Development and Characterization of Azithromycin-Loaded Microemulsions: A Promising Tool for the Treatment of Bacterial Skin Infections

Abstract

In recent years, the treatment of bacterial skin infections has been considered a major healthcare issue due to the growing emergence of antibiotic-resistant strains of Staphylococcus aureus. The incorporation of antibiotics in appropriate nanosystems could represent a promising strategy, able to overcome several drawbacks of the topical treatment of infections, including poor drug retention within the skin. The present work aims to develop microemulsions containing azithromycin (AZT), a broad-spectrum macrolide antibiotic. Firstly, AZT solubility in various oils, surfactants and co-surfactants was assessed to select the main components. Subsequently, microemulsions composed of vitamin E acetate, Labrasol^® and Transcutol^® P were prepared and characterized for their pH, viscosity, droplet size, zeta potential and ability to release the drug and to promote its retention inside porcine skin. Antimicrobial activity against S. aureus methicillin-resistant strains (MRSA) and the biocompatibility of microemulsions were evaluated. Microemulsions showed an acceptable pH and were characterized by different droplet sizes and viscosities depending on their composition. Interestingly, they provided a prolonged release of AZT and promoted its accumulation inside the skin. Finally, microemulsions retained AZT efficacy on MRSA and were not cytotoxic. Hence, the developed AZT-loaded microemulsions could be considered as useful nanocarriers for the treatment of antibiotic-resistant infections of the skin.

Keywords: azithromycin; methicillin-resistant Staphylococcus aureus; microemulsions; permeation/retention studies; skin infections; topical delivery.

Figure 1

Solubility (mg/mL) of AZT in various types of oils, surfactants and co-surfactants at +25 °C. Data are expressed as means ± SD, n = 3.

Figure 2

Pseudoternary phase diagrams of the systems composed of oil, surfactant/co-surfactant (S_mix) and water. The oil phase is vitamin E acetate; S_mix is a mixture of Labrasol^®/Transcutol^® P 2:1 v/v (a) and 3:1 v/v (b). The grey regions indicate transparent and homogenous microemulsions; in the remaining regions, coarse turbid emulsions or phase separated systems were observed.

Figure 3

Appearance of loaded microemulsions M1, M2 and M3.

Figure 4

Size variation in unloaded and loaded microemulsions M1 (a), M2 (b) and M3 (c) during 180 days of storage at +4–8 °C and +25 °C. Data are expressed as means ± SD, n = 3.

Figure 4

Size variation in unloaded and loaded microemulsions M1 (a), M2 (b) and M3 (c) during 180 days of storage at +4–8 °C and +25 °C. Data are expressed as means ± SD, n = 3.

Figure 5

Cumulative drug amount (expressed as fractional amount Mt/M0, where Mt represents the amount of AZT released at each time and M0 the total AZT mass) released from microemulsions and control (Ctrl) plotted as a function of time. Data are expressed as means ± SD, n = 3.

Figure 6

Percentage amount of AZT in the receptor compartment, within the skin and in the donor compartment obtained after 24 h from the application of AZT solution (control, Ctrl) or microemulsions. Data are expressed as means ± SD, n = 5.

Figure 7

Fibroblasts viability (%) after 24 h of incubation at 37 °C with loaded microemulsions and AZT solution at different drug concentrations. The values denote the mean ± S.D. (n = 4). All treatments were analyzed against control (Ctrl), *** p < 0.001.