Determination of enrofloxacin stability and in vitro efficacy against Staphylococcus pseudintermedius and Pseudomonas aeruginosa in four ear cleaner solutions over a 28day period

Chemical stability and in vitro bactericidal efficacy of 0.9% enrofloxacin-compounded solutions were evaluated following storage at room temperature for 28days. Chemical stability of enrofloxacin was determined by high-performance liquid chromatography (HPLC) in five compounded solutions, including sterile water. Bactericidal efficacy was determined by spiral plating serial 10-fold dilutions of bacteria and solutions followed by colony counts. Tris-EDTA [TrizEDTA ^® (TE)], Tris-EDTA and 0.15% chlorhexidine [TrizChlor ^® (TC)], 2.5% lactic acid, 0.1% salicylic acid and 0.1% parachlorometaxylenol [Epi-Otic (EO)], and 0.1% free salicylic acid, 0.1% parachlorometaxylenol and 0.5% EDTA [Epi-Otic Advanced (EA)] were used. High-performance liquid chromatography was carried out with one-step liquid/liquid extraction to detect and quantify enrofloxacin stability. Mean recoveries for compounded samples run in triplicate at 28days were 97.7% (TE), 99.9% (TC), 98.1% (EO) and 97.8% (EA). Kruskal-Wallis analysis showed no significant difference in the percentage recovery (H=0.0539, df=3, P=0.9967). American Type Culture Collection strains of Staphylococcus pseudintermedius and Pseudomonas aeruginosa were used to evaluate in vitro efficacy following 30min incubation on days0, 14 and 28. Consistent in vitro bactericidal efficacy of all compounded solutions, indicated by killing >2.3×10 ⁷ colony-forming units/mL, was seen; however, bactericidal efficacy decreased for compounded TC on day14. Pseudomonas aeruginosa was more sensitive to the ear cleaners and enrofloxacin than S. pseudintermedius. The HPLC and in vitro data suggest that 0.9% enrofloxacin compounded with sterile water, TE, EO and EA maintains chemical stability and bactericidal efficacy for 28days.