Ocular Pharmacokinetics of Topical Chloramphenicol in Dogs: Effects of Concentration and Vehicle Formulation

This study compared tear film exposure and retention of four chloramphenicol ophthalmic formulations in dogs, evaluating the effects of drug concentration (0.2% to 5%) and vehicle type (solution vs. ointment). Eight healthy dogs were enrolled in a randomized paired-eye crossover study consisting of two sessions with a 7-day washout. Tear samples were collected via microcapillary tubes at intervals up to 480 min post-instillation, and concentrations were quantified using UV-Vis spectrophotometry. Results showed that chloramphenicol exposure was significantly enhanced by higher concentrations and ointment vehicles. The 0.8% solution achieved a 2.2-fold increase in AUC_0-480 over the 0.2% solution, while the 5% ointment provided a 3.0-fold increase over the 1% ointment. Tear concentration at 480 min was superior for ointments, with the 5% formulation maintaining 88.7 ± 6.8 μg/mL, whereas both solutions fell below quantification limits by the end of the interval. Overall exposure did not differ significantly between the 0.8% solution and 1% ointment. In conclusion, chloramphenicol exposure on the ocular surface depends on both concentration and formulation. High-concentration ointments maximize exposure and prolong retention, providing a pharmacokinetic framework for optimizing therapy in canine bacterial keratitis.