Rational Design of a Synthetic Galliphore for the Diagnostic Nuclear Imaging and Treatment of Bacterial Infections

We introduce a solid-phase synthesis platform for the rapid, modular, and scalable synthesis of the synthetic metallophore KO-NH₂, which is readily recognized by bacterial siderophore transporters, but has optimized affinity for Ga³⁺ over Fe³⁺. Four synthetic protochelin (PC) analogues, KK-COOH, KO-NH₂, KO-COOH, and OK-COOH were constructed. Growth recovery, radioactive uptake, and covalent photocrosslinker studies in Escherichia coli identified CirA and Fiu as the dominant outer membrane transport proteins capable of binding KO-NH₂-linked, bifunctional constructs. Computational studies further supported that the structural framework of PC can be effectively mimicked by KO or OK peptides. Thermodynamic stability studies revealed an atypical inversion of thermodynamic stability trends, with logK_GaL of 39.9 and a logK_FeL of 36, demonstrating improved stability with Ga³⁺. Studies in naïve mice with ⁶⁸Ga-labeled derivatives identify the lead [⁶⁸Ga][Ga(KO-NH₂)]^3-, which remained intact in vivo and demonstrated selective, enhanced uptake in an E. coli soft tissue model of infection (1.28 % ID/g, with a 3:1 ratio of infected to inflamed/normal muscle tissue). Linkage of KO-NH₂ to the antibiotic ampicillin produces an efficacious antibiotic, which supersedes the potency of the parent antibiotic ampicillin 4-fold (MIC₉₈ = 1.9 µM) effectively treats a soft tissue infection of ampicillin-resistant E. coli.