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A Monoclonal Antibody against Rat Calcitonin Inhibits the Growth of a Rat Medullary Thyroid Carcinoma Cell Line in Vitro

Thyroid Study Unit, Department of Medicine, University of Chicago, Chicago, Illinois 60637

Address all correspondence and requests for reprints to: Leslie J. DeGroot, M.D., Thyroid Study Unit, Mail Code 3090, University of Chicago, 5841 South Maryland Avenue, Chicago, Illinois 60637.

Medullary thyroid carcinoma (MTC) cells synthesize large amounts of calcitonin (CT), which serves clinically as a useful tumor marker. To examine the possibility of CT serving as a target in immunotherapy for MTC, we raised and characterized more than 40 monoclonal antibodies (mAbs) against rat CT (rCT). The affinity constants for the mAbs were between 2.8 x 10⁹ and 1.8 x 10¹¹ M^-1. Some mAbs react preferentially with solid phase rat CT, but not with liquid phase ¹²⁵I-labeled rCT. Thirty-nine mAbs cross-react with human CT.

We evaluated the antitumor effect of the mAbs in vitro by analysis of [³H]thymidine incorporation into the rat MTC cell line CRL-1607. Some antibodies show an antiproliferative effect, but most are inactive. One mAb (2E5G5, IgG2b), which preferentially reacts with solid phase rCT, but not with liquid phase ¹²⁵I-labeled rCT, exerts an antiproliferative activity on CRL-1607. At 6.25 x 10^-7 M, 2E5G5 killed all of the tumor cells independently of complement in a cytotoxicity assay. We explored the cytotoxic mechanisms by assays for cell cycle arrest and DNA fragmentation. The antitumor effect was manifested by apoptosis and cell cycle arrest. Hence, a secreted peptide may serve as a target in tumor immunotherapy. Therapeutically antibodies may exert antitumor activity by a variety of mechanisms. The antitumor effect of this mAb in a rat animal tumor model is being tested.

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