Efficacious Intermittent Dosing of a Novel JAK2 Inhibitor in Mouse Models of Polycythemia Vera

A high percentage of patients with the myeloproliferative disorder polycythemia vera (PV) harbor a Val617→Phe activating mutation in the Janus kinase 2 (JAK2) gene, and both cell culture and mouse models have established a functional role for this mutation in the development of this disease. We describe the properties of MRLB-11055, a highly potent inhibitor of both the WT and V617F forms of JAK2, that has therapeutic efficacy in erythropoietin (EPO)-driven and JAK2V617F-driven mouse models of PV. In cultured cells, MRLB-11055 blocked proliferation and induced apoptosis in a manner consistent with JAK2 pathway inhibition. MRLB-11055 effectively prevented EPO-induced STAT5 activation in the peripheral blood of acutely dosed mice, and could prevent EPO-induced splenomegaly and erythrocytosis in chronically dosed mice. In a bone marrow reconstituted JAK2V617F-luciferase murine PV model, MRLB-11055 rapidly reduced the burden of JAK2V617F-expressing cells from both the spleen and the bone marrow. Using real-time in vivo imaging, we examined the kinetics of disease regression and resurgence, enabling the development of an intermittent dosing schedule that achieved significant reductions in both erythroid and myeloid populations with minimal impact on lymphoid cells. Our studies provide a rationale for the use of non-continuous treatment to provide optimal therapy for PV patients

Effect of MRLB-11055 on major lymphoid populations in spleen of WT B6 mice.

<p>MRLB-11055 was given for either 3 or 6 days (on), followed up by either a 0 or 4 day holiday (off), for up to 4 cycles. Effects on NK, B and T cells were measured by flow cytometry.</p

Effect of MRLB-11055 in a Darbepoetin-Induced PV Efficacy Model.

<p>The ability of MRLB-11055 to prevent darbepoetin-induced increases in hematocrit (Hct) and spleen mass (SPL) over 7 days is shown, as is the impact of MRLB-11055 on white blood cells (WBC) and its concentration in blood (PK). Dashed line indicates <i>in vivo</i> IC50 value.</p

Exposure and target engagement of MRLB-11055 in the peripheral blood of C57BL/6 mice stimulated with darbepoetin.

<p>A. Effect of MRLB-11055 on phosphoSTAT5 levels at various times post-dose. B. Calculation of IC50 value for inhibition of phosphoSTAT5. C. PK of 3 doses of MRLB-11055 in mouse blood, with calculated IC50 superimposed (dashed line).</p