Zirconium and copper immunoPET imaging agents for the diagnosis of cancer

© 2018 Dr. Stacey Erin RuddPositron emission tomography (PET) is a medical diagnostic imaging technique that is particularly useful in cancer diagnostics. In addition to tumour burden assessment, the technology can also provide molecular information, such as specific receptor expression profiles of cancer cells. The technique requires administration of an imaging agent to a patient, and the imaging agent must incorporate a positron emitting radionuclude. The positron-emitting radionuclide may be tethered to a targeting motif for accumulation at disease-afflicted tissues in the patient. Tight control over the biodistribution of the radionuclide is essential for both patient safety and image quality, and the extremely high affinity and specificity of monoclonal antibodies (mAbs) for their target antigen makes them an excellent candidate for delivering a radioisotope to a site of interest in vivo. The long pharmacokinetic half-life of antibodies, however, necessitates the use of a long-lived radionuclide such as 89Zr (t1/2 79 h) or 64Cu (t1/2 12.7 h). This thesis presents the development of novel PET imaging agents using 89Zr and 64Cu. Bifunctional chelators that can be used to attach these metal-based radionuclides to antibodies are described. The synthesis, bioconjugation, radiolabelling and preclinical imaging of various 89Zr immunoPET imaging agents using desferrioxamine (DFO) squaramide are presented. Our DFO-squaramide-based imaging agents outmatched the commercially-available isothiocyanate analogue. A DFO squaramide derivative suitable for use in enzyme-assisted, site-specific bioconjugations was developed, and successfully employed in the modification of antibody fragments using the bacterial transpeptidase Sortase A. The various forms of desferrioxamine squaramide conjugates were then assessed for suitability with various targeting agents, including mAbs and antibody fragments. These conjugates were evaluated as imaging agents in mice models of various cancer types that exhibit overexpression of the human epidermal growth factor receptors. The sarcophagine MeCOSar was also evaluated for [64Cu]Cu2+ labelling of antibodies and fragments. The bioconjugation of the ligand was performed using both an activated ester derivative and a Sortase A mediated coupling. The ligand was assessed in mice models of cancer using various antibodies and fragments as each conjugate exhibited unique biodistribution profiles. The most effective immunoPET agent for imaging of HER2-positive tumours was based on a full IgG construct with trastuzumab, although in a clinical setting the practical and logistical benefits of the F(ab’)2 imaging agent may outweigh its comparatively reduced uptake in tumour tissue