<i>In Vivo</i> Molecular Imaging

In vivo molecular imaging is the visualization, characterization, and measurement of biological processes at the molecular and cellular levels in humans and other living systems. Among the methodologies used in in vivo molecular imaging, two methodologies are of great interest from the view of high sensitivity. One is nuclear medical imaging, and distribution and kinetics of a radiolabeled molecular probe are measured using positron emission tomography (PET) and single-photon emission computed tomography (SPECT). The other is optical molecular imaging, and distribution and kinetics of a fluorescent probe are measured using a fluorescent imaging instrument. In this review, the development of imaging probes for these two methodologies is briefly discussed. In nuclear medical molecular imaging, based on structure-activity-biodistribution relationship studies for small molecule and the concept of "functional unit-binding multifunctional molecular probe" containing 3 functional units (target recognition unit, signal-releasing unit, linker unit) for peptides and proteins, we developed radiolabeled probes with high and specific accumulation to the target for neuroreceptors, β-amyloid plaques, and tau aggregates in the brain, tumors, atherosclerotic plaques, pancreatic β-cell, myocardial sympathetic nerves, and so on. We also discuss the progression of molecular imaging toward therapy (radiotheranotics). In in vivo optical molecular imaging, taking into account the characteristics of optical imaging, we designed tumor-specific optical imaging probes with characteristic imaging mechanism, including near-infrared (NIR) fluorescent probes and activatable probes. Furthermore, we developed a photoacoustic imaging probe, which enables highly sensitive and high-resolution imaging in deep tissues.