How is 68Ga Labeling of Macrocyclic Chelators Influenced by Metal Ion Contaminants in 68Ge/68Ga Generator Eluates?



To assess the influence of Zn2+, Cu2+, Fe3+, Al3+, TiIV, and SnIV on incorporation of 68Ga3+ into pendant-arm macrocyclic chelators, the 68Ga labeling of 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA), 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), 1,4,7-triazacyclononane-1,4,7-tris[methyl(2-carboxyethyl)phosphinic acid]) (TRAP), and 1,4,7-triazacyclononane-1-[methyl(2-carboxyethyl)phosphinic acid]-4,7-bis[methyl(2-hydroxymethyl)phosphinic acid] (NOPO), as well as their peptide conjugates, was investigated in the presence of varying concentrations of these metal ions. The 68Ga labeling yield for carboxylate-type chelators NOTA and DOTA is decreased at lower metal ion contaminant concentrations compared with phosphinate-type chelators TRAP and NOPO. The latter are able to rapidly exchange coordinated ZnII with 68Ga3+, as confirmed by mass spectrometry and 31P NMR spectroscopy. 68Ga labeling of ZnII complexes of TRAP and NOPO proceeds as efficient as labeling of neat NOTA; this applies also to the corresponding peptide conjugates of these chelators. This behavior results in substantially improved selectivity for Ga3+ and, therefore, in more robust and reliable 68Ga labeling procedures. In addition, none of the investigated chelators binds 68Ge, rendering post-labeling purification protocols, for example, solid-phase extraction, a reliable means of removal of 68Ge contamination from 68Ga radiopharmaceuticals.