Chloramphenicol is a broad-spectrum bacteriostatic antimicrobial that is frequently used in equine patients. Its spectrum of activity is against gram-negative, gram-positive, anaerobic, and intracellular organisms, and the drug is formulated into injectable, oral, ophthalmic, and topical solutions. Oral formulations include chloramphenicol base and chloramphenicol palmitate.1,2 Given the relative lack of microbial resistance against chloramphenicol because of its ban from use in the food animal and human health industries and the availability of oral formulations of the drug, it has become an attractive option in oral antimicrobial treatment regimens for many species. Serious adverse effects associated with chloramphenicol administration in humans include bone marrow suppression, gray baby syndrome, encephalitis, and fatal aplastic anemia.3,4 Bone marrow suppression is usually associated with plasma chloramphenicol concentrations exceeding 25 μg/mL and is typically reversible after drug administration is stopped.3,4 Gray baby syndrome in neonates and encephalitis in adults can develop when plasma chloramphenicol concentrations exceed 40 to 200 μg/mL.3,4 Aplastic anemia is idiosyncratic and fatal; it can develop weeks to months after exposure to chloramphenicol and is estimated to effect 1/25,000 to 1/40,000 humans exposed.3,4 Because of these human health risks, chloramphenicol is banned for use in food animals in the United States. Horses are not generally used as food animals in the United States (although the FDA does recognize that horses can be food-producing animals); however, horse meat is consumed regularly in some cultures and human exposure cannot be completely eliminated.5 There are few studies that have investigated the absorption of chloramphenicol base after PO administration in adult horses from which food has not been withheld. Current dosages of chloramphenicol administered PO in clinical settings range from 44 to 50 mg/kg (20 to 22.7 mg/lb) every 6 to 8 hours; these dosages are based on results of studies done prior to advances in analytical analysis (eg, high-performance liquid chromatography) that are available today6–11 The purpose of the study reported here was to determine the pharmacokinetics of chloramphenicol base after oral administration at a dose of 50 mg/kg in adult horses from which food was not withheld.
The authors thank Celia Hurley and Lea Valentine for technical assistance.
Area under the plasma concentration–time curve
Area under the plasma concentration–time curve from time 0 to infinity
Area under the first moment curve
Maximum plasma concentration
Minimum inhibitory concentration
Time to maximum plasma concentration
Apparent volume of distribution
Apparent volume of distribution at steady state
MEDISCA Inc, Plattsburg, NY.
APP Pharmaceuticals LLC, Lake Zurich, Ill.
Ora-blend, Perigo Company PLC, Dublin, Ireland.
2695 separations module, Waters, Milford, Mass.
2487 UV detector, Waters, Milford, Mass.
WinNonlin 5.2, Pharsight Corp, Mountain View, Calif.
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