Scintigraphy and distribution of labeled antibodies in rats with tumors

Radioiodinated antitumor (Ab-gamma globulins), non-tumor-specific Ab, and R¹³¹ISA were used for imaging radiation-induced intestinal tumors in rats. Each agent detected tumors larger than 2 g, but labeled Ab were most efficient in detecting smaller tumors. Tissue distribution studies showed that while 'purified' Ab localized specifically in tumors, 'unpurified' Ab concentrated in the tumor by a mechanism not considered immunological. Localization was variable and the concentration of antitumor Ab reached useful levels only in a small number of cases. The use of high specific activity purified Ab unexpectedly decreased the concentrations of label observed in the tumors when compared with the use of the same activity of low specific activity purified Ab. These results indicated the presence of circulating tumor antigens which were capable of binding the injected Ab. Subsequently, these findings have been substantiated. Thus the animal-to-animal variability could be explained on the basis of differing degrees of interaction of injected Ab with circulating tumor antigens. The usefulness of labeled purified or monospecific antitumor antibodies for tumor imaging and therapy would thus be influenced by the extent of such interactions.