The detection of virally induced tumors by 131I- and 125I-labeled syngeneic monoclonal antibodies.

The binding of the syngeneic monoclonal antibodies IC5F5 and 4D2B4 to Rauscher virus-induced myeloid leukemic (RMB-1) cells was analyzed in vivo in tumor-bearing BALB/c mice. To verify it these antibodies bind specifically to RMB-1 cells, purified antibodies were iodinated with the isotopes 125I and 131I. Mice previously inoculated with tumor cells were injected with these labeled monoclonal antibodies and the plasma clearance and the tissue distribution were determined. The clearance in tumor-bearing animals was faster than in control mice. The tissue distribution was corrected for nonspecific accumulation by scoring for an unrelated antibody. Calculation of a localization index showed that IC5F5 binds at least 4.5 times more specifically to tumor cells than to other tissues. A preferential localization of radioactivity in s.c. tumor tissue was seen in the scanning of animals injected with 131I-labeled antibodies. The most direct proof of specific binding was observed in autoradiograms of animals treated with 125I-labeled antibodies. Small islands of tumor cells in the livers of mice inoculated i.v. had a high density of grains compared to other tissues and also compared to tumor cells in mice treated with unrelated monoclonal antibodies. These results show efficient targeting of these monoclonal antibodies and make immunotherapy of these myeloid leukemic cells possible.