1. ¹Department of Pediatric Hematology/Oncology, VU University Medical Center, De Boelelaan 1117, Postbus 7057, 1007 MB, Amsterdam, The Netherlands
2. ²Department of Medical Oncology, VU University Medical Center, De Boelelaan 1117, Postbus 7057, 1007 MB, Amsterdam, The Netherlands
3. ³Department of Clinical Epidemiology and Biostatistics, VU University Medical Center, De Boelelaan 1117, Postbus 7057, 1007 MB, Amsterdam, The Netherlands
4. ⁴Department of Pediatric Oncology/Hematology, Erasmus MC/Sophia Children's Hospital, Rotterdam, The Netherlands
5. ⁵The Dutch Childhood Oncology Group (DCOG), The Hague, The Netherlands
6. ⁶MRC Childhood Leukaemia Working Party, UK
7. ⁷AML BFM-Study Group, Münster, Germany

Correspondence: Dr I Hubeek, E-mail: isabelle.hubeek@vumc.nl

⁸These authors contributed equally to the work.

Received 25 May 2005; Revised 20 September 2005; Accepted 27 October 2005.

Abstract

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 LC₅₀ 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 LC₅₀ values for gemcitabine (r_p=-0.45, P=0.001) and the dCK/CDA ratio seemed to correlate with LC₅₀ 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.