Human IgG2 constant region enhances in vivo stability of anti-tenascin antibody 81C6 compared with its murine parent.

The in vivo properties of radiolabeled chimeric monoclonal antibodies (mAbs) with human IgG1 and IgG3 constant regions generally are similar to those of their corresponding murine construct. In contrast, we have observed that chimeric anti-tenascin mAb 81C6, which contains IgG2 constant regions, exhibits significantly higher localization in s.c. D-54 MG xenografts and prolonged retention in most normal tissues compared with its IgG2b murine parent. The purpose of the present study was to determine whether substitution of the murine IgG2b constant region domains in mAb 81C6 with those from human IgG2 enhanced the in vivo stability of the 81C6 mAb. Both mAbs were radioiodinated using Iodogen and administered to athymic mice bearing s.c. D-54 MG human glioma xenografts. The nature of the labeled species present in tumor and normal tissues over a 144-h period was investigated by trichloroacetic acid precipitation and SDS PAGE. In tumor and most normal tissues, a greater fraction of chimeric compared with murine 81C6 was present as intact IgG. For example, in tumor at 144 h, the fraction of radioactivity present as intact IgG was twice as high for chimeric compared with murine 81C6. A substantial fraction of murine but not chimeric 81C6 was present as a Mr 70,000-90,000 molecule, which could represent the generation of Fab/Fc monomers through the reduction of the interchain disulfide bonds in the murine IgG2b molecule. These results suggest that the higher tumor and normal tissue levels of chimeric compared with murine 81C6 can be attributed in part to the enhanced in vivo stability of the IgG2 chimeric mAb. The chimeric construct also was demonstrated to be more stable than murine after incubation with cyst fluid obtained from glioma resection cavity patients. Chimeric mAbs containing human IgG2 constant region domains could be of particular value for certain radioimmunotherapeutic applications.