Azithromycin-loaded vesicles for the treatment of skin infections: a comparative study of different formulations

In recent years, the rise of bacterial skin infections together with the lack of adequate therapies have urged the scientific community to focus on the development of new and more efficient treatment strategies [1]. The administration of antibiotics through the topical route is the first-choice treatment for these infections. In fact, localized therapy improves drug safety profile and limits systemic exposure, responsible for the development of resistance [2]. However, limited drug skin retention can lead to an insufficient concentration of the active molecule at the site of action, which in turn requires multiple and frequent administrations, with a negative impact on patient compliance [3]. In this context, the development of new drug delivery systems able to improve topical therapy still remains an important challenge. Indeed, the aim of this study was to prepare niosomes (NS) and liposomes (LP), containing a broad-spectrum antibiotic such as azithromycin (AZT), for the treatment of bacterial skin infections. NS (vesicles formed by the self-assembly of non-ionic surfactants) were prepared by employing sorbitan monopalmitate (Span 40) or sorbitan monostearate (Span 60), while LP (vesicles composed of phospholipid bilayer shells) were obtained with phosphatidylcholine from egg yolk (PC; 80.1% L-α-phosphatidylcholine) or from soybean lecithin (SL; not less than 94% L-α-phosphatidylcholine). Two widely used preparation methods have been chosen: 1) thin film hydration followed by extrusion (TFH) and 2) ethanol injection (EI). The formulations were then characterized for their size and polydispersity index (photon correlation spectroscopy-PCS; Brookhaven 90-PLUS instrument), zeta potential (Malvern Zetasizer 3000 HS instrument), encapsulation efficiency (dialysis method; Visking Tubo Dialysis membrane, Medicell International Ltd., 14,000 Da molecular weight cut-off), stability at room temperature (RT) and at 4-8 °C for 180 days (PCS), and ability to release the drug (Franz cell diffusion system, PermeGear; MF-Millipore® Membrane Filter, 0.22 µm pore size; T = 32 °C) and promote its retention inside the skin (Franz cell diffusion system; porcine ear skin; T = 32 °C). In addition, the antimicrobial activity toward the most representative pathogens (S. aureus, C. acnes) of the skin infections was tested using the broth dilution test. Finally, to determine the cytotoxicity, fibroblasts were exposed to the formulations and their metabolic activity and viability were assessed by using the Resazurin reduction based-assay and by counting living cells. Vesicles prepared through TFH showed higher sizes with respect to the corresponding formulations obtained by the EI method. All the vesicles presented a low polydispersity index, an adequate encapsulation efficiency and a negative zeta potential, which ensured the stability of the vesicle suspensions during the storage period (except for liposomes composed of SL). Formulations prepared through TFH provided a sustained AZT release than the vesicles obtained by EI method, because of the smaller surface area. Moreover, all the vesicles increased drug retention inside the skin in comparison to the control. The percentage amount of AZT in the receptor compartment, within the skin and in the donor compartment obtained after 24 hours from the application of control, LP or NS (means ± SD, n = 5). In general, the lower was the released drug amount, the higher was the retention of AZT inside the skin. Among all the formulations, NS-Span 40-TFH and LP-PC-TFH guaranteed the retention of the greatest amount of AZT inside the skin. The latter capability could ensure the minimization of dosage and frequency of administration and limit the risk of antibiotic drug overuse. Finally, NS-Span 40-TFH and LP-PC-TFH showed appreciable antimicrobial activity against the selected pathogens, and did not exhibit any cytotoxicity, thus representing promising nanocarriers able to improve the effectiveness of skin infection treatment. References [1] Esposito S et al; Expert Rev Anti Infect Ther 17, 17 (2019) [2] Pham TN et al, Med Res Rev 39, 2343 (2019) [3] Gelmetti C, Dermatol Ther 21, 187 (2008)