Molecular Imaging of Cancer with Copper-64 Radiopharmaceuticals and Positron Emission Tomography (PET)

Abstract

Molecular imaging has evolved over the past several years into an important tool for diagnosing, understanding, and monitoring disease. Molecular imaging has distinguished itself as an interdisciplinary field, with contributions from chemistry, biology, physics, and medicine. The cross-disciplinary impetus has led to significant achievements, such as the development of more sensitive imaging instruments and robust, safer radiopharmaceuticals, thereby providing more choices to fit personalized medical needs. Molecular imaging is making steadfast progress in the field of cancer research among others. Cancer is a challenging disease, characterized by heterogeneity, uncontrolled cell division, and the ability of cancer cells to invade other tissues. Researchers are addressing these challenges by aggressively identifying and studying key cancer-specific biomarkers such as growth factor receptors, protein kinases, cell adhesion molecules, and proteases, as well as cancer-related biological processes such as hypoxia, apoptosis, and angiogenesis. Positron emission tomography (PET) is widely used by clinicians in the United States as a diagnostic molecular imaging tool. Small-animal PET systems that can image rodents and generate reconstructed images in a noninvasive manner (with a resolution as low as 1 mm) have been developed and are used frequently, facilitating radiopharmaceutical development and drug discovery. Currently, [¹⁸F]-labeled 2-fluorodeoxyglucose (FDG) is the only PET radiotracer used for routine clinical evaluation (primarily for oncological imaging). There is now increasing interest in nontraditional positron-emitting radionuclides, particularly those of the transition metals, for imaging with PET because of increased production and availability. Copper-based radionuclides are currently being extensively evaluated because they offer a varying range of half-lives and positron energies. For example, the half-life (12.7 h) and decay properties (β⁺, 0.653 MeV, 17.8%; β⁻, 0.579 MeV, 38.4 %; the remainder is electron capture) of ⁶⁴Cu make it an ideal radioisotope for PET imaging and radiotherapy. In addition, the well-established coordination chemistry of copper allows for its reaction with a wide variety of chelator systems that can potentially be linked to antibodies, proteins, peptides, and other biologically relevant molecules. New chelators with greater in vivo stability, such as the cross-bridged (CB) versions of tetraazamacrocyclic 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid (TETA), are now available. Finally, one of the major aspects of successful imaging is the identification and characterization of a relevant disease biomarker at the cellular and subcellular level and the ensuing development of a highly specific targeting moiety. In this Account, we discuss specific examples of PET imaging with new and improved ⁶⁴Cu-based radiopharmaceuticals, highlighting the study of some of the key cancer biomarkers, such as epidermal growth-factor receptor (EGFR), somatostatin receptors (SSRs), and integrin α_vβ₃.