Development of an Aqueous Ophthalmic Solution with an Enhanced-Solubility Enrofloxacin Crystal, and its Clinical Evaluation in Dogs

Background: The concern about the frequent use of ciprofloxacin in veterinary medicine is linked to increased antimicrobial resistance. The corresponding fluoroquinolone for veterinary use is enrofloxacin. A new solvate form of enrofloxacin, as dihydrate-hydrochloride (enro-C) with higher water solubility than the parent compound, was formulated as an ophthalmic solution (pH 5). A multicentre, longitudinal, non-inferiority clinical study in a non-hospital environment was designed to treat 36 dogs affected by tobramycin-unresponsive conjunctivitis with either the experimental 0.5% enro-C ophthalmic preparation (enro-CG) or a commercial preparation of ciprofloxacin (cipro-G). Other causes of conjunctivitis were ruled out. Pathogens were isolated and minimum inhibitory concentration (MIC) studies of tobramycin were carried out. Three blocks of bacterial resistance were set up, beginning at the established breakpoint i.e., 4 μg/mL; 8 μg/mL and 16 μg/mL. Eighteen dogs were randomly assigned to each block. The enro-CG group was treated with two drops of the referred preparation (10 mg/eye) twice a day for 7 days, and the cipro-G group was treated with four drops of a 0.3% commercially available ciprofloxacin eye-drop preparation (9 mg/eye) twice a day, also for 7 days. Clinical and bacteriological cure rates were evaluated. Results: Enro-C-treated dogs achieved a clinical cure one day earlier than ciprofloxacin-treated dogs, and unlike this latter group, enro-CG achieved bacteriological cure in all cases. No side effects were observed in either group, but dogs treated with enro-C showed no discomfort, allowing easier treatment-compliance. Conclusion: This is the first study reported on the successful formulation of enrofloxacin as an ophthalmic solution. Clinical assessment reveals outstanding clinical efficacy. It is necessary to conduct further research on clinical efficacy and toxicity, with the chronic use of this preparation under different clinical challenges.