Effect of probenecid on disposition kinetics of ampicillin in horses

The effect of an oral dose of probenecid on the disposition kinetics of ampicillin was determined in four horses. An intravenous bolus dose (10 mg/kg) of ampicillin sodium was administered to the horses on two occasions. On the first occasion the antibiotic was administered on its own, and on the second occasion it was administered one hour after an oral dose of 75 mg/kg probenecid. The plasma concentration of probenecid reached a mean (+/- se) maximum concentration (Cmax) of 188-6 +/- 19.3 micrograms/ml after 120.0 +/- 21.2 minutes and concentrations greater than 15 micrograms/ml were present 25 hours after it was administered. The disposition kinetics of ampicillin were altered by the presence of probenecid and as a result the antibiotic had a slower body clearance (ClB; 109.4 +/- 6.71 ml/kg hours compared with 208.9 +/- 26.2 ml/kg hours) a longer elimination half-life (t1/2 beta 1.198 hours compared with 0.701 hours) and consequently a larger area under the plasma concentration versus time curve (AUC 92.3 +/- 5.09 mg/ml hours compared with 35.95 +/- 3.45 mg/ml hours) when compared with animals to which ampicillin was administered alone. The ampicillin concentrations observed suggest that the dosing interval for horses may be increased from between six and eight hours to 12 hours when probenecid is administered in conjunction with the ampicillin.