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

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 10(9) and 1.8 x 10(11) 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.