Radiolabeled Streptavidin: A Potential Breast Tumor Imaging Agent

Backgrounds: The reported high concentration of biotin in tumors coupled with the high affinity of streptavidin and avidin for biotin raises the possibility that radiolabeled streptavidin or avidin could be used for tumor imaging. This study examines the tumor localizing potential of radio labeled streptavidin and avidin and compares it to that achieved following the administration of radiopharmaceuticals currently in use. Methods: Nude mice bearing either the MCF-7 or theBT-20 human breast tumor were injected intravenously with(a)111In-streptavidin,(b)111In-avidin,(c)111In-Octreoscan,(d)( superscript 99m)Tc-MDP,(e)(superscript 99m)Tc-sestamibi,(f)18F-FDG and(g)111In-AE-3 anti-breast cancer monoclonal antibody. Animals were sacrificed at various time periods corresponding with accepted clinical protocols and tissues were obtained for determination of radioactivity. Results: Both 111In-streptavidin and 111In-avidin demon-strated superior tumor to normal organ rations in tissues located above the diaphragm. The cholesterol pellet implanted subcutaneously to sustain MCF-7 tumor growth and the low-grade inflammatory reaction surrounding the pellet did not demonstrate increased uptake of radiolabeled streptavidin.Conclusion:111In-streptavidin and 111In-avidin administered as single agents demonstrate significant localization in both estrogen dependent and estrogen independent human breast tumors xenografted in mice. The overall superior tumor to normal organ ratios obtained with 111In-streptavidin and 111In-avidin compared to radiopharmaceuticals currently in use warrants further evaluation of these agents for breast tumor imaging. Since the natural human antibodies to avidin will result in fast clearance of the avidin from the circulation and limited access of the tumor to avidin,111In-streptavidin appears to be more suitable agent to pursue.