A Streptavidin−Biotin Binding System That Minimizes Blocking by Endogenous Biotin

Abstract

Pretargeted radioimmunotherapy specifically targets radiation to tumors using antibody−streptavidin conjugates followed by radiolabeled biotin. A potential barrier to this cancer therapy is the presence of endogenous biotin in serum, which can block the biotin-binding sites of the antibody−streptavidin conjugate before the administration of radiolabeled biotin. Serum-derived biotin can also be problematic in clinical diagnostic applications. Due to the extremely slow dissociation of the biotin−streptavidin complex, this endogenous biotin can irreversibly block the biotin-binding sites of streptavidin and reduce therapeutic efficacy, as well as reduce sensitivity in diagnostic assays. We tested a streptavidin mutant (SAv-Y43A), which has a 67-fold lower affinity for biotin than wild type streptavidin, and three bivalent bis-biotin constructs as replacements for wild-type streptavidin and biotin used in pretargeting and clinical diagnostics. Biotin dimers were engineered with certain parameters including water solubility, biotinidase resistance, and linker lengths long enough to span the distance between two biotin-binding sites of streptavidin. The bivalent biotins were compared to biotin in exchange, retention, and off-rate assays. The faster off-rate of SAv-Y43A allowed efficient exchange of prebound biotin by the biotin dimers. In fluorescent competition experiments, the biotin dimer ligands displayed high avidity binding and essentially irreversible retention with SAv-Y43A. The off-rate of a biotinidase-stabilized biotin dimer from SAv-Y43A was 4.36 × 10^-6 s^-1, over 640 times slower compared to biotin. These findings strongly suggest that employing a mutant streptavidin in concert with a bivalent biotin can mitigate the deleterious impact of endogenous biotin, by allowing exchange of bound biotin and retention of the biotin dimer carriers.