Due to their high affinity for biotin, avidin (Av) and streptavidin (SAv) are used to bridge pretargeted antibody molecules and radiolabeled biotin derivatives in vivo.
Methods: We compared uptake of 125I-labeled Av or SAv (approximately 10-500 micrograms) in tumor and normal tissues 3 days after a biotinylated B72.3 monoclonal antibody (100 micrograms) injection in nude mice. The animals were killed 24 hr later and the biodistribution of 125I was determined.
Results: The percent injected dose per gram of tumor remained constant over the range of injected doses for Av while that for SAv varied. As larger amounts of Av/SAv were injected, the number of moles of each trapped within tumor increased, with the values for SAv being much higher. While the injection of larger doses of Av led to an increase in tumor-to-normal tissue ratios, that of SAv did not.
Conclusion: SAv (2.5 mg/kg) is the preferred "second-step" reagent. At this dose, the number of receptors available for targeting by radiolabeled biotin derivatives is approximately 1.8 times the number of antigen-binding sites accessible for targeting by radiolabeled antibody. Additional targeted-signal amplification should be possible by the successive and repeated administration of such polymeric reagents, each exhibiting high affinity to and forming a specific binding pair with the last-targeted molecule.