Atividade da alexidina contra espécies de Scedosporium e Lomentospora: potenciais alvos na célula fúngica

Abstract

Scedosporium and Lomentospora species are emerging fungal pathogens capable of causing localized to disseminated infections in immunocompromised individuals, cystic fibrosis patients, and, occasionally, in immunocompetent individuals subjected to trauma or near-drowning incidents. The pharmacological management of these infections is hindered by the limited number of effective antifungal agents and by resistance. Therefore, the development of novel therapeutic approaches becomes fundamental. In recent years, several studies have highlighted the activity of alexidine against clinically relevant fungal pathogens, including species of Scedosporium and Lomentospora. Yet, its mechanism of action is not fully understood. In this context, the present work aimed to characterize the mechanism of action of alexidine and to evaluate its effects on S. aurantiacum, S. boydii, and L. prolificans. The susceptibility of these species to alexidine was determined by the broth microdilution method. The minimum inhibitory concentration (MIC), defined as the lowest concentration of alexidine capable of inhibiting 80% of fungal growth, was 1.5 μg/mL. At this concentration, alexidine exhibited a fungistatic effect, whereas a fungicidal effect was observed at 3 μg/mL for all evaluated species. During the first 24 hours of incubation, subinhibitory concentrations of alexidine were able to alter the growth curve and affect the germination of S. aurantiacum, S. boydii, and L. prolificans. To investigate whether the mechanism of action of alexidine involves interaction with components of the fungal cell membrane, a lipid affinity assay was performed in the presence of ergosterol, phosphatidylcholine, and glucosylceramide. Supplementation of the culture medium with these lipids increased the MIC of alexidine against S. aurantiacum by 16-fold (in the presence of phosphatidylcholine) and by 8-fold (in the presence of glucosylceramide and ergosterol), suggesting that these lipids act as potential targets for alexidine. Subsequently, the filipin staining revealed decreased fluorescence in S. aurantiacum cells exposed to alexidine, reinforcing the hypothesis of ergosterol as a potential target for this compound. At 2× and 4× MIC, alexidine altered the membrane permeability of S. aurantiacum conidia and hyphae, as indicated by SYTOX Green kinetics and fluorescence microscopy. Finally, the combination of alexidine and amphotericin B against S. aurantiacum, S. boydii and L. prolificans was evaluated by the Fractional Inhibitory Concentration Index (FICI), which indicated an additive effect, and by surface response analysis based on the Bliss independence model, which revealed predominantly synergistic regions. The obtained results indicate that alexidine exhibits potent activity against Scedosporium and Lomentospora species, interacting with multiple components of the fungal cell membrane and inducing alterations in its permeability. Moreover, the combination of alexidine and amphotericin B results in a potentially favorable interaction against S. aurantiacum, S. boydii, and L. prolificans.