The selective SYK inhibitor P505-15 (PRT062607) inhibits B cell signaling and function in vitro and in vivo and augments the activity of fludarabine in chronic lymphocytic leukemia

Development of SYK NanoBRET Cellular Target Engagement Assays for Gain-of-Function Variants

Spleen tyrosine kinase (SYK) is a non-receptor tyrosine kinase that is activated by phosphorylation events downstream of FcR, B-cell and T-cell receptors, integrins, and C-type lectin receptors. When the tandem Src homology 2 (SH2) domains of SYK bind to phosphorylated immunoreceptor tyrosine-based activation motifs (pITAMs) contained within these immunoreceptors, or when SYK is phosphorylated in interdomain regions A and B, SYK is activated. SYK gain-of-function (GoF) variants were previously identified in six patients that had higher levels of phosphorylated SYK and phosphorylated downstream proteins JNK and ERK. Furthermore, the increased SYK activation resulted in the clinical manifestation of immune dysregulation, organ inflammation, and a predisposition for lymphoma. The knowledge that the SYK GoF variants have enhanced activity was leveraged to develop a SYK NanoBRET cellular target engagement assay in intact live cells with constructs for the SYK GoF variants. Herein, we developed a potent SYK-targeted NanoBRET tracer using a SYK donated chemical probe, MRL-SYKi, that enabled a NanoBRET cellular target engagement assay for SYK GoF variants, SYK(S550Y), SYK(S550F), and SYK(P342T). We determined that ATP-competitive SYK inhibitors bind potently to these SYK variants in intact live cells. Additionally, we demonstrated that MRL-SYKi can effectively reduce the catalytic activity of SYK variants, and the phosphorylation levels of SYK(S550Y) in an epithelial cell line (SW480) stably expressing SYK(S550Y).

Preclinical Pharmacology Characterization of Sovleplenib (HMPL-523), an Orally Available Syk Inhibitor

Spleen tyrosine kinase (Syk) is an intracellular tyrosine kinase involved in the signal transduction in immune cells mainly. Its aberrant regulation is associated with diversified allergic disorders, autoimmune diseases and B cell malignancies. Therefore, inhibition of Syk is considered a reasonable approach to treat autoimmune/inflammatory diseases and B cell malignancies. Here we described the preclinical characterization of sovleplenib, a novel, highly potent and selective, oral Syk inhibitor, in several rodent autoimmune disease models. Sovleplenib potently inhibited Syk activity in a recombinant enzymatic assay and Syk-dependent cellular functions in various immune cell lines and human whole blood in vitro. Furthermore, sovleplenib, by oral administration, demonstrated strong in vivo efficacies in murine models of immune thrombocytopenia (ITP), autoimmune hemolytic anemia (AIHA), and chronic graft-versus-host disease (cGVHD), and a rat model of collagen induced arthritis (CIA) respectively, in a dose-dependent manner. Collectively, these results clearly supported sovleplenib as a therapeutic agent in the treatment of autoimmune diseases. Sovleplenib is being globally developed for ITP (Phase III, NCT05029635, Phase Ib/II, NCT03951623), wAIHA (Phase II/III, NCT05535933) and B-cell lymphoma (Phase I, NCT02857998, NCT03779113). SIGNIFICANCE STATEMENT: Syk is a key mediator of signaling pathways downstream of a wide array of receptors important for immune functions, including the B cell receptor, immunoglobulin receptors bearing Fc receptors. Inhibition of Syk could provide a novel therapeutic approach for autoimmune diseases and hematologic malignancies. The manuscript describes the preclinical pharmacology characterization of sovleplenib, a novel Syk inhibitor, in enzymatic and cellular assays in vitro and several murine autoimmune disease models in vivo.

Phosphorylation of spleen tyrosine kinase at Y346 negatively regulates ITAM-mediated signaling and function in platelets

Spleen tyrosine kinase (Syk) is expressed in a variety of hemopoietic cells. Upon phosphorylation of the platelet immunoreceptor-based activation motif of the glycoprotein VI (GPVI)/Fc receptor gamma chain collagen receptor, both the tyrosine phosphorylation and activity of Syk are increased leading to downstream signaling events. Although it has been established that the activity of Syk is regulated by tyrosine phosphorylation, the specific roles of individual phosphorylation sites remain to be elucidated. We observed that Syk Y346 in mouse platelets was still phosphorylated when GPVI-induced Syk activity was inhibited. We then generated Syk Y346F mice and analyzed the effect this mutation exerts on platelet responses. Syk Y346F mice bred normally, and their blood cell count was unaltered. We did observe potentiation of GPVI-induced platelet aggregation and ATP secretion as well as increased phosphorylation of other tyrosines on Syk in the Syk Y346F mouse platelets when compared to WT littermates. This phenotype was specific for GPVI-dependent activation, since it was not seen when AYPGKF, a PAR4 agonist, or 2-MeSADP, a purinergic receptor agonist, was used to activate platelets. Despite a clear effect of Syk Y346F on GPVI-mediated signaling and cellular responses, there was no effect of this mutation on hemostasis as measured by tail-bleeding times, although the time to thrombus formation determined using the ferric chloride injury model was reduced. Thus, our results indicate a significant effect of Syk Y346F on platelet activation and responses in vitro and reveal its complex nature manifesting itself by the diversified translation of platelet activation into physiological responses.

