Chemically triggered bioorthogonal activation of antimicrobial peptide mimic prodrugs

The rise of antimicrobial resistance has necessitated the urgent development of new and effective antimicrobial agents. One promising class of compounds is antimicrobial peptides (AMPs) and their mimics, although this type of membrane-active agent is also prone to cause toxicity issues. To circumvent the associated toxicities of AMPs, we herein developed a novel AMP mimic prodrug where the cationic ammonium residues are caged with trans-cyclooctene (TCO) units that can be bioorthogonally activated by tetrazine via an inverse-electron-demand Diels-Alder click-to-release reaction. The prodrug, TCO-Dendron, was inactive but exhibited antimicrobial activity against Gram-negative pathogens, especially Pseudomonas aeruginosa, in the presence of dimethyl tetrazine. Crucially, through detailed correlation of NMR, MS and antimicrobial assays, we obtained strong indications that the intermediate dihydropyridazine tautomer that persists due to incomplete release of the original AMP mimic also demonstrates antimicrobial activity, thus overcoming the often rate-limiting step in many click-to-release systems. In addition, TCO-Dendron was significantly less toxic to red blood cells and mouse embryonic fibroblast cells than the original AMP mimic, further demonstrating the on-demand control of toxicity enabled by the bioorthogonal trigger. Overall, this study thus not only reports the development of a new click chemistry-responsive AMP mimic prodrug but also reveals important mechanistic insights that help to inform the development of future antimicrobial prodrug systems.