The Bruton tyrosine kinase inhibitor PCI-32765 ameliorates autoimmune arthritis by inhibition of multiple effector cells

Use of signaling pathways as therapeutic targets for blood cancer

SUMMARY 

Until recently, the treatment of blood cancers has rested exclusively on chemotherapy, radiation and, in select cases, stem cell transplantation, giving rise to frequent and sometimes life-threatening side effects. The past 10 years have witnessed dramatic clinical advances due to the development of novel therapies, hereafter referred to as targeted therapies, which specifically inhibit molecules that are essential to the pathophysiology of individual malignancies. In this article, we will discuss the general concept of targeting signaling pathways in cancers and the limitations of this strategy, with a particular focus on the emergence of resistant cancer clones under the selective pressures exerted by targeted therapies. Finally, we will examine a number of targeted therapies with immediate application in contemporary clinical practice.

Current understanding of tyrosine kinase BMX in inflammation and its inhibitors

Tec family kinases, which include tyrosine kinase expressed in hepatocellular carcinoma (TEC), Bruton’s tyrosine kinase (BTK), interleukin (IL)-2-inducible T-cell kinase (ITK), tyrosine-protein kinase (TXK), and bone marrow tyrosine kinase on chromosome X (BMX), are the second largest group of non-receptor tyrosine kinases and have a highly conserved carboxyl-terminal kinase domain. BMX was identified in human bone marrow cells, and was demonstrated to have been expressed in myeloid hematopoietic lineages cells, endothelial cells, and several types of cancers. Significant progress in this area during the last decade revealed an important role for BMX in inflammation and oncologic disorders. This review focuses on BMX biology, its role in inflammation and possible signaling pathways, and the potential of selective BMX inhibitors.

Targeted biopharmaceuticals for cancer treatment

Cancer is a complex invasive genetic disease that causes significant mortality rate worldwide. Protein-based biopharmaceuticals have significantly extended the lives of millions of cancer patients. This article reviews the biological function and application of targeted anticancer biopharmaceuticals. We first discuss the specific antigens and core pathways that are used in the development of targeted cancer therapy. The innovative monoclonal antibodies, non-antibody proteins, and small molecules targeting these antigens or pathways are then reviewed. Finally, the current challenges in anticancer biopharmaceuticals development and the potential solutions to address these challenges are discussed.

Bruton's tyrosine kinase: from X-linked agammaglobulinemia toward targeted therapy for B-cell malignancies

Discovery of Bruton's tyrosine kinase (BTK) mutations as the cause for X-linked agammaglobulinemia was a milestone in understanding the genetic basis of primary immunodeficiencies. Since then, studies have highlighted the critical role of this enzyme in B-cell development and function, and particularly in B-cell receptor signaling. Because its deletion affects mostly B cells, BTK has become an attractive therapeutic target in autoimmune disorders and B-cell malignancies. Ibrutinib (PCI-32765) is the most advanced BTK inhibitor in clinical testing, with ongoing phase III clinical trials in patients with chronic lymphocytic leukemia and mantle-cell lymphoma. In this article, we discuss key discoveries related to BTK and clinically relevant aspects of BTK inhibitors, and we provide an outlook into clinical development and open questions regarding BTK inhibitor therapy.

Modeling the clinical phenotype of BTK inhibition in the mature murine immune system

Inhibitors of Bruton's tyrosine kinase (BTK) possess much promise for the treatment of oncologic and autoimmune indications. However, our current knowledge of the role of BTK in immune competence has been gathered in the context of genetic inactivation of btk in both mice and man. Using the novel BTK inhibitor PF-303, we model the clinical phenotype of BTK inhibition by systematically examining the impact of PF-303 on the mature immune system in mice. We implicate BTK in tonic BCR signaling, demonstrate dependence of the T3 B cell subset and IgM surface expression on BTK activity, and find that B1 cells survive and function independently of BTK. Although BTK inhibition does not impact humoral memory survival, Ag-driven clonal expansion of memory B cells and Ab-secreting cell generation are inhibited. These data define the role of BTK in the mature immune system and mechanistically predict the clinical phenotype of chronic BTK inhibition.

