The global aim of the project will be to perform detailed computational dosimetry of targeted and untargeted tissues after targeted radionuclide therapy with 131I- and 211At-labelled anti-HER-2 Nanobodies with a major focus on kidney dosimetry.
General internal dosimetry tools following the MIRD schema use biokinetics of radiopharmaceuticals to calculate estimates of the radiation dose to all organs and the entire body. This organ-level or single-region dosimetry is based on the assumption of a uniform distribution of radioactivity across the entire organ. The average absorbed dose to the entire organ predicted by such models can misrepresent local regional doses to specific substructures (6). Variations in absorbed dose can be of great importance in the case of radiolabelled Nanobodies which are partially trapped in the cortical region of the kidneys.
Moreover, this importance will increase when Nanobodies are labelled with alpha-emitters for which smaller scale dosimetry methods should be applied. For alpha-particle emitters, accurate dosimetry calculations require knowledge of the activity distribution as a function of time at the cellular and even subcellular levels. To tackle the problem of preclinical dosimetry of 131I- and 211At-labelled anti-HER-2 Nanobodies several steps will be taken:
In a first phase, in vivo quantitative imaging at different time intervals of mice treated with 131I-labelled Nanobodies will be performed using the MILabs micro SPECT/PET/CT/MR imaging platform, recently installed at ICMI-Lab VUB. This platform allows imaging of anatomical tissues and of the biodistribution of 131I-labelled Nanobodies with a sub-mm spatial resolution. Measured Time-Activity Curves (TACs) will be used to obtain total cumulated activities in different tissues of interest per unit of administered activity of radiopharmaceutical. Additional spatial information at higher resolution will be obtained by parallel performed ex vivo autoradiography. Image segmentation and processing will allow generating a series of voxel phantoms of different targeted and untargeted tissues which will be used for Monte Carlo simulations to calculate S-factors for specific substructures of individual organs. This multiregional approach will be compared to histological analysis and renal function studies, which will be performed in parallel at ICMI.
In the next step of the project, the time and spatial biodistribution of the 131I-labelled Nanobodies can be used to perform multiregional computational dosimetry after a simulated radionuclide exchange of 131I by its alpha-emitting surrogate 211At. Validation of this proof of principle method should be done by studying the feasibility of in vivo quantitative imaging using micro SPECT/CT of mice after injection with 211At-labelled Nanobodies and ex vivo autoradiography. Again, this multiregional approach for 211At-labelled Nanobodies will be compared to histological analysis and renal function studies parallel performed at ICMI.
Finally the focus of the project will shift to smaller scale dosimetry of 211At-labelled Nanobodies. The input for this part will be delivered by a parallel ongoing study in which time-activity distributions of 211At-labelled Nanobodies in cell clusters of targeted and untargeted organs will be determined through ex vivo imaging using the Alpha Camera technology at the University of Gothenburg. This technology allows ex vivo imaging with a spatial resolution of 35 µm. Detailed information of the spatial distribution of 211At-labelled Nanobodies at this small scale should be used as input for dedicated computational dosimetry codes enabling to calculate absorbed doses to isolated cells and cell clusters of 211At-labelled Nanobodies for a simulated location at the cell surface or in the cytoplasm.
Results of the smaller scale and cell-level dosimetry approach will be evaluated against the more macroscopic multiregional approach.