Determination of the Immunoreactivity of Radiolabeled Monoclonal Antibodies

Radiolabeled monoclonal antibodies (mAbs) are in use for numerous immunoassays and localization studies both in vitro and in vivo. It is often important to determine to what extent the radiolabeling procedure has affected the immunoreactivity of the antibody. If, for example, the radioisotope was inserted in the antigen-binding site, this would adversely affect the ability of the antibody to bind to its antigen. It is possible that, by changing the labeling technique or the radioisotope, this loss of immunoreactivity could be minimized. For example, if the antigen-binding site contains tyrosine, then the usual oxidation methods of labeling with radioiodine (1 ,2 ) may iodinate this important tyrosine residue. Other labeling methods may be more appropriate for these antibodies. For example, Bolton-Hunter reagent (3 ) can be used to iodinate antibodies on a lysine residue. Most chelating agents used for radiolabeling with metals are linked to the protein via a lysine residue, so if there is lysine in the antigen-binding site of the antibody, radiometals may not be the most appropriate isotope to use. However, the antibody is likely to contain a number of tyrosine and lysine residues that can be labeled, so the reduction in immunoreactivity caused by radiolabeling may not be too great. The magnitude of the effect will depend on the particular antibody.