Comparison of two antibody-based methods for elimination of breast cancer cells from human bone marrow

Three monoclonal antibodies reactive with antigens abundantly expressed on human carcinoma cells were used to develop and compare the efficacy of immunotoxins (ITs) and immunobeads for purging breast cancer cells from bone marrow. ITs constructed as conjugates of the monoclonal antibodies and Pseudomonas exotoxin A showed high specific cytotoxicity against three breast cancer cell lines, inhibiting protein synthesis by 50% at concentrations of 4 x 10(-13) M to 1 x 10(-10) M. Tested in a reproducible clonogenic assay, two of the ITs used at a concentration of 0.1 microgram/ml killed > 5 log units of MCF7 cells, the maximal sensitivity for assessing cytotoxic effects, and 1.5 log of T-47D tumor cells. At 1 microgram/ml, each of the three ITs eliminated > 5 log of both cell lines. The immunobead procedure removed 2.0-4.1 log of tumor cells with one purging cycle and up to 6.0 log with two cycles. The mixture of the three ITs or immunobeads was not clearly superior in efficacy, compared to the use of individual molecules, probably reflecting an overlap in expression of the respective antigens in these cell lines. For both methods, the purging efficacy was not reduced when the tumor cells were admixed with normal bone marrow cells at a ratio of 1:10. The survival of colony-forming units, granulocyte/macrophage, was 49-86% with the immunobeads and 44-75% even at high concentrations (up to 2.5 micrograms/ml x 3) of the ITs. The results indicate that each of the two immunological methods can be safely used for effective elimination of tumor cells from the graft of breast cancer patients undergoing autologous bone marrow transplantation.