Microbial-inspired antidotes to repurpose toxic compounds as antibiotics

Antibiotic-resistant (AMR) bacterial infections are a major global health threat. Despite the critical need for new antimicrobials, progress is constrained by protracted development timelines, as well as the requirement for chemical novelty to avoid cross-resistance. Repurposing therapeutics approved for indications other than infection offers a potential shortcut to rapidly develop new antibiotics that are outside of existing antibiotic classes and therefore less susceptible to resistance. Cancer therapeutics, which often disrupt essential cellular processes that are evolutionarily conserved within bacteria, are a promising source of novel antimicrobial candidates. However, the narrow therapeutic index of these compounds limits their application for infection. Here, we establish a two-component strategy to repurpose highly toxic therapeutics as antimicrobials by improving their safety profile, using the cytotoxin calicheamicin as a proof-of-concept. In the first arm, we engineer a conditionally-active drug conjugate that restricts calicheamicin activity to infected tissue, improving its therapeutic index. In the second arm, we administer a re-engineered self-resistance enzyme from Micromonospora echinospora, the natural producer of calicheamicin, as an antidote to neutralize calicheamicin released outside of infected tissue, further widening its therapeutic index. The calicheamicin conjugate exhibits activity against a panel of Gram-negative and Gram-positive pathogens in response to a protease present within the infected microenvironment. Delivered in combination with the antidote, we demonstrate that antibacterial efficacy is maintained, and off-target toxicity is reduced in a mouse model of bacterial pneumonia. We anticipate that our dual strategy, which enhances drug safety by combining conditional activation of drug activity with neutralization of off-target effects, provides a generalizable framework for repurposing other promising but toxic compounds as antimicrobials.