Radiolabelled antibody-targeted gold nanoparticles for cancer imaging : effects of nanoparticle conjugation on the biological properties of the antibody and tumor targeting

The last fifty years have seen remarkable progress in the prevention, detection and treatment of cancer. However, the current methods for cancer management including radiation, surgery and chemotherapy suffer from many disappointments, such as non-specific suppression of proliferating cancer cells, non-specific systemic distribution, inadequate drug concentration at the target site, developpment of resistance, and a limited ability to monitor the response to therapy. The concept of the “magic bullet”, proposed by Paul Ehrlich more than 100 years ago, postulated that the ideal drug should selectively target and destroy the cause of disease. Therefore, due to their high specificity, the monoclonal antibodies have shown a great promise in the treatment of cancer. However, the effectiveness of antibodies against cancer, and specifically against solid tumours, which are harder to treat, might be improved. In this context, antibodies are being administered in combination with other therapeutic strategies, such as chemotherapy or radiation, or developed as targeted vehicles by attaching toxins or radionuclides. An extension of these approaches is the study of antibody-modified nanomaterials, and particularly antibody-functionalized gold nanoparticles, which use gold nanoparticles as therapeutic agents and antibodies as targeting ligands, offering a promise of selectively conducting the nanoparticles to tumour cells, and thus a focused targeting thanks to the antibody selectivity. These gold nanoparticles may be used for photothermal therapy, as sensitizers in radiation therapy, and offer more flexibility of design in providing platforms for binding of several therapeutic agents in a single structure for effective improvement of cancer therapy. The general goal of this thesis was to explore the efficiency of targeting cancer cells by using antibodies directed against EGFR, or anti-CD105 antibodies directed against endoglin after their conjugation to gold nanoparticles. It is a proof of concept, where these immunonanoconjugates were evaluated as an imaging tool with the hope of operating them in future therapies and even in theranostic purposes. The approach of conjugating bioactive anticancer molecules, such as antibodies, to nanoparticles has some limitations, notably the risk of losing the antibody specificity for the target after the conjugation reaction. Thus, the assessment of target recognition properties and comparative biodistribution studies of antibodies before and after conjugation to gold nanopaticles were performed. In order to trace these immunonanoconjugates in vivo, the antibodies were firstly radiolabelled before nanoparticle conjugation. For conjugation reaction to gold nanoparticles, we chose two types of monoclonal antibodies directed against EGFR or endoglin (CD105). In this work, our efforts focused on the development of a tool that could be used as an imaging probe in a first step. The first specific aim consisted in the assessment of the effect of nanoparticle conjugation on the antibody biodistribution and on the antibody specificity for its target. Regarding antibody recognition properties, we studied the binding properties of these immunanoconjugates in vitro and in vivo through blocking experiments, and we tried to explain the reasons of the possible loss of antibody immunoreactivity through structural characterization of these conjugates. Another aspect that was treated in this work consisted in justifying the choice of the radiolabel for tracking these immunoconjugates in vivo. We initially directly radiolabelled the antibodies with radioiodine, a common and easy procedure used in laboratory. The unexpected tumour uptake profile of iodinated anti-CD105 antibodies, compared with the known kinetics of most intact antibodies, led us to reconsider the choice of the radiolabel. We studied the biological distribution of anti-CD105 antibodies in mice bearing tumours (two tumour models), and we compared the pharmacokinetic profile of iodinated anti-CD105 antibodies and that of 89Zr-labelled anti-CD105 antibodies, as validated tracers, in order to highlight the limitations of the direct anti-CD105 antibody radioiodination on stable antibody tracking. Besides the validation of these immunonanoconjugates as tracers suitable for cancer imaging, we have high expectations of investigating their therapeutic potential. As highly potent and selective drugs are still lacking, we hope that these assemblies, as novel agents for cancer management, will provide an effective strategy to improve cancer therapy through combining the targeting properties of antibodies and the additional therapeutic properties of gold nanoparticles.