Myelofibrosis (MF) is the deadliest form of myeloproliferative neoplasms (MPN) characterized by bone marrow fibrosis, splenomegaly, aberrant hematopoiesis and frequent transformation to acute myeloid leukemia. The overall median survival for patients with MF is ~5 years. The oncogenic JAK2V617F mutation has been found in 50-60% patients with MF. Mutations in the thrombopoietin receptor (MPL) and calreticulin (CALR) also have been observed in MF. JAK2, MPL and CALR mutations are considered as MPN driver mutations and they lead to hyperactivation of the JAK/STAT signaling. Currently approved JAK inhibitors, Ruxolitinib and Fedratinib, can reduce splenomegaly and alleviate constitutional symptoms but they do not offer significant improvement of bone marrow (BM) fibrosis. While JAK inhibitor therapy represents a major advancement in the management of MF, treatment failure and transformation to acute myeloid leukemia are observed in many cases. Therefore, there is an unmet need to develop novel targeted therapies for MF that can effectively reduce BM fibrosis.
We found that Pim1 expression is significantly increased in hematopoietic progenitors of Jak2V617F knock-in mice and MF patients. So, we hypothesize that Pim1 might play a role in the pathogenesis of MF. In a recent article published in Leukemia, we show that genetic ablation of Pim1 blocked the development of myelofibrosis induced by Jak2V617F and MPLW515L. We found that deletion of Pim1 significantly reduced WBC and neutrophil counts, normalized spleen size and abrogated BM fibrosis in Jak2V617F and MPLW515L mouse models of MF. These results establish an important role of Pim1 in the development of MF.
Data from our Pim1 genetic deletion study prompted us to investigate the effects of pharmacologic inhibition of Pim1 in preclinical models of MF. We used TP-3654, a novel second-generation Pim kinase inhibitor, which is more potent but less toxic than the first-generation Pim kinase inhibitor SGI-1776. TP-3654 also has a greater selectivity for Pim1 than Pim2 or Pim3. We observed that treatment of TP-3654 significantly reduced proliferation of hematopoietic cells and progenitors expressing JAK2V617F and MPLW515L mutants. We also tested the in vivo efficacy of Pim inhibitor TP-3654 in Jak2V617F and MPLW515L mouse models of MF. We observed that inhibition of Pim1 by TP-3654 alone significantly reduced leukocytosis and splenomegaly, and improved BM fibrosis in both Jak2V617F and MPLW515L murine models of MF. Notably, TP-3654 treatment alone exhibited more inhibition of BM fibrosis than Ruxolitinib treatment. Combined treatment of TP-3654 and Ruxolitinib resulted in greater inhibition of MPN cells and abrogation of BM fibrosis in Jak2V617F and MPLW515L mice. We further show that TP-3654 preferentially inhibits Jak2V617F mutant hematopoietic progenitors in mice.
Mechanistically, we show that treatment with TP-3654 inhibited mTORC1, MYC and TGF-β signaling in Jak2V617F mutant hematopoietic cells and attenuated the expression of fibrotic markers (such as, collagen I and III, Snail and α-smooth muscle actin) inthe bone marrow. Overall, our results suggest that TP-3654 alone or in combination with Ruxolitinib may have therapeutic potential against MF. Based on the results from these studies, a Phase I clinical trial of TP-3654 (NCT04176198) in patients with MF has been initiated.
These findings demonstrate that Pim1 plays an important role in the pathogenesis of myelofibrosis, and inhibition of Pim1 with TP-3654 alone or in combination with JAK inhibitor Ruxolitinib could be a useful therapeutic approach for myelofibrosis.
Avik Dutta, Ph.D., Postdoctoral Fellow, National Institutes of Health, Bethesda, MD
Golam Mohi, Ph.D., Professor, University of Virginia, Charlottesville, VA