The Jak2 Kinase Inhibitor, G6, Reduces the Mutant Burden and Reverses Marrow Fibrosis in a Mouse Model of Jak2-V617F Mediated PMF,
Authors: Sayeski Peter P. , Kirabo Annet , Park Sung , Gali Meghanath , Reinhard Mary , Wamsley Heather L , Zhao Zhizhuang Joe , Bisht Kirpal S , Cogle Christopher R , Keseru Gyorgy M 
Published: October 2019

Abstract
Abstract 3858

Introduction:
Philadelphia chromosome negative myeloproliferative neoplasms (MPNs) encompassing polycythemia vera (PV), essential thrombocytosis (ET) and primary myelofibrosis (PMF) are disorders characterized by abnormal hematopoiesis. Among all the MPNs, PMF has the most unfavorable prognosis and there are no known treatments available to reverse its natural history. We recently identified a Jak2 small molecule inhibitor called G6. This compound inhibits Jak2 kinase in vitro and ex vivo as well as in a HEL cell xenograph model, in vivo. However, it's efficacy in Jak2-medaited PMF has not yet been examined. Therefore, the purpose of this investigation was to determine the efficacy of G6 in Jak2-V617F mediated PMF, with particular emphasis on bone marrow efficacy.

Methods:
Transgenic mice, 11–12 months of age and expressing the human Jak2-V617F cDNA under the control of the vav promoter, were employed for this study. The mice exhibit a marked PMF phenotype and recapitulate many of the same pathologies observed in humans. The mice were given either DMSO vehicle control solution (n=7) or 10 mg/kg/day of G6 (n=6) for 28 days and then euthanized. Efficacy was determined my measuring parameters within the skin, peripheral blood, liver, spleen, and bone marrow.

Results:
We found that G6 treatment completely eliminated the pruritus and reduced extramedullary hematopoiesis in the liver of the PMF mice by an average of 45%, when compared to mice that received vehicle control solution. In the peripheral blood, mice receiving vehicle control solution experienced an anemic effect during the 28 day treatment period characterized by a 38% decrease in red cell counts, a 30% decrease in hemoglobin, and a 36% decrease in hematocrit. While mice receiving G6 also experienced anemia over the same 28 day treatment period, the effect was significantly less severe from that observed in mice receiving vehicle control solution; namely, an 18% decrease in red cell counts, a 9% decrease in hemoglobin, and a 15% decrease in hematocrit. In the spleen, G6 treatment resulted in a 42% reduction in spleen size and a 54% reduction in the number of megakaryocytes per high power field. In the critically important bone marrow, G6 significantly reduced the megakaryocytic hyperplasia by an average of 70%, resulting in a significant correction of the M:E ratio. G6 treatment also resulted in a 53% reduction in Jak/STAT signaling in the marrow as well as a 68% reduction in the Jak2 mutant burden. Lastly, G6 treatment decreased the intensity of reticulin staining in the marrow by an average of 67% resulting in a significant reversal of the myelofibrosis.

Conclusions:
Our results indicate that G6 provides significant therapeutic efficacy in a mouse model of Jak2-V617F mediated PMF. Furthermore, by reducing the mutant burden and reversing the fibrosis in the bone marrow, our results suggest that G6 may alter the natural history of myelofibrosis. Given the inability of current Jak2 inhibitors to alter the natural history of PMF, G6 may be a viable candidate for progression into clinical trials for treatment of this complex disorder.

Disclosures:
No relevant conflicts of interest to declare.