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 degrees 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.