Abstract: Background - Chloramphenicol is a broad-spectrum antibiotic used in equine practice, but hyporexia/anorexia may occur with oral administration. Administration per rectum (PR) could decrease appetite suppression seen with oral use and allow its use in horses unable to receive oral medications while altering hepatic metabolism ratios. The pharmacokinetics of chloramphenicol per rectum have not been studied in horses.Hypothesis/Objectives - The objectives of this study were to evaluate the relative bioavailability of chloramphenicol administered via nasogastric tube (NGT) versus PR and determine relevant pharmacokinetic parameters and metabolic ratios. Animals - Ten healthy, adult horses from the university teaching herd. Methods - All horses received chloramphenicol tablets (50 mg/kg) dissolved in water either via NGT, or PR via red rubber catheter, using a 2-way, randomized crossover design with 14-day washout period. Blood samples were collected at predetermined times over 24 hours and plasma concentrations of chloramphenicol and metabolite were analyzed by UPLC-MS/MS and noncompartmental pharmacokinetics.Results - Chloramphenicol was rapidly metabolized to inactive chloramphenicol glucuronide following both routes of administration. Administration PR resulted in a relative bioavailability of 0.56±0.86% with a maximum concentration (Cmax; µg/mL) of 0.119±0.135 versus 11.7 ± 5.8 with NGT administration. The metabolic ratio of chloramphenicol glucuronide: chloramphenicol was 20.2±6.19 for PR and 5±1.88 for NGT.Conclusions and clinical importance - Results of this study show PR administration of chloramphenicol at 50 mg/kg does not reach therapeutic concentrations in horses and does not slow hepatic metabolism. Administration via NGT produced total chloramphenicol concentrations > 2 µg/mL for 3.93±0.44hrs.
