Design of an Irreversible Inhibitor to Overcome Efflux Pump-Mediated Resistance in Fungal Species

Abstract

Terbinafine and tolnaftate, the primary fungicides used to treat dermatophyte infections, target squalene epoxidase (SQLE), thereby inhibiting ergosterol biosynthesis. Despite relatively similar intrinsic sensitivities with dermatophytes, non-dermatophytes exhibit substantially lower susceptibility to terbinafine and tolnaftate due to their strong efflux pump activity. To address this pump-driven resistance, we propose the development of irreversible versions of terbinafine and tolnaftate. Target cysteine residues exclusively lining the binding pocket of several fungal strains, but not human SQLE, were identified, and a representative model of non-dermatophyte Candida albicans was generated from the crystal structure of human SQLE (RCSB ID: 6C6N).  In this study, we consider various attachment points of an acrylamide warhead to terbinafine and tolnaftate, finding promising irreversible inhibitor designs with selectivity against human SQLE. Nine other non-dermatophyte, terbinafine-resistant fungal species conserve the cysteine residue (C192), so the designed inhibitors may function as antifungals against these fungal species as well. This study elucidates the potential and limitations of an irreversible inhibitor designed to overcome efflux pump-mediated resistance, serving to guide next-generation drug design against resistant fungal strains.