Btk inhibition suppresses agonist-induced human macrophage activation and inflammatory gene expression in RA synovial tissue explants

OBJECTIVES

Bruton's tyrosine kinase (Btk) is required for B lymphocyte and myeloid cell contributions to pathology in murine models of arthritis. Here, we examined the potential contributions of synovial Btk expression and activation to inflammation in rheumatoid arthritis (RA).

MATERIALS AND METHODS

Btk was detected by immunohistochemistry and digital image analysis in synovial tissue from biologically naive RA (n=16) and psoriatic arthritis (PsA) (n=12) patients. Cell populations expressing Btk were identified by immunofluorescent double labelling confocal microscopy, quantitative (q-) PCR and immunoblotting. The effects of a Btk-specific inhibitor, RN486, on gene expression in human macrophages and RA synovial tissue explants (n=8) were assessed by qPCR, ELISA and single-plex assays.

RESULTS

Btk was expressed at equivalent levels in RA and PsA synovial tissue, restricted to B lymphocytes, monocytes, macrophages and mast cells. RN486 significantly inhibited macrophage IL-6 production induced by Fc receptor and CD40 ligation. RN486 also reduced mRNA expression of overlapping gene sets induced by IgG, CD40 ligand (CD40L) and RA synovial fluid, and significantly suppressed macrophage production of CD40L-induced IL-8, TNF, MMP-1 and MMP-10, LPS-induced MMP-1, MMP-7 and MMP-10 production, and spontaneous production of IL-6, PDGF, CXCL-9 and MMP-1 by RA synovial explants.

CONCLUSIONS

Btk is expressed equivalently in RA and PsA synovial tissue, primarily in macrophages. Btk activity is needed to drive macrophage activation in response to multiple agonists relevant to inflammatory arthritis, and promotes RA synovial tissue cytokine and MMP production. Pharmacological targeting of Btk may be of therapeutic benefit in the treatment of RA and other inflammatory diseases.

Bruton's tyrosine kinase (BTK) inhibitors in clinical trials

BTK is a cytoplasmic, non-receptor tyrosine kinase that transmits signals from a variety of cell-surface molecules, including the B-cell receptor (BCR) and tissue homing receptors. Genetic BTK deletion causes B-cell immunodeficiency in humans and mice, making this kinase an attractive therapeutic target for B-cell disorders. The BTK inhibitor ibrutinib (PCI-32765, brand name: Imbruvica) demonstrated high clinical activity in B-cell malignancies, especially in patients with chronic lymphocytic leukemia (CLL), mantle cell lymphoma (MCL), and Waldenstrom's macroglobulinemia (WM). Therefore, ibrutinib was granted a 'breakthrough therapy' designation for these indications and was recently approved for the treatment of relapsed MCL by the U.S. Food and Drug Administration. Other BTK inhibitors in earlier clinical development include CC-292 (AVL-292), and ONO-4059. In CLL and MCL, ibrutinib characteristically induces redistribution of malignant B cells from tissue sites into the peripheral blood, along with rapid resolution of enlarged lymph nodes and a surge in lymphocytosis. With continuous ibrutinib therapy, growth- and survival-inhibitory activities of ibrutinib result in the normalization of lymphocyte counts and remissions in a majority of patients. This review discusses the clinical advances with BTK inhibitor therapy, as well as its pathophysiological basis, and outlines perspectives for future use of BTK inhibitors.

The orally available Btk inhibitor ibrutinib (PCI-32765) protects against osteoclast-mediated bone loss

Bone-resorbing osteoclasts play an essential role in normal bone homeostasis, as well as in various bone disorders such as osteoporosis and rheumatoid arthritis. Previously we showed that the Tec family of tyrosine kinases is essential for the differentiation of osteoclasts and the inhibition of Btk is a promising strategy for the prevention of the bone loss in osteoclast-associated bone disorders. Here we demonstrate that an orally available Btk inhibitor, ibrutinib (PCI-32765), suppresses osteoclastic bone resorption by inhibiting both osteoclast differentiation and function. Ibrutinib downregulated the expression of NFATc1, the key transcription factor for osteoclastogenesis, and disrupted the formation of the actin ring in mature osteoclasts. In addition, genome-wide screening revealed that Btk regulates the expression of the genes involved in osteoclast differentiation and function in both an NFATc1-dependent and -independent manner. Finally, we showed that ibrutinib administration ameliorated the bone loss that developed in a RANKL-induced osteoporosis mouse model. Thus, this study suggests ibrutinib to be a promising therapeutic agent for osteoclast-associated bone diseases.

