In Vitro Affinity Constant Estimation of Two IgG2a Immunoglobulins
Affinity constants were estimated for two radioiodinated IgG2a monoclonal antibodies against their complementary antigen receptor cells. Mouse antibodies B 1 and 5G6.4 were targeted against a lyophilized lymphoma cell line and live ovarian carcinoma cell line respectively. Radiolabeling by the iodogen method (Fraker and Speck, 1978) yielded preparations of 131 I-B 1 and 1251-5G6.4 whose immunoreactivity was determined by radioimmunoassay plotting maximal antibody binding as a function of increasing cell concentration. The affinity constant at equilibrium of each antibody was then similarly determined by varying the amount of antibody against a constant number of cells and plotted as such (Stanley and Lew et aI, 1983). Graphical analysis showed 131 I-B 1 to have an equilibrium constant of ~1.40 x 10 10 M -1 while 125 I-5G6.4 had a value of ~9.95 x 10 8 M -1. These data provide a more rigorous assessment of the radiopharmaceutical quality of radioiodinated monoclonal antibodies used in clinical studies.