout interval of at least (21 days). Plasma samples were obtained over a 288-h interval after each oral administration of digoxin. The present method utilizes microenzyme particle immunoassay technology, in which the digoxin in the sample binds to anti-digoxin-coated microparticles and after separation; digoxin-alkaline phosphatase conjugate binds to the available sites remaining. Digoxin concentrations are calculated from the fluorescent products generated as a result of substrate (4-methylumbelliferyl) passage through the matrix cell.

The method was shown to be specific and sensitive with good accuracy and precision. The geometric mean and 90% confidence intervals (CI) for the Digox/Digoxin ratio were 107.62% (96.71 - 119.80%) for AUC0-t, 97.15% (80.54 - 117.19%) for AUC0-inf, and 91.23% (83.55 - 99.62%) for Cmax.

Since the 90% CI for the parameters were all within the 80 - 125% interval proposed by the US Food and Drug Administration Agency, the two formulations were considered bioequivalent in terms of rate and extent of absorption.