Disease progression in patients with chronic myeloid leukemia (CML) while on imatinib therapy has been reported, and ex-vivo studies have suggested that this resistance can be seen despite adequate suppression and deactivation of the BCR-ABL fusion protein (

Donato et al.
Blood
.
2003
;
15
:
690
). Whether the levels of BCR-ABL phosphorylation differ between imatinib-resistant and imatinib-sensitive patients is not known. We compared the relative phosphorylation of BCR-ABL protein (ratios of phospho-tyr245-BCR-Abl:total BCR-ABL protein and phospho-thr735-BCR-ABL:total BCR-ABL protein) in 54 previously untreated CML patients and 62 CML patients with progressive disease who were resistant to imatinib. Resistant patients had significantly lower levels of phospho-thr735-BCR-ABL protein than previously untreated patients, with a median ratio of 0.64 (range, 0.03–1.63) vs. 0.80 (range, 0.40–1.67) (Wilcoxon test, P <0.001). Ratios of phospho-tyr245-BCR-ABL were also lower in patients with resistant progressive disease, with a median of 0.71 (range, 0.02–2.18) vs. 0.80 (range, 0.30–2.15) in previously untreated patients (P=0.03). Furthermore, when quantitative flow cytometry (QuantiBRITE) was used to measure CrkL phosphorylation, resistant patients had significantly lower levels of phosphorylated CrkL/cell than did previously untreated patients (median of 4204 mol/cell Vs. 5982 mol/cell; P=0.01), despite an increase in the overall percentage of p-CrkL-positive cells due to an increase in blasts. Similarly, the levels of AKT phosphorylation as measured by quantitative flow cytometry showed significantly lower intensity p-AKT in resistant patients than in previously untreated patients (median of 4131 mol/cell vs. 5949 mol/cell; P=0.008), again despite an increased percentage of p-AKT-positive cells due to an increase in blasts. There was no significant difference in Stat5 phosphorylation as measured by quantitative intensity, but the percentage of p-Stat5-positive cells was significantly higher in resistant patients than in previously untreated patients (median 22% vs. 8%; P=0.02). This data confirms that in imatinib-resistant CML patients, the disease resistance is not necessarily dependent on higher activity in the BCR-ABL and AKT pathways, but is more likely due to inability of imatinib to bind and suppress BCR-ABL (possibly due to mutations) or to the activation of other pathways that render the disease resistant. Further studies are needed to focus on specific mutations and the activation of other pathways.

Topics:
bcr-abl tyrosine kinase, imatinib mesylate, phosphorylation, drug binding, fusion proteins, bcr-abl, proto-oncogene proteins c-akt, flow cytometry, progressive neoplastic disease, disease progression, leukemia, myelocytic, chronic

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2005, The American Society of Hematology
2005
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