The Pan-PIM Inhibitor INCB053914 Displays Potent Synergy at Low Doses in Combination with Ruxolitinib in Pre-Clinical Models of MPNs

Classic Philadelphia chromosome-negative myeloproliferative neoplasms (MPNs) are hematopoietic stem cell disorders that affect about 300,000 people in the U.S. and can progress to bone marrow failure and acute myeloid leukemia. MPNs are driven by aberrant activation of the JAK2 tyrosine kinase, which leads to activation of downstream signaling molecules including STAT5, ERK, and AKT, among others. While the JAK inhibitor ruxolitinib is approved for use with patients with intermediate and high risk myelofibrosis and patients with polycythemia vera who are resistant or intolerant to hydroxyurea based on compelling clinical and/or hematologic efficacy, molecular responses to ruxolitinib therapy occur only rarely. In order to identify new MPN regimens that may enhance patient benefits, we investigated the effect of combining the JAK1/JAK2 inhibitor ruxolitinib with a new inhibitor (INCB053914) of the PIM family of Ser/Thr kinases, which are downstream targets of JAK2/STAT signaling. Members of the PIM family of kinases are proto-oncogenes that can cooperate with cMyc to induce lymphomagenesis in mice. PIM kinase activity regulates a variety of cell processes, including cell proliferation and apoptosis. For example, PIM kinases phosphorylate and inactivate the pro-apoptotic protein BAD. In addition, PIM activity can regulate translation initiation via activation of the mTORC1 complex, leading to inactivation of the translational initiation repressor 4EBP1. Triple PIM1/2/3 knockout mice demonstrate no major deleterious abnormalities suggesting specific targeting of PIM kinase activity may have limited adverse effects. However, the elimination of PIM1 in mice has suggested a role of PIM1 in the function of hematopoietic stem cells. Thus, deregulated PIM activity could affect hematopoietic stem cell disorders such as MPNs. We have recently demonstrated that PIM family members are elevated in MPN cells that persistently grow in the presence of ruxolitinib and that exogenous expression of PIM1 can induce ruxolitinib resistance. Taken together, PIMs appear to be strong candidates as therapeutic targets in MPNs.

INCB053914 (PIMi) exhibits biochemical IC50s against Pim1/2/3 of 0.24 nM/30 nM/0.12 nM (at 1 mM ATP). In this study, we demonstrate that PIMi exhibits potent combination effects with ruxolitinib in pre-clinical models of MPNs. PIMi displayed synergistic effects in combination with ruxolitinib in MPN model cell lines, including BaF3/EpoR-JAK2-V617F, UKE1, and SET2. These effects include growth inhibition and, importantly, synergistic induction of apoptosis. The phosphorylation of biomarkers for Pim activity, including BAD, p70S6K, S6, and 4EBP1 was significantly diminished with combination of low dose ruxolitinib (0.1 uM) and PIMi (0.1 uM). Cells that were generated to persistently grow in ruxolitinib remained sensitive to PIMi. Importantly, neoplastic growth of erthyroid colonies from JAK2-V617F-positive MPN patients was sensitive to low dose PIMi (5 nM), with colony formation from some patient samples displaying sensitivity to as low as 1 nM of PIMi. In combination with ruxolitinib (25-50 nM), low dose PIMi displayed impressive synergy in inhibiting neoplastic colony formation of primary patient cells. Finally, in an in vivo model of JAK2-V617F-driven MPN cell growth, the combined treatment of ruxolitinib and PIMi suppressed tumor formation in CB17-SCID mice at doses that had little anti-tumor effect as single agents. The combination therapy was well tolerated in mice.

Together, these data suggest that the pan-PIM inhibitor INCB053914 may provide a highly effective novel anti-MPN therapeutic, particularly in combination with ruxolitinib. INCB053914 is currently under investigation in a Phase 1/2 study in patients with advanced hematological malignancies (ClinicalTrials.gov: NCT02587598). Our findings support a clinical study investigating the combination of INCB053914 and ruxolitinib in MPN patients.