A Targeted Click-to-Release Activation of the Last-Resort Antibiotic Colistin Reduces its Renal Cell Toxicity

The use of highly potent but very toxic antibiotics such as colistin has become inevitable due to the rise of antimicrobial resistance. We aimed for a chemically-triggered, controlled release of colistin at the infection site to lower its systemic toxicity by harnessing the power of click-to-release reactions. Kinetic experiments with nine tetrazines and three dienophiles demonstrated a fast release via an inverse-electron-demand Diels–Alder reaction between trans-cyclooctene (TCO) and the amine-functionalised tetrazine Tz7. The antibiotic activity of colistin against Escherichia coli was masked by TCO units, but restored upon reaction with d-Ubi−Tz, a tetrazine functionalised with the bacterial binding peptide d-Ubi_29–41. While standard TCO did not improve toxicity against human proximal tubular kidney HK-2 cells, the installation of an aspartic acid-modified TCO masking group reduced the overall charge of the peptide and entry to the kidney cells, thereby dramatically lowering its toxicity. The analog Col−(TCO-Asp)₁ had favourable pharmacokinetic properties in mice and was successfully activated locally in the lung by d-Ubi−Tz in an in vivo infection model, whereas it remained inactive and non-harmful without the chemical trigger. This study constitutes the first example of a systemically acting two-component antibiotic with improved drug tolerability.