Figure 3.

Tumor targeting Strategies: a Passive targeting relies on the selective accumulation of contrast agents in the tumor tissue, via leaky vasculature and absent lymph drainage. The contrast agents used in this targeting strategy are fluorescent at injection, and can result in non-specific binding when the contrast agent is deposited outside the confines of the tumor. b Active targeting probes rely on a targeting moiety conjugated to a contrast agent to specify fluorescence. c Activatable probes exhibit quenched fluorescence when they are injected, or topically applied. Binding to specific antigens, or cleavage by a tumor protease results in the activation of these probes. Therefore, only cells that are targeted fluoresce. d There are many moieties available to use for active targeting, including but not limited to; antibodies, antibody derivatives, nanoparticle scaffolds, peptides, ligands, and aptamers.