Erythromycin ethosomal systems: Physicochemical characterization and enhanced antibacterial activity

The rationale behind this work was that a permeation enhancing carrier could facilitate the transport of antibacterial molecules through the two biological barriers: stratum corneum of the skin and bacterial membrane/cell wall. To this end, erythromycin ethosomes (EE) were designed and characterized, and their antibacterial efficiency was evaluated in vitro and in vivo. TEM, CLSM, DLS, DSC and ultracentrifugation tests indicate that EE are small unilamellar soft vesicles encapsulating 78.6% erythromycin. The compositions were stable for at least one year at room temperature. In live/dead viability/cytotoxicity tests, EE systems were nontoxic to cultured 3T3 dermal fibroblasts. Susceptibility studies conducted on three bacterial strains (B. subtilis ATCC 6633, S. aureus ATCC 29213 and S. aureus clinically resistant to erythromycin) showed significantly larger inhibition zones for EE as compared to erythromycin in hydroethanolic solutions. Moreover, EE reduced erythromycin MIC as compared to control solution: from 2.5 to 1.25μg/ml for S. aureus ATCC 29213 and from 12.5 to 5.0μg/ml for clinically isolated resistant S. aureus strain. Ethosomal erythromycin applied to the skin of ICR mice inoculated with 107cfu S. aurues ATCC 29213 resulted in complete inhibition of infection. On the contrary, when hydroethanolic solution of erythromycin was applied, deep dermal and subcutaneous abscesses developed within five days after challenge. On day seven, a similar number of S. aureus colonies (1.06×10⁷ vs. 0.90×10⁷ cfu/g tissue) were isolated from the untreated wounds or treated with hydroethanolic erythromycin. For these animals, histopathological examination showed necrosis, destroyed skin structures and dense infiltrates of neutrophils and macrophages. These findings show that ethosomes are efficient carriers for erythromycin delivery to bacteria localized within the deep skin strata for eradication of staphylococcal infections.