Efficacy of NS-018, a potent and selective JAK2/Src inhibitor, in primary cells and mouse models of myeloproliferative neoplasms. Blood Cancer J. 2011;1:e29. [PMID: 22829185 PMCID: PMC3255248 DOI: 10.1038/bcj.2011.29]

Regulation of STAT5 phosphorylation and interaction with SHP1 by lnc-AC004893, a long non-coding RNA overexpressed in myeloproliferative neoplasms

OBJECTIVES

Constitutive activation of Janus kinase 2 (JAK2)/signal transducer and activator of transcription (STAT) signaling pathway is central to the pathogenesis of myeloproliferative neoplasms (MPNs). Long noncoding RNAs (lncRNAs) regulate diverse biological processes. However, the role of lncRNAs in MPN pathogenesis is not well studied.

METHODS

The expression of lnc-AC004893 in MPN patients was measured by quantitative real-time PCR (qRT-PCR). Gene-specific short hairpin RNAs (shRNAs) were designed to inhibit the expression of lnc-AC004893, and western blot was performed to explore the role of lnc-AC004893 via regulating the JAK2/STAT5 signaling pathway. Furthermore, co-IP was performed to determine the binding ability of lnc-AC004893 and STAT5 protein. Finally, the BaF3-JAK2V617F-transplanted mouse model was used to assess the biological role of lnc-ac004893 in vivo.

RESULTS

We report that lnc-AC004893, a poorly conserved pseudogene-209, is substantially upregulated in MPN cells compared with normal controls (NCs). Knockdown of lnc-AC004893 by specific shRNAs suppressed cell proliferation and decreased colony formation. Furthermore, the knockdown of lnc-AC004893 reduced the expression of p-STAT5 but not total STAT5 in HEL and murine IL-3-dependent Ba/F3 cells, which present constitutive and inducible activation of JAK2/STAT5 signaling. In addition, inhibition of murine lnc-ac004893 attenuated BaF3-JAK2V617F-transplanted phenotypes and extended the overall survival. Mechanistically, knockdown of lnc-AC004893 enhanced the binding ability of STAT5 and protein tyrosine phosphatase SHP1. Furthermore, knockdown of lnc-AC004893 decreased STAT5-lnc-AC004893 interaction but not SHP1-lnc-AC004893 interaction.

CONCLUSION

Lnc-AC004893 regulates STAT5 phosphorylation by affecting the interaction of STAT5 and SHP1. Lnc-AC004893 might be a potential therapeutic target for MPN patients.

Synthesis and clinical application of small-molecule inhibitors of Janus kinase

Janus kinase (JAK) plays a crucial role in intracellular signaling pathways, particularly in cytokine-mediated signal transduction, making them attractive therapeutic targets for a wide range of diseases, including autoimmune disorders, myeloproliferative neoplasms, and inflammatory conditions. The review provides a comprehensive overview of the development and therapeutic potential of small-molecule inhibitors targeting JAK family of proteins in various clinical trials. It also discusses the mechanisms of action, specificity, and selectivity of these inhibitors, shedding light on the challenges associated with achieving target selectivity while minimizing off-target effects. Moreover, the review offers insights into the clinical applications of JAK inhibitors, summarizing the ongoing clinical trials and the Food and Drug Administration (FDA)-approved JAK inhibitors currently available for various diseases. Overall, this review provides a thorough examination of the synthesis and clinical use of typical small-molecule JAK inhibitors in different clinical stages and offers a bright future for the development of novel small-molecule JAK inhibitors.

JAK2 inhibitors for the treatment of Philadelphia-negative myeloproliferative neoplasms: current status and future directions

The overactivation of Janus kinases 2 (JAK2) by gain-of-function mutations in the JAK2, Myeloproliferative leukemia virus oncogene, or Calreticulin genes are the most important factor in the development of Philadelphia-negative myeloproliferative neoplasms (MPNs). The discovery of the JAK2V617F mutation is a significant breakthrough in understanding the pathogenesis of MPNs, and inhibition of JAK2 abnormal activation has become one of the most effective strategies against MPNs. Currently, three JAK2 inhibitors for treating MPNs have been approved, and several are being evaluated in clinical trials. However, persistent challenges in terms of drug resistance and off-target effects remain unresolved. In this review, we introduce and classify the available JAK2 inhibitors in terms of their mechanisms and clinical considerations. Additionally, through an analysis of target points, binding modes, and structure-activity inhibitor relationships, we propose strategies such as combination therapy and allosteric inhibitors to overcome specific challenges. This review offers valuable insights into current trends and future directions for optimal management of MPNs using JAK2 inhibitors.

