Localization of human colorectal tumor xenografts in the nude mouse with the use of radiolabeled monoclonal antibody.

For the evaluation of the clinical usefulness of monoclonal antibodies as diagnostic or therapeutic reagents, tumor localization must be clearly demonstrated in an experimental model. In this report, nude mice carrying two human tumor xenografts--a colon carcinoma (Colo 205) and a melanoma (Colo 239)--were given ip injections of radiolabeled monoclonal antibodies. Monoclonal antibody 250-30.6, which reacted specifically with the colon carcinoma but not with the melanoma, was labeled with 125I, while a second monoclonal antibody of similar immunoglobulin subclass, but unreactive with either cell type, was labeled with 131I. Both antibodies were injected simultaneously, and either the mice were scanned with a gamma camera or their tissues were removed and the localization of radiolabeled antibody was calculated with the use of localization index (LI)--the ratio of the tissue to blood distribution for each isotope. The studies showed that specific localization had occurred, there being a colon tumor LI of 6 at 2 days. Tumors of 150-300 mg (mean diameter, 6 mm) and with an LI as low as 1.5 could be successfully imaged after computer-assisted background subtraction. This study demonstrated that relatively small human tumor xenografts in the nude mouse can be specifically detected with the use of paired monoclonal antibodies, each labeled with a different isotope.