Serum Concentrations of Trimethoprim and Sulfadiazine Following Oral Paste Administration to the Horse

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Abstract
SUMMARY AND CONCLUSIONS: Two fasted and 2 fed horses were dosed orally with a combined trimethoprim and sulfadiazine paste formulation at a dose of 35 mg (1:5 combined active ingredients)/kg. Serum concentrations of each drug were determined periodically for 3 consecutive days for the 4 horses. The extent and rate of absorption for trimethoprim were variable, but peak serum concentrations occurred generally within 3 hours; sulfadiazine absorption was slower, reaching peak concentrations by 6 hours. Fasting did not have a consistent effect on the serum concentration profiles for either drug. Both drugs achieved serum concentrations that equaled or exceeded the minimum inhibitory concentrations necessary to inhibit the growth of certain pathogens common to the horse. Thus, the paste formulation provides an effective means of dose administration to horses.