Thwarting Isomerization through Rigidity: A Promising HBED Derivative for the Chelation of Gallium-68

The hexadentate acyclic ligand, N,N'-di(2-hydroxybenzyl)-(1,2-cyclohexanediamine)-N,N'-diacetic acid (HBCD) designed for the chelation of the positron-emitting radiometal ⁶⁸Ga was developed by replacing the flexible ethylenediamine backbone of its parent ligand, N,N'-di(2-hydroxybenzyl)ethylenediamine-N,N'-diacetic acid (HBED), with a more rigid cyclohexane diamine scaffold (DACH). This aims to hinder the formation of multiple isomers upon Ga³⁺-complexation as observed in HBED-containing molecules, which could affect the in vivo behavior of ⁶⁸Ga-labeled radiopharmaceuticals. To this end, we report the synthesis of HBCD, a comprehensive investigation of its acid-base behavior, its Ga³⁺ coordination chemistry, its labeling performances with generator-produced ⁶⁸Ga, and the stability of the corresponding radioactive complex in physiological media. Our findings confirm that the DACH scaffold promotes the formation of a hexacoordinated single-isomer Ga³⁺ complex. Although Ga³⁺-HBCD resulted less thermodynamically stable than Ga³⁺-HBED, it is by far more stable than the Ga³⁺ complex formed with the clinical workhorse DOTA chelator. HBCD demonstrated the ability to bind [⁶⁸Ga]Ga³⁺ under extremely diluted radiochemical conditions (C_L = 10^-6 M, 90 °C, pH 4.5 and 7). Notably, [⁶⁸Ga][Ga(HBCD)]^- shows exceptional stability in biological media. These results position HBCD as a highly attractive chelator for the development of next-generation PET radiotracers, effectively addressing the issue of isomerization in its parent ligand HBED.