Janus Kinases in Leukemia

Janus kinases (JAKs) transduce signals from dozens of extracellular cytokines and function as critical regulators of cell growth, differentiation, gene expression, and immune responses. Deregulation of JAK/STAT signaling is a central component in several human diseases including various types of leukemia and other malignancies and autoimmune diseases. Different types of leukemia harbor genomic aberrations in all four JAKs (JAK1, JAK2, JAK3, and TYK2), most of which are activating somatic mutations and less frequently translocations resulting in constitutively active JAK fusion proteins. JAKs have become important therapeutic targets and currently, six JAK inhibitors have been approved by the FDA for the treatment of both autoimmune diseases and hematological malignancies. However, the efficacy of the current drugs is not optimal and the full potential of JAK modulators in leukemia is yet to be harnessed. This review discusses the deregulation of JAK-STAT signaling that underlie the pathogenesis of leukemia, i.e., mutations and other mechanisms causing hyperactive cytokine signaling, as well as JAK inhibitors used in clinic and under clinical development.

Janus kinases (JAKs): The efficient therapeutic targets for autoimmune diseases and myeloproliferative disorders

The Janus kinases or JAKs are a family of intracellular tyrosine kinases that play an essential role in the signaling of numerous cytokines that have been implicated in the pathogenesis of autoimmune diseases and myeloproliferative disorders. JAKs are activated upon ligand induced receptor homo- or heterodimerization, which results in the immediate phosphorylation of tyrosine residues and the phosphotyrosines then serve as docking sites for cytoplasmic signal transducer and activator of transcription (STAT) proteins which become phosphorylated by the JAKs upon recruitment to the receptor complex. The phosphorylated STAT proteins dimerize and travel to the cellular nucleus, where they act as transcription factors. Interfering in the JAK-STAT pathway has yielded the only approved small molecule kinase inhibitors for immunological indications. Numerous medicinal chemistry studies are currently aimed at the design of novel and potent inhibitors for JAKs. Additionally, whether the second-generation inhibitors which possessed selectivity for JAKs are more efficient are under research. This Perspective summarizes the progress in the discovery and development of JAKs inhibitors, including the potential binding site and approaches for identifying small-molecule inhibitors, as well as future therapeutic perspectives in autoimmune diseases and myeloproliferative disorders are also put forward in order to provide reference and rational for the drug discovery of novel and potent JAKs inhibitors.

The role of driver mutations in myeloproliferative neoplasms: insights from mouse models

High frequency of JAK2V617F or CALR exon 9 mutations is a main molecular feature of myeloproliferative neoplasms (MPNs). Analysis of mouse models driven by these mutations suggests that they are a direct cause of MPNs and that the expression levels of the mutated genes define the disease phenotype. The function of MPN-initiating cells has also been elucidated by these mouse models. Such mouse models also play an important role in modeling disease to investigate the effects and action mechanisms of therapeutic drugs, such as JAK2 inhibitors and interferon α, against MPNs. The mutation landscape of hematological tumors has already been clarified by next-generation sequencing technology, and the importance of functional analysis of mutant genes in vivo should increase further in the future.

Targeted therapies for myeloproliferative neoplasms

The discovery of JAK2V617F and the demonstration that BCR-ABL-negative myeloproliferative neoplasms (MPNs) are driven by abnormal JAK2 activation have led to advances in diagnostic algorithms, prognosis and ultimately also treatment strategies. The JAK 1/2 inhibitor ruxolitinib was a pivotal moment in the treatment of MPNs, representing the first targeted treatment in this field. Despite a weak effect on the cause of the disease itself in MPNs, ruxolitinib improves the clinical state of patients and increases survival in myelofibrosis. In parallel, other JAK inhibitors with potential for pathologic and molecular remissions, less myelosuppression, and with greater selectivity for JAK1 or JAK2, and the ability to overcome JAK inhibitor persistence are in various stages of development. Moreover, many novel classes of targeted agents continue to be investigated in efforts to build on the progress made with ruxolitinib. This article will discuss some of the advances in the targeted therapy in this field in recent years and explore in greater detail some of the most advanced emerging agents as well as those with greatest potential.

Targeting STATs in neuroinflammation: The road less traveled!

