Equilibrium, Kinetic and Structural Properties of Gallium(III) and Some Divalent Metal Complexes Formed with the New DATAm and DATA5m Ligands

The development of Ge-68/Ga-68 generators has made the positron-emitting Ga-68 isotope widely accessible and raised interest in new chelate complexes of Ga3+. The hexadentate 1,4-di(acetate)-6-methyl[ amino(methyl) acetate]perhydro- 1,4-diazepane (DATA(m)) ligand and its bifunctional analogue, 1,4-di(acetate)-6-pentanoic acid[ amino(methyl) acetate] perhydro-1,4-diazepane (DATA(5m)), rapidly form complexes with Ga-68 in high radiochemical yield. The stability constants of DATAm and DATA5m complexes formed with Ga3+, Zn2+, Cu2+, Mn2+ and Ca2+ have been determined by using pH potentiometry, spectrophotometry (Cu2+) and H-1 and 71Ga NMR spectroscopy (Ga3+). The stability constants of Ga(DATAm) and Ga(DATA(5m)) complexes are slightly higher than those of Ga(AAZTA). The species distribution calculations indicated the predominance of Ga(L) OH mixed-hy-droxo complexes at physiological pH. The H-1 and Ga-71 NMR spectroscopy studies provided information about the coordinated functional groups of ligands and on the kinetics of exchange between the Ga(L) and Ga(L) OH complexes. The transmetalation reactions between the Ga(L) complexes and Cu2+ citrate (6< pH< 8.5) occur through both spontaneous and OH--assisted dissociation of the Ga(L) OH species. At pH 7.4 and 25 degrees C, the half-lives of the dissociation of Ga(DATAm), Ga(DATA(5m)) and Ga(AAZTA) were 11, 44 and 24 h, respectively. Similar half-lives have been obtained for the ligand-exchange reactions between the Ga(L) OH complexes and transferrin. The equilibrium and kinetic data indicate that the Ga(DATA(5m)) complex is a good Ga-68-based radiodiagnostic candidate.