Design, synthesis and evaluation of antifungal activity of 8-hydroxyquinolin-5-ylidene thiosemicarbazone derivatives

Over the past decades, the incidence and modality of invasive fungal infections have been significantly underestimated. And the limited availability of clinically effective antifungals further underscores the need for novel therapeutic agents. In this study, a series of novel 8-hydroxyquinolin-5-ylidene thiosemicarbazone derivatives (<b>A5</b>-<b>A32</b>) were designed and synthesized via a condensation reaction between 5-formyl-8-hydroxyquinoline and substituted hydrazinecarbothioamides. The <i>in vitro</i> antifungal activity of all synthesized compounds was evaluated against a panel of clinically relevant fungal pathogens using the Clinical and Laboratory Standards Institute (CLSI) broth microdilution method, with minimum inhibitory concentrations (MICs) ranging from 4 to ≤ 0.0313 μg/mL. Notably, compound <b>A14</b> was the most active, demonstrating the lowest MICs among the compounds against each of the tested seven fungal pathogens. Specifically, <b>A14</b> exhibited remarkable potency against <i>Cryptococcus gattii, C. neoformans, C. glabrata</i>, and <i>C. auris</i>, with MICs ranging from ≤ 0.0313 to 2 μg/mL. This potency is significantly higher than that of the lead compound <b>5r</b> (MICs: 0.25- >16 μg/mL) and fluconazole (MICs: 2->16 μg/mL). Furthermore, <b>A14</b> inhibited hyphal formation in <i>C. albicans</i> SC5314 at 8 μg/mL, and remarkably inhibited biofilm formation in both <i>C. albicans</i> SC5314 and <i>C. neoformans</i> H99, achieving >90 % suppression at 32 μg/mL. The cytotoxic effects of compound <b>A14</b> on the viability of HUVECs and Caco-2 cells were evaluated using the CCK-8 assay, demonstrating acceptable safety profiles. As an innovative antifungal agent, 8-hydroxyquinolin-5-ylidene thiosemicarbazone <b>A14</b> warrants further investigation.