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68Ga-Labeled radiopharmaceuticals for pretargeted PET imaging : synthesis of 68Ga-HBED-CC-tetrazines

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Title: 68Ga-Labeled radiopharmaceuticals for pretargeted PET imaging : synthesis of 68Ga-HBED-CC-tetrazines
Author(s): Honkaniemi, Elina
Contributor: University of Helsinki, Faculty of Science, Department of Chemistry
Discipline: Radiochemistry
Language: English
Acceptance year: 2017
An extremely high demand of 68Ga-radiotracers has ascended during the last decade in the field of nuclear medicine. 68Ga is a positron emitting radionuclide which is widely used in positron emission tomography (PET) studies due to its short half-live of 68 minutes and good labeling properties. 68Ga can be produced on site using 68Ge/68Ga-generator which makes it a viable option in comparison to the cyclotron-based PET isotopes such as 18F. Gallium’s coordination chemistry demands it to be coordinated with a ligand for it to be stable enough to be introduced to target peptide. For that reason, chelating agents are commonly used to stabilize gallium, such as DOTA, NOTA or TRAP. Major of the chelators are bifunctional which means they can also bind to the target biomolecule such as peptide or antibody. Tetrazine molecule is widely used in bioorthogonal cycloaddition reactions in live cell labeling. Bioorthogonal reactions are used in pretargeting approach where first the unlabeled antibody is administrated to localize the tumour and after that, small rapid-clearing radiolabeled compound is injected to attach to the antibody. In this work, bifunctional chelating agent HBED-CC was first coupled with tetrazine and after successful synthesis, the compound was labeled with 68Ga. Aim of the study was to discover the potential of this compound to pass the cell membrane and to determinate its properties. The synthesis of HBED-CC-tetrazine was successfully optimized with good yields in a range of 65-85 %. Different synthesis conditions were tested including temperature, reaction time and the choice of a coupling reagent. Optimized conditions for the synthesis of HBED-CC-tetrazine were 48 hours in room temperature using the coupling reagent HATU. Radiosynthesis of [68Ga] Ga-HBED-CC-tetrazine was also optimized using different temperatures, reaction times and precursor amounts. All conditions resulted in good radiochemical yields. Optimized conditions for radiolabeling turned out to be in 85 degrees for 20 minutes which resulted in 97 % of radiochemical yield with over 98 % radiochemical purity. The properties of the labeled compound [68Ga] Ga-HBED-CC-tetrazine were tested, such as lipophilicity and the stability of the compound in a presence of iron.

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