Rhodium(II) Proximity-Labeling Identifies a Novel Target Site on STAT3 for Inhibitors with Potent Anti-Leukemia Activity.

Nearly 40 % of children with acute myeloid leukemia (AML) suffer relapse arising from chemoresistance, often involving upregulation of the oncoprotein STAT3 (signal transducer and activator of transcription 3). Herein, rhodium(II)-catalyzed, proximity-driven modification identifies the STAT3 coiled-coil domain (CCD) as a novel ligand-binding site, and we describe a new naphthalene sulfonamide inhibitor that targets the CCD, blocks STAT3 function, and halts its disease-promoting effects in vitro, in tumor growth models, and in a leukemia mouse model, validating this new therapeutic target for resistant AML.

POL5551, a novel and potent CXCR4 antagonist, enhances sensitivity to chemotherapy in pediatric ALL.

The importance of the cell surface receptor CXCR4 and the chemokine stromal cell-derived factor-1 (SDF-1/CXCL12) is well-established in normal and malignant hematopoiesis. The Protein Epitope Mimetic POL5551 is a novel and potent antagonist of CXCR4. POL5551 efficiently mobilizes hematopoietic stem and progenitor cells, but its effects in acute lymphoblastic leukemia (ALL) have not been reported. Here, we demonstrate that POL5551 is a potent antagonist of CXCR4 in pre-B and T cell ALL cell lines and pediatric ALL primary samples. POL5551 has activity at nanomolar concentrations in decreasing CXCR4 antibody binding, blocking SDF-1α-mediated phosphorylation of ERK1/2, inhibiting SDF-1α-induced chemotaxis, and reversing stromal-mediated protection from chemotherapy. POL5551 is significantly more effective at inhibiting CXCR4 antibody binding than the FDA-approved CXCR4 inhibitor plerixafor in ALL cell lines and primary samples. We also show that treatment with POL5551 in vitro and cytarabine +/- POL5551 in vivo modulates surface expression of adhesion molecules, findings that may guide the optimal clinical use of POL5551. Finally, we demonstrate that POL5551 increases sensitivity to cytarabine in a xenograft model of a high-risk pediatric ALL, infant MLL-rearranged (MLL-R) ALL. Therefore, disruption of the CXCR4/SDF-1 axis with POL5551 may improve outcomes in children with high-risk ALL.

Plerixafor as a chemosensitizing agent in pediatric acute lymphoblastic leukemia: efficacy and potential mechanisms of resistance to CXCR4 inhibition.

In spite of advances in the treatment of pediatric acute lymphoblastic leukemia (ALL), a significant number of children with ALL are not cured of their disease. We and others have shown that signaling from the bone marrow microenvironment confers therapeutic resistance, and that the interaction between CXCR4 and stromal cell-derived factor-1 (SDF-1 or CXCL12) is a key mediator of this effect. We demonstrate that ALL cells that upregulate surface CXCR4 in response to chemotherapy treatment are protected from chemotherapy-induced apoptosis when co-cultured with bone marrow stroma. Treatment with the CXCR4 inhibitor plerixafor diminishes stromal protection and confers chemosensitivity. Using xenograft models of high-risk pediatric ALL, plerixafor plus chemotherapy induces significantly decreased leukemic burden, compared to chemotherapy alone. Further, treatment with plerixafor and chemotherapy influences surface expression of CXCR4, VLA-4, and CXCR7 in surviving ALL blasts. Finally, prolonged exposure of ALL blasts to plerixafor leads to a persistent increase in surface CXCR4 expression, along with modulation of surface expression of additional adhesion molecules, and enhanced SDF-1α-induced chemotaxis, findings that may have implications for therapeutic resistance. Our results suggest that while CXCR4 inhibition may prove useful in ALL, further study is needed to understand the full effects of targeting the leukemic microenvironment.

Does hematopoietic stem cell transplantation benefit infants with acute leukemia?

A 6-month-old girl was diagnosed with acute lymphoblastic leukemia (ALL). She has completed induction therapy and is currently in first complete remission (CR1). You are asked by your resident if hematopoietic stem cell transplantation (HSCT) would benefit infants with acute leukemia.

Dynamic chemotherapy-induced upregulation of CXCR4 expression: a mechanism of therapeutic resistance in pediatric AML.

