Characterisation of a humanised bispecific monoclonal antibody for cancer therapy.

A humanised bispecific monoclonal antibody (bsMAb) with binding specificity for carcinoembryonic antigen (CEA) on one arm and a radiolabelled chelate (DTPA-90Y) on the other arm was generated by consecutively transfecting the humanised genes of an anti-CEA MAb and the chimerised genes of an anti-chelate MAb into eucaryotic BHK cells using the calcium-phosphate coprecipitation technique. The antibodies secreted were of IgG3 isotype with a shortened hinge region (delta gamma 3) and light chains. Double transfectomas were screened for the secretion of bsMAbs using a double determinant enzyme-linked immunosorbent assay (ELISA) based on solid phase attached HSA-benzyl-DTPA and an anti-idiotypic MAb selective for the CEA-specific arm. After purification on two immunoaffinity chromatography columns, the humanised bsMAbs were characterised by SDS-PAGE and a quantitative binding assay in antigen excess. The purification procedure resulted in 95% reactive bispecific MAb. This humanised bsMAb may be employed in two phase radioimmunotherapy, binding to the tumour via the anti-CEA arm and localising a radiolabelled chelate with the other arm, without inducing a strong immune response observed sometimes with murine MAbs.