Novel approaches to therapy in CML.

Treatment with tyrosine kinase inhibitors (TKIs) results in remission and prolongation of survival in most chronic myeloid leukemia (CML) patients but fails to eliminate the leukemia stem cells (LSCs) responsible for disease development and propagation. This accounts for the clinical observation that TKI discontinuation leads to rapid leukemia relapse. Most patients require continued treatment to prevent relapse, with associated risk of relapse, toxicity, teratogenic effects, financial burden, and noncompliance. Understanding LSC resistance to TKI and development of strategies to increase the proportion of CML patients achieving treatment-free remissions is a critical area of investigation in CML. In addition, LSCs are the source of TKI resistance, relapse, or disease progression, which is another major area of need in CML treatment. It is now understood that BCR-ABL kinase-independent mechanisms are responsible for retention of LSC subpopulations. It is likely that both cell-intrinsic and microenvironmental mechanisms contribute to LSC maintenance. Here, we review the current understanding of mechanisms underlying persistence of CML LSCs during TKI treatment, recently described approaches to target these cells and emerging clinical trials, and the challenges impeding more rapid progress in achieving cures for a greater number of CML patients.