Direct Binding of Radioiodinated Monoclonal Antibody to Tumor Cells: Significance of Antibody Purity and Affinity for Drug Targeting or Tumor Imaging

Abstract

For MoAb [monoclonal antibodies] to be used efficiently for drug targeting and tumor imaging, the fraction of antibody binding to tumor cells must be maximized. The binding of 125I MoAb in 3 different tumor systems was studied. The fraction of antibody that bound to the cell surface was directly proportional to the antibody purity. The affinity constant also limited the fraction of antibody that bound to cells at a given antigen concentration. Rearrangement of the standard expression for univalent equilibrium binding between 2 reactants allowed calculation of the maximum fraction of antibody that can bind in antigen excess. Binding data using 4 different MoAb with 3 cell systems confirmed this relationship. Estimates for reasonable concentrations of tumor antigens in vivo indicated that antibodies with binding constants < 108 M-1 are not likely to be useful for drug targeting or tumor imaging.