what concentration of compounded cefazolin ophthalmic drops is appropriate for bacterial keratitis in dogs?

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Compounded cefazolin ophthalmic drops for canine bacterial keratitis are used at 5.5% (55 mg/mL) concentration. This is the concentration evaluated in canine tear film pharmacokinetic work and is consistent with the fortified cefazolin preparation (33 mg/mL) described as a minimum for stromal or melting ulcers, with the 5.5% formulation representing a higher-concentration compounded option.12 The 5.5% cefazolin formulation compounded in 0.75% cross-linked hyaluronic acid (XHA) achieves significantly higher tear film concentrations than the same antibiotic compounded in 1.4% polyvinyl alcohol (PVA) at all measured time points beyond baseline. Tear film kinetics follow a biphasic pattern: concentrations decline from 0 to 120 minutes, rise modestly between 120 and 360 minutes, then decline again through 480 minutes.1 Stability of the XHA-compounded formulation is maintained for 30 days under refrigeration, with concentrations remaining between 90% and 110% of baseline.1 Cefazolin's spectrum is predominantly gram-positive (staphylococci and streptococci), making it appropriate as part of combination therapy for stromal or melting ulcers rather than monotherapy. It is recommended in combination with an aminoglycoside or fluoroquinolone to extend gram-negative coverage.2 This is clinically relevant given that Pseudomonas aeruginosa — which is resistant to cefazolin — accounts for 15.55% of isolates in canine ulcerative keratitis and 31% in referral-level bacterial keratitis cases.58 Enterobacteriaceae also show 47.06% resistance to cefazolin in culture data from dogs with ulcerative keratitis.5 High concentrations and excessively frequent topical application of cefazolin are toxic to corneal epithelial cells and slow healing — this is a meaningful constraint when dosing the fortified or 5.5% formulation.2 | Formulation | Concentration | Excipient | Tear Film Exposure | Stability | |---|---|---|---|---| | Cefazolin-XHA | 5.5% (55 mg/mL) | 0.75% cross-linked hyaluronic acid