The human equilibrative nucleoside transporter 1 mediates in vitro cytarabine sensitivity in childhood acute myeloid leukaemia

Cytarabine (ara-C) is the most effective agent for the treatment of acute myeloid leukaemia (AML). Aberrant expression of enzymes involved in the transport/metabolism of ara-C could explain drug resistance. We determined mRNA expression of these factors using quantitative-real-time-PCR in leukemic blasts from children diagnosed with de novo AML. Expression of the inactivating enzyme pyrimidine nucleotidase-I (PN-I) was 1.8-fold lower in FAB-M5 as compared to FAB-M1/2 (P=0.007). In vitro sensitivity to deoxynucleoside analogues was determined using the MTT-assay. Human equilibrative nucleoside transporter-1 (hENT1) mRNA expression and ara-C sensitivity were significantly correlated (r(p)=−0.46; P=0.001), with three-fold lower hENT1 mRNA levels in resistant patients (P=0.003). hENT1 mRNA expression also seemed to correlate inversely with the LC(50) values of cladribine (r(p)=−0.30; P=0.04), decitabine (r(p)=−0.29; P=0.04) and gemcitabine (r(p)=−0.33; P=0.02). Deoxycytidine kinase (dCK) and cytidine deaminase (CDA) mRNA expression seemed to correlate with in vitro sensitivity to gemcitabine (r(p)=−0.31; P=0.03) and decitabine (r(p)=0.33; P=0.03), respectively. The dCK/PN-I ratio correlated inversely with LC(50) values for gemcitabine (r(p)=−0.45, P=0.001) and the dCK/CDA ratio seemed to correlate with LC(50) values for decitabine (r(p)=−0.29; 0.04). In conclusion, decreased expression of hENT1, which transports ara-C across the cell membrane, appears to be a major factor in ara-C resistance in childhood AML.