JAK/STAT transduction pathway is a highly conserved pathway implicated in regulating cellular proliferation, differentiation, survival and apoptosis. Dysregulation of this pathway is involved in the onset of autoimmune, haematological, oncological, metabolic and neurological diseases. Over the last few years, the research of anti-neuroinflammatory agents has gained considerable attention. The ability to diminish the STAT-induced transcription of inflammatory genes is documented for both natural compounds (such as polyphenols) and chemical drugs. Among polyphenols, quercetin and curcumin directly inhibit STAT, while Berberis vulgaris L. and Sophora alopecuroides L extracts act indirectly. Also, the Food and Drug Administration has approved several JAK/STAT inhibitors (direct or indirect) for treating inflammatory diseases, indicating STAT can be considered as a therapeutic target for neuroinflammatory pathologies. Considering the encouraging data obtained so far, clinical trials are warranted to demonstrate the effectiveness and potential use in the clinical practice of STAT inhibitors to treat inflammation-associated neurodegenerative pathologies.

Beyond JAK-2: potential targets for myeloproliferative neoplasm therapy

INTRODUCTION

The outlook for patients with myeloproliferative neoplasms, particularly myelofibrosis, has improved in recent years, with greater understanding of the pathogenesis and the subsequent development of a plethora of new agents. Areas covered: This article will discuss some of the advances in the field in recent years and explore in greater detail some of the most advanced emerging agents as well as those with greatest potential. An extensive literature review has been performed to identify recent clinical trials and any relevant pre-clinical work. Expert commentary: Important discoveries regarding molecular pathogenesis have led to advances in diagnostic algorithms, prognosis and ultimately also treatment strategies. However, the therapeutic armamentarium for MPN is still largely inadequate to cope with significant challenges including normalization of life span, reduction of cardiovascular complications, prevention of hematological progression and improved quality of life. Sadly, no currently available drugs have shown clear evidence of disease-modifying activity and results of early phase I and II clinical trials have been quite disappointing to date, with toxicities sometimes limiting and a lack of meaningful biological surrogate end points.

JAK inhibitors for the treatment of myeloproliferative neoplasms and other disorders

JAK inhibitors have been developed following the discovery of the JAK2V617F in 2005 as the driver mutation of the majority of non- BCR-ABL1 myeloproliferative neoplasms (MPNs). Subsequently, the search for JAK2 inhibitors continued with the discovery that the other driver mutations ( CALR and MPL) also exhibited persistent JAK2 activation. Several type I ATP-competitive JAK inhibitors with different specificities were assessed in clinical trials and exhibited minimal hematologic toxicity. Interestingly, these JAK inhibitors display potent anti-inflammatory activity. Thus, JAK inhibitors targeting preferentially JAK1 and JAK3 have been developed to treat inflammation, autoimmune diseases, and graft-versus-host disease. Ten years after the beginning of clinical trials, only two drugs have been approved by the US Food and Drug Administration: one JAK2/JAK1 inhibitor (ruxolitinib) in intermediate-2 and high-risk myelofibrosis and hydroxyurea-resistant or -intolerant polycythemia vera and one JAK1/JAK3 inhibitor (tofacitinib) in methotrexate-resistant rheumatoid arthritis. The non-approved compounds exhibited many off-target effects leading to neurological and gastrointestinal toxicities, as seen in clinical trials for MPNs. Ruxolitinib is a well-tolerated drug with mostly anti-inflammatory properties. Despite a weak effect on the cause of the disease itself in MPNs, it improves the clinical state of patients and increases survival in myelofibrosis. This limited effect is related to the fact that ruxolitinib, like the other type I JAK2 inhibitors, inhibits equally mutated and wild-type JAK2 (JAK2WT) and also the JAK2 oncogenic activation. Thus, other approaches need to be developed and could be based on either (1) the development of new inhibitors specifically targeting JAK2V617F or (2) the combination of the actual JAK2 inhibitors with other therapies, in particular with molecules targeting pathways downstream of JAK2 activation or the stability of JAK2 molecule. In contrast, the strong anti-inflammatory effects of the JAK inhibitors appear as a very promising therapeutic approach for many inflammatory and auto-immune diseases.

JAK2 inhibitors for myeloproliferative neoplasms: what is next?

Since its approval in 2011, the Janus kinase 1/2 (JAK1/2) inhibitor ruxolitinib has evolved to become the centerpiece of therapy for myelofibrosis (MF), and its use in patients with hydroxyurea resistant or intolerant polycythemia vera (PV) is steadily increasing. Several other JAK2 inhibitors have entered clinical testing, but none have been approved and many have been discontinued. Importantly, the activity of these agents is not restricted to patients with JAK2 V617F or exon 12 mutations. Although JAK2 inhibitors provide substantial clinical benefit, their disease-modifying activity is limited, and rational combinations with other targeted agents are needed, particularly in MF, in which survival is short. Many such combinations are being explored, as are other novel agents, some of which could successfully be combined with JAK2 inhibitors in the future. In addition, new JAK2 inhibitors with the potential for less myelosuppression continue to be investigated. Given the proven safety and efficacy of ruxolitinib, it is likely that ruxolitinib-based combinations will be a major way forward in drug development for MF. If approved, less myelosuppressive JAK2 inhibitors such as pacritinib or NS-018 could prove to be very useful additions to the therapeutic armamentarium in MF. In PV, inhibitors of histone deacetylases and human double minute 2 have activity, but their role, if any, in the future treatment algorithm is uncertain, given the availability of ruxolitinib and renewed interest in interferons. Ruxolitinib is in late-phase clinical trials in essential thrombocythemia, in which it could fill an important void for patients with troublesome symptoms.

