Spectroscopic, radiochemical, and theoretical studies of the Ga 3+ ‐ N ‐2‐hydroxyethyl piperazine‐ N ′‐2‐ethanesulfonic acid (HEPES buffer) system: evidence for the formation of Ga 3+ ‐ HEPES complexes in 68 Ga labeling reactions

Recent reports have claimed a superior performance of HEPES buffer in comparison to alternative buffer systems for 67/68  Ga labeling in aqueous media. In this paper we report spectroscopic ( 1 H and 71  Ga NMR), radiochemical, mass spectrometry and theoretical modeling studies on the Ga 3+ /HEPES system (HEPES =  N ‐2‐hydroxyethylpiperazine‐ N ′‐2‐ethanesulfonic acid) performed with the aim of elucidating a potential contribution of HEPES in the 68/67  Ga radiolabeling process. Our results demonstrate that HEPES acts as a weakly but competitive chelator of Ga 3+ and that this interaction depends on the relative Ga 3+ : HEPES concentration. A by‐product formed in the labeling mixture has been identified as a [ 68  Ga]Ga(HEPES) complex via chromatographic comparison with the nonradioactive analog. The formation of this complex was verified to compete with [ 68  Ga]Ga(NOTA) complexation at low NOTA concentration. Putative chelation of Ga 3+ by the hydroxyl and adjacent ring nitrogen of HEPES is proposed on the basis of 1 H NMR shifts induced by Ga 3+ and theoretical modeling studies. Copyright © 2013 John Wiley & Sons, Ltd.