Comparing physical and biological DNA Dosimetry after treatment with radiopharmaceuticals

• Applicants: Professor Dr. Michael Laßmann; Professor Dr. Harry Scherthan
• Subject Area: Nuclear Medicine, Radiotherapy, Radiobiology ; Medical Physics, Biomedical Technology
• Term: from 2012 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 204090103

Radiation-induced DNA double strand breaks (DSBs) cause the phosphorylation of the histone H2AX (then called gamma-H2AX) in the surrounding chromatin as well as the accumulation of the 53BP1 protein that binds to and signals damaged chromatin at a DSB site. This leads to the formation of microscopically visible nuclear foci containing both markers which thereby mark radiation-induced DSBs, especially in low dose scenarios. The latter makes this DNA-Damage-Focus (DDF) Assay an ideal endpoint for studying the dose response in patients after radionuclide therapy and to establish a correlation to physical dosimetry. We have initiated the latter approach in a previous project, whose aim was to determine the intra- and interpatient variability in the formation of colocalized gamma-H2AX und 53BP1 DNA-damage foci in the nuclei of lymphocytes in thyroid carcinoma patients after the first ablation therapy using I-131 (n=20) and to compare these results to the corresponding patient-specific absorbed doses to the blood. We also determined DNA-damage foci after Lu-177 labelled DOTA radiopeptide therapy (n=19) and after one Ra-223-dichloride therapy (four or more samples/patient).In a follow up study we will now calibrate the DDF-Assay in-vitro for low absorbed doses to the blood (< 100 mGy) by exposure to radionuclides using blood samples taken from volunteers. This calibration will be performed with beta/gamma emitting short-lived nuclides. Additionally, alpha particle emitters will be included for determining the relative biological effectiveness (RBE) in-vitro. The independent in-vitro calibration data will then be used to compare the data of the previous project. We aim to establish a method that is capable to reduce the intra-patient in-vivo variations observed in preceding projects.Furthermore, we will investigate whether addition of a phosphatase inhibitor to blood samples exposed to radionuclides can increase the DDF numbers at the first blood draw due to inhibition of repair processes. We will then develop a calculation method of the patient-specific absorbed dose to the blood for radiopharmaceuticals which are bound to the blood. For the determination of the RBE in-vivo for 20 patients treated with alpha-emitters will be included in the study. We will compare the results to the data of the patients of the previous study treated with beta emitters. In addition, we will scrutinize up to 15 patients after previously not included radionuclide therapies and 20 patients after diagnostics with positron emitters (e.g. Ga-68) for their DNA-Damage response using our DDF assay.Overall, the aim of this project is to the first time systematically calibrate the DDF-assay after internal radiation by radionuclides, to check if the assay is capable to determine the RBE for different radiation qualities and energies, and to test if the system is suited to compare absorbed doses to the blood after various radionuclide therapies and diagnostic procedures.

• DFG Programme: Research Grants