Combination of guanine arabinoside and Bcl‐2 inhibitor YC 137 overcomes the cytarabine resistance in HL ‐60 leukemia cell line

Cytarabine (ara‐C) is the key agent for treating acute myeloid leukemia. After being transported into leukemic cells, ara‐C is phosphorylated, by several enzymes including deoxycytidine kinase ( dCK ), to ara‐C triphosphate (ara‐CTP), an active metabolite, and then incorporated into DNA, thereby inhibiting DNA synthesis. Therefore, the cytotoxicity of ara‐C depends on the production of ara‐CTP and the induction of apoptosis. Here, we established a new ara‐C‐resistant acute myeloid leukemia cell line (HL‐60/ara‐C60) with dual resistance characteristics of the anti‐antimetabolic character of decreased ara‐CTP production and an increase in the antiapoptotic factors Bcl‐2 and Bcl‐XL. We further attempted to overcome resistance by augmenting ara‐CTP production and stimulating apoptosis. A relatively new nucleoside analog, 9‐β‐ d ‐arabinofuranosylguanine (ara‐G), and the small molecule Bcl‐2 antagonist YC137 were used for this purpose. HL‐60/ara‐C60 was 60‐fold more ara‐C‐resistant than the parental HL‐60 cells. HL‐60/ara‐C60 cells exhibited low dCK protein expression, which resulted in decreased ara‐CTP production. HL‐60/ara‐C60 cells were also refractory to ara‐C‐induced apoptosis due to overexpression of Bcl‐2 and Bcl‐XL. Combination treatment of ara‐C with ara‐G augmented the dCK protein level, thereby increasing ara‐CTP production and subsequent cytotoxicity. Moreover, the combination of ara‐C with YC137 produced a greater amount of apoptosis than ara‐C alone. Importantly, the three‐drug combination of ara‐C, ara‐G and YC137 provided greater cytotoxicity than ara‐C+ara‐G or ara‐C+YC137. These findings suggest possible combination strategies for overcoming ara‐C resistance by augmenting ara‐CTP production and reversing refractoriness against the induction of apoptosis in ara‐C resistant leukemic cells.