Convenient preparation of 68Ga-based PET-radiopharmaceuticals at room temperature

A straightforward labeling using generator produced positron emitting Ga-68, 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 Ga-68-complexation at room temperature would simplify even further tracer preparation and open wide possibilities for Ga-68-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, [NODAGAO(0), Tyr(3)] Octreotate) were labeled with Ge-68/Ga-68-generator produced positron emitting 68Ga. Formation kinetics of Ga-68-NOTA was studied as a function of pH and formation kinetics of Ga-68-NODAGA-TATE was studied as a function of the bioconjugate concentration. The nearly quantitative radioactivity incorporation (RAI > 95%) for Ga-68-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 mu M. In both cases the purification of the Ga-68-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 Ga-nat 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 Ga-68 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 Ga-68-based radiopharmaceuticals.