Comprehensive profiling of clinical JAK inhibitors in myeloproliferative neoplasms

Functional and Structural Characterization of Clinical-Stage Janus Kinase 2 Inhibitors Identifies Determinants for Drug Selectivity

Janus kinase 2 (JAK2) plays a critical role in orchestrating hematopoiesis, and its deregulation leads to various blood disorders, most importantly myeloproliferative neoplasms (MPNs). Ruxolitinib, fedratinib, momelotinib, and pacritinib are FDA-/EMA-approved JAK inhibitors effective in relieving symptoms in MPN patients but show variable clinical profiles due to poor JAK selectivity. The development of next-generation JAK2 inhibitors is hampered by the lack of comparative functional analysis and knowledge of the molecular basis of their selectivity. Here, we provide mechanistic profiling of the four approved and six clinical-stage JAK2 inhibitors and connect selectivity data with high-resolution structural and thermodynamic analyses. All of the JAK inhibitors potently inhibited JAK2 activity. Inhibitors differed in their JAK isoform selectivity and potency for erythropoietin signaling, but their general cytokine inhibition signatures in blood cells were comparable. Structural data indicate that high potency and moderate JAK2 selectivity can be obtained by targeting the front pocket of the adenosine 5'-triphosphate-binding site.

In vivo ablation of NF-κB cascade effectors alleviates disease burden in myeloproliferative neoplasms

ABSTRACT

Hyperactivation of the NF-κB cascade propagates oncogenic signaling and proinflammation, which together augments disease burden in myeloproliferative neoplasms (MPNs). Here, we systematically ablate NF-κB signaling effectors to identify core dependencies using a series of primary samples and syngeneic and patient-derived xenograft (PDX) mouse models. Conditional knockout of Rela attenuated Jak2V617F- and MPLW515L-driven onset of polycythemia vera and myelofibrosis disease hallmarks, respectively. In PDXs, RELA knockout diminished leukemic engraftment and bone marrow fibrosis while extending survival. Knockout of upstream effector Myd88 also alleviated disease burden; conversely, perturbation of negative regulator miR-146a microRNA induced earlier lethality and exacerbated disease. Perturbation of NF-κB effectors further skewed the abundance and distribution of hematopoietic multipotent progenitors. Finally, pharmacological targeting of interleukin-1 receptor-associated kinase 4 (IRAK4) with inhibitor CA-4948 suppressed disease burden and inflammatory cytokines specifically in MPN without inducing toxicity in nondiseased models. These findings highlight vulnerabilities in MPN that are exploitable with emerging therapeutic approaches.

Altered erythropoiesis via JAK2 and ASXL1 mutations in myeloproliferative neoplasms

Myeloproliferative neoplasms (MPNs) are driven by hyperactivation of JAK-STAT signaling but can demonstrate skewed hematopoiesis upon acquisition of additional somatic mutations. Here, using primary MPN samples and engineered embryonic stem cells, we demonstrate that mutations in JAK2 induced a significant increase in erythroid colony formation, whereas mutations in additional sex combs-like 1 (ASXL1) led to an erythroid colony defect. RNA-sequencing revealed upregulation of protein arginine methyltransferase 6 (PRMT6) induced by mutant ASXL1. Furthermore, genetic perturbation of PRMT6 exacerbated the MPN disease burden, including leukemic engraftment and splenomegaly, in patient-derived xenograft models, highlighting a novel tumor-suppressive function of PRMT6. However, augmented erythroid potential and bone marrow human CD71+ cells following PRMT6 knockdown were reserved only for primary MPN samples harboring ASXL1 mutations. Last, treatment of CD34+ hematopoietic/stem progenitor cells with the PRMT6 inhibitor EPZ020411 induced expression of genes involved in heme metabolism, hemoglobin, and erythropoiesis. These findings highlight interactions between JAK2 and ASXL1 mutations and a unique erythroid regulatory network in the context of mutant ASXL1.

Changes in bone marrow fibrosis during momelotinib or ruxolitinib therapy do not correlate with efficacy outcomes in patients with myelofibrosis

Bone marrow fibrosis (BMF) is a pathological feature of myelofibrosis, with higher grades associated with poor prognosis. Limited data exist on the association between outcomes and BMF changes. We present BMF data from Janus kinase (JAK) inhibitor-naive patients from SIMPLIFY-1 (NCT01969838), a double-blind, randomized, phase 3 study of momelotinib vs ruxolitinib. Baseline and week 24 bone marrow biopsies were graded from 0 to 3 as per World Health Organization criteria. Other assessments included Total Symptom Score, spleen volume, transfusion independence status, and hemoglobin levels. Paired samples were available from 144 and 160 patients randomized to momelotinib and ruxolitinib. With momelotinib and ruxolitinib, transfusion independence was achieved by 87% and 44% of patients with BMF improvement of ≥1 grade and 76% and 56% of those with stable/worsening BMF; there was no association between BMF changes and transfusion independence for either arm (momelotinib, p = .350; ruxolitinib, p = .096). Regardless of BMF changes, hemoglobin levels also generally increased on momelotinib but decreased on ruxolitinib. In addition, no associations between BMF changes and spleen (momelotinib, p = .126; ruxolitinib, p = .407)/symptom (momelotinib, p = .617; ruxolitinib, p = .833) outcomes were noted, and no improvement in overall survival was observed with ≥1-grade BMF improvement (momelotinib, p = .395; ruxolitinib, p = .407). These data suggest that the anemia benefit of momelotinib is not linked to BMF changes, and question the use of BMF assessment as a surrogate marker for clinical benefit with JAK inhibitors.

Idasanutlin and navitoclax induce synergistic apoptotic cell death in T-cell acute lymphoblastic leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematologic malignancy in which activating mutations in the Notch pathway are thought to contribute to transformation, in part, by activating c-Myc. Increased c-Myc expression induces oncogenic stress that can trigger apoptosis through the MDM2-p53 tumor suppressor pathway. Since the great majority of T-ALL cases carry inactivating mutations upstream in this pathway but maintain wildtype MDM2 and TP53, we hypothesized that T-ALL would be selectively sensitive to MDM2 inhibition. Treatment with idasanutlin, an MDM2 inhibitor, induced only modest apoptosis in T-ALL cells but upregulated the pro-apoptotic BH3 domain genes BAX and BBC3, prompting us to evaluate the combination of idasanutlin with BH3 mimetics. Combination treatment with idasanutlin and navitoclax, a potent Bcl-2/Bcl-xL inhibitor, induces more consistent and potent synergistic killing of T-ALL PDX lines in vitro than venetoclax, a Bcl-2 specific inhibitor. Moreover, a marked synergic response to combination treatment with idasanutlin and navitoclax was seen in vivo in all four T-ALL xenografts tested, with a significant increase in overall survival in the combination treatment group. Collectively, these preclinical data show that the combination of idasanutlin and navitoclax is highly active in T-ALL and may merit consideration in the clinical setting.