A semi rigid novel hydroxamate AMPED-based ligand for⁸⁹ Zr PET imaging

In this work, we designed, developed, characterized, and investigated a new chelator and its bifunctional derivative for⁸⁹ Zr labeling and PET-imaging. In a preliminary study, we synthesized two hexadentate chelators named AAZTHAS and AAZTHAG, based on the seven-membered hetero-cycle AMPED (6-amino-6-methylperhydro-1,4-diazepine) with the aim to increase the rigidity of the⁸⁹ Zr complex by using N-methyl-N-(hydroxy)succinamide or N-methyl-N-(hydroxy)glutaramide pendant arms attached to the cyclic structure. N-methylhydroxamate groups are the donor groups chosen to efficiently coordinate⁸⁹ Zr. After in vitro stability tests, we selected the chelator with longer arms, AAZTHAG, as the best complexing agent for⁸⁹ Zr presenting a stability of 86.4 ± 5.5% in human serum (HS) for at least 72 h. Small animal PET/CT static scans acquired at different time points (up to 24 h) and ex vivo organ distribution studies were then carried out in healthy nude mice (n = 3) to investigate the stability and biodistribution in vivo of this new⁸⁹ Zr-based complex. High stability in vivo, with low accumulation of free⁸⁹ Zr in bones and kidneys, was measured. Furthermore, an activated ester functionalized version of AAZTHAG was synthesized to allow the conjugation with biomolecules such as antibodies. The bifunctional chelator was then conjugated to the human anti-HER2 monoclonal antibody Trastuzumab (Tz) as a proof of principle test of conjugation to biologically active molecules. The final⁸⁹ Zr labeled compound was characterized via radio-HPLC and SDS-PAGE followed by autoradiography, and its stability in different solutions was assessed for at least 4 days.