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.

Management of thrombotic thrombocytopenic purpura: current perspectives

Thrombotic thrombocytopenic purpura (TTP) is a rare, life-threatening thrombotic microangiopathy which causes significant morbidity and mortality unless promptly recognized and treated. The underlying pathogenesis of TTP is a severe deficiency in ADAMTS13 activity, a metalloprotease that cleaves ultralarge von Willebrand factor multimers. This deficiency is either autoantibody mediated (acquired TTP) or due to deleterious mutations in the gene encoding ADAMTS13 (congenital TTP). The elucidation of this disease mechanism has reinforced the rationale and place of current therapies (eg, plasma exchange) as well as providing a basis for the prospective evaluation of immunotherapy with rituximab in addition to classic immunosuppression (eg, corticosteroid) in autoantibody-mediated TTP. This review discusses the current evidence base for therapeutic interventions in acquired and congenital TTP as well as providing a practical approach to the other aspects of investigation and management for which a firm evidence base is lacking. Novel agents that are currently being evaluated in prospective trials and future directions of therapy are also discussed which are expected to make an important contribution to improving outcomes in patients with TTP.

Bruton's tyrosine kinase regulates TLR9 but not TLR7 signaling in human plasmacytoid dendritic cells

Plasmacytoid dendritic cells (PDCs) represent a key cell type for both innate and adaptive immunity. PDCs express both TLR7 and TLR9 and the recognition of nucleic acids by these two receptors triggers the production of a large amount of type-I IFN and the induction of PDC maturation into APCs. This unique feature of PDCs is at the basis of clinical development of both TLR7 and TLR9 agonists for infectious diseases, allergy, cancer, and asthma. However, TLR7 and TLR9 recognition of self-nucleic acids is linked to many autoimmune diseases including lupus, and a better understanding of the signaling pathways of these two receptors in PDCs is thus important. We have identified Bruton's tyrosine kinase (Btk) as an important player for TLR9 but not TLR7 signaling in human PDCs. Blocking Btk using a specific inhibitor leads to the reduction of all TLR9-induced responses in PDCs, including cytokine production and expression of costimulatory molecules, while this has no impact on the TLR7 response. This identifies Btk as a key molecule in TLR9 signaling in PDCs and is the first demonstration that the TLR7 and TLR9 pathways can be dissociated in human PDCs.

Ibrutinib

Abnormal B-cell receptor (BCR) signaling is a key mechanism of disease progression in B-cell malignancy. Bruton's tyrosine kinase (BTK) has a pivotal role in BCR signaling. Ibrutinib (PCI-32765) is a novel agent which serves as a covalent irreversible inhibitor of BTK. It is characterized by high selectivity for BTK and high potency. Preliminary data from phase I and ongoing phase II trials have proven very promising so far. It suggests the substance has high efficacy in B-cell malignancies such as chronic lymphocytic leukemia (CLL); diffuse large B-cell lymphoma (DLBCL), follicular lymphoma (FL), mantel cell lymphoma (MCL), and multiple myeloma (MM) and is very well tolerable. Most notably, the substance does not cause myelosuppression. This chapter discusses structure, mechanism of action, and toxicities of ibrutinib and also presents important preclinical and clinical data. Phase III trials will have to determine the definite role of ibrutinib in clinical practice but the data available so far suggests it may be a powerful new weapon in the battle against B-cell malignancies.

