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Selective Cytotoxicity of the SM1 Monoclonal Antibody towards Small Cell Carcinoma of the Lung¹

Divisions of Medicine and Medical Oncology, Dana-Farber Cancer Institute and Harvard Medical School, Boston, Massachusetts 02115

A murine monoclonal antibody, SM1, is strongly reactive with the surface membrane of small cell carcinoma of the lung (1). SM1 antibody is unreactive with most other cancers and various normal tissues including bone marrow cells. We now find that SM1 antibody is selectively cytotoxic to small cell carcinoma (SCC) in vitro. The antibody is present in high titers in supernatant fluids or ascites obtained by i.p. injection of SM1 hybridoma cells into pristaned BALB/c mice. The cytotoxic effect of the antibody is reduced to one-half maximal activity only at dilutions greater than 1:40,000. The efficiency of tumor cell lysis is greatly enhanced by repeated treatments with antibody and complement. Using three treatments with antibody and complement, 99.9% of SCC cells are lysed, as determined by the chromium release. Similar efficiency of SCC cell kill was observed by clonogenic assays. SM1 antibody produces no significant antibody-dependent lysis of cell lines derived from non-SCC lung carcinomas and leukemia cells. The results from chromium release assay and clonogenic assays also indicate that the effect of SM1 antibody and complement on bone marrow cells is minimal and could be accounted for by the effect of complement alone.

¹ Supported in part by National Cancer Institute Grants CA 36498 and CA 33847.

² To whom requests for reprints should be addressed.

Received 4/16/84. Accepted 12/ 5/84.