The interaction of a complex and highly regulated marrow microenvironment with hematopoietic stem cells (HSCs) is crucial for normal HSC survival and function. HSCs and AML blasts share many of the same surface molecules with adhesive properties that are critical for their retention in the marrow niche, where the microenvironment provides antiapoptotic and self-renewal signals and resistance to chemotherapy.1, 2, 3 The sanctuary provided by the marrow microenvironment is illustrated by the relative ease with which standard chemotherapy often clears leukemic cells from the peripheral blood relative to the marrow.
The chemokine receptor CXCR4 is expressed on normal HSCs and leukemia blasts. Its ligand, CXCL12, is expressed on marrow stromal cells, and the receptor/ligand interaction is essential for HSC homing to the marrow.2 The adverse prognostic impact of higher leukemic cell expression of CXCR44 highlights the importance of the CXCR4/CXCL12 interaction in protection of leukemic blasts by the marrow microenvironment. The CXCR4 antagonist bicyclam AMD3100 (plerixafor) blocks CXCR4/CXCL12 signaling and stromal/HSC and stromal/leukemia cell interactions, fostering dislodgement of not only HSCs, but also of leukemic cells, from their structurally and functionally defined marrow niche. Displacement of leukemic cells from the marrow by CXCR4 inhibition has been demonstrated in vitro,5 in murine models5, 6 and in patients with AML.7 Enhanced recruitment of leukemic cells from the marrow into the bloodstream appears to make them more susceptible to chemotherapy. Uy et al.7 have demonstrated the safety and feasibility of combining plerixafor with chemotherapy and shown chemosensitization in patients with relapsed or refractory AML. However, the extent of blast mobilization was modest, there was no evidence of preferential mobilization of leukemic blasts over normal cells, and although the remission rates were favorable, survival was short.
Two avenues for refining this approach to eradication of leukemia are apparent: more forceful disruption of the niche/leukemia blast cell interaction and administration of more effective chemotherapy. Both were attempted in the trial by Konopleva and colleagues8 published in this issue of Bone Marrow Transplantation.
In order to more effectively disrupt the stromal/leukemia cell interaction, granulocyte-CSF (G-CSF) was used in combination with plerixafor. G-CSF disrupts the stromal interaction with normal HSCs and with leukemic blasts by inhibiting the same pathway as plerixafor, as well as additional pathways, including cleavage of multiple retention molecules9 and depletion of niche macrophages.10 The use of G-CSF prior to and concurrently with induction chemotherapy has been reported to improve outcomes in AML,11 although the mechanisms have not been defined. Furthermore, the combination of G-CSF and plerixafor appears to be synergistic in mobilizing HSCs for autologous transplantation.12, 13
Myeloablative doses of chemotherapy, as administered before allogeneic hematopoietic cell transplantation (HCT), provide a more potent method, compared with standard chemotherapy, to kill vulnerable leukemia cells which have been evicted from the marrow niche. Furthermore, allogeneic transplantation circumvents the potential problem of significant injury to normal mobilized HSCs by chemotherapy. Cure of a patient with AML with chemotherapy requires eradication of not only bulk leukemic cells, but also leukemia stem cells (LSCs) which are directly derived from, and closely resemble, normal hematopoietic stem/progenitor cells. Current approaches cannot achieve selective eradication of LSCs without injury to normal HSCs. Allogeneic HCT permits administration of myeloablative chemotherapy without the concern of damage to chemotherapy-exposed normal HSCs because of rescue by unexposed donor HSCs. HCT thus provides a sensible and pragmatic platform for innovative approaches to eradication of LSCs, including the strategy reported by Konopleva et al.8 in this issue.
They demonstrate the safety and feasibility of disrupting the interaction between the marrow niche and leukemic blast cells by G-CSF and plerixafor followed by myeloablative HCT. The failure, in this phase I/II study, to improve relapse rates compared with historical controls should not dampen enthusiasm for this tactic; however, further improvements will be required. In contrast to patients with relapsed AML receiving plerixafor prior to chemotherapy,7 combining G-CSF with plerixafor achieves preferential mobilization of CD34+ cells, CXCR4+ cells, and FISH+ (leukemia) cells compared with normal WBCs. The synergistic mobilization of CD34+ cells, including HSCs, observed with plerixafor and G-CSF, suggests that this combination might effectively mobilize LSCs and thereby increase the susceptibility of these particularly chemotherapy-resistant cells that are responsible for the initiation of leukemia and relapse owing to their persistence.
Standard chemotherapy contributes to enhanced drug resistance of surviving leukemia cells. Recent reports suggest that the interaction of the marrow niche with its resident stem cells is bidirectional, and leukemia cells which survive standard chemotherapy contribute to remodeling the marrow niche to make it even more protective and drug-resistant.14 In addition, standard chemotherapy used for induction and maintenance of remission leads to the development of new mutations that confer additional drug resistance to remaining leukemic clones.15 These discoveries bolster an already strong theoretical basis for the importance of early definitive therapy in improving cure rates in AML.
Attention to important biologic issues, including pursuit of better mechanisms of mobilization (for example, the use of existing more effective CXCR4 inhibitors and inhibitors of the VLA-4/VCAM-1 axis) and specific analysis of the extent to which LSCs are effectively mobilized and killed provide compelling pathways for further investigation. Techniques for identification of clinically relevant LSCs have been reported16 and could be utilized for such studies. The initial work in targeting the HSC marrow niche in animal models, in AML patients receiving standard chemotherapy, and now with allogeneic transplantation provides a foundation for further study by demonstrating feasibility and safety. The next challenge is to translate our growing understanding of normal and malignant stem cell interactions with the microenvironment into meaningful therapeutic advances.
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The author declars no conflict of interest.
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Copelan, E. Leukemia cell mobilization: a road to eradication?. Bone Marrow Transplant 50, 905–906 (2015) doi:10.1038/bmt.2015.78