The effects of trace metal impurities on Ga-68-radiolabelling with a tris(3-hydroxy-1,6-dimethylpyridin-4-one) (THP) chelator

Abstract

GMP-grade ⁶⁸Ge/⁶⁸Ga generators provide access to positron-emitting ⁶⁸Ga, enabling preparation of Positron Emission Tomography (PET) tracers and PET imaging at sites that do not have access to cyclotron-produced radionuclides. Radiotracers based on tris(3-hydroxy-1,6-dimethylpyridin-4-one) (THP) chelators enable simple one-step preparations of ⁶⁸Ga PET radiopharmaceuticals from pre-fabricated kits without pre-processing of generator eluate or post-purification. However, trace metal impurities eluted along with ⁶⁸Ga could compete for THP and reduce radiochemical yields (RCY). We have quantified trace metal impurities in ⁶⁸Ga eluate from an Eckert & Ziegler (E&Z) generator using ICP-MS. The metals Al, Fe, ^natGa, Pb, Ti and ^natZn were present in generator eluate in significantly higher concentrations than in the starting eluent solution. Concentrations of Fe and ^natGa in eluate were in the range of 0.01–0.1 μM, Al, Zn and Pb in the range of 0.1–1 μM, and Ti in the range of 0.9–1.5 μM. To assess the ability of THP to chelate ⁶⁸Ga in the presence of such metal ions, radiolabelling reactions were undertaken in which selected metal ions were added to make them equimolar with THP, or higher. Al³⁺, Fe³⁺, ^natGa³⁺ and Ti⁴⁺ reduced RCY at concentrations equimolar with THP and higher, but at lower concentrations they did not affect RCY. Pb²⁺, Zn²⁺, Ni²⁺ and Cr³⁺ had no effect on RCY (even under conditions in which each metal ion was present in 100-fold molar excess over THP). The multi-sample ICP-MS analysis reported here is (to date) the most comprehensive and robust quantification of metal impurities in the widely used E&Z ⁶⁸Ga generator. ⁶⁸Ga from an E&Z generator enables near-quantitative radiolabelling of THP at chelator concentrations as low as 5 μM (lower than other common gallium chelators) without pre-processing. The combination of Al³⁺, Fe³⁺, ^natGa³⁺ and Ti⁴⁺ in unprocessed ⁶⁸Ga eluate is likely to decrease RCY of ⁶⁸Ga radiolabelling if a lower amount of THP chelator is used, and future kit design should take this into account. To increase specific activities by using even lower THP concentrations, purification of ⁶⁸Ga from trace metal ions will likely be required.