Extraction, Solubility and Antimicrobial Activity of (-) Usnic Acid in Ethanol, a Pharmaceutically Relevant Solvent

摘要

Usnic acid (UA) is the most studied bioactive secondary metabolite from lichens. Recently, UA research has focused on its antibacterial activity. However, increased application in healthcare products of UA is hampered by low solubility in aqueous solutions. Here, we report ethanol both as an alternative extraction solvent to acetone and as a pharmaceutically relevant solvent for future formulations. Dried samples of the abundant reindeer lichen (Cladonia stellaris) were extracted in a 23 factorial design using either ethanol or acetone, varying both time and solvent-to-dry mass ratio. In both extracts, UA was the predominant compound as shown by two-dimensional thin-layer chromatography (TLC). In a disc diffusion assay, both extracts demonstrated similar antibacterial susceptibility towards Gram-positive (Bacillus subtilis and Staphylococcus amens), while showing little to no activity against Gram-negative bacteria (Escherichia coli and Pseudomonas aerugtnosa). Among a range of pharmaceutically relevant solvents tested, UA was only soluble in ethanol (at concentrations up to 1-2 mg mL(-1)).