The Jak1/Jak2 Inhibitor Momelotinib Inhibits Alk2, Decreases Hepcidin Production and Ameliorates Anemia of Chronic Disease (ACD) in Rodents

Many patients with myelofibrosis (MF) develop anemia and are, or become, dependent on frequent red blood cell transfusions [1]. Results from the phase 2 studies for the treatment of myelofibrosis (MF) with the Jak1/2 inhibitor momelotinib (MMB) demonstrated that MMB treatment provided an anemia benefit, in addition to providing a durable spleen response and improvement in constitutional symptoms in MF patients [2]. MMB's anemia benefit was an unexpected outcome as erythropoietin (EPO)-mediated JAK2 signaling is essential for erythropoiesis [3, 4]. Systemic iron homeostasis is controlled by the peptide hormone hepcidin produced by the liver. Hepcidin reduces iron export from duodenal enterocytes and splenic and hepatic macrophages by binding to and down-regulating the iron exporter ferroportin [5-7]. In chronic disease, there is a significant increase in hepcidin levels which can result in severe anemia; this pathologic condition is termed anemia of chronic disease (ACD). Recently, MF patients have been shown to have elevated serum hepcidin levels and this increase is associated with inferior overall survival in these patients [8].

This study aimed to determine whether MMB's clinical anemia benefit is driven by direct activity of MMB on the hepcidin pathway. We assayed MMB inhibitory activity on the BMP-receptor kinase pathway (the central driver of hepcidin transcription in hepatocytes) and assessed the activity of MMB in a rodent model of anemia of chronic disease (ACD). We demonstrate that MMB inhibits BMP6-induced in vitro production of hepcidin in cultured hepatocytes (HepG2 cells) with an EC50 = 651 +/- 203 nM (n=3). This inhibitory activity is mediated by direct suppresion of the BMP-receptor kinase Alk2 as MMB inhibits Alk2 enzymatic activity with an IC50 = 8.4 +/- 1.5 nM (n=3). Ruxolitinib has no activity on either Alk2 or the BMP-receptor kinase pathway. To understand whether MMB could modulate hepcidin levels in vivo and ameliorate anemia in vivo we assessed the effect of MMB in a peptidoglycan-polysaccharide fragment (PG-APS)-induced rat ACD model. Treatment with clinically relevant exposure levels of MMB for 3 days resulted in a dose dependent reduction in both liver RNA and serum protein hepcidin levels and caused an increase in serum iron. Furthermore, long-term treatment with MMB for 21 days increased the numbers and percent of reticulocytes and mature red blood cells in the bone marrow and increased Hgb and hematocrit to normal levels in the blood.

Our data suggest that MMB's clinical anemia benefit results from inhibition of ALK2-mediated expression of hepcidin in the liver, which results in increased release of iron from sequestered cellular stores and enhanced erythropoiesis. A phase 2 translational study in anemic subjects with MPNs is scheduled to confirm this mechanism. The Alk2-mediated activity on iron metabolism through hepcidin could prove beneficial in a number of additional indications, and facilitate the combination of MMB with myelosuppressive agents.

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8. Pardanani, A., et al., Associations and prognostic interactions between circulating levels of hepcidin, ferritin and inflammatory cytokines in primary myelofibrosis. Am J Hematol, 2013. 88 (4): p. 312-6.