Disposition of ampicillin sodium in horses, ponies and donkeys after intravenous administration

Pharmacokinetics of sulfamethoxazole and trimethoprim in donkeys, mules, and horses

OBJECTIVE

To compare serum disposition of sulfamethoxazole and trimethoprim after IV administration to donkeys, mules, and horses.

ANIMALS

5 donkeys, 5 mules, and 3 horses.

PROCEDURE

Blood samples were collected before (time 0) and 5, 15, 30, and 45 minutes and 1, 1.25, 1.5, 1.75, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 8, 10, and 24 hours after IV administration of sulfamethoxazole (12.5 mg/kg) and trimethoprim (2.5 mg/kg). Serum was analyzed in triplicate with high-performance liquid chromatography for determination of sulfamethoxazole and trimethoprim concentrations. Serum concentration-time curve for each animal was analyzed separately to estimate noncompartmental pharmacokinetic variables.

RESULTS

Clearance of trimethoprim and sulfamethoxazole in donkeys was significantly faster than in mules or horses. In donkeys, mean residence time (MRT) of sulfamethoxazole (2.5 hours) was less than half the MRT in mules (6.2 hours); MRT of trimethoprim in donkeys (0.8 hours) was half that in horses (1.5 hours). Volume of distribution at steady state (Vdss) for sulfamethoxazole did not differ, but Vdss of trimethoprim was significantly greater in horses than mules or donkeys. Area under the curve for sulfamethoxazole and trimethoprim was higher in mules than in horses or donkeys.

CONCLUSIONS AND CLINICAL RELEVANCE

Dosing intervals for IV administration of trimethoprim-sulfamethoxazole in horses may not be appropriate for use in donkeys or mules. Donkeys eliminate the drugs rapidly, compared with horses. Ratios of trimethoprim and sulfamethoxazole optimum for antibacterial activity are maintained for only a short duration in horses, donkeys, and mules.

Pharmacokinetics and applications of ampicillin sodium as an intravenous infusion in the horse

A regime for administration of ampicillin sodium by continuous intravenous infusions to horses was designed. The aim was to achieve plasma ampicillin concentrations between 5 and 10 micrograms/ml over a 4-h period. A 2 mg/kg bodyweight loading dose of ampicillin sodium was administered intravenously at the beginning of the infusion in order to achieve steady-state plasma concentrations rapidly. The infusion system subsequently administered ampicillin at a rate of approximately 19.2 micrograms/min/kg bodyweight. The plasma concentrations obtained over the infusion period correlated very well with predicted calculations based on pharmacokinetic parameters. A mean +/- SEM steady-state plasma concentration (Cpss) of 5.94 +/- 0.33 was obtained and ampicillin was shown to have an apparent steady-state volume of distribution (Vdss) of 175.43 +/- 13.63 ml/kg. When the pump was disconnected the concentrations declined over the following 4 h in an exponential way with an elimination half-life (t1/2 beta) of 0.62 h. In addition, three different infusion dose rates (13.78, 19.34 and 24.48 micrograms/min/kg) were administered to a single animal showing that a good correlation (correlation coefficient > 0.99) existed between the dose administered the steady-state plasma concentrations and the corresponding areas under the plasma concentration versus time curve.