Figure 1.

Strategies for the delivery of nanoparticle drug carriers and/or radioisotopes to tumor cells include a) non-targeted, b & c) directly targeted (1-step), and d & e) pretargeted (multistep) approaches. a) Passively targeted nanoparticles coated solely with stealth polymers typically do not exhibit specific interactions with tumor cells. b) Radioimmunotherapy (RIT) uses radiolabeled tumor receptor-specific antibodies to deliver therapeutic doses of radiation to target cells. c) Modification with receptor-specific ligands allows the active targeting of nanoparticles (NPs) to tumor cells, which commonly induces receptor-dependent internalization. d) Pretargeted radioimmunotherapy (PRIT) splits tumor targeting and radioisotope delivery into sequential steps: 1) binding of bispecific proteins (BsPs) to target receptors and 2) binding of radiolabeled effector molecules to the BsPs. e) For pretargeted drug delivery systems, 1) bispecific proteins (BsPs) bind target receptors, and 2) a drug-loaded effector nanoparticle binds to the BsPs, which should ideally result in internalization.