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⁺, Ti^IV, and Sn^IV 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 Zn^II with ⁶⁸Ga³⁺, as confirmed by mass spectrometry and ³¹PNMR spectroscopy. ⁶⁸Ga labeling of Zn^II 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.