Hematopoietic Stem Cells Actively Extrude Imatinib-Mesylate Due to High ABCG2-Transporter Activity.

The tyrosine kinase inhibitor imatinib mesylate (STI571) is a potent therapeutic agent for treatment of chronic myeloid leukemia (CML) due to its specific inhibition of bcr-abl kinase. However, patients with CML always relapse after withdrawal of imatinib therapy. With CML being the paradigm for a stem cell disease, we sought to investigate the influence of imatinib on the most primitive stem cell compartment.

Imatinib has recently been reported to be a high affinity substrate for the ABC-transporters ABCG2/BCRP (breast cancer resistance protein) and MDR1 (P-Glycoprotein). Given the high expression of these ABC-transporters in Hematopoietic Stem Cells (HSC), we examined the influence of imatinib on ABCG2- and MDR1-activity in human and murine HSC by using Hoechst- and Rhodamine-efflux assays. MDR1-mediated Rhodamine-efflux was only mildly influenced by Imatinib. However, addition of imatinib at therapeutic dosages completely abrogated the SP phenotype of total bone marrow after Hoechst 33342 staining. This effect was even more pronounced in cells of the HSC-phenotype, i.e., lineage-negative/AC133+ or c-kit+/Sca-1+/lin− (KSL) of human and murine marrow, respectively. In order to determine the effect of imatinib on stem cells in vivo, we isolated SP cells from 13 CML patients at various stages of disease using FACS. We employed quantitative RT-PCR-analysis for bcr-abl and demonstrated that the majority of CML patients had bcr-abl-negative SP cells, while peripheral blood mononuclear cells were mostly bcr-abl-positive. However, two patients in complete cytogenetic and molecular-genetic remission proved to be bcr-abl-positive within the SP cell population. This supports the notion that continuous imatinib-therapy may not eradicate the malignant stem cell pool and the leukemic clone may be able to expand even during permanent imatinib-therapy.

Furthermore, these data imply that the detoxifying function of ABC transporters on stem cells may be altered under imatinib treatment. Using HEK-293 cells transfected with ABCG2, we demonstrate that imatinib reverses mitoxantrone-resistance - potentially due to high affinity substrate inhibition.

In summary, our data suggest the novel resistance mechanism of imatinib being actively extruded from primitive leukemic stem cells, and that addition of chemotherapeutic agents to imatinib therapy may facilitate the eradication of bcr-abl-positive stem cells in chronic myeloid leukemia.