Artificial Siderophores with a Trihydroxamate-DOTAM Scaffold Deliver Iron and Antibiotic Cargo into the Bacterial Pathogen Escherichia coli

Infections with multidrug-resistant Gram-negative bacteria constitute a silent pandemic threat that is increasing globally. A major technical and scientific hurdle hampering the development of efficient antibiotics against Gram-negative species is the low permeability of their outer membrane that prevents the entry of most small molecules into the cells. This can be overcome by targeting active iron transport systems of the pathogens in a Trojan-Horse strategy that makes use of drug-loaded artificial siderophores. While we utilized catechols as iron-binding motifs in previous work, this study reports the design, synthesis and characterization of siderophores with a DOTAM scaffold that was substituted with three hydroxamate arms allowing for a hexacoordination of iron. Their iron-chelating capabilities were shown colorimetrically, and the ability of compound 1 to deliver iron into Escherichia coli in a chelation-specific manner was proven by a growth recovery assay. A covalent siderophore-ciprofloxacin conjugate exerted antibiotic effects against E. coli, albeit it was less potent than the free drug. The study qualifies artificial DOTAM siderophores with hydroxamate binders as scaffolds for bacterial Trojan Horses. This contribution for honoring my mentor Helmut Schwarz echoes two motifs of my work with him: Hydroxylamin, the topic of my first paper ever, and the fascinating properties of iron ions, studied in the gas phase during my Ph.D. Thesis, became a core subject of our current chemical biology research on antiinfectives.