Considerations on the Advantages of Small Tracers for Optical Molecular Imaging

Molecular imaging is nowadays gaining a complementary role in cancer
therapy, specifically in diagnosis, selection of therapy, monitoring of therapeutic
efficacy, and also in the surgical setting. Many tumor-targeted tracers have been
developed for these applications. Optical molecular imaging has been gaining
more importance, particularly due to the absence of ionizing radiation, rendering
its use friendlier and safer for patients and medical personnel. Recently, clinical
trials have been initiated for tumor optical imaging with monoclonal antibodies
conjugated to the near-infrared fluorophore IRDye 800CW. Yet, preclinical
studies clearly indicate several aspects that favor smaller tracer formats over
antibodies, namely, their rapid distribution, efficient penetration, and the rapid
clearance of unbound tracers. Altogether, these small tracers lead to adequate
contrast within minutes or hours after administration, instead of days. Here, the
discussion will be centered on the properties of the different formats of tracers,
in particular the small formats. Although, the conventional antibody format has
already reached clinical trials, optical imaging trials with smaller formats are
eagerly awaited to clarify whether their advantages over antibodies are also
evident in patients.