Exploring the chemistry of H₂DEDPA derivatives for [^nat/68Ga]Ga³⁺ complexation

We report a detailed study of the coordination chemistry of acyclic hexadentate H₂DEDPA derivatives towards [^nat/68Ga]Ga³⁺ (H₂DEDPA = 6,6'-((ethane-1,2-diylbis(azanediyl))bis(methylene))dipicolinic acid). Three structural modifications were considered, involving the substitution of the central ethylene spacer of H₂DEDPA by 1,2-cyclohexyldiamine, 1,2-cyclopentyldiamine or 1,3-cyclobutyldiamine linkers, affording chelators H₂CHXDEDPA, H₂CpDEDPA and H₂CBuDEDPA, respectively. The X-ray structures of [Ga(CpDEDPA)](PF₆)·3H₂O and [Ga(CBuDEDPA)](ClO₄)·H₂O evidence hexadentate binding of the ligands to Ga³⁺, which displays a distorted octahedral coordination environment. Solution structures compare well to those observed in the solid state, as demonstrated by NMR studies and DFT calculations. The stability constants of the complexes were determined using spectrophotometric titrations (25 °C, 1 M NaCl), affording log K_GaL values of 24.94, 21.90 and 19.50 for the complexes of CHXDEDPA^2-, CpDEDPA^2- and CBuDEDPA^2-, respectively. Both CHXDEDPA^2- and CpDEDPA^2- can be quantitatively radiolabeled with 10 nmol [⁶⁸Ga]Ga³⁺ at room temperature (0.5 M ammonium acetate at pH 5) with no significant difference between 15, 30 and 60 min labeling time, whereas CBuDEDPA^2- produced a < 50 % radiochemical yield. The radiolabeled complexes of CHXDEDPA^2- and CpDEDPA^2- showed high stability in PBS buffer and in the presence of 1000 equivalents of DTPA^5-, with CpDEDPA^2- providing slightly better results. However, in vivo PET imaging and biodistribution studies evidence dissociation of the CpDEDPA^2- complex, while the [⁶⁸Ga][Ga(CHXDEDPA)]⁺ complex remains stable and demonstrates mixed, renal and hepatic clearance.