AAZTA-Like Ligands Bearing Phenolate Arms as Efficient Chelators for Ga-68 Labelling in vitro and in vivo

The introduction of a phenolate pendant arm in place of an acetate on AAZTA- and DATA-like ligands resulted in hepta- and hexadentate chelators able to form Ga(III) complexes with thermodynamic stability and kinetic inertness higher than that of other Ga(III) complexes based on the parent 6-amino-6-methylperhydro-1,4-diazepine scaffold. In particular, the heptadentate AAZ3A-endoHB with a phenolate arm on an endocyclic N-atom shows a logK(GaL) of 27.35 and a remarkable resistance to hydroxide coordination up to basic pH (pH > 9). This behaviour allows to also improve the kinetic inertness of the complex showing a dissociation half-life (t(1/2)) at pH 7.4 of 76 h. Although also the hexadentate AAZ2A-exoHB chelator forms a stable (logK(GaL)=24.69) and inert (t(1/2)=33 h at pH 7.4) Ga(III) complex, the Ga-68 labelling showed a better radiochemical yield with AAZ3A-endoHB, especially at room temperature. Thus, a bifunctional chelator of AAZ3A-endoHB was synthesized bearing an isothiocyanate group that was conjugated to the N-terminus of a c(RGD) peptide for integrin receptor targeting. Finally, the conjugate was successfully labelled with Ga-68 isotope, and the resulting radiotracer tested for its stability in human serum and then in vivo for targeting B16-F10 tumours with miniPET imaging.