Acute myeloid leukaemia (AML) is a heterogeneous malignant disease in which Bcl-2 is expressed simultaneously with several putative drug resistance parameters in AML cells. Bcl-2 over-expression is associated with CD34 positivity, poor response to chemotherapy and reduced overall survival in AML patients. The role of Bcl-2 in determining the response of AML cells to two chemotherapy drugs used in treatment of AML: cytarabine and fludarabine was investigated using human leukaemia cell lines expressing different levels of Bcl-2: U937 CD34 negative expressing low levels of Bcl-2 and MHH225 CD34 positive expressing high levels of Bcl-2. Apoptosis was significantly more in CD34 negative cells with low Bcl-2 expression than in CD34 positive cells with high Bcl-2 expression. The IC50 of cytarabine and fludarabine were significantly higher in CD34 positive cells with high Bcl-2 than in CD34 negative cells with low Bcl-2. Using a quantitative ELISA assay, the results revealed a 2-log higher Bcl-2 concentrations in CD34 positive (144.7 13.3 Units per 105 cells) than in CD34 negative (6.3 0.01 Units per 104 cells) leukaemia cells. Both cytarabine and fludarabine have reduced Bcl-2 concentrations in both cell types. However, the significantly high basal level of Bcl-2 concentrations in CD34 positive leukaemia cells has resulted in a persistent high Bcl-2 concentration levels remaining after treatment with the anti-leukaemia drugs in these cells. Whereas in CD34 negative leukaemia cells the low basal level of Bcl-2 concentrations was significantly reduced by the anti-leukaemia drugs to extremely low levels. Therefore, the high Bcl-2 concentration levels remaining after treatment with anti-leukaemia drugs can be responsible for resistance to chemotherapy by protecting CD34 positive AML cells from induced apoptosis.

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