Pre-targeting increases the effectiveness of radiation with immunotherapy (radio-immunotherapy, or RIT) by improving the slow uptake of radioactively labeled antibodies by tumor tissue while minimizing radiation of normal tissue. A multi-step process, pre-targeting begins with injecting biotinylated antibodies that target the tumor cells. Avidin molecules are then injected that bind to the biotin attached to the antibodies in the tumor. Since each avidin molecule is able to bind to multiple biotin molecules, radioactively labeled biotin molecules injected afterwards seek out and attach to the avidin already attached to the tumor - delivering the radiation phase of the therapy directly to the cancerous cells.30
Administering avidin directly to tumor beds after surgery is also an effective means of pre-targeting malignant cells for post-operative radiation. Inflammatory conditions in the tissue (due to surgery) serve to bind avidin to the tumor site. When the radioactively-tagged biotin is administered, it hones in on the tumor bed and delivers radiation to any lingering cancer cells.31
In both these pre-targeting mechanisms any avidin-conjugated or biotinylated therapeutic molecules in the blood and not bound to the tumor or tumor site are cleared out prior to injection with the radioactive molecules. This is easily accomplished by administration of nonradioactive biotin or avidin to flush these loose molecules out, thus avoiding the chance that these may attract the radioactive molecules away from the cancer cells.30-31
Clinical trials have demonstrated how well using an avidin/biotin system as a pre-targeting mechanism works to enhance delivery of radio-immunotherapy and radiotherapy in different types of cancers:30-31
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Attached to a biotin molecule.9
Attached to an avidin molecule.9