Preclinical evaluation of SPECT imaging with I-131-labeled monoclonal antibody SZ39 in nude mice bearing human glioma xenografts

The imaging characteristics of monoclonal antibody SZ39 against glioma were evaluated in glioma-bearing nude mice. Monoclonal antibody SZ39 is a murine IgG2a that reacts with a glycoprotein epitope (molecular weight 180,000), a human glioma-associated membrane antigen. Monoclonal antibody SZ39 was labeled with I-131 using a modified chloramine T method. Each glioma-bearing nude mouse was given 50 mu Ci/40 mu g of the experimental agent, I-131-labeled monoclonal antibody, or 50 mu Ci/46 mu g of a control agent, I-131-labeled monoclonal antibody C50, an antibody against colon cancer. Single-photon emission computed tomography (SPECT) was performed every 24 hours in the first week after administration. Glioma-bearing nude mice were killed in groups of 3 at 24 hours and daily up to 72 hours. The ratio of radioactivity uptake in glioma to normal organs was calculated. After administration of the labeled SZ39, glioma was visualized with SPECT on days 1 to 7, particularly at 72 hours. There was no accumulation of radioactivity in glioma with the labeled C50 antibody. Ali glioma-organ ratios increased with time. At 72 hours, the ratio of glioma to brain was 22.46 and of the other organs was 2.64 on average. SZ39 had a relatively low endocytosis rate and was favorable for I-131 labeling. These characteristics were helpful to reduce free I-131 in the blood and reduce the uptake by other organs. The results suggest that I-131-labeled SZ39 selectively accumulates in glioma, representing a potential strategy for SPECT imaging of these lesions.