Bruton tyrosine kinase inhibitor ibrutinib (PCI-32765)

Over the past 3 years, ibrutinib (PCI-32765) has emerged as a breakthrough in targeted therapy for patients with certain types of B cell malignancies. Early stage clinical trials found ibrutinib to be particularly active in chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL), providing the rationale for ongoing phase 3 trials. In contrast to conventional chemo-immunotherapy, ibrutinib is not myelosuppressive, and responses are not affected by disease features that predict failure to respond to or short remission durations after chemo-immunotherapy, such as del17p. In CLL, ibrutinib characteristically causes an early redistribution of tissue-resident CLL cells into the blood, with rapid resolution of enlarged lymph nodes, along with a surge in lymphocytosis. Later, after weeks to months of continuous ibrutinib therapy, the growth- and survival-inhibitory activities of ibrutinib result in the normalization of lymphocyte counts and remissions in a majority of patients. This review discusses the discovery, preclinical and clinical development of ibrutinib, its pathophysiological basis, and outlines perspectives for future use of ibrutinib.

Egress of CD19(+)CD5(+) cells into peripheral blood following treatment with the Bruton tyrosine kinase inhibitor ibrutinib in mantle cell lymphoma patients

Ibrutinib (PCI-32765) is a highly potent oral Bruton tyrosine kinase (BTK) inhibitor in clinical development for treating B-cell lymphoproliferative diseases. Patients with chronic lymphocytic leukemia (CLL) often show marked, transient increases of circulating CLL cells following ibrutinib treatments, as seen with other inhibitors of the B-cell receptor (BCR) pathway. In a phase 1 study of ibrutinib, we noted similar effects in patients with mantle cell lymphoma (MCL). Here, we characterize the patterns and phenotypes of cells mobilized among patients with MCL and further investigate the mechanism of this effect. Peripheral blood CD19(+)CD5(+) cells from MCL patients were found to have significant reduction in the expression of CXCR4, CD38, and Ki67 after 7 days of treatment. In addition, plasma chemokines such as CCL22, CCL4, and CXCL13 were reduced 40% to 60% after treatment. Mechanistically, ibrutinib inhibited BCR- and chemokine-mediated adhesion and chemotaxis of MCL cell lines and dose-dependently inhibited BCR, stromal cell, and CXCL12/CXCL13 stimulations of pBTK, pPLCγ2, pERK, or pAKT. Importantly, ibrutinib inhibited migration of MCL cells beneath stromal cells in coculture. We propose that BTK is essential for the homing of MCL cells into lymphoid tissues, and its inhibition results in an egress of malignant cells into peripheral blood. This trial was registered at www.clinicaltrials.gov as #NCT00114738.

Ibrutinib is an irreversible molecular inhibitor of ITK driving a Th1-selective pressure in T lymphocytes

Given its critical role in T-cell signaling, interleukin-2-inducible kinase (ITK) is an appealing therapeutic target that can contribute to the pathogenesis of certain infectious, autoimmune, and neoplastic diseases. Ablation of ITK subverts Th2 immunity, thereby potentiating Th1-based immune responses. While small-molecule ITK inhibitors have been identified, none have demonstrated clinical utility. Ibrutinib is a confirmed irreversible inhibitor of Bruton tyrosine kinase (BTK) with outstanding clinical activity and tolerability in B-cell malignancies. Significant homology between BTK and ITK alongside in silico docking studies support ibrutinib as an immunomodulatory inhibitor of both ITK and BTK. Our comprehensive molecular and phenotypic analysis confirms ITK as an irreversible T-cell target of ibrutinib. Using ibrutinib clinical trial samples along with well-characterized neoplastic (chronic lymphocytic leukemia), parasitic infection (Leishmania major), and infectious disease (Listeria monocytogenes) models, we establish ibrutinib as a clinically relevant and physiologically potent ITK inhibitor with broad therapeutic utility. This trial was registered at www.clinicaltrials.gov as #NCT01105247 and #NCT01217749.

X-linked immunodeficient mice exhibit enhanced susceptibility to Cryptococcus neoformans Infection

