Efficacy of Dual Inhibition of p38 Mitogen Activated Protein Kinase (MAPK) and Tie-2 Kinase in Myelodysplastic Syndromes (MDS) and Acute Myeloid Leukemia (AML)

Introduction- MDS and AML are hematologic malignancies that are characterized by malignant cell expansion and suppression of normal hematopoietic activity. An inflammatory cytokine milieu in the marrow has been implicated in suppression of normal hematopoietic activity in MDS and AML. Since cytopenias are a major source of morbidity, these incurable diseases need agents that can target the malignant clones while also relieving the repression of healthy hematopoietic stem cells (HSCs) in the marrow. Angiopoietin -1 (Ang-1) is a cytokine that is implicated in vascular development and angiogenesis and has also been shown to regulate stem cell quiescence.

Methods and Results- We determined Angiopoietin-1 levels in a large cohort of MDS CD34+ stem cells (n=183) and found its expression to be significantly elevated when compared to marrow CD34+ cells from healthy controls (n=17) (P Value <0.05). We also observed that expression of the receptor for Angiopoietin-1, Tie-2, was significantly elevated in MDS stem cells. Furthermore, Angiopoietin-1 levels were found to be strongly predictive of poor survival in MDS (Log Rank P value = 0.001). Using The Cancer Genome Atlas (TCGA) dataset, we found that AML patients with higher Angiopoietin-1 also showed a trend for adverse overall survival (N=200, Log Rank P Value = 0.05), thus demonstrating its biological significance in these diseases.

To test the functional role of Angiopoietin-1 signaling in MDS/AML, we designed specific siRNAs against Tie-2 and determined that receptor knockdown led to decreased leukemic cell proliferation in multiple cell lines. Next, we tested the efficacy of ARRY-614, a novel inhibitor of Tie-2 that also inhibits p38 MAPK. ARRY-614 is being presently tested in clinical trials in MDS and has led to hematologic responses in preliminary analyses. Treatment of leukemic cells with ARRY-614 led to dose dependant decrease in proliferation. Immunoblotting showed that ARRY-614 inhibited p38 MAPK effectively in leukemic cells and also blocked downstream activation of effector kinases MapKapK2 and EIF4E in these cells upon inhibitory cytokine stimulation. Functionally, ARRY-614 treatment was able to inhibit TNF-alpha mediated inhibitory affects on normal hematopoietic stem cells in colony assays in vitro. Finally treatment of primary MDS samples with the dual inhibitor led to stimulation of erythroid and myeloid colonies.

Conclusion- These data demonstrate elevated levels of Angiopoietin-1 and its receptor Tie-2 in MDS and show that these are prognostic for worse overall survival. A dual inhibitor of Tie-2 and p38 MAPK is able to inhibit leukemic cell growth and reverse inhibitory effects of cytokines on healthy hematopoiesis.