Developmental Therapeutics in Myeloproliferative Neoplasms

The unprecedented success of the Janus kinase (JAK) 1/2 inhibitor ruxolitinib in myelofibrosis (MF) provided much-needed impetus for clinical drug development for the Philadelphia chromosome-negative myeloproliferative neoplasms. The survival benefit conferred by this agent, along with its marked efficacy with regard to spleen volume and symptom reduction, have made ruxolitinib the cornerstone of drug therapy in MF. However, there remain significant unmet needs in the treatment of patients with MF, and many novel classes of agents continue to be investigated in efforts to build on the progress made with ruxolitinib. These include inhibitors of histone deacetylases (HDACs) and DNA methyltransferases, phosphatidylinositol-3-kinase isoforms, heat shock protein 90, cyclin-dependent kinases 4/6, and Hedgehog signaling, among others. In parallel, other JAK inhibitors with potential for less myelosuppression or even improvement of anemia, greater selectivity for JAK1 or JAK2, and the ability to overcome JAK inhibitor persistence are in various stages of development. First-in-class agents such as the activin receptor IIA ligand trap sotatercept (for anemia of MF), the telomerase inhibitor imetelstat, and the antifibrotic agent PRM-151 (recombinant human pentraxin-2) are also in clinical trials. In polycythemia vera, a novel interferon administered every 2 weeks is being developed for front-line therapy in high-risk individuals, and inhibitors of human double minute 2 (HDM2) have shown promise in preclinical studies, as have HDAC inhibitors such as givinostat (both in the laboratory and in the clinic). Ruxolitinib is approved for second-line therapy of polycythemia vera and is being developed for essential thrombocythemia.

Investigational Janus kinase inhibitors in development for myelofibrosis

INTRODUCTION

Since the discovery of the activating V617F mutation in Janus kinase 2 (JAK2), a number of pharmacologic inhibitors of JAK2 have entered clinical trials for patients with myelofibrosis. However, ruxolitinib, approved in 2011, remains the only one currently available for treatment of myelofibrosis, with many others having been discontinued for toxicity, and considerable uncertainty surrounding the future of those still in development. Areas covered: The available clinical data on pacritinib and momelotinib, the two agents in the most advanced phases of clinical testing in myelofibrosis, are examined in detail. NS-018 and INCB039110, selective inhibitors of JAK2 and JAK1, respectively, are also discussed. Finally, the JAK2 inhibitors no longer in clinical development are summarized in tabular form. Expert opinion: The different agents evaluated clearly differ in their kinomes, toxicity profiles and potential for myelosuppression. If approved, the JAK2-specific non-myelosuppressive inhibitor pacritinib could fulfill a major unmet need, that of patients with significant cytopenias. However, toxicity concerns persist. The data from the pivotal trials of momelotinib do not support its approval, although improvement of anemia is an important benefit. Selective JAK1 inhibition alone is unlikely to succeed in myelofibrosis. In these circumstances, rational ruxolitinib-based combinations may represent the best way forward.

A phase I, open-label, dose-escalation, multicenter study of the JAK2 inhibitor NS-018 in patients with myelofibrosis

NS-018 is a Janus-activated kinase 2 (JAK2)-selective inhibitor, targeting the JAK–signal transducer and activator of transcription (STAT) pathway that is deregulated in myelofibrosis. In this phase I, dose-escalation portion of a phase I/II study, patients with myelofibrosis received oral NS-018 in continuous 28-day cycles. The primary study objective was to evaluate safety, tolerability and clinically active dose of NS-018. Forty-eight patients were treated; 23 (48%) had previously received a JAK inhibitor (JAKi). The most common drug-related adverse events were thrombocytopenia (27%)/anemia (15%) for hematologic events, and dizziness (23%)/nausea (19%) for non-hematologic events. Once daily NS-018 at 300 mg was chosen as the phase II study dose based on improved tolerability compared with higher doses. A ⩾50% reduction in palpable spleen size was achieved in 56% of patients (47% of patients with prior JAKi treatment), and improvements were observed in myelofibrosis-associated symptoms. Bone marrow fibrosis grade (local assessment) improved from baseline in 11/30 evaluable patients (37%) after 3 cycles of NS-018. JAK2 allele burden was largely unchanged. Changes in cytokine/protein levels were noted after 4 weeks of treatment. NS-018 reached peak plasma concentration in 1–2 h and did not accumulate with multiple dosing. NS-018 will be assessed in patients with previous JAKi exposure in the phase II portion.

