Selective killing of carcinoembryonic-antigen (CEA)-producing cells in vitro by the immunoconjugate cytorhodin-S and CEA-reactive cytorhodin-S antibody CA208.

Cytorhodin-S, an anthracycline derivative, was covalently coupled to a monoclonal antibody (mAb) CA208, against carcinoembryonic antigen (CEA) in order to achieve selective killing of a CEA-producing colon carcinoma cell line, COLO 205. The conjugate (15 molecules of drugs/antibody) retained substantial antibody activity as well as drug activity as assessed by enzyme-linked immunosorbent assay and 24-h L1210 proliferation assay, respectively. Furthermore, the conjugate inhibited the growth of COLO 205 cells in a short-term cytostatic assay. This cytostatic effect of the immunoconjugate on COLO 205 cells was inhibited in a dose-dependent manner by pretreatment of the cells with unconjugated CA208 mAb. In addition, chloroquine, a lysosomotropic agent, inhibited the cytostatic effect of the immunoconjugate, indicating the involvement of lysosomal enzymes in releasing drugs from the immunoconjugate. The antibody (CA208) was significantly incorporated into the cytoplasm of COLO 205 cells as demonstrated by immuno-electron microscopy. These in vitro results indicate that cytorhodin-S may be a good partner in immunoconjugates. However, in vivo animal experiments with the immunoconjugate revealed that the immunoconjugate was not so effective in prolonging survival. Thus, in vivo efficacy of this immunoconjugate remains to be further improved in application to cancer immunotherapy.