Advancing targeting radiopharmaceuticals for theranostic applications

In the past few decades, targeted radionuclide therapy has emerged as a potential strategy for combatting cancer. Through the delivery of alpha, beta-minus, or Auger electron emitting radionuclides to diseased tissue via targeting vectors, targeting radiopharmaceuticals can deal cytotoxic damage to cancerous cells while minimizing damage to healthy tissue. Furthermore, applying the theranostic approach, where a diagnostic radionuclide is delivered using the same targeting vector, allows targeted radionuclide therapy to become more personalized, potentially making these treatments more effective. This strategy allows for using the same targeting radiopharmaceutical for both diagnostics and therapy. This enables patient-specific dosimetry, the ability to predict the response for a given therapy, improved treatment planning, and monitoring of the ongoing treatment. Unfortunately, widespread use of the theranostic approach faces a variety of challenges in order to make theranostics more obtainable. This dissertation seeks to advance targeting radiopharmaceuticals for theranostic applications by investigating a novel companion diagnostic radionuclide, 134Ce, as a companion diagnostic for both 225Ac and 227Th. In addition, the use of the chelator 3,4,3-LI(1,2-HOPO) (HOPO) was investigated with 134Ce, the 90Y/86Y theranostic pair, and 225Ac. HOPO has the potential to be a “theranostic chelator” due to its affinity for a variety of strategic trivalent and tetravalent radiometals. Furthermore, a novel elution strategy for 224Ra/212Pb generators was benchmarked which could facilitate the use of the 212Pb/203 Pb theranostics in clinical settings. Lastly, the use of Siderocalin fusion proteins, which could pave the doorway for the development of antibody-based “cold-kits” thereby allowing for the rapid, room temperature labeling of monoclonal antibodies for targeted radionuclide therapy, was explored. The actinide radionuclides 225Ac and 227Th have recently shown great clinical and preclinical success for targeted radionuclide therapy due to their long decay chain which emits 4 or 5 α particles and their lack of a long-lived decay product. Unfortunately, neither of these radionuclides can be suitably imaged by contemporary molecular imaging modalities including positron emission tomography (PET) or single photon emission computed tomography (SPECT). Furthermore, there is not an actinium or thorium isotope that allows for PET or SPECT imaging. To address this need a surrogate diagnostic radionuclide with chemically similar properties using the in vivo generator system 134Ce/134La was investigated since it can be imaged by PET and has an unique redox chemistry that allows 134Ce to be stabilized as 134CeIII to mimic 225AcIII or as 134CeIV to mimic 227ThIV. The surrogacy of 134Ce/134La is investigated by assaying the biodistribution and in vivo stability of 134Ce complexes (134CeIII-DTPA, 134CeIII-DOTA, and 134CeIV-HOPO) through microPET imaging of a murine model. All of these complexes displayed high in vivo stability and rapid pharmacokinetics, and they all had negligible residual activity 24 hours after administration. Furthermore, the long half-life of 134Ce (3.2 days) allows for it to be compatible with antibody drug conjugates. This compatibility is investigated through microPET imaging 134CeIII-DOTA-Trastuzumab in a SK-OV-3 tumor-bearing murine model. Because of the high in vivo stability of 134Ce-DOTA and long half-life of 134Ce, 134Ce-DOTA-Trastuzumab displayed elevated tumor uptake for up to 9 days after administration and has a similar biodistribution to 225Ac -DOTA -Trastuzumab. These proof of concept studies open the doorway for the development of targeting radiopharmaceuticals incorporating 134Ce. 86YIII (a positron emitter) and 90YIII (a β- emitter) are rare earth metal theranostic pairs that facilitate the implementation of the theranostic approach due to both the therapeutic and diagnostic radionuclides having identical chemistry. Due to HOPO’s affinity for trivalent radiometals, HOPO can rapidly form highly stable yttrium complexes at room temperature, unlike DTPA or DOTA which are commonly used for 90Y/86Y theranostics. Through in vivo microPET imaging, the biodistribution and in vivo stability of 86Y-HOPO are assayed in a murine model. 86Y-HOPO displayed high in vivo stability and rapid pharmacokinetics, and it had negligible residual activity 24 hours after administration which is valuable for future work investigating yttrium-based targeting radiopharmaceuticals. 212Pb, another promising α emitting radionuclide, is available in a convenient 224Ra/212Pb generator and has a diagnostic matched pair available, 203Pb, which allows for SPECT imaging. A new elution strategy that directly yields 212Pb in a more favorable electrolytic solution (1.0 M sodium acetate) for radiolabeling, circumventing the need for long evaporation and redissolution processes, is benchmarked by a series of radiolabeling experiments comparing this new strategy to the conventional one. Through two weeks of labeling experiments, this elution strategy minimized the time needed to elute the 225Ra/212Pb, and it maintained a high radiochemical yield and radiochemical purity while labeling 212Pb to TCMC-Trastuzumab conjugates. This novel elution strategy can facilitate the use of 224Ra/212Pb generators in clinical settings allowing a more widespread implementation of 212Pb/203Pb theranostics. Siderocalin fusion proteins allow for a novel way to deliver therapeutic radionuclides to diseased sites. Because of Siderocalin’s ability to non-covalently bind to highly stable, negatively charged metal-HOPO complexes, these fusion proteins can allow for rapid, room temperature radiolabeling compared to DOTA which requires high temperatures in order for quantitative radiolabeling. In order to investigate their tumor targeting and tumor control properties, ex vivo biodistribution and tumor control experiments are performed with 225Ac and 90Y labeled fusion proteins in a SK-OV-3 tumor-bearing murine model. While 225Ac labeled fusion proteins were not able to display elevated uptake in tumors due to the low in vivo stability of 225Ac-HOPO, the 90Y fusion proteins were able to display tumor uptake within 50 hours. With further development, these Siderocalin fusion proteins can potentially become the blueprint for antibody-based “cold-kits”.

Advisor: Abergel, Rebecca J