Evaluation of a reducible disulfide linker for siderophore-mediated delivery of antibiotics.

Bacterial iron uptake machinery can be hijacked for the targeted delivery of antibiotics into pathogens by attaching antibiotics to siderophores, iron chelators that are employed by bacteria to obtain this essential nutrient. We synthesized and evaluated Ent-SS-Cipro, a siderophore-antibiotic conjugate comprised of the triscatecholate siderophore enterobactin and the fluoroquinolone antibiotic ciprofloxacin that contains a self-immolative disulfide linker. This linker is designed to be cleaved after uptake into the reducing environment of the bacterial cytoplasm. We show that the disulfide bond of Ent-SS-Cipro is cleaved by reducing agents, including the cellular reductant glutathione, which results in release of the unmodified fluoroquinolone antibiotic. Antibacterial activity assays against a panel of Escherichia coli show that Ent-SS-Cipro exhibits activity against some, but not all, E. coli. This work informs the design of siderophore-antibiotic conjugates, particularly those carrying antibiotics with cytoplasmic targets that require release after uptake into bacterial cells, and indicates that disulfide linkers may not be generally applicable for conjugation strategies of antibiotics.