The binding parameters of radiolabelled monoclonal F (ab′)2 and Fab′ fragments relative to immunoglobulin G in reactions with surface-bound antigens

The binding parameters of iodine-125-labelled intact monoclonal immunoglobulin G (IgG), F(ab′)2 and Fab′ fragments were compared. The study was carried out with the two monoclonal antibodies (MoAbs) K13 and K16 specific for human Ig light chains K and λ, respectively. When testing the 125I-MoAbs against monodisperse polymer particles coated with the specific antigens, the Ka for the F(ab′)2 fragments were similar to that for IgG, while the Ka for the Fab′ fragments were reduced to 10%–20% of that for IgG. The number N of effective target sites revealed with Fab′ was higher than with F(ab′)2 and IgG, presumably because less surface area is occupied by the small Fab′ molecules. The immunoreactive fraction F ranged according to IgG > F(ab′)2 > Fab′. The explanation of the moderate difference between the Ka of the monoclonal Fab′ and the divalent IgG and F(ab′)2 was that the divalent molecules were not divalently attached to the particles. When testing the same antibody preparations against human lymphoma cells producing Ig with light chains K or λ, the binding results were less reliable than when particles were utilised, presumably due to antigen shedding. Different MoAbs vary in their loss of immunoreactivity due to enzymatic degradation and the radiolabelling procedure. The preparation of the radiolabelled fragments should therefore be optimized for each MoAb, and evaluation is necessary before injection. Artificial targets with a low leakage of antigen, like the monodisperse polymer particles here applied, are recommended for the in vitro evaluation of the immunoreactivity of labelled MoAb preparations.