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Preclinical Evaluation of an "Instant" ^99mTc-labeling Kit for Antibody Imaging¹

Immunomedics, Inc. and Center for Molecular Medicine and Immunology, University of Medicine and Dentistry of New Jersey, Newark, New Jersey 07103

This communication presents information on the quality and stability of a new conjugate of ^99mTc to an anticancer antibody fragment and results of animal imaging and distribution studies. Clinical studies have proved that this labeled antibody can image tumors within 2 hours of i.v. injection. It is produced using an "instant kit," merely requiring addition of [^99mTc]pertechnetate to a lyophilized vial containing the fragment and a pertechnetate-reducing agent. Within 5 min of addition of pertechnetate to the vial containing the fragment, [^99mTc]pertechnetate is reduced and quantitatively bound to an intrinsic high-affinity receptor present in the Fab'. The Fab'-bound ^99mTc is completely stable to challenge with diethylenetriaminepentaacetic acid and human serum albumin. Minimal oxidation and release of ^99mTc bound to the fragment were observed when the labeled fragment was incubated for 24 h at 37°C. Animal biodistribution studies were consistent with the known metabolic elimination of a radiolabeled Fab' fragment by the kidney. Tumor localization studies in athymic nude mice bearing xenografts of human colonic carcinoma (GW-39) predicted the positive results later demonstrated in clinical studies.