An Immunotoxin Cytotoxic for Breast Cancer Cells in Vitro1

A potent immunotoxin was formed by conjugating the murine mono clonal antibody 323/A, to the A chain of ricin. The 323/A} antibody recognizes an antigen expressed by most human breast cancers. When binding of 323/A3 is examined by enzyme-linked immunosorption assay, three human breast cell lines displayed strong binding, whereas two human breast cell lines and three non-breast cell lines displayed little or no binding. When the cell lines were tested at a concentration of 0.1 MK/ ml, those cell lines which displayed an abundance of antigen by enzymelinked immunosorption assay were most sensitive to the effects of the 323/A) immunotoxin. On the other hand, cell lines which displayed little or no antigen by enzyme-linked immunosorption assay were not inhibited by the immunotoxin at this concentration. Further examination of the effects of immunotoxin concentration on protein synthesis confirmed the sensitivity of those cell lines rich in the 323/A3 antigen over a broad dose range. Similarly, three cell lines which displayed little of the 323/A3 antigen demonstrated little inhibition of protein synthesis with various concentrations of 323/A3 immunotoxin. However, two cell lines which displayed little antigen were intermediate in their sensitivity to the 323/ A., immunotoxin. Preclinical evaluation of immunotoxins as potential therapeutic agents will require accurate and sensitive screening of a wide variety of cell types. The 323/A3 remains of interest in studying the effects of immunotoxin in a defined in vivo model system.