A peptide-based bifunctional chelating agent for 99mTc- and 186Re-labeling of monoclonal antibodies

Background. The development of new bifunctional chelating agents for the labeling of monoclonal antibodies with radiometals is a desirable goal in the area of radioimmunodetection and radioimmunotherapy of cancer. The authors have developed a new N3S-ligand, N-(S-acetylmercaptoacetyl) (p-NCS)phenylalanylglycylglycine ethyl ester (MAIPGG) for technetium-99m ([sup.99m.Tc]) or rhenium-186 ([sup.186.Re]) labeling of monoclonal antibody D612 reactive with human colon cancer. The biodistribution of [sup.99m.Tc/186Re]-MAIPGG-D612 conjugates was studied in nude mice bearing human colon cancer xenografts. Methods. MAIPGG was synthesized from Boc-p-nitrophenylalanine and was coupled to antibody D612, and the conjugate was labeled with [sup.99m.]Tc using a Glucoscan kit (DuPont, North Billerico, MA) as the reducing system. [sup.186.Re]-MAIPGG-D612 was prepared by radiolabeling MAIPGG with [sup.186.Re], with sodium citrate/SnCl2, used as a reducing system, after which the raiolabeled MAIPGG was coupled to monoclonal antibody D612. The biodistribution of these radioimmunoconjugates in athymic nude mice bearing LS174T human colon cancer xenografts was studied. Results. The cold precursor MAIPGG was prepared easily from Boc-p-nitro phenylaianine with an overall yield of 12-15%, which, when coupled to monoclonal antibody D612, did not affect its binding activity to LS174T cells in vitro. The biodistribution and imaging results demonstrated that these radioimmunoconjugates localized preferentially in tumor. Conclusions. These preliminary results suggest that MAIPGG may be useful for the radiolabeling of a variety of monoclonal antibodies with [sup.99m.Tc] or [sup.186.Re,] which then can be used for radioimmunodetection and radioimmunotherapy of cancer.