Th^IV-Desferrioxamine: characterization of a fluorescent bacterial probe

Diversifying our ability to guard against emerging pathogenic threats is essential for keeping pace with global health challenges, including those presented by drug-resistant bacteria. Some modern diagnostic and therapeutic innovations to address this challenge focus on targeting methods that exploit bacterial nutrient sequestration pathways, such as the desferrioxamine (DFO) siderophore used by Staphylococcus aureus (S. aureus) to sequester Fe^III. Building on recent studies that have shown DFO to be a versatile vehicle for chemical delivery, we show proof-of-principle that the Fe^III sequestration pathway can be used to deliver a potential radiotherapeutic. Our approach replaces the Fe^III nutrient sequestered by H₄DFO⁺ with Th^IV and made use of a common fluorophore, FITC, which we covalently bonded to DFO to provide a combinatorial probe for simultaneous chelation paired with imaging and spectroscopy, H₃DFO_FITC. Combining insight provided from FITC-based imaging with characterization by NMR spectroscopy, we demonstrated that the fluorescent DFO_FITC conjugate retained the Th^IV chelation properties of native H₄DFO⁺. Fluorescence microscopy with both [Th(DFO_FITC)] and [Fe(DFO_FITC)] complexes showed similar uptake by S. aureus and increased intercellular accumulation as compared to the FITC and unchelated H₃DFO_FITC controls. Collectively, these results demonstrate the potential for the newly developed H₃DFO_FITC conjugate to be used as a targeting vector and bacterial imaging probe for S. aureus. The results presented within provide a framework to expand H₄DFO⁺ and H₃DFO_FITC to relevant radiotherapeutics (like ²²⁷Th).