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²¹¹At-labeled and Biotinylated Effector Molecules for Pretargeted Radioimmunotherapy Using Poly-L- and Poly-D-Lysine as Multicarriers¹

Departments of Radiation Physics [S. L., B. K., L. J.] and Oncology [H. A., R. H.], Göteborg University, Sahlgrenska University Hospital, SE-413 45 Göteborg, Sweden, and Department of Nuclear Chemistry, Chalmers University of Technology, SE-412 96 Göteborg Sweden [G. S.]

Poly-L- and poly-D-lysine were evaluated as carriers of astatine and biotin for prospective use as effector molecules in pretargeted radioimmunotherapy of micrometastases. The precursor polylysine was derivatized in a three-step, single-pot procedure, including biotinylation with biotin amidocaproic N-hydroxysuccinimide, astatination via the intermediate reagent N-succinimidyl 3-(trimethylstannyl)benzoate, and, finally, charge modification using succinic anhydride. The chemistry was shown to be very facile, with a biotinylation efficiency of 75 ± 5%, and overall radiochemical yields in the range of 50–70%. After charge modification, no amines could be detected in the final product. The biotin function was unaffected by the chemistry and the radiation, as confirmed by almost complete binding of the effector molecule to avidin beads using a convenient filter tube assay. The effector molecules were evaluated in tumor-free female nude mice with regard to whole-body retention and tissue distribution after i.p. administration. The distribution of the L-isomer effector molecule showed rapid whole-body clearance with low uptake in all tissues, whereas the D-isoform showed whole-body clearance related to uptake in the kidneys. Both D-isomer and L-isomer showed faster blood clearance and generally lower tissue uptakes than labeled antibodies. The normal tissue distribution after the peritoneal administration implies that pretargeting using L-structure polylysine as the effector molecule may give a higher therapeutic index than that achieved in conventional radioimmunotherapy.