Preclinical Molecular Imaging using Multi-Isotope Digital Autoradiography - Techniques and Applications

Abstract: Molecular imaging, both in vivo and ex vivo, is playing an increasingly important role in preclinical medical research. When using radionuclide-labeled tracers, e.g. in the development of radiopharmaceuticals for diagnostic imaging or for radionuclide therapy, quantitative in vivo imaging can be performed using emission tomography. Quantitative autoradiography of thin tissue sections is employed to obtain high-resolution images of the radioactivity distribution ex vivo, commonly using photo¬graphic film or a storage phosphor screen. The aim of the work presented in this thesis was to evaluate the potential of digital auto-radiography with multi-radionuclide imaging capabilities to contribute to pre¬clinical, small-animal, research studies using radiolabeled targeting molecules for diagnosis or therapy. The characteristics of a digital autoradiography system employing a double-sided silicon strip detector were investigated with regard to spatial resolution, detection sensitivity, background and noise, system dead time, and the ability to resolve energy spectra. This system, although it has a smaller field of view, was found to perform favorably compared to a storage phosphor system. Methods of separating the contributions from several radionuclides imaged simultaneously were developed and evaluated. The intratumoral distribution of 177Lu-labeled monoclonal antibodies (mAbs) over time was studied in a syngeneic rat model of colon carcinoma at both therapeutic and lower activity levels. The activity was initially found in the tumor periphery, then in areas of viable, antigen-expressing cells, and at 24-48 h and later in areas of granulation tissue and low antigen expression. A point-dose kernel was used to calculate the absorbed-dose rate distribution in tumor sections, which was found to be twice the section mean in areas with high activity accumulation. The distributions of 111In- and 177Lu-labeled mAbs targeting intercellular adhesion molecule 1 were studied in a xenograft model of human prostate cancer, and compared with a control mAb as well as clinically used 18F-labelled tracers using small animal SPECT/CT and PET/CT, as well as multi-radionuclide digital auto¬radio¬graphy. Results from this study, and those from an 125I-labelled mAb compared with a co-injected smaller antibody fragment labeled with 131I, both targeting carcino¬embryonic antigen, confirmed that barriers to tumor penetration by macro-molecules are a major problem, especially in radioimmunotherapy. A mAb against free prostate-specific antigen was also studied. Using multi-radionuclide digital autoradiography, the plaque-to-aorta contrast of a mAb targeting oxidized low-density lipoprotein was observed to be higher than that of 18FDG in slide mounted aortas from atherosclerotic mice. These studies show that digital autoradiography, including multi-radionuclide imaging, has the potential to provide novel data for preclinical imaging studies, but that further development and optimiz¬ation of the method are needed.