Biotin Reagents for Antibody Pretargeting. 2. Synthesis and in Vitro Evaluation of Biotin Dimers and Trimers for Cross-Linking of Streptavidin

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Abstract
Polymerization and/or cross-linking of recombinant streptavidin (r-SAv) with biotin derivatives containing two biotin moieties (biotin dimers) or three biotin moieties (biotin trimers) has been investigated as a model for reagents to be used to increase the amount of radioactivity on cancer cells in tumor pretargeting protocols. In the investigation, six biotin dimers and three biotin trimers were synthesized. Most biotin derivatives synthesized had ether containing linker molecules incorporated to improve their aqueous solubility. The synthesized biotin dimers contained linker moieties which provided distances (when fully extended) of 13−49 Å between biotin carboxylate carbon atoms, and the biotin trimers contained linker moieties which provided distances of 31−53 Å between any two biotin carboxylate atoms. All of the biotin derivatives were evaluated for their ability to polymerize r-SAv in solution. When the biotin derivatives were mixed with r-SAv, none of the biotin dimers caused polymerization, but all of the biotin trimers resulted in complete polymerization. Some of the biotin dimers did cross-link r-SAv (to form r-SAv dimers, trimers, etc.), but the percentage of cross-linking was low (≤40%). The length of the linker molecule was important in cross-linking of biotin dimers. While linkers which provided distances of 13 and 19 Å between biotin carboxylate carbon atoms did not result in cross-linking, a linker which provided a 17 Å distance resulted in a small (≤10%) amount of cross-linking. Also, cross-linking was increased in biotin dimers with linkers which provided distances between biotin carboxylate carbon atoms of ≥23 Å. Cross-linking of streptavidin bound in polystyrene wells with biotin dimers and trimers was also examined. In those experiments, an excess of each biotin derivative was incubated at 37 °C for 10−30 min in polystyrene wells containing bound SAv. After the excess biotin derivative was rinsed from the wells, an excess of r-[125I]SAv was incubated for another 10−30 min. The amount of r-[125I]SAv bound after rinsing the excess from the wells was an indicator of the extent of cross-linking that occurred. The process of alternating additions of reagents was repeated four times to demonstrate that bound radioactivity could be increased with each addition of [125I]SAv. The results of cross-linking r-SAv in polystyrene wells paralleled results from cross-linking in solution.