Detection of minimal residual disease in breast cancer patients undergoing high-dose chemotherapy and stem cell rescue

High-dose chemotherapy with autologous stem cell rescue has become a standard treatment modality for women with breast cancer. An important, unresolved issue whether the stem cell product used for transplantation is contaminated with residual tumor cells that are capable of giving rise to disease when reinfused. Detection of residual breast tumor cells in stem cell products is complicated by their low frequency and lack of a suitable assay. The goal of this study was to develop a sensitive assay for detection of rare tumor cells and to determine the significance of tumor cell contamination on clinical outcome. Two assays, immonuhistochemistry (IHC) and flow cytometry (FCM), were developed using antibodies to cytokeratin (CK) proteins in cells of epithelial origin. The patient cohort included 45 stage IV and five stage II/III breast cancer patients, 13 nonepithelial cell cancer patients and three normal stem cell donors. CK-positive cells were detected by both assays in breast cancer patients with a median incidence of 2.17 (IHC) or 1.34 (FCM) per 10 (5) mononuclear cells (MNC). CK-positive cells were also detected in aptients with nonepithelial cell tumors in in normal donors, albeit at a lower frequency [0.38 (IHC) / 0.48 (FCM) and 0.45 (IHC) / 0.7 (FCM), respectively]. A treshold of _> 3 CK-positive cells wer 10(5) MNC was established as a positive result for tumor contamination. Based upon these criteria, 17 of 45 (38%) stage IV and 1 of 5 (20%) stage II/III patients were considered positive for tumor. No significant survival advantage was observed regardless of the CK status in this patient cohort. These results demonstrate that sensitive assay for the detection of CK-positive cells was successfully developed to evaluate tumor burden in patients with breast cancer. The lack of correlation between the incidence of CK-positive cells and survival in stage IV patients most likely results from unresolved tumor in vivo following treatment. Future studies will focus on evaluation of a cohort of patients (stage II/III) with less advanced disease and the addition of more specific tumor markers.