Enhanced targeting specificity to tumor cells by simultaneous recognition of two antigens.

Radioimmunotherapy recently afforded convincing results for B-cell non-Hodgkin's lymphoma treatment with antibody specific for B-cell differentiation antigens. High doses of unlabeled or labeled antibodies are necessary to saturate specific sites on normal B-cells. We thus developed a new targeting strategy, taking advantage of dual binding cooperativity, to enhance the specificity of the radioactive uptake by tumor cells. This approach was evaluated using human Burkitt lymphoma cells (Ramos) which express both CD10 and CD20 antigens. Most normal cells express at most one of these two differentiation antigens but many hematological tumors, including most human B type acute lymphoblastic leukemia cells, express both. Cells pretargeted with two bispecific antibodies, one recognizing CD10 and a histamine derivative (HSG), the other recognizing CD20 and the DTPA-indium complex, bind cooperatively radiolabeled mixed-haptens (DTPA-HSG). Increased binding (about 5-fold compared to binding to only one of CD10 or CD20 antigens) is observed at 37 degrees C, demonstrating the feasibility of the technique. This binding enhancement is a slow process, not observed at 4 degrees C. Such a binding enhancement will increase specificity for targeting isotopes to double antigen positive tumor cells compared to nontumor tissue cells bearing only one of them. This approach might be used to increase tumor irradiation with minimal irradiation of normal cells.