[Antitumor effect of lidamycin-containing monoclonal antibody immunoconjugate with downsized-molecule].

OBJECTIVE: To develop a novel monoclonal antibody (mAb) immunoconjugate with downsized-molecule and highly potent antitumor effects. METHODS: The mAb conjugate was prepared by linking lidamycin (LDM), an antitumor antibiotic with extremely potent cytotoxicity, to mAb Fab' fragment. The molecular weight of Fab'-LDM was determined by non-reduced SDS-PAGE. The immunoreactivity of Fab'-LDM conjugate with cancer cells was detected by ELISA. The antitumor activity of the conjugate was determined by MTT assay, clonogenic assay, and animal model of transplantable colon carcinoma 26' (C26) in mice. RESULTS: The relative molecular mass of Fab'-LDM conjugate was approximately 65,000, suggesting a molar ratio of 1:1 between Fab' fragment and LDM in the conjugate. Fab'-LDM was reactive with hepatoma BEL-7402 and colon carcinoma 26 cells, but not reactive with KB cells. The cytotoxicity of Fab'-LDM conjugate to BEL-7402 cells, the antigen relevant cancer cells, was 13-fold more potent than that of free LDM, while the cytotoxicity of Fab'-LDM conjugate against C26 cells was 5.5-fold more potent than that of free LDM. However, the cytotoxicity of Fab'-LDM conjugate to KB cells, the antigen irrelevant cells, was similar to that of free LDM. Given by 3 intravenous injections, on day 1, 4 and 7, Fab'-LDM conjugate at doses of 0.05 mg/kg, 0.1 mg/kg and 0.2 mg/kg markedly suppressed the growth of colon carcinoma 26 in mice by 80%, 92% and 94%, respectively, whereas free LDM at 0.1 mg/kg suppressed the growth by 77%. The survival time of tumor-bearing mice was also increased by Fab'-LDM conjugate treatment. Fab'-LDM conjugate was more effective than equivalent unconjugated LDM (P < 0.01). CONCLUSIONS: The immunoconjugate composed of LDM and Fab' fragment that is characterized by downsized-molecule shows remarkable effectiveness against tumor growth.