Immunohistochemical analysis of tumor antigen saturation following injection of monoclonal antibody G250

Clinical tumor targeting studies with monoclonal antibody (mAb) G250 showed excellent targeting of primary renal cell carcinomas (RCC). However, tumor uptake decreased at higher mAb G250 doses, suggesting saturation of the G2 50 antigenic determinants In this immunohistochemical study we investigated the saturability of G250 antigen sites after i.v. administration of mAb G2 50 at various protein doses in nude mice with RCC xenografts. Five groups o f mice received five different protein doses (1, 3 10, 30 or 100 mu g) of m urine mAb G250. Three days post injection mice were killed and the tumors w ere removed. Free G250 antigen sites, ie., not targeted by the i.v. injecte d murine mAb G250 were determined by immunohistochemical staining with chim eric mAb G250. Distinct staining of the G250 antigen was observed only at t he 1 and 3 mu g dose whereas G250 antigen staining at higher doses was virt ually negative. The results of this study indicate that saturation of antig en occurs at relatively low doses of i.v. administered mAb G250. Apparently , all antigenic determinants present on the RCC tumor cells were targeted w hile previous preclinical studies suggested that iv. administration of mAb G250 only saturated the accessible antigen sites.