A Salmochelin S4-Inspired Ciprofloxacin Trojan Horse Conjugate

A novel ciprofloxacin-siderophore Trojan Horse antimicrobial was prepared by incorporating key design features of salmochelin, a stealth siderophore that evades mammalian siderocalin capture via its glycosylated catechol units. Assessment of the antimicrobial activity of the conjugate revealed that attachment of the salmochelin mimic resulted in decreased potency, compared to ciprofloxacin, against two Escherichia coli strains, K12 and Nissle 1917, in both iron replete and deplete conditions. This observation could be attributed to a combination of reduced DNA gyrase inhibition, as confirmed by in vitro DNA gyrase assays, and reduced bacterial uptake. Uptake was monitored using radiolabeling with iron-mimetic ⁶⁷Ga³⁺, which revealed limited cellular uptake in E. coli K12. In contrast, previously reported staphyloferrin-based conjugates displayed a measurable uptake in analogous ⁶⁷Ga³⁺ labeling studies. These results suggest that, in the design of Trojan Horse antimicrobials, the choice of siderophore and the nature and length of the linker remain a significant challenge.