Pharmacokinetic analysis of topical tobramycin in equine tears by automated immunoassay

The mean tobramycin concentrations in the tears at 5, 10, 15, 30 minutes and 1, 2, 4, 6 hours after administration were 759 (±414), 489 (±237), 346 (±227), 147 (±264), 27.6 (±28.4), 14.8 (±66.6), 6.7 (±18.6), and 23.4 (±73.4) mg/L. Mean tobramycin concentration was maintained above the MIC90 for commonly isolated bacteria for 68.5 min.A single dose of topical tobramycin resulted in therapeutic concentrations of tobramycin in the tears for 1 h after administration. Therapeutic levels of tobramycin remained in equine tears 6 times longer than was reported in rabbit tears.Ulcerative keratitis commonly occurs in horses. Their prominent eye profile and active nature likely predispose them to ocular trauma [1]. Breaks in the corneal surface allow commensal conjunctival and corneal organisms to colonize the epithelium or the stroma. Primary corneal bacterial pathogens, such as Pseudomonas, can establish infection despite an intact corneal epithelium [1-3]. Bacteria most frequently cultured from the corneas of horses with ulcerative keratitis include Streptococcus equi subspecies zooepidemicus, Pseudomonas aeruginosa, and Staphylococcus spp. [2,4,5].Although general recommendations are to use a broad-spectrum initial antimicrobial therapy for ulcerative keratitis, the antibiotic choice and frequency of administration are empirical and guided by the clinical examination and cytological evaluation of corneal swabs [1]. Therapy is modified based on bacterial and fungal culture and sensitivities, when they become available in the following days to weeks [1,6].Appropriate antimicrobials to prevent infection include triple antibiotic ointment (neomycin, polymyxin B, bacitracin), tetracyclines, and aminoglycosides [1,2]. Fluoroquinolones are often used to treat established infections [1]. Treatment intervals for ulcerative keratitis range from q.6 h to q.8 h for prophylaxis of infection, and may be as frequent as q.2 h to q.4 h for corneas that are infected [1].Tobramycin is commo