Imatinib Mesylate (Gleevec or Glivec from NOVARTIS) is widely used for the treatment of CML inhibiting the BCR-ABL tyrosine kinase activity. Although, 90% of the patients respond to this treatment, resistance to this drug has been seen, mainly due mutations in the tyrosine kinase domain of BCR-ABL. Recently, has been reported that imatinib is a substrate for the ABC-transporters BCRP (breast cancer resistance protein) and MDR1 (P-Glycoprotein). Based in that report, we examined the expression of those two ABC-transporter, by quantitative PCR (Q-PCR) technology, in 29 CML patients who were resistant to IM (9 CP, 17 AP, and 3 BC), in 34 untreated CML patients (23 CP, 10 AP, and 1 BC) and 6 normal blood donors. cDNA was synthesized for RQ-PCR, which was performed on peripheral blood samples drawn after loss of CHR or MCyR for those with resistance to imatinib and prior to start of treatment with imatinib and those untreated patients. All samples were analyzed using SYBER Green technology and reported as the 2−Δ Δ Ct where it assumes that the amplification efficiency of the target gene (BCRP or MDR1) and the internal control gene (ABL) are the same. Resistant patients had a significantly higher levels of BCRP expression than untreated patients, and normal blood donors with a median ratio of 2.1 (range, 0.04–34.2) vs. 0.64 (range, 0–18.6) vs. 0.6 (range, 0.07–1.41) (Wilcoxon test, P<0.0002). For the MDR1 gene expression, patients with resistance to imatinib had also a significantly higher level, median ratio of 1.65 (range, 0.09–10.8) vs. 0.68 (range, 0.01–2.63) in untreated patients and normal blood donors median ratio 0.76 (range, 0.02–1.92) (P < 0.0017). Nineteen of 29 resistant patients (65.5%) overexpressed BCRP and 41.3% (12/29) overexpressed MDR1, meantime only 26.5% (9/34) and 11.8% (4/34) of the untreated CML patients overexpressed BCRP and MDR1 respectively (X2 test P=0.004 and P=0,022). Overexpression of BCRP was more frequent in AP of the resistant group (70%) than the untreated patients (30%) (Fisher exact test P=0.05). When comparing the expression of MDR1 between the resitant group in AP (35,3%) and the untreated one in the same phase (40%) no statistic difference was seen (P=1). Patients in CP and BC do not show any difference in the expression of BCRP and MDR1 regardless the presence or not of resistance to imatinib. These data confirms that high expression of BCRP or MDR1 may contribute to imatinib resistance phenotype leading to a reduction in intracellular imatinib concentrations.Further follow-up is necessary for those untreated CML patients that overexpress, at diagnosis, those ABC-transporters BCRP and MDR1. These may reveal correlations between the expression and clinical prognosis and may allow the development of new starategies to overcome imatinib mesylate resistance.

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