Competitive enzyme‐linked immunoassay of monoclonal immunoglobulin G anti‐D preparations

The development of monoclonal immunoglobulin G (IgG) anti-D for prophylaxis necessitates estimation of their potency in terms of their red cell-binding ability, but it is unclear which quantification methodology is most suitable for this. The aim of this study was to assess 50 monoclonal anti-D from the 4th International Workshop for quantitative and qualitative binding to red cells in a competitive enzyme-linked immunoassay (EIA) in which varying amounts of monoclonal antibodies (MoAbs) and a constant amount of biotinylated monoclonal anti-D (BRAD-5) compete for red cell binding. The potencies of the MoAb were estimated against the International Reference Preparation (IRP) for anti-D Ig. Two MoAbs as supplied were insufficiently inhibitory of biotinylated BRAD-5 binding to be quantified; the potencies of the remainder ranged from 4 to 343 IU mL-1 and included 11 MoAbs showing dose-responses that were nonparallel to those of other MoAb and the IRP. Estimation of the concentration of antibody in the supernatants by radial immunodiffusion ranged from 0.5 to 61 micro g mL-1, giving specific activities of <1-24 IU micro g-1. The results show that competitive EIA is suitable for quantitating most monoclonal anti-D for development and quality control purposes, regardless of their D epitope reactivity.