Bruton’s tyrosine kinase (Btk) is a signaling molecule that plays important roles in B-1 B cell development and innate myeloid cell functions and has recently been identified as a target for therapy of B cell lymphomas. We examined the contribution of B-1 B cells to resistance to Cryptococcus neoformans infection by utilizing X-linked immunodeficient (XID) mice (CBA-CaHN-XID), which possess a mutation in Btk. XID mice had significantly higher brain fungal burdens than the controls 6 weeks after infection with C. neoformans strain 52D (CN52D); however, consistent with the propensity for greater virulence of C. neoformans strain H99 (CNH99), CNH99-infected XID mice had higher lung and brain fungal burdens than the controls 3 weeks after infection. Further studies in a chronic CN52D model revealed markedly lower levels of total and C. neoformans-specific serum IgM in XID mice than in the control mice 1 and 6 weeks after infection. Alveolar macrophage phagocytosis was markedly impaired in CN52D-infected XID mice compared to the controls, with XID mice exhibiting a disorganized lung inflammatory pattern in which Gomori silver staining revealed significantly more enlarged, extracellular C. neoformans cells than the controls. Adoptive transfer of B-1 B cells to XID mice restored peritoneal B-1 B cells but did not restore IgM levels to those of the controls and had no effect on the brain fungal burden at 6 weeks. Taken together, our data support the hypothesis that IgM promotes fungal containment in the lungs by enhancing C. neoformans phagocytosis and restricting C. neoformans enlargement. However, peritoneal B-1 B cells are insufficient to reconstitute a protective effect in the lungs.

Cryptococcus neoformans is a fungal pathogen that causes an estimated 600,000 deaths per year. Most infections occur in individuals who are immunocompromised, with the majority of cases occurring in those with HIV/AIDS, but healthy individuals also develop disease. Immunoglobulin M (IgM) has been linked to resistance to disease in humans and mice. In this article, we found that X-linked immunodeficient (XID) mice, which have markedly reduced levels of IgM, were unable to contain Cryptococcus in the lungs. This was associated with reduced yeast uptake by macrophages, an aberrant tissue inflammatory response, an enlargement of the yeast cells in the lungs, and fungal dissemination to the brain. Since XID mice have a mutation in the Bruton’s tyrosine kinase (Btk) gene, our data suggest that treatments aimed at blocking the function of Btk could pose a higher risk for cryptococcosis.

Role of Bruton's tyrosine kinase in myeloma cell migration and induction of bone disease

Myeloma cells typically grow in bone, recruit osteoclast precursors and induce their differentiation and activity in areas adjacent to tumor foci. Bruton's tyrosine kinase (BTK), of the TEC family, is expressed in hematopoietic cells and is particularly involved in B-lymphocyte function and osteoclastogenesis. We demonstrated BTK expression in clinical myeloma plasma cells, interleukin (IL)-6- or stroma-dependent cell lines and osteoclasts. SDF-1 induced BTK activation in myeloma cells and BTK inhibition by small hairpin RNA or the small molecule inhibitor, LFM-A13, reduced their migration toward stromal cell-derived factor-1 (SDF-1). Pretreatment with LFM-A13 also reduced in vivo homing of myeloma cells to bone using bioluminescence imaging in the SCID-rab model. Enforced expression of BTK in myeloma cell line enhanced cell migration toward SDF-1 but had no effect on short-term growth. BTK expression was correlated with cell-surface CXCR4 expression in myeloma cells (n = 33, r = 0.81, P < 0.0001), and BTK gene and protein expression was more profound in cell-surface CXCR4-expressing myeloma cells. BTK was not upregulated by IL-6 while its inhibition had no effect on IL-6 signaling in myeloma cells. Human osteoclast precursors also expressed BTK and cell-surface CXCR4 and migrated toward SDF-1. LFM-A13 suppressed migration and differentiation of osteoclast precursors as well as bone-resorbing activity of mature osteoclasts. In primary myeloma-bearing SCID-rab mice, LFM-A13 inhibited osteoclast activity, prevented myeloma-induced bone resorption and moderately suppressed myeloma growth. These data demonstrate BTK and cell-surface CXCR4 association in myeloma cells and that BTK plays a role in myeloma cell homing to bone and myeloma-induced bone disease. Am. J. Hematol. 88:463-471, 2013. © 2013 Wiley Periodicals, Inc.

