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Design, Synthesis, And Evaluation Of Novel Positron Emission Tomography Radiotracers, Christopher Philip Surdock

Theses and Dissertations (ETD)

Neuroblastoma (NB) is the most common extracranial tumor in patients under 1 year of age and it constitutes about 8-10% of all childhood cancer. It originates from neural crest cells that normally differentiate to form the sympathetic ganglia, adrenal medulla and other paraspinal sites where sympathetic nervous system tissue is present. Even with an extensive treatment regimen that typically includes surgery, chemotherapy, total body irradiation and autologous stem cell transplantation, the 5-year event-free survival is <50% for high risk patients, and there are numerous long-term side effects associated with treatment. This body of work investigated two projects for improving patient outcomes through the development of positron emission tomography (PET) radiotracers that could be used for therapy planning. The goal of the first project was to design, synthesize, and evaluate PET radiotracers that could measure the enzymatic activation of Irinotecan (CPT-11), a potent chemotherapeutic used in the treatment of colon cancer and several pediatric solid tumors. CPT-11 itself is a prodrug which is converted in vivo to SN-38, via metabolism by carboxylesterase (CE) enzymes. St. Jude Children’s Research Hospital researchers have designed a two-pronged protocol of tumor-targeted CPT-11 chemotherapy combining the complementary approaches of a) specific modulation of human CE in normal tissues to improve drug delivery, and b) tumor-targeted activation of prodrug using neural progenitor cells (NPC) transfected with a mutant human CE cDNA. The tumor-selective trafficking of NPC allows over-expression of CE within the tumor. This prodrug/activating enzyme therapeutic approach has shown extremely encouraging preclinical results in the treatment of NB (90% 1-year survival in mice). However, successful translation of this novel therapeutic approach into general clinical practice requires a better understanding of progenitor cell trafficking, duration and intensity of enzymatic activity and the ultimate biological fate of the therapeutic construct. Toward this end, PET radiotracers were developed based on extensive structure-activity relationship (SAR) studies of CE binding. The goal of the second project was to design, synthesize, and evaluate PET radiotracers that could identify the presence of the tropomyosin receptor kinase B (TrkB). TrkB is not normally found in sympathetic nervous tissue, which is the tissue NB develops from, and thus is a potential target for imaging and therapy. The presence of TrkB and its neurotrophin, brain derived neurotrophic factor (BDNF), have been reported to protect neuroblastoma tumor cells from chemotherapy-induced apoptosis via a phosphatidylinositol 3’-kinase pathway. Radiotracers were synthesized and evaluated for their ability to identify TrkB both in vitro and in vivo. PET radiosynthetic procedures were optimized to synthesize novel radiotracers for imaging targets that could help clinicians monitor therapy or identify markers that would aid in therapy planning for NB patients. The method development could be applied to future compounds that show improved chemical characteristics for synthesis and selectivity.