Antibody targeting to the murine lymphoma ESb-MP: increased accumulation due to reduced internalization into lymphoma cells as compared to normal lymphoid cells.

Using rat monoclonal antibody (MAb) 12-15A against the spontaneously metastasizing mouse lymphoma variant ESb-MP, we elaborated conditions for targeting. In vitro, high binding of labelled antibody to ESb-MP cells and low binding to lymphoid cells (e.g., spleen cells) was noted. In vivo, we observed pronounced accumulation in spleen, lymph nodes and bone marrow, the uptake kinetics indicating high accessibility of the target antigen in these tissues, and rapid clearance of radioactivity from blood and most normal tissues, indicating degradation of the antibody and excretion of the label. Binding to lymphoma tissue was slow but persistent, resulting in high tumor:tissue ratios only after 2-3 days. Biodistribution could be dramatically changed by pre-treatment of animals with excess cold antibody, which reduced trapping of labelled antibody in normal lymphatic tissue, leading to prolonged persistence in the blood and preferential uptake into tumor tissue. Monovalent 12-15A fragments showed less pronounced binding to lymphatic tissue, while being rapidly cleared from the circulation by virtue of their inherent tendency to bind to kidney tissue. Tumor:tissue ratios up to 56:1 were obtained by a combination of pre-treatment with unlabelled 12-15A IgG or Fab fragment, followed by injection of labelled fragment or IgG, respectively. This is interpreted on the basis of differences in the internalization and retention of antigen-antibody complexes. Pre-treatment obviously leads to a temporary blockade or removal of the target antigen, which is much more efficient with normal lymphoid cells than with tumor cells. Thus, it may become possible to target antibodies into the tumor despite concurrent antigen expression on normal tissue.