Modeling tumor-host interactions of chronic lymphocytic leukemia in xenografted mice to study tumor biology and evaluate targeted therapy

Chronic lymphocytic leukemia (CLL) cells depend on microenvironmental factors for proliferation and survival. In particular, the B-cell receptor (BCR) and NF-κB pathways are activated in the lymph node microenvironment. Thus, model systems mimicking tumor-host interactions are important tools to study CLL biology and pathogenesis. We investigated whether the recently established NOD/scid/γc^null (NSG) mouse xenograft model can recapitulate the effects of the human microenvironment. We assessed, therefore, tumor characteristics previously defined in lymph node-resident CLL cells, including proliferation, and activation of the BCR and NF-κB pathways. We found that the murine spleen microenvironment supported CLL cell proliferation and activation to a similar degree than the human lymph node, including induction of BCR and NF-κB signaling in the xenografted cells. Next, we used this model to study ibrutinib, a Bruton's tyrosine kinase inhibitor in clinical development. Ibrutinib inhibited BCR and NF-κB signaling induced by the microenvironment, decreased proliferation, induced apoptosis, and reduced the tumor burden in vivo. Thus, our data demonstrate that the spleen of xenografted NSG mice can, in part, recapitulate the role of the human lymph node for CLL cells. In addition, we show that ibrutinib effectively disrupts tumor-host interactions essential for CLL cell proliferation and survival in vivo.

Novel Bruton's tyrosine kinase inhibitors currently in development

Bruton’s tyrosine kinase (Btk) is intimately involved in multiple signal-transduction pathways regulating survival, activation, proliferation, and differentiation of B-lineage lymphoid cells. Btk is overexpressed and constitutively active in several B-lineage lymphoid malignancies. Btk has emerged as a new antiapoptotic molecular target for treatment of B-lineage leukemias and lymphomas. Preclinical and early clinical results indicate that Btk inhibitors may be useful in the treatment of leukemias and lymphomas.

B-cell biology and development

B cells develop from hematopoietic precursor cells in an ordered maturation and selection process. Extensive studies with many different mouse mutants provided fundamental insights into this process. However, the characterization of genetic defects causing primary immunodeficiencies was essential in understanding human B-cell biology. Defects in pre-B-cell receptor components or in downstream signaling proteins, such as Bruton tyrosine kinase and B-cell linker protein, arrest development at the pre-B-cell stage. Defects in survival-regulating proteins, such as B-cell activator of the TNF-α family receptor (BAFF-R) or caspase recruitment domain-containing protein 11 (CARD11), interrupt maturation and prevent differentiation of transitional B cells into marginal zone and follicular B cells. Mature B-cell subsets, immune responses, and memory B-cell and plasma cell development are disturbed by mutations affecting Toll-like receptor signaling, B-cell antigen receptor coreceptors (eg, CD19), or enzymes responsible for immunoglobulin class-switch recombination. Transgenic mouse models helped to identify key regulatory mechanisms, such as receptor editing and clonal anergy, preventing the activation of B cells expressing antibodies recognizing autoantigens. Nevertheless, the combination of susceptible genetic backgrounds with the rescue of self-reactive B cells by T cells allows the generation of autoreactive clones found in patients with many autoimmune diseases and even in those with primary immunodeficiencies. The rapid progress of functional genomic research is expected to foster the development of new tools that specifically target dysfunctional B lymphocytes to treat autoimmunity, B-cell malignancies, and immunodeficiency.

Back to the future: oral targeted therapy for RA and other autoimmune diseases

The molecular biology revolution coupled with the development of monoclonal antibody technology enabled remarkable progress in rheumatology therapy, comprising an array of highly effective biologic agents. With advances in understanding of the molecular nature of immune cell receptors came elucidation of intracellular signalling pathways downstream of these receptors. These discoveries raise the question of whether selective targeting of key intracellular factors with small molecules would add to the rheumatologic armamentarium. In this Review, we discuss several examples of this therapeutic strategy that seem to be successful, and consider their implications for the future of immune-targeted treatments. We focus on kinase inhibitors, primarily those targeting Janus kinase family members and spleen tyrosine kinase, given their advanced status in clinical development and application. We also summarize other targets involved in signalling pathways that might offer promise for therapeutic intervention in the future.