UNLABELLED: Cure rates in pediatric acute leukemias remain suboptimal. Overexpression of the cell-surface chemokine receptor CXCR4 is associated with poor outcome in acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML). Certain nonchemotherapeutic agents have been shown to modulate CXCR4 expression and alter leukemia interactions with stromal cells in the bone marrow microenvironment. Because chemotherapy is the mainstay of AML treatment, it was hypothesized that standard cytotoxic chemotherapeutic agents induce dynamic changes in leukemia surface CXCR4 expression, and that chemotherapy-induced upregulation of CXCR4 represents a mechanism of acquired therapeutic resistance. Here, it was shown that cell lines variably upregulate CXCR4 with chemotherapy treatment. Those that showed upregulation were differentially protected from chemotherapy-induced apoptosis when cocultured with stroma. The functional effects of chemotherapy-induced CXCR4 upregulation in an AML cell line (MOLM-14, which harbors consistent upregulated CXCR4) and clinical specimens were explored. Importantly, enhanced stromal-cell derived factor-1α (SDF1A/CXCL12)-mediated chemotaxis and stromal protection from additional chemotherapy-induced apoptosis was found. Furthermore, treatment with plerixafor, a CXCR4 inhibitor, preferentially decreased stromal protection with higher chemotherapy-induced upregulation of surface CXCR4. Thus, increased chemokine receptor CXCR4 expression after treatment with conventional chemotherapy may represent a mechanism of therapeutic resistance in pediatric AML. IMPLICATIONS: CXCR4 may be a biomarker for the stratification and optimal treatment of patients using CXCR4 inhibitors.

MLL-rearranged acute lymphoblastic leukaemia stem cell interactions with bone marrow stroma promote survival and therapeutic resistance that can be overcome with CXCR4 antagonism.

Infants with MLL-rearranged (MLL-R) acute lymphoblastic leukaemia (ALL) have a dismal prognosis. While most patients achieve remission, approximately half of patients recur with a short latency to relapse. This suggests that chemotherapy-resistant leukaemia stem cells (LSCs) survive and can recapitulate the leukaemia. We hypothesized that interactions between LSCs and the bone marrow microenvironment mediate survival and chemotherapy resistance in MLL-R ALL. Using primary samples of infant MLL-R ALL, we studied the influence of bone marrow stroma on apoptosis, proliferation, and cytotoxicity induced by the FLT3 inhibitor lestaurtinib. MLL-R ALL were differentially protected by stroma from spontaneous apoptosis compared to non-MLL-R ALL. Co-culture of bulk MLL-R ALL in direct contact with stroma or with stroma-produced soluble factors promoted proliferation and cell cycle entry. Stroma also protected bulk MLL-R ALL cells and MLL-R ALL LSCs from lestaurtinib-mediated cytotoxicity. Previous studies have demonstrated that CXCR4 mediates bone marrow microenvironment signalling. Using a xenograft model of MLL-R ALL, we demonstrated that CXCR4 inhibition with AMD3100 (plerixafor) led to markedly enhanced efficacy of lestaurtinib. Therefore, the bone marrow microenvironment is a mediator of chemotherapy resistance in MLL-R ALL and targeting leukaemia-stroma interactions with CXCR4 inhibitors may prove useful in this high-risk subtype of paediatric ALL.

The bone marrow microenvironment and leukemia: biology and therapeutic targeting.

Multiple studies have demonstrated that interaction with the bone marrow stromal microenvironment contributes to the survival of leukemia cells. One explanation for this phenomenon is the interaction between the cell surface receptors CXCR4 and CXCL12. Through CXCL12/CXCR4-mediated chemotaxis, leukemia cells migrate to microscopic niches within the bone marrow, which leads to increased proliferation and survival. Several studies have suggested that increased CXCR4 expression may portend a poor prognosis in various types of leukemia, possibly due to increased protection of leukemia cells by bone marrow stroma. A potential therapeutic strategy to overcome this stromal-mediated survival advantage is to target CXCR4. Inhibition of CXCR4 may allow leukemia cells to be released from bone marrow niches that confer resistance to chemotherapy and negate the survival benefit imparted by bone marrow stroma.