Endocytosis triggers V-ATPase-SYK-mediated priming of cGAS activation and innate immune response

The current view of nucleic acid-mediated innate immunity is that binding of intracellular sensors to nucleic acids is sufficient for their activation. Here, we report that endocytosis of virus or foreign DNA initiates a priming signal for the DNA sensor cyclic GMP-AMP synthase (cGAS)-mediated innate immune response. Mechanistically, viral infection or foreign DNA transfection triggers recruitment of the spleen tyrosine kinase (SYK) and cGAS to the endosomal vacuolar H⁺ pump (V-ATPase), where SYK is activated and then phosphorylates human cGAS^Y214/215 (mouse cGas^Y200/201) to prime its activation. Upon binding to DNA, the primed cGAS initiates robust cGAMP production and mediator of IRF3 activation/stimulator of interferon genes-dependent innate immune response. Consistently, blocking the V-ATPase-SYK axis impairs DNA virus- and transfected DNA-induced cGAMP production and expression of antiviral genes. Our findings reveal that V-ATPase-SYK-mediated tyrosine phosphorylation of cGAS following endocytosis of virus or other cargos serves as a priming signal for cGAS activation and innate immune response.

Investigational spleen tyrosine kinase (SYK) inhibitors for the treatment of autoimmune diseases

INTRODUCTION

Autoimmune diseases (ADs) are disorders induced by multiple inflammatory mediators, in which immune system attacks healthy tissues and triggers tissue injury. Targeted regulation of the activity of kinases that influence inflammation is one of the major therapies for ADs. Recently, investigational spleen tyrosine kinase (SYK) inhibitors have shown encouraging results in the ADs therapy.

AREAS COVERED

This article provides a background on autoimmune diseases and provides an update on investigational SYK inhibitors. This literature review was conducted by searching publications about investigational Spleen tyrosine kinase inhibitors in the treatment of ADs from experimental to clinical studies. The search terms used were SYK inhibitors, R406, fostamatinib (R788), P505-15 (PRT062607), entospletinib (GS-9973), R112, lanraplenib (GS-9876), cerdulatinib, R343, BAY-61-3606, GSK compound 143 (GSK143), R211, SKI-G-618, SKI-O-85, ER-27319, YM193306, RO9021 in conjunction with autoimmune disease using electronic databases including PubMed, EMBASE, MEDLINE and Google Scholar.

EXPERT OPINION

SYK inhibitors are promising drugs with unique advantages and acceptable tolerability and safety for the treatment of ADs. However, the difficulties in developing highly selective SYK inhibitors and the unknown effects are challenges. Long term and real-world data are essential to determine the risk-benefit ratio and true role of SYK inhibitors in the therapy of ADs.

Some important inhibitors and mechanisms of rheumatoid arthritis

Rheumatoid arthritis is a chronic disease that seriously affects human health and quality of life, and it is one of the main causes of labor loss and disability. Many countries have listed rheumatoid arthritis as one of the national a key diseases to tackle. The pathogenesis of RA in humans is still unknown, and medical researchers believe that the pathogenesis of RA may be the result of a combination of genetic and environmental factors. RA is an incurable condition that can only be controlled and treated with conventional drugs. In this paper, the pathologic features and pathogenesis of RA were introduced, and the research progress of new anti-rheumatoid arthritis chemical drugs in recent years was reviewed.

Syk Inhibitors: New Computational Insights into Their Intraerythrocytic Action in Plasmodium falciparum Malaria

Resistance to antimalarial drugs has spread rapidly over the past few decades. The WHO recommends artemisinin-based combination therapies for the treatment of uncomplicated malaria, but unfortunately these approaches are losing their efficacy in large areas of Southeast Asia. In 2016, artemisinin resistance was confirmed in 5 countries of the Greater Mekong subregion. We focused our study on Syk inhibitors as antimalarial drugs. The Syk protein is present in human erythrocytes, and the membrane of protein band 3 is its major target following activation by oxidant stress. Tyr phosphorylation of band 3 occurs during P. falciparum growth, leading to the release of microparticles containing hemicromes and structural weakening of the host cell membrane, simplifying merozoite reinfection. Syk inhibitors block these events by interacting with the Syk protein’s catalytic site. We performed in vitro proteomics and in silico studies and compared the results. In vitro studies were based on treatment of the parasite’s cellular cultures with different concentrations of Syk inhibitors, while proteomics studies were focused on the Tyr phosphorylation of band 3 by Syk protein with the same concentrations of drugs. In silico studies were based on different molecular modeling approaches in order to analyze and optimize the ligand–protein interactions and obtain the highest efficacy in vitro. In the presence of Syk inhibitors, we observed a marked decrease of band 3 Tyr phosphorylation according to the increase of the drug’s concentration. Our studies could be useful for the structural optimization of these compounds and for the design of novel Syk inhibitors in the future.

