To evaluate the possible effect of quinidine on digoxin bioavailability, the steady state digoxin kinetics was examined with and without concomitant quinidine therapy, in 7 cardiac patients after simultaneous administration of oral digoxin and intravenous [3H]-digoxin. In the presence of quinidine, the absorption rate constant of digoxin (ka) increased from 2.72±1.04 to 3.53±1.34 h−1 (p<0.05), whereas lag time and peak time decreased from 0.16±0.10 to 0.05±0.04 h (p<0.05) and from 0.92±0.27 to 0.69±0.19 h (p<0.02), respectively. Predose plasma digoxin increased from 0.41±0.25 to 0.70±0.31 ng/ml (p<0.02), while peak plasma digoxin increased from 0.93±0.34 to 1.63±0.46 ng/ml (p<0.02). The systemic availability of digoxin increased from 68.48±13.35 to 79.09±14.89% (p<0.05) in the presence of quinidine. Quinidine had no effect on the biotransformation pattern of digoxin, as assessed by thin layer chromatography. Quinidine increases the rate and extent of digoxin absorption, and this interaction contributes significantly to the elevation in plasma digoxin during both its distribution and elimination phases.
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Pedersen, K.E., Christiansen, B.D., Klitgaard, N.A. et al. Effect of quinidine on digoxin bioavailability. Eur J Clin Pharmacol 24, 41–47 (1983). https://doi.org/10.1007/BF00613925