Exploring Design Concepts for Siderophore-Fluoroquinolone Trojan Horse Antimicrobials

Urgent action is required to combat the ongoing threat of antimicrobial resistance. Trojan horse conjugates, where antimicrobials are linked to a nutrient carrier, can evade permeability-related resistance through active transport of drugs into bacterial cells. The work presented herein explores modification of the linker moiety between a citrate siderophore and ciprofloxacin, as well as the addition of glycosyl groups to catecholate siderophore moieties to mimic the salmochelins; stealth siderophores which can evade the mammalian defence protein siderocalin. Two linkers were investigated, one containing a carbamate and the other a disulfide bond neighbouring a carbamate group. These were chosen to give intracellular release of the antimicrobial, either by the action of carboxylesterases or through reduction of the disulfide bond by thiolate anions in the cytoplasm, respectively. A carbamate-linked conjugate was synthesised and screened against wild type E. coli, and demonstrated lower antimicrobial activity than that for the parent drug. Screening against a bacterial strain lacking the outer membrane ferric citrate receptor FecA demonstrated that FecA is not essential for uptake of the synthesised conjugate. The synthesis of a disulfide linked conjugate was unsuccessful, due to release of free ciprofloxacin during the final deprotection step. A salmochelin-inspired conjugate was synthesised. Whilst it demonstrated reduced antimicrobial activity against wild type E. coli in iron rich media, compared to the parent drug, higher antimicrobial activity was observed in iron-limited media, suggesting active uptake. A DNA gyrase assay showed that the inhibitory activity of this conjugate was lower than the free drug, suggesting that the antimicrobial activity observed under iron deficiency may be due extracellular iron sequestration by the siderophore.