The stability and in vitro antibacterial efficacy of enrofloxacin and gentamicin solutions against Staphylococcus pseudintermedius over 28 days.

ABSTRACT

BACKGROUND: Many clinicians prepare compounded otic solutions to treat otitis externa (OE). Research evaluating the stability and antimicrobial efficacy of these solutions is limited. HYPOTHESIS/OBJECTIVES: This study determined the chemical stability and in vitro bactericidal efficacy of compounded solutions of enrofloxacin and gentamicin during storage for 28 days. MATERIALS AND METHODS: Solutions of enrofloxacin (10 mg/mL, 1%) and gentamicin (3 mg/mL, 0.3%) were prepared with normal saline and 1 mg/mL dexamethasone. Solutions were stored at room temperature (25°C) for 0, 14 and 28 days. The chemical stability of the antibiotics and dexamethasone were determined using liquid chromatography tandem mass spectrometry in triplicate. Efficacy assessment was made with 10 isolates of Staphylococcus pseudintermedius obtained from dogs with OE. Serial 10-fold dilutions of the bacteria with the compounded solutions were prepared and the colony count results were converted into colony-forming units (cfus). The mean cfu/mL and cfu/mL reduction rates were compared between Day (D)0, D14 and D28. All of the antimicrobial testing solutions were performed in triplicate. RESULTS: Chromatography showed that both antibiotics and dexamethasone were stable for 28 days. No significant differences were observed in the antibiotic bactericidal efficacy of stored solutions at D0, D14 or D28. CONCLUSIONS AND CLINICAL RELEVANCE Solutions of 1% enrofloxacin and 0.3% gentamicin in normal saline with 0.1% dexamethasone maintained chemical stability and bactericidal efficacy over 28 days. These solutions can be considered as alternatives to commercial preparations for treatment of canine OE when indicated.