Targeting the SYK-BTK axis for the treatment of immunological and hematological disorders: recent progress and therapeutic perspectives

Spleen Tyrosine Kinase (SYK) and Bruton's Tyrosine Kinase (BTK) are non-receptor cytoplasmic tyrosine kinases that are primarily expressed in cells of hematopoietic lineage. Both are key mediators in coupling activated immunoreceptors to downstream signaling events that affect diverse biological functions, from cellular proliferation, differentiation and adhesion to innate and adaptive immune responses. As such, pharmacological inhibitors of SYK or BTK are being actively pursued as potential immunomodulatory agents for the treatment of autoimmune and inflammatory disorders. Deregulation of SYK or BTK activity has also been implicated in certain hematological malignancies. To date, from a clinical perspective, pharmacological inhibition of SYK activity has demonstrated encouraging efficacy in patients with rheumatoid arthritis (RA), while patients with relapsed or refractory chronic lymphocytic leukemia (CLL)/small lymphocytic lymphoma (SLL) have benefited from covalent inhibitors of BTK in early clinical studies. Here, we review and discuss recent insights into the emerging role of the SYK-BTK axis in innate immune cell function as well as in the maintenance of survival and homing signals for tumor cell progression. The current progress on the clinical development of SYK and BTK inhibitors is also highlighted.

Role of phosphatidylinositol 3,4,5-trisphosphate in cell signaling

Many lipids present in cellular membranes are phosphorylated as part of signaling cascades and participate in the recruitment, localization, and activation of downstream protein effectors. Phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P3) is one of the most important second messengers and is capable of interacting with a variety of proteins through specific PtdIns(3,4,5)P3 binding domains. Localization and activation of these effector proteins controls a myriad of cellular functions including cell survival, proliferation, cytoskeletal rearrangement, and gene expression. Aberrations in the production and metabolism of PtdIns(3,4,5)P3 have been implicated in many human diseases including cancer, diabetes, inflammation, and heart disease. This chapter provides an overview of the role of PtdIns(3,4,5)P3 in cellular regulation and the implications of PtdIns(3,4,5)P3 dysregulation in human diseases. Additionally, recent attempts at targeting PtdIns(3,4,5)P3 signaling via small molecule inhibitors are summarized.

Modulating proximal cell signaling by targeting Btk ameliorates humoral autoimmunity and end-organ disease in murine lupus

INTRODUCTION

Systemic lupus erythematosus is a chronic autoimmune disease characterized by an abundance of autoantibodies against nuclear antigens. Bruton's tyrosine kinase (Btk) is a proximal transducer of the BCR signal that allows for B-cell activation and differentiation. Recently, selective inhibition of Btk by PCI-32765 has shown promise in limiting activity of multiple cells types in various models of cancer and autoimmunity. The aim of this study was to determine the effect of Btk inhibition by PCI-32765 on the development of lupus in lupus-prone B6.Sle1 and B6.Sle1.Sle3 mice.

METHODS

B6.Sle1 or B6.Sle1.Sle3 mice received drinking water containing either the Btk inhibitor PCI-32765 or vehicle for 56 days. Following treatment, mice were examined for clinical and pathological characteristics of lupus. The effect of PCI-32765 on specific cell types was also investigated.

RESULTS

In this study, we report that Btk inhibition dampens humoral autoimmunity in B6.Sle1 monocongenic mice. Moreover, in B6.Sle1.Sle3 bicongenic mice that are prone to severe lupus, Btk inhibition also dampens humoral and cellular autoimmunity, as well as lupus nephritis.

CONCLUSIONS

These findings suggest that partial crippling of cell signaling in B cells and antigen presenting cells (APCs) may be a viable alternative to total depletion of these cells as a therapeutic modality for lupus.

