18F is the most important clinically relevant radionuclide. At present 18F is imported from external cyclotrons as [18F] flouride. It is used for research and development but also for the routine synthesis of 18F-labelled PET-radiopharmaceuticals for human studies and medical patient care:
• 18F-labelled benzamides for the dopaminergic system
• 18F-benzodiazepines for the GABA/benzodiazepine system
• 18F-ligands for the opiate system
• 18F-labelled ligands for the NMDA system
• 18F-labelled amino acids
• 18F-labelled choline
• 18F-labelled enzyme-ligands
Synthesis with metallic and longer-lived positron emitters
For special problems it is very interesting to consider at new positron emitters. That refers to radionuclides which are used for the study of longer-lasting physiological processes in the range of several hours or days. For these studies the longer-lived radionuclides 86Y, 90Nb, 72As and the system of 140Nd/Pr. This includes measurements of nuclear data, targetry, radiochemical separation and syntheses of new labelling synthesis and labelled compounds. Irradiations are executed in cooperation with our external partners.
• 86Y (T1/2 = 14.7 h, 33% β+)
• 68Ga (T1/2 = 68 min, 98% β+)
• 72As (T1/2 = 26,0 h, 88% β+)
The 86Y is used as a chemical analogue of the therapentic radionuclide 90Y and is used to determine the potential of quantitative PET for the determination of the radiation-dosimetry of analogue 90Y-labelled radiotherapeutics. The 68Ga is a product of the 68Ge/Ga-radionuclide generator, which was optimized in our group. It is used currently as [68Ga]DOTATOC or [68Ga]DOTANOC for the diagnosis of tumors and metastases using PET/CT especially in cooperation with the Katharinenhospital Stuttgart, the Zentralklinik Bad Berka and many others.
Antibodies, fragments of antibodies and particularly “customized” peptides are of a certain interest for specific oncological relevant research. These tumor-affine molecules show a biological half-live in the range of many hours of days. For the synthesis of corresponding radiopharmaceuticals, positron-emitters of a longer physically half-live are required. Candidates are radionuclides like:
• 72As (T1/2 = 26.0 h, 88% β+)
• 90Nb (T1/2 = 14.6 h, 53% β+)
For these isotopes the nuclear parameters are measured, effective radiochemical separation from a target which was irradiated in a cyclotron is developed and different radiopharmaceuticals are synthesized. Those syntheses concern in principle the binding of the radio metals via a bifunctional chelating agent (coordinative binding of the metal to the chelating agent and a covalent coupling of the complex by a free functional group of the chelate ligand to a suitable functional group of the targeting vector). In case of the radioarsenic isotopes alternative functions of coupling have to be created.