Pacritinib: a new agent for the management of myelofibrosis?

INTRODUCTION

Myelofibrosis (MF) is a clonal haematological disease associated with recurrent somatic gene mutations (JAK2V617F, MPL, CALR) and constitutive activation of the Janus kinase (JAK)/Signal Transducer and Activator of Transcription pathway. MF is often characterised by debilitating symptoms and JAK inhibitors (JAKIs) have revolutionised available therapeutic options. Ruxolitinib, a JAK1 and 2 inhibitor, is the only currently approved agent. Several other JAKIs are undergoing evaluation in the clinical trial setting and Pacritinib , a novel JAK2 and FLT3 inhibitor, is at an advanced stage of investigation with recent completion of a Phase III trial and another ongoing.

AREAS COVERED

Within this article we focus on pacritinib, summarising the development, preclinical and up-to-date results from the Phase I - III trials. We present the most recent data on efficacy and safety and indirectly compare this novel JAKI with ruxolitinib.

EXPERT OPINION

The kinome array data for pacritinib suggests that it has a range of targets differing to those for ruxolitinib. Pacritinib appears to be an effective agent for the control of MF-related symptoms and splenomegaly with potentially fewer haematological side-effects when compared with ruxolitinib and seems a particularly promising agent for anaemic and thrombocytopenic patients. It is also an attractive drug for potential combination studies due to its good tolerability.

Reducing symptom burden in patients with myeloproliferative neoplasms in the era of Janus kinase inhibitors

Philadelphia chromosome-negative myeloproliferative neoplasms (MPNs) are clonal stem cell-derived malignancies that include primary myelofibrosis, polycythemia vera and essential thrombocythemia and are characterized by dysregulated Janus kinase-signal transducers and activators of transcription (JAK-STAT) signaling. Clinical manifestations include splenomegaly, cytopenias and/or systemic inflammation. Patients have a heterogeneous symptom profile that includes fatigue, loss of appetite, pruritus and night sweats, which significantly impact quality of life (QoL) and lead to poor survival outcomes. With the introduction of JAK inhibitors, improvement in disease-related symptoms has emerged as a realistic expectation of therapy and an integral measure of clinical efficacy. The JAK1/JAK2 inhibitor ruxolitinib is approved for the treatment of myelofibrosis and is currently under clinical development for polycythemia vera. Ruxolitinib has demonstrated significant reductions in symptom burden, with consequent improvements in QoL measures. With the potential to improve QoL, recognition of the impact and burden of symptoms on patients with MPNs is critical.

Protein kinase small molecule inhibitors for rheumatoid arthritis: Medicinal chemistry/clinical perspectives

Medicinal chemistry strategies have contributed to the development, experimental study of and clinical trials assessment of the first type of protein kinase small molecule inhibitor to target the Janus kinase/Signal Transducers and Activators of Transcription (JAK/STAT) signaling pathway. The orally administered small molecule inhibitor, tofacitinib, is the first drug to target the JAK/STAT pathway for entry into the armamentarium of the medical therapy of rheumatoid arthritis. The introduction of tofacitinib into general rheumatologic practice coupled with increasing understanding that additional cellular signal transduction pathways including the mitogen-activated protein kinase and phosphatidylinositide-3-kinase/Akt/mammalian target of rapamycin pathways as well as spleen tyrosine kinase also contribute to immune-mediated inflammatory in rheumatoid arthritis makes it likely that further development of orally administered protein kinase small molecule inhibitors for rheumatoid arthritis will occur in the near future.

Selective JAK inhibitors

Consisting of four members, JAK1, JAK2, JAK3 and TYK2, the JAK kinases have emerged as important targets for proliferative and immune-inflammatory disorders. Recent progress in the discovery of selective inhibitors has been significant, with selective compounds now reported for each isoform. This article summarizes the current state-of-the-art with a discussion of the most recently described selective compounds. X-ray co-crystal structures reveal the molecular reasons for the observed biochemical selectivity. A concluding analysis of JAK inhibitors in the clinic highlights increased clinical trial activity and diversity of indications. Selective JAK inhibitors, as single agents or in combination regimens, have a very promising future in the treatment of oncology, immune and inflammatory diseases.

