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

Abstract

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.

The Race for Hydroxamate-Based Zirconium-89 Chelators

“…Fast pharmacokinetics with predominant renal excretion was observed. The results were confirmed by PET/CT imaging [55].…”

“…Assessment of the 89 Zr-complexes and conjugates by transchelation experiments in the presence of 1000-fold excess EDTA at different pH revealed that all FSC-based chelators performed better than DFO with the highest stability for 89 Zr-TAFC and 89 Zr-FSC(succ) 3 (e.g., at pH 7 89 Zr-DFO lost close to 60% of the metal ion, where the next weakest ligand 89 Zr-FSC(succ-RGD) 3 lost only 4%.) [54,55]. As seen in previously discussed examples, this can be ascribed to both the proper denticity of the chelator and the macrocyclic effect.…”

“…In 2015, Zhai et al reported on the 89 Zr-labeling of triacetyl fusarinine C (TAFC) and tumor targeting trimer FSC(succ-RGD) 3 with RGD being a small cyclic peptide [54]. In 2019, further examples including FSC(succ) 2 Ac and FSC(succ) 3 followed (Ac = acetate, succ = −COCH 2 CH 2 COOH) which expanded the hexadentate FSC to an octa-and potentially nonadentate chelator able to saturate the coordination sphere of Zr 4+ [55]. Radiolabeling of the novel chelators with 89 Zr-oxalate was achieved in quantitative yields at RT and neutral pH.…”

Predicting the Thermodynamic Stability of Zirconium Radiotracers

“…In total, optimised structures and reaction energetics for 23 different Zr 4+ complexes, using 17 different ligands (Figure 11) were calculated. 2,10,38,[46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62] Structures of the optimised Zr-complexes are presented in Figure 12 and key thermodynamic data are given in Table 2. The ligands chosen were selected based on their reported experimental potential to form stable complexes with 89 Zr 4+…”

Current Perspectives on 89Zr-PET Imaging

“…As most dechelated forms of 89 Zr ( 89 Zr-chloride, 89 Zr-citrate, and 89 Zr-oxalate) accumulate in the bone marrow, this process is attributed to the instability of 89 Zr-DFO chelation [ 30 ]. Recent studies in radiochemistry are paving the way toward improving stability by changing the structure of DFO or exploiting new chelators [ 114 , 115 , 116 , 117 , 118 , 119 , 120 ]. A decrease in bone marrow uptake is good for increasing sensitivity to lesions, as well as decreasing bone marrow toxicity for patients.…”