PHARMACOKINETICS OF TRIMETHOPRIM-SULFAMETHOXAZOLE IN 2-DAY-OLD FOALS AFTER A SINGLE INTRAVENOUS-INJECTION

Six healthy two‐day‐old foals (3 pony foals and 3 horse foals) were given a single intravenous (iv) injection of trimethoprim (TMP) ‐ sulphamethoxazole (SMZ) at a dosage of 2.5 mg of TMP/kg bodyweight (bwt) and 12.5 mg of SMZ/kg bwt. Serum TMP and SMZ concentrations were measured serially during a 24 hour period. The overall elimination rate constant (K) for TMP in the pony and horse foals was 0.45/h, whereas the K values for SMZ for the pony and horse foals were 0.12/h and 0.07/h, respectively (no significant difference; P > 0.05). Based on published minimum inhibitory concentration values for equine pathogens (Adamson et al 1985), the primary indication for the use of TMP/SMZ in foals may be in the treatment of infections caused by gram‐positive bacteria. A dosage of 2.5 mg of TMP/kg bwt and 12.5 mg of SMZ/kg bwt, given iv at 12 h intervals would be appropriate. © 1990 EVJ Ltd