How many JAK inhibitors in myelofibrosis?

The discovery of the activating mutation JAK2 V617F ushered a new era in MPN which included new diagnostic and prognostic criteria as well as a potential therapeutic target. JAK2 inhibition became a reality with first patients receiving drugs that targeted JAK2 in 2007 and was marked by the first approval in 2011 of Ruxolitinib a JAK 1 and 2-inhibitor to treat myelofibrosis (MF). In this article entitled "How many JAK inhibitors for myelofibrosis" we discuss JAK2 as a target, review briefly the benefits to patients with MF of JAK inhibition and highlight some of the differences between the number of JAK inhibitors currently being evaluated. Reflecting upon what we have learnt from the chronic myeloid leukaemia field and for MF regarding disease complexity as well as individual patient factors including resistance we discuss why it is likely we will need several different agents with JAK inhibitory activity. The next chapter discusses combination therapies for myelofibrosis which is a logical step in both trying to cure this disease and improve patient outcome and toxicities with JAK inhibitors.

Preclinical models for drug selection in myeloproliferative neoplasms

The discovery that an abnormally activated JAK-STAT signaling pathway is central to the pathogenesis of myeloproliferative neoplasms has promoted the clinical development of small-molecule JAK2 inhibitors. These agents have shown remarkable efficacy in disease control, but do not induce molecular remission; on the other hand, interferon holds the promise to target the putative hematopoietic progenitor cell initiating the disease. The presence of additional molecular abnormalities indicates a high molecular complexity of myeloproliferative neoplasms, and the need for simultaneously targeting different targets. Several drugs are currently under study as single agents and in combination. This review briefly describes the several in vitro and in vivo models of myeloproliferative neoplasms that are being used as preclinical models for drug development.

Janus kinase inhibitors for the treatment of myeloproliferative neoplasms

INTRODUCTION

Disordered signaling through the JAK/STAT pathway is a hallmark of myeloproliferative neoplasms (MPNs). Targeted therapies that inhibit and regulate this pathway are reasonable strategies for disease management. Only one JAK1/JAK2 inhibitor has gained FDA approval for treatment of myelofibrosis. Despite significant reductions in splenomegaly and disease-associated symptoms, additional agents are necessary to manage disease in those that do not respond.

AREAS COVERED

A review of the currently available literature and meeting abstracts for JAK inhibitors in myeloproliferative neoplasms identified studies aimed at improving outcomes and establishing alternative therapies in MPNs. Development of specific JAK inhibitors and ongoing trials involving ruxolitinib, CYT387, SAR302503, CEP701, SB 1518, XL-019, LY2784544, BMS-911453, NS-018, AZD1480 and INCB039110 are reviewed.

EXPERT OPINION

The identification of JAK2V617F mutation and its link to MPNs has revolutionized treatment options. Resultant research in targeting the JAK/STAT pathway led to the approval of ruxolitinib, a JAK1/JAK2 inhibitor with activity in MPNs. While ruxolitinib produces durable reductions in splenomegaly and improvement of symptoms, and prolongs survival, there is room for new and more specific agents to be developed. Minimizing toxicity and avoiding drug resistance are challenges that lie ahead. Combining agents with different mechanisms seems to be a rational strategy.

Effect of NS-018, a selective JAK2V617F inhibitor, in a murine model of myelofibrosis

A single somatic mutation, V617F, in Janus kinase 2 (JAK2) is one of the causes of myeloproliferative neoplasms (MPNs), including primary myelofibrosis, and the JAK2V617F mutant kinase is a therapeutic target in MPN. However, inhibition of wild-type (WT) JAK2 can decrease the erythrocyte or platelet (PLT) count. Our selective JAK2 inhibitor, NS-018, suppressed the growth of Ba/F3 cells harboring JAK2V617F more strongly than that of cells harboring WT JAK2. The 4.3-fold JAK2V617F selectivity of NS-018 is higher than the 1.0- to 2.9-fold selectivity of seven existing JAK2 inhibitors. NS-018 also inhibited erythroid colony formation in JAK2V617F transgenic mice at significantly lower concentrations than in WT mice. In keeping with the above results, in a JAK2V617F bone marrow transplantation mouse model with a myelofibrosis-like disease, NS-018 reduced leukocytosis and splenomegaly, improved bone marrow fibrosis and prolonged survival without decreasing the erythrocyte or PLT count in the peripheral blood. By exploring the X-ray co-crystal structure of NS-018 bound to JAK2, we identified unique hydrogen-bonding interactions between NS-018 and Gly993 as a plausible explanation for its JAK2V617F selectivity. These results suggest that NS-018 will have therapeutic benefit for MPN patients through both its efficacy and its reduced hematologic adverse effects.

