Role of p38 MAPK and Tie2 in the Pathogenesis of MDS and Their Inhibition by Dual Inhibitor ARRY-614.

Abstract 2825

Myelodysplastic syndromes (MDS) are a heterogeneous collection of disorders characterized by dysfunctional bone marrow progenitors leading to peripheral cytopenias. The molecular mechanisms underlying MDS pathophysiology are unclear, but emerging data support a role for both p38 MAPK (p38) and TEK (Tie2). Over-activation of the p38 pathway and increased apoptosis have been reported in the bone marrow of MDS patients, and dysregulation of Tie2 signaling, a potential regulator of hematopoiesis via maintenance of normal hematopoietic stems cell (HSC) quiescence, is associated with worse outcome.

To better understand the role of Tie2 in hematopoiesis, CD34+ cells from human cord blood were separated by multi-parameter flow cytometry and cell sorted into HSCs (i.e., CD34⁺, CD38⁻) or the more differentiated common myeloid progenitors (CMPs; i.e., CD38⁺, IL-3Rα^lo, CD45RA⁻), myeloid erythroid progenitors (MEPs; i.e., CD38⁺, IL-3Rα⁻, CD45RA⁻), and granulocyte myeloid progenitors (GMPs; i.e., CD38⁺, IL-3Rα^lo, CD45RA⁺). Expression of Tie2, Tie1, Ang-1 (Tie2 agonist) and Ang-2 (Tie2 antagonist) was determined by qPCR. The data showed that Tie2, Tie1, and Ang-1 were expressed in CD34+ cells and appeared to be regulated during differentiation, with reduced Tie2 expression observed in the GMP population.

To further understand the roles of p38 and Tie2 in MDS, bone marrow and plasma samples were analyzed from a Phase 1 clinical study conducted with ARRY-614, a p38/Tie2 inhibitor, in patients with IPSS Low/Int-1 Risk MDS. At baseline, 65% (13/20) of the MDS patients showed aberrant p38 activation (≥5% phospho-p38 positive cells). Following treatment with ARRY-614, the median percent of cells positive for phospho-p38 was decreased by 85% through up to 4 cycles of treatment (∼112 days). Apoptosis in patient bone marrow samples was reduced as well (monitored by cleaved caspase-3).

In cell-based assays, ARRY-614 inhibits both p38-mediated HSP27 phosphorylation (IC₅₀ = 1 nM) and Tie2-dependent AKT phosphorylation (IC₅₀ = 13 nM). Cellular IC₅₀ values, corrected for plasma protein binding, and preliminary pharmacokinetic parameters were used to predict inhibition of these targets in patients. Analysis of the highest administered dose (1200 mg QD) of ARRY-614 as a powder-in-capsule (PiC) formulation in the Phase 1 clinical study predicted robust suppression of phospho-p38 (≥ 50% for 24 hours), consistent with bone marrow and plasma biomarker findings. However, partial inhibition of Tie2 (≥ 50% for 17.1 hours) was predicted. In a second Phase 1 clinical study, an optimized ARRY-614 formulation has demonstrated decreased intra- and inter-patient variability and increased peak plasma concentrations. With this new formulation, peak plasma concentrations of the 400 mg QD cohort were ∼50% higher than those of the 1200 mg QD PiC formulation cohort, possibly affording more extensive Tie2 inhibition.

In summary, these observations suggest that inhibition of both p38 and Tie2 may be important for the effects of ARRY-614 in MDS patients. The ongoing Phase 1 dose escalation trial of the optimized ARRY-614 formulation may further our understanding of the contributions of these targets to the pathogenesis of MDS.