Tuning the Properties of Rigidified Acyclic DEDPA 2- Derivatives for Application in PET Using Copper-64

We present a detailed investigation of the coordination chemistry toward [ nat Cu/ 64 Cu]copper of a series of HDEDPA derivatives (HDEDPA = 6,6'-((ethane-1,2-diylbis(azanediyl))bis(methylene))dipicolinic acid) containing cyclohexyl (HCHXDEDPA), cyclopentyl (HCpDEDPA) or cyclobutyl (HCBuDEDPA) spacers. Furthermore, we also developed a strategy that allowed the synthesis of a HCBuDEDPA analogue containing an additional NHBoc group at the cyclobutyl ring, which can be used for conjugation to targeting units. The X-ray structures of the Cu(II) complexes evidence distorted octahedral coordination around the metal ion in all cases. Cyclic voltammetry experiments (0.15 M NaCl) evidence quasi-reversible reduction waves associated with the reduction of Cu(II) to Cu(I). The complexes show a high thermodynamic stability, with log K values of 25.11(1), 22.18(1) and 20.19(1) for the complexes of CHXDEDPA 2-, CpDEDPA 2- and CBuDEDPA 2-, respectively (25 °C, 1 M NaCl). Dissociation kinetics experiments reveal that both the spontaneous- and proton-assisted pathways operate at physiological pH. Quantitative labeling with 64 CuCl was observed at 0.1 nmol for CHXDEDPA 2- and CpDEDPA 2-, 0.025 nmol for CBuDEDPA 2- and 1 nmol for CBuDEDPA-NHBoc 2-, with no significant differences observed at 15, 30, and 60 min. The radio-complexes are stable in PBS over a period of 24 h. The nature of the rigid central spacer in acyclic chelators has a significant impact in the properties of their toward [ nat Cu/ 64 Cu]copper complexes