"Off-Label Use" of the Siderophore Enterobactin Enables Targeted Imaging of Cancer with Radioactive Ti^(IV)

The development of inert, biocompatible chelation methods is required to harness the emerging positron emitting radionuclide ⁴⁵Ti for radiopharmaceutical applications. Herein, we evaluate the Ti^(IV)-coordination chemistry of four catechol-based, hexacoordinate chelators using synthetic, structural, computational, and radiochemical approaches. The siderophore enterobactin (Ent) and its synthetic mimic TREN-CAM readily form mononuclear Ti^(IV) species in aqueous solution at neutral pH. Radiolabeling studies reveal that Ent and TREN-CAM form mononuclear complexes with the short-lived, positron-emitting radionuclide ⁴⁵Ti^(IV), and do not transchelate to plasma proteins in vitro and exhibit rapid renal clearance in naïve mice. These features guide efforts to target the ⁴⁵Ti isotope to prostate cancer tissue through the design, synthesis, and evaluation of Ent-DUPA, a small molecule conjugate composed of a prostate specific membrane antigen (PSMA) targeting peptide and a monofunctionalized Ent scaffold. The [⁴⁵Ti][Ti(Ent-DUPA)]^2- complex forms readily at room temperature. In a tumor xenograft model in mice, selective tumor tissue accumulation (8±5 %, n=5), and low off-target uptake in other organs is observed. Overall, this work demonstrates targeted imaging with ⁴⁵Ti^(IV), provides a foundation for advancing the application of ⁴⁵Ti in nuclear medicine, and reveals that Ent can be repurposed as a ⁴⁵Ti-complexing cargo for targeted nuclear imaging applications.