Acquisition of multidrug resistance in recurrent breast cancer demonstrated by the histoculture drug response assay

Recurrent breast cancer has a very poor response rate to chemotherapy. To understand the degree of acquisition of multidrug resistance in recurrent disease, 24 recurrent breast tumors and 127 primary tumors were evaluated and compared for chemosensitivity in the histoculture drug response assay (HDRA). The evaluation rate was 98.8%. The HDRA utilizes 3-dimensional culture of human tumors on collagen-gel rafts. Doxorubicin (DXR), 5-fluorouracil (5-FU) and mitomycin C (MMC) were tested as standard agents and cisplatin (CDDP) as a candidate agent on surgical specimen of breast cancer in the HDRA. In vitro drug exposure in the HDRA was for 7 days. At the end of the assay, tumor response was assessed by the reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). The mean inhibition rates of primary tumors vs. recurrent tumors were 57.9% and 38.6% for DXR (p<0.0005); 59.9% and 42.8% for MMC (p<0.01); 49.0% and 33.4% for 5-FU (p<0.01); and 34.5% and 16.0% for CDDP (p<0.005), respectively. The recurrent cases were pretreated clinically with CAF (cyclophosphamide, DXR and 5-FU), CEF (cyclophosphamide, epirubicin and 5-FU) or CMF (cyclophosphamide, methotrexate and 5-FU). In the CAF and CEF group, the HDRA sensitivity to CDDP was significantly lower in recurrent disease (p<0.005) than that of primary breast cancer suggesting that one agent can induce resistance to another. This is further suggested by the fact that 64.7% of the recurrent cases were resistant to all 4 agents tested as opposed to 27% of the primary cases and that only 5.9% of the recurrent cases were sensitive to three or more agents as opposed to 18% of the primary cases. The correlation of the HDRA results to clinical outcome in the study was 80.0% with 15 cases evaluated consisting of 5 true positives, 3 false positives, 7 true negatives and no false negatives. Thus, the HDRA gives useful clinical information, in particular for the specific individualized treatment design necessary to overcome the multidrug resistance problem of recurrent breast cancer.