Rational Design, Synthesis and Preliminary Evaluation of Novel Fusarinine C-Based Chelators for Radiolabeling with Zirconium-89

Fusarinine C (FSC) has recently been shown to be a promising and novel chelator for ⁸⁹Zr. Here, FSC has been further derivatized to optimize the complexation properties of FSC-based chelators for ⁸⁹Zr-labeling by introducing additional carboxylic groups. These were expected to improve the stability of ⁸⁹Zr-complexes by saturating the 8-coordination sphere of [⁸⁹Zr] Zr⁴⁺, and also to introduce functionalities suitable for conjugation to targeting vectors such as monoclonal antibodies. For proof of concept, succinic acid derivatization at the amine groups of FSC was carried out, resulting in FSC(succ)₂ and FSC(succ)₃. FSC(succ)₂ was further derivatized to FSC(succ)₂ AA by reacting with acetic anhydride (AA). The Zr⁴⁺ complexation properties of these chelators were studied by reacting with ZrCl₄. Partition coefficient, protein binding, serum stability, acid dissociation, and transchelation studies of ⁸⁹Zr-complexes were carried out in vitro and the results were compared with those for ⁸⁹Zr-desferrioxamine B ([⁸⁹Zr]Zr-DFO) and ⁸⁹Zr-triacetylfusarinine C ([⁸⁹Zr]Zr-TAFC). The in vivo properties of [⁸⁹Zr]Zr-FSC(succ)₃ were further compared with [⁸⁹Zr]Zr-TAFC in BALB/c mice using micro-positron emission tomography/computer tomography (microPET/CT) imaging. Fusarinine C (succ)₂AA and FSC(succ)₃ were synthesized with satisfactory yields. Complexation with ZrCl₄ was achieved using a simple strategy resulting in high-purity Zr-FSC(succ)₂AA and Zr-FSC(succ)₃ with 1:1 stoichiometry. Distribution coefficients of ⁸⁹Zr-complexes revealed increased hydrophilic character compared to [⁸⁹Zr]Zr-TAFC. All radioligands showed high stability in phosphate buffered saline (PBS) and human serum and low protein-bound activity over a period of seven days. Acid dissociation and transchelation studies exhibited a range of in vitro stabilities following the order: [⁸⁹Zr]Zr-FSC(succ)₃ > [⁸⁹Zr]Zr-TAFC > [⁸⁹Zr]Zr-FSC(succ)₂AA >> [⁸⁹Zr]Zr-DFO. Biodistribution studies of [⁸⁹Zr]Zr-FSC(succ)₃ revealed a slower excretion pattern compared to [⁸⁹Zr]Zr-TAFC. In conclusion, [⁸⁹Zr]Zr-FSC(succ)₃ showed the best stability and inertness. The promising results obtained with [⁸⁹Zr]Zr-FSC(succ)₂AA highlight the potential of FSC(succ)₂ as a monovalent chelator for conjugation to targeted biomolecules, in particular, monoclonal antibodies.