Resistance Mechanisms to SYK Inhibition in Acute Myeloid Leukemia

Spleen tyrosine kinase (SYK) is a nonmutated therapeutic target in acute myeloid leukemia (AML). Attempts to exploit SYK therapeutically in AML have shown promising results in combination with chemotherapy, likely reflecting induced mechanisms of resistance to single-agent treatment in vivo. We conducted a genome-scale open reading frame (ORF) resistance screen and identified activation of the RAS-MAPK-ERK pathway as one major mechanism of resistance to SYK inhibitors. This finding was validated in AML cell lines with innate and acquired resistance to SYK inhibitors. Furthermore, patients with AML with select mutations activating these pathways displayed early resistance to SYK inhibition. To circumvent SYK inhibitor therapy resistance in AML, we demonstrate that a MEK and SYK inhibitor combination is synergistic in vitro and in vivo. Our data provide justification for use of ORF screening to identify resistance mechanisms to kinase inhibitor therapy in AML lacking distinct mutations and to direct novel combination-based strategies to abrogate these. SIGNIFICANCE: The integration of functional genomic screening with the study of mechanisms of intrinsic and acquired resistance in model systems and human patients identified resistance to SYK inhibitors through MAPK signaling in AML. The dual targeting of SYK and the MAPK pathway offers a combinatorial strategy to overcome this resistance.This article is highlighted in the In This Issue feature, p. 161.

How Biophysical Forces Regulate Human B Cell Lymphomas

The role of microenvironment-mediated biophysical forces in human lymphomas remains elusive. Diffuse large B cell lymphomas (DLBCLs) are heterogeneous tumors, which originate from highly proliferative germinal center B cells. These tumors, their associated neo-vessels, and lymphatics presumably expose cells to particular fluid flow and survival signals. Here, we show that fluid flow enhances proliferation and modulates response of DLBCLs to specific therapeutic agents. Fluid flow upregulates surface expression of B cell receptors (BCRs) and integrin receptors in subsets of ABC-DLBCLs with either CD79A/B mutations or WT BCRs, similar to what is observed with xenografted human tumors in mice. Fluid flow differentially upregulates signaling targets, such as SYK and p70S6K, in ABC-DLBCLs. By selective knockdown of CD79B and inhibition of signaling targets, we provide mechanistic insights into how fluid flow mechanomodulates BCRs and integrins in ABC-DLBCLs. These findings redefine microenvironment factors that regulate lymphoma-drug interactions and will be critical for testing targeted therapies.

IgE-mediated mast cell activation promotes inflammation and cartilage destruction in osteoarthritis

Osteoarthritis is characterized by articular cartilage breakdown, and emerging evidence suggests that dysregulated innate immunity is likely involved. Here, we performed proteomic, transcriptomic, and electron microscopic analyses to demonstrate that mast cells are aberrantly activated in human and murine osteoarthritic joint tissues. Using genetic models of mast cell deficiency, we demonstrate that lack of mast cells attenuates osteoarthritis in mice. Using genetic and pharmacologic approaches, we show that the IgE/FcεRI/Syk signaling axis is critical for the development of osteoarthritis. We find that mast cell-derived tryptase induces inflammation, chondrocyte apoptosis, and cartilage breakdown. Our findings demonstrate a central role for IgE-dependent mast cell activation in the pathogenesis of osteoarthritis, suggesting that targeting mast cells could provide therapeutic benefit in human osteoarthritis.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Non-receptor tyrosine kinase signaling in autoimmunity and therapeutic implications

Autoimmune diseases are characterized by impaired immune tolerance towards self-antigens, leading to enhanced immunity to self by dysfunctional B cells and/or T cells. The activation of these cells is controlled by non-receptor tyrosine kinases (NRTKs), which are critical mediators of antigen receptor and cytokine receptor signaling pathways. NRTKs transduce, amplify and sustain activating signals that contribute to autoimmunity, and are counter-regulated by protein tyrosine phosphatases (PTPs). The function of and interaction between NRTKs and PTPs during the development of autoimmunity could be key points of therapeutic interference against autoimmune diseases. In this review, we summarize the current state of knowledge of the functions of NRTKs and PTPs involved in B cell receptor (BCR), T cell receptor (TCR), and cytokine receptor signaling pathways that contribute to autoimmunity, and discuss their targeting for therapeutic approaches against autoimmune diseases.

