In clinical trials, the tyrosine kinase inhibitor STI571 has proven highly effective in reducing leukemic cell burden in chronic myeloid leukemia (CML). The overall sensitivity of CML CD34⁺ progenitor cells to STI571 and the degree to which cell death was dependent on cell cycle status were determined. Stem cells (Lin⁻CD34⁺) from the peripheral blood of patients with CML in chronic phase and from granulocyte–colony-stimulating factor–mobilized healthy donors were labeled with carboxy-fluorescein diacetate succinimidyl diester dye to enable high-resolution tracking of cell division. Then they were cultured for 3 days with and without growth factors ± STI571. After culture, the cells were separated by fluorescence-activated cell sorting into populations of viable quiescent versus cycling cells for genotyping. For healthy controls, in the presence of growth factors, STI571 affected neither cell cycle kinetics nor recovery of viable cells. In the absence of growth factors, normal cells were unable to divide. For CML samples, in the presence or absence of growth factors, the response to STI571 was variable. In the most sensitive cases, STI571 killed almost all dividing cells; however, a significant population of viable CD34⁺ cells was recovered in the undivided peak and confirmed to be part of the leukemic clone. STI571 also appeared to exhibit antiproliferative activity on the quiescent population. These studies confirm that CML stem cells remain viable in a quiescent state even in the presence of growth factors and STI571. Despite dramatic short-term responses in vivo, such in vitro insensitivity to STI571, in combination with its demonstrated antiproliferative activity, could translate into disease relapse after prolonged therapy.

Chronic myeloid leukemia (CML) stem and progenitor cells overexpress BcrAbl and are insensitive to imatinib mesylate (IM). We therefore investigated whether these cells were efficiently targeted by nilotinib. In K562, the inhibitory concentration (IC50) of nilotinib was 30 nM versus 600 nM for IM, consistent with its reported 20-fold-higher potency. However, in primary CD34+ CML cells, nilotinib and IM were equipotent for inhibition of BcrAbl activity, producing equivalent but incomplete reduction in CrkL phosphorylation at 5 μM. CML CD34+ cells were still able to expand over 72 hours with 5 μM of either drug, although there was a concentration-dependent restriction of amplification. As for IM, the most primitive cells (CFSEmax) persisted and accumulated over 72 hours with nilotinib and remained caspase-3 negative. Furthermore, nilotinib with IM led to further accumulation of this population, suggesting at least additive antiproliferative effects. These results confirmed that, like IM, the predominant effect of nilotinib is antiproliferative rather than proapoptotic.

Key Points: The JAK2/STAT5 pathway is a relevant therapeutic target in CML SPCs. Targeting the JAK2/STAT5 pathway by nilotinib and RUX in combination leads to enhanced eradication of primitive CML stem cells.

Acute myeloid leukemia (AML) is a highly heterogeneous hematopoietic malignancy characterized by excessive proliferation and accumulation of immature myeloid blasts in the bone marrow. AML has a very poor 5-year survival rate of just 16% in the UK; hence, more efficacious, tolerable, and targeted therapy is required. Persistent leukemia stem cell (LSC) populations underlie patient relapse and development of resistance to therapy. Identification of critical oncogenic signaling pathways in AML LSC may provide new avenues for novel therapeutic strategies. The phosphatidylinositol-3-kinase (PI3K)/Akt and the mammalian target of rapamycin (mTOR) signaling pathway, is often hyperactivated in AML, required to sustain the oncogenic potential of LSCs. Growing evidence suggests that targeting key components of this pathway may represent an effective treatment to kill AML LSCs. Despite this, accruing significant body of scientific knowledge, PI3K/Akt/mTOR inhibitors have not translated into clinical practice. In this article, we review the laboratory-based evidence of the critical role of PI3K/Akt/mTOR pathway in AML, and outcomes from current clinical studies using PI3K/Akt/mTOR inhibitors. Based on these results, we discuss the putative mechanisms of resistance to PI3K/Akt/mTOR inhibition, offering rationale for potential candidate combination therapies incorporating PI3K/Akt/mTOR inhibitors for precision medicine in AML.

It was previously shown that patients with chronic myeloid leukemia (CML) have a rare but consistently detectable population of quiescent (G₀) leukemic (Philadelphia chromosome–positive and BCR-ABL–positive [BCR-ABL⁺]) CD34⁺ cells. In the study described here, most such cells expressed a primitive phenotype (CD38⁻, CD45RA⁻, CD71⁻, and HLA-DR^lo) and cultures of these cells containing growth factors produced ultimately larger, but initially more slowly growing clones than do cultures of initially cycling CD34⁺ leukemic cells. Initially quiescent leukemic cells expressing BCR-ABLproliferated in single-cell cultures in the absence of added growth factors, thereby demonstrating their ability to spontaneously exit G₀ and enter a continuously cycling state. Interestingly, on isolation, few of these quiescentBCR-ABL⁺ cells contained either interleukin-3 (IL-3) or granulocyte colony-stimulating factor (G-CSF) transcripts, whereas both were present in most cyclingBCR-ABL⁺ CD34⁺ cells. However, after 4 days of culture in the absence of added growth factors and in association with their entry into the cell cycle (as indicated by up-regulation of Ki-67 and cdc25 transcripts), IL-3 transcripts became detectable. These findings show that entry of leukemic (BCR-ABL–expressing) progenitors into a quiescent (G₀) state in vivo is highest among the most primitive leukemic cell populations, associated with a down-regulation of IL-3 and G-CSF gene expression, and spontaneously reversible in association with up-regulation of IL-3 expression. These results highlight the potential physiologic relevance of quiescent CML progenitors, even in treated patients, in whom these cells would be predicted to have a proliferative advantage over their quiescent normal counterparts when cytokine concentrations are low.

Imatinib mesylate (IM) therapy for chronic myeloid leukemia (CML) has transformed the treatment of this disease. However, the vast majority of patients, despite major responses, still harbor Philadelphia chromosome–positive (Ph⁺) cells. We have described a population of primitive Ph⁺ cells that are insensitive to IM and may be a source of IM resistance. Cell line studies have suggested that the drug transporter ABCG2 may be a mediator of IM resistance, however there is considerable debate about whether IM is an ABCG2 substrate or inhibitor. We demonstrate here that primitive CML CD34⁺ cells aberrantly overexpress functional ABCG2 but that cotreatment with IM and an ABCG2 inhibitor does not potentiate the effect of IM. We definitively show that IM is an inhibitor of, but not a substrate for, ABCG2 and that, therefore, ABCG2 does not modulate intracellular concentrations of IM in this clinically relevant cell population.