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Abstract:
The new cyclohexane-fused CyAAZTA ligand was synthesized to increase the structural rigidity of the heptadentate chelator AAZTA with the aim of improving the overall stability of its GaIII complex. The stability constant of Ga(CyAAZTA)−, determined both by pH-potentiometry (logKGaL=21.39) and by 71Ga NMR (logKGaL=21.92), was found similar to that of GaAAZTA (logKGaL=22.18). The kinetic inertness of Ga(CyAAZTA)− was investigated by following its transmetallation and ligand exchange reactions with Cu2+ and human serum transferrin, respectively. The formation of a hydroxido-complex near pH 7 decreases the half-life (t1/2) of the dissociation reactions for Ga(CyAAZTA)− compared to Ga(AAZTA)− (8.5 h vs 21 h, pH 7.4). However, at pH < 7 the t1/2 of Ga(CyAAZTA)− is much longer (234 h at pH 6). Finally, CyAAZTA was successfully radiolabelled with 68Ga in acetate buffer at pH 3.8, in 15 minutes at room temperature at [CyAAZTA]=10 μM, with a labelling yield higher than 80 . A 1 μM solution of CyAAZTA was successfully labelled (L.Y.: 97.4 ) in 5 minutes at 90 °C. Stability tests in human serum and in the presence of 50 mM DTPA showed no significant decomposition of 68GaCyAAZTA over 90 minutes.
