Comparison between 131I and 111In as radiolabels for monoclonal antibodies in immunoscintigraphy of tumor bearing nude mice.

F(ab')2 fragments of monoclonal antibody (MoAb) 19-9 with specificity for human colorectal adenocarcinomas were labeled with 111In or 131I and infused into nude mice bearing the human adenocarcinoma HT 29 in order to compare their preferential biodistribution according to the radiolabel used. Animal tissue distribution measured one day and five days after infusion showed that tumor accumulation was greater for 111In than for 131I. However, non specific binding of 111In labeled MoAb 19-9 was also greater in normal tissue than 131I labeled antibody, except in blood. Therefore, the tumor/normal tissue ratios were to the advantage of 131I MoAb 19-9 and a better contrast was obtained on imaging with 131I as compared to 111In labeled MoAb 19-9. Based on this experimental model 111In does not seem to be the optimal candidate for tumor imaging using radiolabeled MoAb.