Adjuvant 131I-anti-CEA-antibody radioimmunotherapy inhibits the development of experimental colonic carcinoma liver metastases

Adjuvant radioimmunotherapy (RIT) for human colonic cancer was performed in a nude rat model of experimental liver metastases. Thirty-three rats were injected intraportally through a mesenteric vein with 5 x 106 cells from the human colonic cancer cell line LS174T. Within half an hour, 20 MBq (n=2), 75 MBq (n=5), or 150 MBq (n=10) of the 131 I-labelled anti- carcinoembryonic antigen (CEA) monoclonal antibody (MAb) 38S1 was administered intravenously (i.v.), whereas control groups received either i.v. saline injections (n=12) or 150 MBq of the irrelevant 131 I-labelled MAb 79C (n =4). Decay corrected whole-body data showed that more than 80% of the initially MAb-bound radioiodine was excreted during the first 2 weeks. Whole- body clearance and blood clearance of 131 I-38S1 and 131 I-79C were essentially similar. At sacrifice 5-7 weeks after administration, neither 20 MBq nor 75MBq 131 I-38SI significantly prevented the development of liver metastases. By contrast, with 150 MBq, no metastases formed in the animals treated with MAb 131 I-38S1 or 131 I-79C. A radiation induced effect on the haematopoietic system was found in the 150MBq dosage groups. It is concluded that the inhibition of tumour induction was not strictly dependent on a radiation dose delivered by a tumour-specific MAb. Since a non-tumour-specific 131 I-MAb, in a smaller group of animals, proved equally efficacious in preventing tumour growth, the total body 131 I dose was probably the major contributing factor.