Pharmacokinetic Studies on Quinidine Sulfate Orally Administered in Horses

Abstract

Five healthy adult horses weighing 480 to 505 kg were administered orally with quinidine sulfate (40 mg/kg) to determine pharmacokinetic values of the drug (Study 1). The mean peak plasma quinidine concentration was 2mg/l 2h after oral administration. The plasma half-life (T_1/2) of quinidine was 8.1±0.91h, the elimination rate (k) of quinidine 0.09±0.010h^-l, and the apparent volume of distribution (Vd), (15.1±1.54)⋅f 1/kg (f was bioavailability). Next, two dosing trials werec arried out to attain two kinds of average steady-state plasma quinidine concentration (1.4 and 1.8mg/l) based on the pharmacokinetic values obtained from Study 1 (Study 2). In both studies substantially steady states of plasma quinidine concentration were obtained. The predicted level was attained in one trial (1.8mg/l) and a level a little lower than the predicted one in the other (1.4 mg/l). Finally, the plasma quinidine concentration was ascertained in a dosage regimen (10g of quinidine sulfate, 3-times at 3-h intervals) adopted widely in the equine practice (Study 3). In this dosage regimen it was not in a steady state, but was raised stepwise with each administration. Discussion was made briefly on the application of these pharmacokinetic determinations to the clinical use of quinidine in the horse.