Convenient Preparation of 68Ga-Based PET-Radiopharmaceuticals at Room Temperature

Abstract

A straightforward labeling using generator produced positron emitting ⁶⁸Ga, which provides high quality images, may result in kit type production of PET radiopharmaceuticals and make PET examinations possible also at centers lacking accelerators. The introduction of macrocyclic bifunctional chelators that would provide fast ⁶⁸Ga-complexation at room temperature would simplify even further tracer preparation and open wide possibilities for ⁶⁸Ga-labeling of fragile and potent macromolecules. Gallium-68 has the potential to facilitate development of clinically practical PET and to promote PET technique for individualized medicine. The macrocyclic chelator, 1,4,7-triazacyclononanetriacetic acid (NOTA), and its derivative coupled to an eight amino acid residue peptide (NODAGA-TATE, [NODAGA⁰, Tyr³]Octreotate) were labeled with ⁶⁸Ge/⁶⁸Ga-generator produced positron emitting ⁶⁸Ga. Formation kinetics of ⁶⁸Ga-NOTA was studied as a function of pH and formation kinetics of ⁶⁸Ga-NODAGA-TATE was studied as a function of the bioconjugate concentration. The nearly quantitative radioactivity incorporation (RAI > 95%) for ⁶⁸Ga-NOTA was achieved within less than 10 min at room temperature and pH 3.5. The concentrations of NODAGA-TATE required for RAI of >90% and >95% were, respectively, 2–5 and 10 µM. In both cases the purification of the ⁶⁸Ga-labeled products was not necessary since the radiochemical purity was >95% and the preparation buffer, 4-(2-hydroxyethyl) piperazine-1-ethanesulfonic acid (HEPES) is suitable for human use. In order to confirm the identity of the products, complexes comprising ^natGa were synthesized and analyzed by mass spectrometry. The complex was found to be stable in the reaction mixture, phosphate buffer, and human plasma during 4.5 h incubation. Free and peptide conjugated NOTA formed stable complexes with ⁶⁸Ga at room temperature within 10 min. This might be of special interest for the labeling of fragile and potent macromolecules and allow for kit type preparation of ⁶⁸Ga-based radiopharmaceuticals.