Breast cancer resistance protein expression and resistance to daunorubicin in blast cells from patients with acute myeloid leukaemia.

Breast cancer resistance protein (BCRP) is a recently described member of the ATP binding cassette transporter superfamily. It has been shown to confer resistance to mitoxantrone, topotecan, doxorubicin and daunorubicin in human tumour cell lines. We describe a study of BCRP expression in blast cells derived from 20 patients with acute myeloid leukaemia (AML). Twelve samples were from patients who had received previous cytotoxic therapy. BCRP expression was measured by immunocytochemistry using the BXP-34 monoclonal antibody. In vitro drug sensitivity was assessed using the methyl thiazol tetrazoliumbromide assay. BCRP expression varied between patients, and six out of 22 (27%) samples had > 10% cells staining positively (median 37%, range 13-95%). BCRP positivity was seen in both de novo samples and those from previously treated patients. There was a marked variation in the effect of all drugs tested between patients. Although there was no correlation between BCRP positivity and the effect of mitoxantrone, topotecan or doxorubicin, the median daunorubicin LC(50) value of BCRP(+) cells was fourfold higher than that of BCRP- cells (0.89 micromol/l compared with 0.21 micromol/l, P < 0.05). These results suggest that BCRP may be involved in resistance to the agents commonly used in AML and may explain some of the anomalous results found when studying other membrane transporters, such as P-gp or MRP.