ENHANCING RIGIDITY TO RULE ISOMERIZATION: A PROMISING HBED DERIVATIVE FOR CHELATING GALLIUM-68

Aim/Introduction The hexadentate acyclic ligand, N,N’-di(2-hydroxybenzyl)-(1,2-cyclohexanediamine)-N,N’-diacetic acid (HBCD) designed for the chelation of 68Ga was developed by replacing the flexible ethylenediamine backbone of its parent ligand HBED with a more rigid cyclohexane diamine scaffold (DACH), with the aim of hindering the formation of multiple isomers upon Ga3+-complexation as observed for HBED. To this end, we report the synthesis of HBCD, a comprehensive investigation of its acid-base behavior, its Ga3+ coordination chemistry, its labelling performances with generator-produced 68Ga, and the stability of the corresponding radioactive complex in physiological media. Materials and Methods UV-Vis pH-spectrophotometric titrations were conducted at T = 25°C and I = 0.15 M NaCl (CGa = CHBCD = 20-50 μM). A 1850 MBq 68Ge/68Ga generator (GalliaPharm, Ezag, Berlin) was manually eluted with 0.1 M HCl (5 mL). HBCD and HBED stock solutions were prepared in ultrapure water at 1.0 × 10−3 M and diluted appropriately to give a serial dilution series (1.0 × 10−4−1.0 × 10−8 M). Labelling reactions were evaluated in term of temperature, pH and ligand concentration. RCI was determined via radio-TLC on silica plates (CH3OH/NH4OAc 1 M 1/1 V/V). Results Our work confirms that the DACH scaffold promotes the formation of a hexacoordinated single-isomer Ga3+ complex (Fig. 1A). As for Ga3+-HBED, the prevailing species at physiological pH (7.4) is [GaL]-, and the two systems have comparable thermodynamic stability. Ga3+-HBCD is significantly more stable than the Ga3+ complex formed by the clinical workhorse DOTA chelator. HBCD also demonstrates the ability to bind [68Ga]Ga3+ under extremely diluted radiochemical conditions (CL = 10−6 M, 90°C, pH 4.5/7 and 10−5 M, RT) (Fig. 1B). Conclusions Notably, [68Ga][Ga(HBCD)]− shows exceptional stability in biological media, feature that positions 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.