Comparative imaging and biodistribution studies with an anti-CEA monoclonal antibody and its F(ab)2 and Fab fragments in mice with colon carcinoma xenografts.

An IgG1 mouse monoclonal antibody directed against CEA has been digested with papain to yield F(ab)2 and Fab fragments. Following radioiodination, intact antibody and fragments showed specific binding to cells of a CEA-producing tumour, although the immune reactivities of the fragments were lower than that of intact antibody. Gamma scintigraphy of nude mice bearing CEA-producing human tumour xenografts and injected with 131I-labelled fragments showed earlier and superior imaging of tumours than did 131I-intact antibody, and this was most marked with the Fab fragment. Sequential dissection analyses showed that this was due to earlier and higher tumour-to-blood ratios with fragments than with intact antibody, but in absolute terms the degree of localization of both fragment types was significantly lower than that of intact antibody.