Carbohydrate-dependent B cell activation by fucose-binding bacterial lectins

Bacterial lectins are typically multivalent and bind noncovalently to specific carbohydrates on host tissues to facilitate bacterial adhesion. Here, we analyzed the effects of two fucose-binding lectins, BambL from Burkholderia ambifaria and LecB from Pseudomonas aeruginosa, on specific signaling pathways in B cells. We found that these bacterial lectins induced B cell activation, which, in vitro, was dependent on the cell surface expression of the B cell antigen receptor (BCR) and its co-receptor CD19, as well as on spleen tyrosine kinase (Syk) activity. The resulting release of intracellular Ca²⁺ was followed by an increase in the cell surface abundance of the activation marker CD86, augmented cytokine secretion, and subsequent cell death, replicating all of the events that are observed in vitro upon canonical and antigen-mediated B cell activation. Moreover, injection of BambL in mice resulted in a substantial, BCR-independent loss of B cells in the bone marrow with simultaneous, transient enlargement of the spleen (splenomegaly), as well as an increase in the numbers of splenic B cells and myeloid cells. Together, these data suggest that bacterial lectins can initiate polyclonal activation of B cells through their sole capacity to bind to fucose.

Cerdulatinib Pharmacodynamics and Relationships to Tumor Response Following Oral Dosing in Patients with Relapsed/Refractory B-cell Malignancies

PURPOSE

Preclinical studies suggest SYK and JAK contribute to tumor-intrinsic and microenvironment-derived survival signals. The pharmacodynamics of cerdulatinib, a dual SYK/JAK inhibitor, and associations with tumor response were investigated.

PATIENTS AND METHODS

In a phase I dose-escalation study in adults with relapsed/refractory B-cell malignancies, cerdulatinib was administered orally to sequential dose-escalation cohorts using once-daily or twice-daily schedules. The study enrolled 8 patients with chronic lymphocytic leukemia (CLL)/small lymphocytic lymphoma (SLL), 13 with follicular lymphoma, 16 with diffuse large B-cell lymphoma (DLBCL), and 6 with mantle cell lymphoma. Correlation of tumor response with pharmacodynamic markers was determined in patients with meaningful clinical responses.

RESULTS

Following cerdulatinib administration, complete SYK and JAK pathway inhibition was achieved in whole blood of patients at tolerated exposures. Target inhibition correlated with serum cerdulatinib concentration, and IC₅₀ values against B-cell antigen receptor (BCR), IL2, IL4, and IL6 signaling pathways were 0.27 to 1.11 μmol/L, depending on the phosphorylation event. Significant correlations were observed between SYK and JAK pathway inhibition and tumor response. Serum inflammation markers were reduced by cerdulatinib, and several significantly correlated with tumor response. Diminished expression of CD69 and CD86 (B-cell activation markers), CD5 (negative regulator of BCR signaling), and enhanced expression of CXCR4 were observed in 2 patients with CLL, consistent with BCR and IL4 suppression and loss of proliferative capacity.

CONCLUSIONS

Cerdulatinib potently and selectively inhibited SYK/JAK signaling at tolerated exposures in patients with relapsed/refractory B-cell malignancies. The extent of target inhibition in whole-blood assays and suppression of inflammation correlated with tumor response. (ClinicalTrials.gov ID:NCT01994382).

Discovery of potent and selective Spleen Tyrosine Kinase inhibitors for the topical treatment of inflammatory skin disease

The discovery and lead optimisation of a novel series of SYK inhibitors is described. These were optimised for SYK potency and selectivity against Aurora B. Compounds were profiled in a human skin penetration study to identify a suitable candidate molecule for pre-clinical development. Compound 44 (GSK2646264) was selected for progression and is currently in Phase I clinical trials.

Myeloid cell deficiency of p38γ/p38δ protects against candidiasis and regulates antifungal immunity

Candida albicans is a frequent aetiologic agent of sepsis associated with high mortality in immunocompromised patients. Developing new antifungal therapies is a medical need due to the low efficiency and resistance to current antifungal drugs. Here, we show that p38γ and p38δ regulate the innate immune response to C. albicans We describe a new TAK1-TPL2-MKK1-ERK1/2 pathway in macrophages, which is activated by Dectin-1 engagement and positively regulated by p38γ/p38δ. In mice, p38γ/p38δ deficiency protects against C. albicans infection by increasing ROS and iNOS production and thus the antifungal capacity of neutrophils and macrophages, and by decreasing the hyper-inflammation that leads to severe host damage. Leucocyte recruitment to infected kidneys and production of inflammatory mediators are decreased in p38γ/δ-null mice, reducing septic shock. p38γ/p38δ in myeloid cells are critical for this effect. Moreover, pharmacological inhibition of p38γ/p38δ in mice reduces fungal burden, revealing that these p38MAPKs may be therapeutic targets for treating C. albicans infection in humans.