JAK of all trades: JAK2-STAT5 as novel therapeutic targets in BCR-ABL1+ chronic myeloid leukemia

The transcription factor signal transducers and activators of transcription 5 (STAT5) has an important and unique role in Breakpoint Cluster Region - Abelson 1 (BCR-ABL1)-driven neoplasias. STAT5 is an essential component in the signaling network that maintains the survival and growth of chronic myeloid leukemia (CML) cells. In contrast, the function of the prototypical upstream kinase of STAT5, the Janus kinase JAK2, in CML is still under debate. Although there is widespread agreement that JAK2 is part of the signaling network downstream of BCR-ABL1, it is unclear whether and under what circumstances JAK2 inhibitors may be beneficial for CML patients. Recent studies in murine models have cast doubt on the importance of JAK2 in CML maintenance. Nevertheless, JAK2 has been proposed to have a central role in the cytokine signaling machinery that allows the survival of CML stem cells in the presence of BCR-ABL1 tyrosine kinase inhibitors. In this review, we summarize the current debate and provide an overview of the arguments on both sides of the fence. We present recent evidence showing that CML stem cells do not depend on BCR-ABL1 kinase activity but require the continuous support of the hematopoietic niche and its distinct cytokine environment and suggest that it has the potential to resolve the dispute.

JAK2 mutants (e.g., JAK2V617F) and their importance as drug targets in myeloproliferative neoplasms

The Janus kinase 2 (JAK2) mutant V617F and other JAK mutants are found in patients with myeloproliferative neoplasms and leukemias. Due to their involvement in neoplasia and inflammatory disorders, Janus kinases are promising targets for kinase inhibitor therapy. Several small-molecule compounds are evaluated in clinical trials for myelofibrosis, and ruxolitinib (INCB018424, Jakafi®) was the first Janus kinase inhibitor to receive clinical approval. In this review we provide an overview of JAK2V617F signaling and its inhibition by small-molecule kinase inhibitors. In addition, myeloproliferative neoplasms are discussed regarding the role of JAK2V617F and other mutant proteins of possible relevance. We further give an overview about treatment options with special emphasis on possible combination therapies.

Targeting the JAK-STAT pathway in lymphoma: a focus on pacritinib

INTRODUCTION

The Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway mediates signaling by cytokine, chemokine and growth factor receptors on cell surface to the nucleus. JAK/STAT pathway is aberrantly activated in a variety of lymphomas, with a dual role of promoting cell survival/proliferation and immune evasion.

AREAS COVERED

This review describes the preclinical rationale behind the development of JAK inhibitors in lymphoma, some of which are being evaluated in Phase I/II studies, and summarizes the characteristics and clinical results of different JAK inhibitors in clinical development. Available preclinical and clinical data about JAK inhibition in lymphoid malignancies were reviewed using a PubMed access. To date, pacritinib (SB1518), a selective JAK2/FLT3 inhibitor is the first and only JAK inhibitor that has been evaluated in patients with relapsed lymphoma.

EXPERT OPINION

The preclinical rationale behind the development of pacritinib in lymphoproliferative neoplasms is strong, as the deregulation of the JAK/STAT pathway is involved in the pathogenesis of multiple lymphoma subtypes, although with different mechanisms. Pacritinib demonstrated safety and early clinical efficacy in a variety of lymphoma histologic types, providing the first proof of principle of the potential clinical value of targeting JAK/STAT pathway in lymphoma.

Investigational Janus kinase inhibitors

INTRODUCTION

Dysregulation of the Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway is central to the pathophysiology of myeloproliferative neoplasms (MPN). Small molecule inhibitors of JAK family members are currently under investigation for the treatment of MPN. Of these, ruxolitinib has received approval for clinical use in myelofibrosis in the United States and Europe.

AREAS COVERED

The clinical results and future development program of major JAK inhibitors, including ruxolitinib, CYT387, SAR302503, lestaurtinib, pacritinib, XL-019, LY2784544, BMS-911453, AZD1480 and NS-018 are reviewed.

EXPERT OPINION

JAK inhibitors are effective in relieving organomegaly (splenomegaly and hepatomegaly) and constitutional symptoms of myelofibrosis and some modulate inflammatory cytokines. However, they have little impact on disease burden and bone marrow fibrosis. The relationship between clinical efficacy, toxicity profile and specificity of JAK family member inhibition (i.e., JAK2 specific vs JAK1/JAK2 active) is poorly defined. Novel resistance mechanisms including heterodimerization of JAK2 with other JAK family members have been described. It is likely that the future lies in the use of rational drug combinations that target multiple signaling pathways.

