Because otitis externa can be difficult to treat, noncommercial products with minimal supporting evidence, particularly compounded ear medications (made either in compounding pharmacies or the veterinary clinic), are often used. Stability and efficacy of these products can be influenced by several factors (eg, pH, storage temperature).
This study examined ceftazidime compounded into solutions with 3 base diluents (ie, 0.9% sodium chloride, tris-EDTA, 0.02% phytosphingosine hydrochloride) and stored at 3 temperatures (ie, 77°F [25°C], 39.2°F [4°C], −4°F [−20°C]). Solutions were evaluated against a standard Pseudomonas aeruginosa strain over 28 days for changes in pH, drug concentration, and antimicrobial activity.
All drug concentrations decreased over time; decrease was more significant when solutions were stored at room temperature compared with being refrigerated or frozen. Drug concentration of the tris-EDTA solution significantly decreased as early as within 7 days, likely due to the higher pH of tris-EDTA compared with other diluents.
Antimicrobial activity of each solution was affected by time and temperature. Stability was best achieved with the 0.9% sodium chloride solution when refrigerated or frozen.
This study illustrates the importance of evidence-based medicine in establishing the efficacy of compounded ear medications. For example, ceftazidime diluted into tris-EDTA and stored at room temperature would become unstable within several days, resulting in ineffective treatment.