CPX-351 exhibits potent and direct ex vivo cytotoxicity against AML blasts with enhanced efficacy for cells harboring the FLT3-ITD mutation

PURPOSE

Identify AML patients most likely to respond to CPX-351, a nano-scale liposome formulation containing cytarabine and daunorubicin co-encapsulated at a 5:1 molar ratio.

METHODS

We examined the ex vivo cytotoxic activity of CPX-351 against leukemic cells isolated from 53 AML patients and an additional 127 samples including acute lymphoblastic leukemia, myelodysplastic syndrome/myeloproliferative neoplasms, or chronic lymphocytic leukemia/lymphoma. We assessed activity with respect to common molecular lesions and used flow cytometry to assess CPX-351 cellular uptake.

RESULTS

AML specimen sensitivity to CPX-351 was similar across conventional risk groups. FLT3-ITD cases were five-fold more sensitive to CPX-351. CPX-351 was active across other indications with nearly all cases exhibiting IC₅₀ values markedly lower than reported 72-h plasma drug concentration in patients receiving CPX-351. The range and distribution of CPX-351 IC₅₀ values were comparable for AML, CLL, and ALL, whereas MDS/MPN cases were less sensitive. CPX-351 uptake analysis revealed a correlation between uptake of CPX-351 and cytotoxic potency.

CONCLUSIONS

Our findings are consistent with clinical data, in which CPX-351 activity is retained in high-risk AML patients. Ex vivo analysis of cytotoxic potency may provide a means to identify specific AML subsets, such as FLT3-ITD, that benefit most from CPX-351 and warrant additional clinical evaluation.

PRT062607 Achieves Complete Inhibition of the Spleen Tyrosine Kinase at Tolerated Exposures Following Oral Dosing in Healthy Volunteers

The spleen tyrosine kinase (SYK) regulates immune cell activation in response to engagement of a variety of receptors, making it an intriguing target for the treatment of inflammatory and autoimmune disorders as well as certain B‐cell malignancies. We have previously reported on the discovery and preclinical characterization of PRT062607, a potent and highly selective inhibitor of SYK that exhibits robust anti‐inflammatory activity in a variety of animal models. Here we present data from our first human studies aimed at characterizing the pharmacokinetics (PK), pharmacodynamics (PD), and safety of PRT062607 in healthy volunteers following single and multiple oral administrations. PRT062607 demonstrated a favorable PK profile and the ability to completely inhibit SYK activity in multiple whole‐blood assays. The PD half‐life in the more sensitive assays was approximately 24 hours and returned to predose levels by 72 hours. Selectivity for SYK was observed at all dose levels tested. Analysis of the PK/PD relationship indicated an IC₅₀ of 324 nM for inhibition of B‐cell antigen receptor‐mediated B‐cell activation and 205 nM for inhibition of FcεRI‐mediated basophil degranulation. PRT062607 was safe and well tolerated across the entire range of doses. Clinical PK/PD was related to in vivo anti‐inflammatory activity of PRT062607 in the rat collagen‐induced arthritis model, which predicts that therapeutic concentrations may be safely achieved in humans for the treatment of autoimmune disease. PRT062607 has a desirable PK profile and is capable of safely, potently, and selectively suppressing SYK kinase function in humans following once‐daily oral dosing.

RPPA-based protein profiling reveals eIF4G overexpression and 4E-BP1 serine 65 phosphorylation as molecular events that correspond with a pro-survival phenotype in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL), the most common adult leukemia, remains incurable despite advancements in treatment regimens over the past decade. Several expression profile studies have been pursued to better understand CLL pathogenesis. However, these large-scale studies only provide information at the transcriptional level. To better comprehend the differential protein changes that take place in CLL, we performed a reverse-phase protein array (RPPA) analysis using 167 different antibodies on B-cell lysates from 18 CLL patients and 6 normal donors. From our analysis, we discovered an enrichment of protein alterations involved with mRNA translation, specifically upregulation of the translation initiator eIF4G and phosphorylation of the cap-dependent translation inhibitor 4E-BP1 at serine 65. Interestingly, 4E-BP1 phosphorylation occurred independently of AKT phosphorylation, suggesting a disconnect between PI3K/AKT pathway activation and 4E-BP1 phosphorylation. Based on these results, we treated primary CLL samples with NVP-BEZ235, a PI3K/mTOR dual inhibitor, and compared its apoptotic-inducing potential against the BTK inhibitor Ibrutinib and the PI3Kδ inhibitor Idelalisib. We demonstrated that treatment with NVP-BEZ235 caused greater apoptosis, greater apoptotic cleavage of eIF4G, and greater dephosphorylation of 4E-BP1 in primary CLL cells. Taken together, these results highlight the potential dependence of eIF4G overexpression and 4E-BP1 phosphorylation in CLL survival.

