Summary
Single-dose pharmacokinetic variables of pyrimethamine were studied in horses. Pyrimethamine (1 mg/kg of body weight) was administered iv and orally to 6 adult horses, and plasma samples were obtained at frequent intervals thereafter. Plasma pyrimethamine concentration was assayed by gas chromatography, and concentration-time data were analyzed, using a pharmacokinetic computer program. The iv and oral administration data were best described by 3-compartment and 1-compartment models, respectively. The median volume of distribution at steady state after iv administration was 1,521 ml/kg and the median elimination half-time was 12.06 hours. Mean plasma concentration after oral administration fluctuated between a maximal concentration of 0.18 µg/ml and 0.09 µg/ml (24 hours after dosing). Bioavailability after oral administration was 56%.
