Cure of Metastatic Human Colonic Cancer in Mice with Radiolabeled Monoclonal Antibody Fragments

There is currently no method to cure patients with disseminated colorectal cancer, which is the third leading cancer killer in the Western World. This report shows that the GW-39 intrapulmonary micrometastatic human colonic cancer model in nude mice can be cured with radiolabeled antibodies against carcinoembryonic antigen, and that this approach of radioimmunotherapy is superior to conventional chemotherapy with 5-fluorouracil and leucovorin (5-FU/LV). Monovalent Fab fragments labeled with 131 I are superior to intact IgG when survival was evaluated 3, 7, and 14 days after implantation, leading to cures in up to 90% of the mice. Histological results provide support for the differences in therapeutic efficacy observed. Microautoradiography was used to evaluate the intratumoral distribution of each form of antibody. The enhanced tumor control by Fab compared with IgG could be explained in part by the homogeneity of radioantibody distribution of Fab. Biodistribution analysis and initial dose rate calculations for all three forms of antibody also help explain the ability of 131 I-labeled Fab to provide better tumor growth control than seen with 131 I-labeled IgG. Thus, radioimmunotherapy may be a new modality to treat metastatic disease, particularly when using small antibody fragments.