Targeting B-cell receptor signaling kinases in chronic lymphocytic leukemia: the promise of entospletinib

The B-cell receptor signaling pathway has emerged as an important therapeutic target in chronic lymphocytic leukemia and other B-cell malignancies. Novel agents have been developed targeting the signaling enzymes spleen tyrosine kinase (SYK), Bruton’s tyrosine kinase, and phosphoinositide 3-kinase delta. This review discusses the rationale for targeting these enzymes, as well as the preclinical and clinical evidence supporting their role as therapeutic targets, with a particular focus on SYK inhibition with entospletinib.

Role of Tyrosine Kinase Inhibitors in Indolent and Other Mature B-Cell Neoplasms

Targeting tyrosine kinases represents a highly specific treatment approach for different malignancies. This also includes non-Hodgkin lymphoma since it is well known that these enzymes are frequently involved in the lymphomagenesis. Hereby, tyrosine kinases might either be dysregulated intrinsically or be activated within signal transduction pathways leading to tumor survival and growth. Among others, Bruton's tyrosine kinase (Btk) is of particular interest as a potential therapeutic target. Btk is stimulated by B-cell receptor signaling and activates different transcription factors such as nuclear factor κB. The Btk inhibitor ibrutinib has been approved for the treatment of chronic lymphocytic leukemia and mantle-cell lymphoma recently. Numerous clinical trials evaluating this agent in different combinations (eg, with rituximab or classical chemotherapeutic agents) as a treatment option for aggressive and indolent lymphoma are under way. Here, we summarize the role of tyrosine kinase inhibitors in the treatment of indolent and other non-Hodgkin lymphomas (eg, mantle-cell lymphoma).

The role of Syk signaling in antifungal innate immunity of human corneal epithelial cells

BACKGROUND

Fungal keratitis is a kind of intractable and sight-threatening diseases. Spleen-tyrosine kinase (Syk) is a non-receptor tyrosine kinase, which plays an important role in the signaling pathway of the receptors. In the current study, we investigate the expression and function of Syk in human corneal epithelial cells with Aspergillus fumigatus (A. fumigatus) infection.

METHODS

Cultured telomerase-immortalized human corneal epithelial cells (THCEs) were treated with A. fumigatus hyphae with or without treatment of Syk inhibitors. Activation of Syk and the role of Syk in regulating inflammatory cytokines and chemokines expression were evaluated. The mRNA expression was determined by real time PCR, and protein activation was measured by western blotting.

RESULTS

Syk protein was detected in THCEs, and its activation was enhanced after treatment of A. fumigatus hyphae. Expression of inflammatory cytokines (IL-1β and IL-6) and chemokines (IL-8 and CXCL1) mRNA were significantly increased after stimulation of A. fumigatus hyphae in THCEs. Activation of Syk and expression of IL-1β, IL-6, IL-8 and CXCL1 by A. fumigatus hyphae were blocked by Syk inhibitors.

CONCLUSION

These findings demonstrate that normal human corneal epithelial cells produce Syk, and Syk activation plays an important role in regulating A. fumigatus hyphae-induced inflammatory responses in THCEs.

Small-molecule inhibitors of spleen tyrosine kinase as therapeutic agents for immune disorders: will promise meet expectations?

Following on the heels of the US FDA approval of tofacitinib (Xeljanz, Pfizer, USA), an inhibitor of the JAK family members, and ibrutinib (Imbruvica, Janssen, Belgium), an inhibitor of BTK, for the treatment of rheumatoid arthritis and chronic lymphocytic leukemia, respectively, there is now renewed interest in the biopharmaceutical industry in the development of orally active small-molecule agents targeting key protein kinases implicated in immune regulation. One such ‘immunokinase’ target is SYK, a non-receptor tyrosine protein kinase critical for transducing intracellular signaling cascades for various immune recognition receptors, such as the B-cell receptor and the Fc receptor. Here, we review and discuss the progress and challenges in the development of small-molecule inhibitors of SYK and their potential as a new class of disease-modifying immunosuppressive agents for certain inflammatory and autoimmune disorders.

