Background: Studies on radiolabeled monoclonal antibodies (MoAb) have dealt mainly with single antibodies. However, major differences may exist among different radiolabeled MoAb that bind to the same antigen and between switch variants of the same antibody. This study evaluates and compares a series of radiolabeled MoAb of different specificities, subclasses, and isotypes applicable in treatment of patients with B cell malignancies.
Methods: MoAb were iodinated with iodogen. Immunoreactivity was determined in cell binding assays. Scatchard analyses were performed to determine association constants of radiolabeled MoAb and to measure antigen density on malignant B cells in various differentiation stages. The fate of the MoAb after antigen binding in vitro was studied by modulation and internalization experiments.
Results: All MoAb tested could be iodinated efficiently and displayed association constants of 0.9 x 10(9)M-1 or higher. Immunoreactivity of radiolabeled MoAb ranged from 62-79%, except for the immunoglobulin (Ig)-M MoAb CLB-MD20.2, which had an immunoreactivity of 43%. The highest number of binding sites was detected for the CD20 MoAb (12 x 10(3) - 355 x 10(3), whereas the expression of antigens recognized by the CD22 MoAb was lowest on all cell types tested (4 x 10(3) - 26 x 10(3)). The MoAb CD19 and CD22 both induced modulation, whereas the CD20 MoAb did not. Modulation induced by the CD19 MoAb was caused by internalization. The rate of internalization was isotype-dependent and, for CD19-IgG1, partly mediated by Fc gamma ReceptorII.
Conclusions: Radiolabeled B cell MoAb tested in this study are promising for use in radioimmunotherapy. For therapy with the radioisotope iodine-131, the IgG2a and IgG2b CD19 MoAb are more suitable than CD19-IgG1, because of their slower modulation and internalization rate.