REACTIVITY OF MONOCLONAL-ANTIBODY FC-2.15 AGAINST DRUG-RESISTANT BREAST-CANCER CELLS - ADDITIVE CYTOTOXICITY OF ADRIAMYCIN AND TAXOL WITH FC-2.15

Monoclonal antibody (MAb) FC-2.15 recognizes Lewis x antigen (Le(x)-Ag ) expressed on the cell surface of most human breast cancer cells. FC- 2.15 displays important human complement (C')-mediated cytotoxicity (C MC) against its target cells. In this study the reactivity of FC-2.15 against drug resistant-breast cancer cells was investigated, as well a s the possibility to combine the antitumor activities of this MAb with adriamycin (Adr) or taxol. Since resistant clones with altered expres sion of tumor-associated antigens usually emerge after chemotherapy, t he expression of Le(x)-Ag was analyzed in Adr(R) MCF-7 breast cancer c ells (Adr resistant subline) and in tumor samples from nine patients w ith locally advanced breast carcinoma who were treated with FEC chemot herapy. A flow cytometry assay showed that most of Adr(R) MCF-7 cells, as well as wild type (WT) cells, expressed Le(x)-Ag; however, the Le( x) epitope is probably bound to different backbones in these cells. Wh en the cytotoxic ability of FC-2.15 against WT and Adr(R) MCF-7 cells was compared, it was found that a 90 min treatment with FC-2.15 plus C ' induced similar CMC against both cell lines. An important cytolysis was obtained at 5 mu g/ml FC-2.15, reaching a plateau at 25 mu g/ml, a t which cell population was diminished to 21.1% for WT and 27.9 for Ad r(R) MCF-7 cells. Regarding human tumors, Le(x)-Ag expression was eval uated in samples obtained before and in most cases after chemotherapy, and it could be observed that: 1) before treatment, tumor samples fro m all patients analyzed (responders and non-responders to chemotherapy ) were FC-2.15-positive; 2) the presence of Le(x)-Ag was not modified after treatment. The combined action of Adr or taxol with FC-2.15 was then evaluated. WT and Adr(R) MCF-7 cells were cultured with Adr or ta xol followed by an incubation with different FC-2.15 concentrations pl us C'. When the effect of Adr alone was determined, ID50 were 1 x 10(- 7) M for WT and 4.2 x 10(-5) M for Adr(R) MCF-7 cells. The cytotoxic a bility of taxol alone was also tested, and ID50 were 6.4 x 10(-9) M fo r WT and 3.1 x 10(-6) M for Adr(R) MCF-7 cells. When FC-2.15 was added to Adr or taxol, the cytotoxicity of the drug-FC-2.15 combined treatm ent was always higher than the isolated effects, showing additive cyto toxicity at the different concentrations tested and with both cell lin es. Our results suggest that FC-2.15 may be a useful agent against bre ast tumor cells which survive chemotherapy with Adr or taxol.