Type II JAK2 Inhibitor NVP-CHZ868 Is Active in Vivo Against JAK2-Dependent B-Cell Acute Lymphoblastic Leukemias (B-ALLs)

Approximately 10% of B-ALLs harbor CRLF2 rearrangements, which may portend a poor prognosis. Although these leukemias are addicted to JAK2 signaling, ATP-competitive type I JAK2 inhibitors have limited activity in vitro or in vivo (Weigert et al. J Exp Med 2012). This may result from heterodimerization of JAK2 with other JAK family members (Koppikar et al. Nature 2012). Type II inhibitors bind JAK2 in the inactive conformation, which may overcome this resistance. When assayed in MHH-CALL4 cells harboring a CRLF2/IGH rearrangement and JAK2 I682F mutation, the type II JAK2 inhibitors NVP-BBT594 and NVP-CHZ868 were 10-35-fold more potent than the type I JAK2 inhibitors NVP-BSK805 and NVP-BVB808. Similarly, in Ba/F3 cells dependent on CRLF2 and the gain-of-function allele JAK2 R683G, the IC50 for CHZ868 was 5-20-fold lower than the IC50s for BSK805 and BVB808. Unlike type I inhibitors, which induce paradoxical hyperphosphorylation of JAK2, CHZ868 completely blocks JAK2 and STAT5 phosphorylation. In addition, the JAK2 Y931C allele that confers 4-6-fold resistance to BSK805 and BVB808 did not alter sensitivity to CHZ868. CHZ868 abrogates STAT5 phosphorylation in Ba/F3 cells expressing CRLF2 with JAK2 R683G/Y931C while BVB808 does not. CHZ868 is the first type II JAK2 inhibitor amenable to in vivo use. We assessed its efficacy in mice transplanted with transgenic (CRLF2/JAK2 R683G/Cdkn2a^-/- or CRLF2/JAK2 R683G/Pax5^+/-/Ts1Rhr) or primary human CRLF2-rearranged B-ALLs. Splenocytes from patient-derived xenografts (PDXs) treated with CHZ868 in vivo for 3 days are more primed for apoptosis as demonstrated by a 2-6-fold EC50 reduction for PUMA permeabilizing activity compared to vehicle. Transcriptional profiling of splenocytes from CHZ868-treated PDXs revealed downregulation of critical survival pathways including E2F1, STAT3, and AKT-mediated signaling. Of note, 2 of the most downregulated genes are STAT targets, PIM1 and Myc. Mice treated for 5-6 days with CHZ868 had significant reductions in spleen size and complete loss of phospho-STAT5 in residual leukemia cells. In both murine leukemias and human xenografts, CHZ868 prolonged survival compared to controls (p<0.001). BH3 profiling of splenocytes from PDXs treated until moribund showed a 2-4-fold increase in the EC50 for BIM compared to vehicle, consistent with decreased priming for apoptosis in the relapsed setting. To study mechanisms of resistance to type II JAK2 inhibitors, we screened a randomly mutagenized JAK2 R683G library expressed in Ba/F3-CRLF2 cells for clones resistant to BBT594. All >30 clones sequenced harbored the same JAK2 L884P mutation. Ba/F3 cells expressing CRLF2 with JAK2 R683G/L884P displayed cross-resistance to CHZ868, while sensitivity to type I inhibitors was not affected. Structural modeling of the JAK2 JH1 domain suggested that L884P alters the binding pocket for type II inhibitors. JAK2 L884P is homologous to an EGFR L747P activating mutation, which destabilizes the P-loop and C-helix portion of the kinase domain (He et al. Clin Cancer Res 2012). The fact that L884P was reported in two B-ALL patients lacking additional JAK2 mutations (Torra et al. Blood (ASH Annual Meeting Abstracts) 2010) raised the possibility it was also an activating mutation. We confirmed L884P is an activating allele, as Ba/F3 cells expressing CRLF2, IL7R, and JAK2 L884P proliferated in the absence of TSLP ligand. To improve CHZ868 efficacy, we tested for synergy with multiple chemotherapy agents currently used in B-ALL treatment. Dexamethasone was the most highly synergistic with CHZ868 in MHH-CALL4 cells. To assess the combination in vivo, we treated mice transplanted with CRLF2/JAK2 R683G/Pax5^+/-/Ts1Rhr murine B-ALL with vehicle, CHZ868, dexamethasone, or CHZ868 + dexamethasone for 14 days post engraftment. CHZ868 treatment prolonged survival compared to vehicle (p<0.0001) or dexamethasone (p<0.01), and the combination prolonged survival beyond CHZ868 monotherapy (p<0.0001). In summary, the type II JAK2 inhibitor CHZ868 potently kills JAK2-dependent B-ALL and overcomes genetic resistance to type I inhibitors. CHZ868 prolongs survival in murine transgenic and human xenograft models and synergizes with dexamethasone in vivo. Thus, combination strategies using dexamethasone with type II JAK2 inhibitors merit testing in patients with relapsed or refractory JAK2-dependent B-ALL.