Bone Marrow-Derived, BCR/ABL⁺ and Flk1⁺CD31⁻CD34⁻ Stem Cells from CML Patients That Are Insensitive to STI571 In Vitro.

To evaluate the effect of imatinib mesylate (STI571, Gleevec) on primitive/committed malignant progenitor cells in chronic myelogenous leukemia (CML) and to further elucidate the mechanisms involved in CML relapse, bone marrow-derived, malignant, BCR/ABL⁺Flk1⁺CD31CD34⁻ cells with hemangioblastic characteristics from CML patients were labeled with carboxy-fluorescein diacetate succinimidyl diester dye to enable high-resolution tracking of cell division. Then they were cultured in Methocult GF+ media with or without STI571. After culture, the cells were separated by fluorescence-activated cell sorting into populations of viable quiescent versus cycling cells. Then the mechanisms involved in CML stem cells that became resistant to STI571 were studied. We found that in the most sensitive cases, STI571 killed almost all dividing cells; however, a significant population of viable BCR/ABL-positive cells were recovered in the undivided peak and confirmed to be the leukemic stem cells. STI571 also appeared to exhibit antiproliferative activity on the quiescent population. We further demonstrate that apoptosis was restricted to dividing cells, whereas nonproliferating BCR/ABL⁺Flk1⁺CD31CD34⁻ cells were resistant to apoptosis induced by imatinib, Ara-C, VP-16, ceramide, or arsenic, either alone or in combination. These studies suggested that CML primitive stem cells remain viable in the presence of STI571 and that inhibition of BCR/ABL tyrosine kinase by STI571 restores normal hematopoiesis by removing the proliferative advantage of CML committed progenitors but that elimination of all CML progenitors may not occur. So despite dramatic short-term responses in vivo, such in vitro insensitive to STI571, might translate into disease relapse after prolonged therapy.