The novel kinase inhibitor PRT062070 (Cerdulatinib) demonstrates efficacy in models of autoimmunity and B-cell cancer

The heterogeneity and severity of certain autoimmune diseases and B-cell malignancies warrant simultaneous targeting of multiple disease-relevant signaling pathways. Dual inhibition of spleen tyrosine kinase (SYK) and Janus kinase (JAK) represents such a strategy and may elicit several benefits relative to selective kinase inhibition, such as gaining control over a broader array of disease etiologies, reducing probability of selection for bypass disease mechanisms, and the potential that an overall lower level suppression of individual targets may be sufficient to modulate disease activity. To this end, we provide data on the discovery and preclinical development of PRT062070 [4-(cyclopropylamino)-2-({4-[4-(ethylsulfonyl)piperazin-1-yl]phenyl}amino)pyrimidine-5-carboxamide hydrochloride], an orally active kinase inhibitor that demonstrates activity against SYK and JAK. Cellular assays demonstrated specific inhibitory activity against signaling pathways that use SYK and JAK1/3. Limited inhibition of JAK2 was observed, and PRT062070 did not inhibit phorbol 12-myristate 13-acetate-mediated signaling or activation in B and T cells nor T-cell antigen receptor-mediated signaling in T cells, providing evidence for selectivity of action. Potent antitumor activity was observed in a subset of B-cell lymphoma cell lines. After oral dosing, PRT062070 suppressed inflammation and autoantibody generation in a rat collagen-induced arthritis model and blocked B-cell activation and splenomegaly in a mouse model of chronic B-cell antigen receptor stimulation. PRT062070 is currently under evaluation in a phase I dose escalation study in patients with B-cell leukemia and lymphoma (NCT01994382), with proof-of-concept studies in humans planned to assess therapeutic potential in autoimmune and malignant diseases.

Targeting the B-cell receptor signaling pathway in B lymphoid malignancies

PURPOSE OF REVIEW

Normal B cells that have failed to productively rearrange immunoglobulin V region genes encoding a functional B-cell receptor (BCR) are destined to die. Likewise, the majority of B-cell malignancies remain dependent on functional BCR signaling, whereas in some subtypes BCR expression is missing and, apparently, counterselected. Here, we summarize the recent experimental evidence for the importance of BCR signaling and clinical concepts to target the BCR pathway in B-cell leukemia and lymphoma.

RECENT FINDINGS

Although the dependency on pre-BCR signaling in pre-B acute lymphoblastic leukemia (ALL) seems to be limited to few ALL subtypes (e.g. TCF3-PBX1), most mature B-cell lymphomas rely on BCR signaling provided by different stimuli, for example tonic B-cell signaling, chronic (auto)-antigen exposure, and self-binding properties of the BCR. The finding that in chronic lymphocytic leukemia, BCRs bind to an epitope on the BCR itself unravels a novel concept for chronic lymphocytic leukemia pathogenesis.

SUMMARY

Targeting of the B-cell receptor tyrosine kinases spleen tyrosine kinase, Bruton's tyrosine kinase, and phosphatidylinositol 3-kinase achieve promising clinical responses in various mature B-cell malignancies and might also be useful in defined subsets of ALL. However, further understanding of the BCR signal integration in the different disease groups is required to accurately predict which groups of patients will benefit from BCR pathway inhibition.

Getting Syk: spleen tyrosine kinase as a therapeutic target

Spleen tyrosine kinase (Syk) is a cytoplasmic protein tyrosine kinase well known for its ability to couple immune cell receptors to intracellular signaling pathways that regulate cellular responses to extracellular antigens and antigen-immunoglobulin (Ig) complexes of particular importance to the initiation of inflammatory responses. Thus, Syk is an attractive target for therapeutic kinase inhibitors designed to ameliorate the symptoms and consequences of acute and chronic inflammation. Its more recently recognized role as a promoter of cell survival in numerous cancer cell types ranging from leukemia to retinoblastoma has attracted considerable interest as a target for a new generation of anticancer drugs. This review discusses the biological processes in which Syk participates that have made this kinase such a compelling drug target.

Pharmacokinetics and efficacy of the spleen tyrosine kinase inhibitor r406 after ocular delivery for retinoblastoma

PURPOSE

Retinoblastoma is a childhood cancer of the retina. Clinical trials have shown that local delivery of broad spectrum chemotherapeutic agents is efficacious. Recent studies characterizing the genomic and epigenomic landscape of retinoblastoma identified spleen tyrosine kinase (SYK) as a promising candidate for targeted therapy. The purpose of this study was to conduct preclinical testing of the SYK antagonist R406 to evaluate it as a candidate for retinoblastoma treatment.

METHODS

The efficacy of the SYK antagonist R406 delivered locally in a human orthotopic xenograft mouse model of retinoblastoma was tested. Intraocular exposure of R406 was determined for various routes and formulations.

RESULTS

There was no evidence of efficacy for subconjunctival. R406. Maximal vitreal concentration was 10-fold lower than the minimal concentration required to kill retinoblastoma cells in vitro. Dosage of R406 subconjunctivally from emulsion or suspension formulations, direct intravitreal injection of the soluble prodrug of R406 (R788), and repeated topical administration of R406 all increased vitreal exposure, but failed to reach the exposure required for retinoblastoma cell death in culture.

