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 Zr. Here, FSC has been further derivatized to optimize the complexation properties of FSC-based chelators for Zr-labeling by introducing additional carboxylic groups. These were expected to improve the stability of Zr-complexes by saturating the 8-coordination sphere of [Zr] Zr, 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 Zr complexation properties of these chelators were studied by reacting with ZrCl₄. Partition coefficient, protein binding, serum stability, acid dissociation, and transchelation studies of Zr-complexes were carried out in vitro and the results were compared with those for Zr-desferrioxamine B ([Zr]Zr-DFO) and Zr-triacetylfusarinine C ([Zr]Zr-TAFC). The in vivo properties of [Zr]Zr-FSC(succ)₃ were further compared with [Zr]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 Zr-complexes revealed increased hydrophilic character compared to [Zr]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: [Zr]Zr-FSC(succ)₃ > [Zr]Zr-TAFC > [Zr]Zr-FSC(succ)₂AA >> [Zr]Zr-DFO. Biodistribution studies of [Zr]Zr-FSC(succ)₃ revealed a slower excretion pattern compared to [Zr]Zr-TAFC. In conclusion, [Zr]Zr-FSC(succ)₃ showed the best stability and inertness. The promising results obtained with [Zr]Zr-FSC(succ)₂AA highlight the potential of FSC(succ)₂ as a monovalent chelator for conjugation to targeted biomolecules, in particular, monoclonal antibodies.