Dysregulation of JAK-STAT pathway in hematological malignancies and JAK inhibitors for clinical application

JAK-STAT (Janus associated kinase-signal transducer and activator of transcription) pathway plays a critical role in transduction of extracellular signals from cytokines and growth factors involved in hematopoiesis, immune regulation, fertility, lactation, growth and embryogenesis. JAK family contains four cytoplasmic tyrosine kinases, JAK1-3 and Tyk2. Seven STAT proteins have been identified in human cells, STAT1-6, including STAT5a and STAT5b. Negative regulators of JAK-STAT pathways include tyrosine phosphatases (SHP1 and 2, CD45), protein inhibitors of activated STATs (PIAS), suppressors of cytokine signaling (SOCS) proteins, and cytokine-inducible SH2-containing protein (CIS). Dysregulation of JAK-STAT pathway have been found to be key events in a variety of hematological malignancies. JAK inhibitors are among the first successful agents reaching clinical application. Ruxolitinib (Jakafi), a non-selective inhibitor of JAK1 & 2, has been approved by FDA for patients with intermediate to high risk primary or secondary myelofibrosis. This review will also summarize early data on selective JAK inhibitors, including SAR302503 (TG101348), lestaurtinib (CEP701), CYT387, SB1518 (pacritinib), LY2784544, XL019, BMS-911543, NS-018, and AZD1480.

Therapy with JAK2 inhibitors for myeloproliferative neoplasms

The development of JAK2 inhibitors followed the discovery of activating mutation of JAK2 (JAK2V617F) in patients with classic Philadelphia-negative myeloproliferative neoplasms (Ph-negative MPN). It is now known that mutations activating the JAK-STAT pathway are ubiquitous in Ph-negative MPN, and that the deregulated JAK-STAT pathway plays a central role in the pathogenesis of these disorders. JAK2 inhibitors thus are effective in patients both with and without the JAK2V617F mutation. This article reviews the rationale for using JAK2 inhibitors in Ph-negative MPN, and the results of more recent clinical trials with these drugs.

Potential use of STAT3 inhibitors in targeted prostate cancer therapy: future prospects

In 2012, prostate cancer will once again be the second-leading cause of cancer death of American males. Although initially treatable, prostate cancer can recur in a hormone refractory form that is not responsive to current available therapies. The mortality rate associated with hormone refractory prostate cancer is high, and there is an urgent need for new therapeutic agents to treat prostate cancer. A common feature of prostate cancer is the dependence on activated signal transducer and activator of transcription 3 (STAT3), a transcription factor, for survival. More important, inhibition of STAT3 has been shown to induce apoptosis in prostate cancer cells. In recent years, inhibitors of STAT3 have emerged as promising molecular candidates for targeted prostate cancer therapy. The aim of this review is to examine the role of STAT3 in prostate cancer and how inhibitors of STAT3 could advance the quest for treatment of the disease. Janus kinase 2 (JAK2)-targeted therapy appears very promising in the treatment of prostate cancer. It has been shown to decrease symptoms associated with myeloproliferative disorders and increase overall survival of patients compared with the best available therapy. In addition to improved outcome, many JAK2 inhibitors have been found to be tolerable with no adverse impact on quality of life. As such, JAK2 inhibitors may play an important role in the management of patients with prostate cancer. Current studies are evaluating the role of JAK2 inhibitors in solid tumors. Pending clinical trial results will determine the future direction of JAK2 inhibitors in the treatment of patients with prostate cancer.

Ruxolitinib, a selective JAK1 and JAK2 inhibitor for the treatment of myeloproliferative neoplasms and psoriasis

Ruxolitinib (INCB-018424) is a potent, orally available, selective inhibitor of both JAK1 and JAK2 of the JAK-STAT signaling pathway, being developed by Incyte Corp and Novartis AG. Ruxolitinib was initially developed to target the constitutive activation of the JAK-STAT pathway in patients with myeloproliferative neoplasms (MPNs). Meaningful reductions in spleen size and constitutional symptoms have been noted in patients with myelofibrosis (both primary and post-essential thrombocythemia/polycythemia vera). Data from a phase I/II clinical trial led to ongoing registration trials in the US and Europe. Toxicity (primarily decreased erythropoiesis and thrombocytopoiesis) has been managed by close control of dosing. The inhibition of inflammatory cytokine signaling through JAK1 inhibition has led to intriguing results in patients with rheumatoid arthritis and psoriasis (using a topical cream formulation). Ruxolitinib is a well tolerated, first-in-class JAK2 inhibitor with various potential clinical indications.