Uptake and Localization of 131I-labeled Anti-calcitonin Immunoglobulins in Rat Medullary Thyroid Carcinoma Tissue

A medullary carcinoma of the thyroid gland (MCT) which has been transplanted repeatedly under the kidney capsule of Wag/Rij rats secretes calcitonin (CT) spontaneously. From 10--20 weeks after transplantation, immunoreactive serum calcitonin (iCT) is abnormally elevated and continues to rise parallel to tumor growth. The immunoglobulin fraction of the rabbit anti-CT antiserum raised against intact synthetic hormone, was purified and iodinated electrolytically. Specific activities of 131I-labeled immunoglobulin of 0.008--0.014 mCi/microgram protein were obtained with 80% preservation of CT binding activity. Wag/Rig rats with MCT tumor and increased serum iCT concentrations received intravenous injections of 131I-labeled immunoglobulins (0.054--0.811 mCi). The distribution of radioactivity in the rats was followed for 14 days using external scintigraphy in combination with radioactivity measurements of blood and different organs at the end of the observation period. The distribution of 113mIn was used as a marker for blood distribution. When the radioactivity ratios (131I/113mIn) in tumor and different organs were related to that of blood which was set equal to unity, tumor tissue contained 3--6 times higher activity. Nonhyperimmune rabbit immunoglobulins or rabbit antirat prolactin immunoglobulins were not concentrated in MCT tissue, nor did anti-CT immunoglobulins localize in rat prolactin adenomas.