Augmenting the activity of antifungal agents against aspergilli using structural analogues of benzoic acid as chemosensitizing agents

A number of benzoic acid analogues showed antifungal activity against strains of Aspergillus flavus, Aspergillus fumigatus and Aspergillus terreus, causative agents of human aspergillosis, in in vitro bioassays. Structure–activity analysis revealed that antifungal activities of benzoic and gallic acids were increased by addition of a methyl, methoxyl or chloro group at position 4 of the aromatic ring, or by esterification of the carboxylic acid with an alkyl group, respectively. Thymol, a natural phenolic compound, was a potent chemosensitizing agent when co-applied with the antifungal azole drugs fluconazole and ketoconazole. The thymol-azole drug combination demonstrated complete inhibition of fungal growth at dosages far lower than the drugs alone. Co-application of thymol with amphotericin B had an additive effect on all strains of aspergilli tested with the exception of two of three strains of A. terreus, where there was an antagonistic effect. Use of two mitogen-activated protein kinase (MAPK) mutants of A. fumigatus, sakAΔ and mpkCΔ, having gene deletions in the oxidative stress response pathway, indicated antifungal and/or chemosensitization activity of the benzo analogues was by disruption of the oxidative stress response system. Results showed that both these genes play overlapping roles in the MAPK system in this fungus. The potential of safe, natural compounds or analogues to serve as chemosensitizing agents to enhance efficacy of commercial antifungal agents is discussed.