Molecular imaging techniques apply sophisticated technologies to monitor, directly or
indirectly, the spatiotemporal distribution of molecular or cellular processes for biomedical, diagnostic,
or therapeutic purposes. For example, Single-Photon Emission Computed Tomography
(SPECT) and Positron Emission Tomography (PET) imaging, the most representative modalities of
molecular imaging, enable earlier and more accurate diagnosis of cancer and cardiovascular
diseases. New possibilities for noninvasive molecular imaging in vivo have emerged with advances
in bioorthogonal chemistry. For example, tetrazine-related Inverse Electron Demand Diels-Alder
(IEDDA) reactions can rapidly generate short-lived radioisotope probes in vivo that provide strong
contrast for SPECT and PET. Here, we review pretargeting strategies for molecular imaging and
novel radiotracers synthesized via tetrazine bioorthogonal chemistry. We systematically describe
advances in direct radiolabeling and pretargeting approaches in SPECT and PET using metal and
nonmetal radioisotopes based on tetrazine bioorthogonal reactions, and we discuss prospects for the
future of such contrast agents.
Keywords: Pretargeting, bioorthogonal chemistry, tetrazine, molecular imaging, Single-Photon Emission Computed
tomography, Positron Emission Tomography.
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