CONCLUSION

Taken together, these data suggest that R406 is not a viable clinical candidate for the treatment of retinoblastoma. This study highlights the importance of pharmacokinetic testing of molecular targeted retinoblastoma therapeutics.

Role of spleen tyrosine kinase in the pathogenesis of chronic lymphocytic leukemia

The antigen-dependent B-cell receptor (BCR) is triggered by binding to external antigens and transmits signals in normal B lymphocytes. Tonic signaling through the BCR plays a crucial role in the pathogenesis and progression of chronic lymphocytic leukemia (CLL). Spleen tyrosine kinase (Syk) is a key component of both BCR signals, and regulates multiple physiological functions of B lymphocytes. Studies have defined enhanced gene expression and protein expression of Syk in CLL cells which are closely related to the status of the immunoglobulin heavy chain variable region genes (IgVH). Recently, abrogating the BCR-induced signaling pathway by Syk inhibitors has represented a novel and active therapeutic approach for CLL. Studies of the correlation between Syk and ZAP-70 expression in CLL cells have brought a new perspective to determining the value of Syk in evaluating the effect of therapy and the prognosis of CLL. Therefore, we here review the role of Syk in the pathogenesis of CLL and provide an update of progress in the clinical development of Syk inhibitors.

B-cell antigen receptor signaling in chronic lymphocytic leukemia: therapeutic targets and translational opportunities

B-cell chronic lymphocytic leukemia (CLL) is characterized by clonally expanded and molecularly heterogeneous populations of B lymphocytes with impaired apoptotic mechanisms. This occurs as a result of multiple genetic and epigenetic abnormalities, including chromosomal aberrations and enhancer region hypomethylation, often impinging on intracellular signaling pathways that are essential to normal B-cell activation, proliferation, and survival. The B-cell antigen receptor (BCR) signaling is one such pathway usurped by malignant B cells, as exemplified by the early phase clinical success achieved by small-molecule agents targeting key players involved in the pathway. Such new targeted agents, including those that inhibit the function of Spleen tyrosine kinase (SYK), Bruton's tyrosine kinase (BTK), phosphatidylinositol 3-kinases (PI3K), and B-cell lymphoma 2 (BCL-2), along with the current standard therapy comprising chemo-immunotherapies with or without B-cell depleting biologic agent rituximab (anti-CD20 monoclonal antibody), should expand the armamentarium for CLL therapy. We review the therapeutic agents currently in clinical development which target different effectors of the malignant BCR signaling, and discuss their overlapping and discriminating translational opportunities in the context of CLL treatment.

B-cell receptor signaling inhibitors for treatment of autoimmune inflammatory diseases and B-cell malignancies

B-cell receptor (BCR) signaling is essential for normal B-cell development, selection, survival, proliferation, and differentiation into antibody-secreting cells. Similarly, this pathway plays a key role in the pathogenesis of multiple B-cell malignancies. Genetic and pharmacological approaches have established an important role for the Spleen tyrosine kinase (Syk), Bruton's tyrosine kinase (Btk), and phosphatidylinositol 3-kinase isoform p110delta (PI3Kδ) in coupling the BCR and other BCRs to B-cell survival, migration, and activation. In the past few years, several small-molecule inhibitory drugs that target PI3Kδ, Btk, and Syk have been developed and shown to have efficacy in clinical trials for the treatment of several types of B-cell malignancies. Emerging preclinical data have also shown a critical role of BCR signaling in the activation and function of self-reactive B cells that contribute to autoimmune diseases. Because BCR signaling plays a major role in both B-cell-mediated autoimmune inflammation and B-cell malignancies, inhibition of this pathway may represent a promising new strategy for treating these diseases. This review summarizes recent achievements in the mechanism of action, pharmacological properties, and clinical activity and toxicity of these BCR signaling inhibitors, with a focus on their emerging role in treating lymphoid malignancies and autoimmune disorders.

SYK is a critical regulator of FLT3 in acute myeloid leukemia

Cooperative dependencies between mutant oncoproteins and wild-type proteins are critical in cancer pathogenesis and therapy resistance. Although spleen tyrosine kinase (SYK) has been implicated in hematologic malignancies, it is rarely mutated. We used kinase activity profiling to identify collaborators of SYK in acute myeloid leukemia (AML) and determined that FMS-like tyrosine kinase 3 (FLT3) is transactivated by SYK via direct binding. Highly activated SYK is predominantly found in FLT3-ITD positive AML and cooperates with FLT3-ITD to activate MYC transcriptional programs. FLT3-ITD AML cells are more vulnerable to SYK suppression than FLT3 wild-type counterparts. In a FLT3-ITD in vivo model, SYK is indispensable for myeloproliferative disease (MPD) development, and SYK overexpression promotes overt transformation to AML and resistance to FLT3-ITD-targeted therapy.