Role of P-glycoprotein in evolution of populations of chronic myeloid leukemia cells treated with imatinib

Abstract

Imatinib mesylate (imatinib) is a new generation preparation that is now successfully used for treatment of cancer, particularly for chemotherapy of chronic myeloid leukemia (CML). Imatinib inhibits the activity of chimeric kinase BCR-ABL, which is responsible for the development of CML. The goal of this study was to investigate the role of a multidrug resistance protein, P-glycoprotein (Pgp), in the evolution of CML treated with imatinib. We demonstrate here that although imatinib is a substrate for Pgp, cultured CML cells (strain K562/i-S9), overexpressing active Pgp, do not exhibit imatinib resistance. Studies of CML patients in the accelerated phase have shown variations in the number of Pgp-positive cells (Pgp+) among individual patients treated with imatinib. During treatment of patients with imatinib for 6-12 months, the number of Pgp-positive cells significantly increased in most patients. The high number of Pgp+ cells remained in patients at least for 4.5 years and correlated with active Rhodamine 123 (Rh123) efflux. Such correlation was not found in the group of imatinib-resistant patients examined 35-60 months after onset of imatinib therapy: cells from the imatinib-resistant patients exhibited efficient Rh123 efflux irrespectively of Pgp expression. We also compared the mode of Rh123 efflux by cells from CML patients who underwent imatinib treatment for 6-24 months and the responsiveness of patients to this therapy. There were significant differences in survival of patients depending on the absence or the presence of Rh123 efflux. In addition to Pgp, patients' cells expressed other transport proteins of the ABC family. Our data suggest that treatment with imatinib causes selection of leukemic stem cells characterized by expression of Pgp and other ABC transporters.

MRP1 and P-glycoprotein expression assays would be useful in the additional detection of treatment non-responders in CML patients without ABL1 mutation

“…In addition, we also found that the simultaneous implementation of two MDR expression assays could additionally detect 41.2% of treatment failed patients in overall, and 51.9% of treatment failed patients with no ABL1 kinase domain mutations specifically. This result supports the findings of a previous study, which reported that resistance mediated by increased expression of the efflux pump is also an important resistance mechanism in CML [10,12,[19][20][21][22]. Our results can underscore the clinical usefulness of two MDR expression assays in the additional detection of treatment failed CML patients, and may suggest that the simultaneous application of two MDR expression assays can be a more useful strategy in the clinical setting than performing a single test.…”

“…By contrast, the rhodamine-123 efflux assay could not efficiently discriminate patients with treatment failure from responders. These results are partly inconsistent with previous reports [10,12,[19][20][21][22] and support the hypothesis that not only Pgp expression assay but also MRP1 expression assay, which has insufficient data about the clinical relevance in CML might be also useful in the discrimination of patients in whom treatment will fail than the rhodamine-123 efflux assay, and that for these two MDR expression assays, reporting the results as a percentage is better than the absolute mcf value or mcf ratio. The finding that the results of the two MDR expression assays correlated well but that those of the rhodamine-123 efflux assay did not correlate with other results also supports this hypothesis.…”

“…In contrast, they also reported that the RFI values of three assays did not vary with respect to different CML phase or Sokal score, although the Pgp expression positive cases was more frequently observed at advanced stage and low Sokal score than the MRP1 expression and the Rhodamine-123 efflux positive cases [10]. A recent study analyzed the role of Pgp in evolution of populations of CML cells treated with imatinib and reported that the high number of Pgp+ cells remained in patients treated with imatinib at least for 4.5 years and correlated well with active rhodamine-123 efflux, but such correlation was not found in the group of imatinib-resistant patients examined 35-60 months after onset of imatinib therapy [22]. Therefore, the clinical usefulness of the MDR expression and functional assays in CML has not been confirmed, especially in the MDR functional assay.…”

Imatinib increases apoptosis index through modulation of survivin subcellular localization in the blast phase of CML cells

“…This finding suggests a cross-resistance between vincristine and imatinib, which can be explained by the fact that Lucena cells overexpress Pgp. Moreover, previous studies have shown that imatinib is a substrate for this drug transporter protein [4,7,26]. Ara-C induced a low apoptosis index in both cell lines, but when the drugs were combined, the apoptosis index in the sensitive cells was similar to that found with imatinib alone.…”

“…CML cells may also acquire resistance to imatinib by other mechanisms, such as P-glycoprotein (Pgp) overexpression [4][5][6]. Pgp is a protein encoded by the ABCB1 gene that acts as a drug efflux pump [7]. Another common resistance mechanism in neoplasms is the survivin overexpression, an inhibitor apoptosis protein (IAP) that exerts a central role in cell division and in apoptosis inhibition by blocking the activation of caspases [6,8].…”