The rise of invasive fungal infections, coupled with limited therapeutic options and increasing antifungal resistance, underscores the urgent need for novel antifungal agents. Antimicrobial peptides (AMPs) might be suitable alternatives due to their membrane-targeting mechanisms and reduced likelihood of cross-resistance with small-molecule antibiotics but clinicl translation is challenging due to their haemolytic activity. Here, we investigate the antifungal and haemolytic activity of the cyclic decapeptide Tyrocidine A (TyrA) and >20 variants.
To probe the structure–activity relationship of TyrA, variants were designed by substituting specific amino acids with residues of varying hydrophobicity or charge. These modifications produced a diverse range of minimum inhibitory concentrations (MICs) across fungal species, which in some cases varied significantly from the parent peptide. In parallel, the variants displayed a broad spectrum of haemolytic (HC50) values, resulting in distinct selectivity indices for the different variants. Notably, this study extends TyrA antifungal testing to clinically relevant pathogens such as Cryptococcus neoformans, Candida auris, and Candida albicans, that have not been widely investigated in previous TyrA studies, which largely focused on plant-pathogenic fungi or fungal biofilms.
In this study, we categorise TyrA variants according to their selectivity indices, thereby identifying those with the most favourable balance between antifungal potency and reduced toxicity. Furthermore, we assess the impact of each variant on membrane fluidity, providing mechanistic insights into how sequence modifications influence both activity and safety. By linking structural variations of TyrA to its safety and anti-fungal activity, this work highlights the development of a safer, more selective variant of TyrA.