Chemical stability and in vitro antimicrobial efficacy of diluted silver sulfadiazine powder and cream over a six‐month period

Abstract

BackgroundSilver sulfadiazine (SSD) is commonly formulated into otic preparations to treat otitis externa, although evidence of stability and antimicrobial efficacy with long‐term storage is lacking.ObjectivesTo evaluate the effect of storage time on chemical stability and in vitro antimicrobial activity of SSD diluted in sterile water, including two 1% suspensions using SSD pharmaceutical‐grade powder stored at room temperature (RT) in plastic or sterile glass bottles, and a 1:9 dilution using prescription SSD 1% cream stored at RT in a sterile glass bottle.Materials and MethodsLiquid chromatography–tandem mass spectrometry (LC–MS/MS) assessed chemical stability. Trimethoprim/sulfamethoxazole‐susceptible and trimethoprim/sulfamethoxazole‐resistant strains of Staphylococcus pseudintermedius (SP), meticillin‐resistant (MR) SP, S. schleiferi (SS), MRSS, Pseudomonas aeruginosa, Proteus mirabilis and Escherichia coli evaluated by 24 h time–kill analysis assessed in vitro antimicrobial efficacy. Each assessment was performed at zero, one, three and six months of storage.ResultsLC–MS/MS showed no significant change in concentration over time for any suspension. When adjusted for time and species/strain, all SSD suspensions showed significant reductions in colony forming units (cfu)/mL at 24 h (p < 0.001). Including all suspensions, a bactericidal effect (minimum 3‐log cfu/mL reduction at 24 h) occurred against 94% of total isolates, with failure against 33 of 552 isolates (6%). Bactericidal failure was more likely with the cream‐based suspension (p < 0.05) and at six months (p < 0.01).Conclusions and Clinical RelevancePowder‐based and cream‐based SSD/sterile water suspensions showed no significant change in concentration and demonstrated in vitro antimicrobial activity for six months. Bactericidal failure was more likely with the cream‐based suspension and after six months of storage.