B-cell selection and the development of autoantibodies

The clearest evidence that B cells play an important role in human autoimmunity is that immunotherapies that deplete B cells are very effective treatments for many autoimmune diseases. All people, healthy or ill, have autoreactive B cells, but not at the same frequency. A number of genes influence the level of these autoreactive B cells and whether they are eliminated or not during development at a central checkpoint in the bone marrow (BM) or at a later checkpoint in peripheral lymphoid tissues. These genes include those encoding proteins that regulate signaling through the B-cell receptor complex such as Btk and PTPN22, proteins that regulate innate signaling via Toll-like receptors (TLRs) such as MyD88 and interleukin-1 receptor-associated kinase 4, as well as the gene encoding the activation-induced deaminase (AID) essential for B cells to undergo class switch recombination and somatic hypermutation. Recent studies have revealed that TLR signaling elements and AID function not only in peripheral B cells to help mediate effective antibody responses to foreign antigens, but also in the BM to help remove autoreactive B-lineage cells at a very early point in B-cell development. Newly arising B cells that leave the BM and enter the blood and splenic red pulp can express both AID and TLR signaling elements like TLR7, and thus are fully equipped to respond rapidly to antigens (including autoantigens), to isotype class switch, and to undergo somatic hypermutation. These red pulp B cells may thus be an important source of autoantibody-producing cells arising particularly in extrafollicular sites, and indeed may be as significant a source of autoantibody-producing cells as B cells arising from germinal centers.

Tec family kinases in inflammation and disease

Over the last decade, the Tec family of nonreceptor tyrosine kinases (Btk, Tec, Bmx, Itk, and Rlk) have been shown to play a key role in inflammation and bone destruction. Bruton's tyrosine kinase (Btk) has been the most widely studied due to the critical role of this kinase in B-cell development and recent evidence showing that blocking Btk signaling is effective in ameliorating lymphoma progression and experimental arthritis. This review will examine the role of TFK in myeloid cell function and the potential of targeting these kinases as a therapeutic intervention in autoimmune disorders such as rheumatoid arthritis.

Bruton's Tyrosine Kinase (Btk)

Bruton's tyrosine kinase (Btk) is a non-receptor tyrosine kinase belonging to the Tec family of kinases. Btk is critical for B-cell development, differentiation and signalling through the B-cell antigen receptor (BCR) as is evident by its genetic association to a human primary immunodeficiency disease known as X-linked Agammaglobulinemia (XLA). Btk is also present in specific cells of the myeloid lineage and contributes to the activation of the FcγR and FcεR signalling pathways in macrophages, neutrophils and mast cells. Because of its key role in these pathways, Btk is considered a promising target for therapeutic intervention in autoimmune and inflammatory disease. Numerous research groups are actively working to identify Btk inhibitors through the targeting of inactive kinase conformations or covalent active site inhibition. Both strategies have benefited from the rapid growth in structural biology insight for the target. Recently discovered potent and orally bioavailable Btk inhibitors have shown promising efficacy in several pre-clinical animal models of rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE). These results, coupled with promising initial findings from the study of Btk inhibitors in human clinical trials for oncology, strongly suggest Btk intervention offers significant potential as a treatment strategy in inflammatory disease.

[New kinase inhibitors]

Treatment of rheumatoid arthritis (RA) has dramatically changed during the last 15 years. A limited number of conventional disease-modifying antirheumatic drugs (DMARD) in combination with non-steroid anti-inflammatory drugs (NSAID) and corticosteroids are facing a variety of biologics that are increasingly being used. Because of the high costs of biologics as well as the necessity for subcutaneous or intravenous administration, there is currently a growing interest in new and potent oral compounds such as the small molecules. Inflammatory pathways and mechanisms in signal transduction have been characterized in detail. Instead of neutralizing the action of a proinflammatory cytokine by antagonizing its biologic effect by an antibody, these small molecules interfere with the intracellular pathways of the inflammatory cascade. Intracellular kinases are among these enzymes which are crucially involved in intracellular signal transduction. Kinase inhibitors have been successfully used within the last few years in the treatment of various hematological malignancies, such as imatinib in patients with chronic myeloid leukemia. More recently, the Janus kinase (JAK) inhibitor tofacitinib has been evaluated as a potential new treatment option in RA and is awaiting approval. While an overview about JAK inhibition will be given elsewhere, other inhibitors such as spleen tyrosine kinase (Syk) inhibitor, mitogen-activated protein kinase (MAPK) inhibitor and Bruton's tyrosine kinase (Btk) inhibitor are currently in preclinical and clinical development.

Bruton tyrosine kinase inhibition is a novel therapeutic strategy targeting tumor in the bone marrow microenvironment in multiple myeloma

Bruton tyrosine kinase (Btk) has a well-defined role in B-cell development, whereas its expression in osteoclasts (OCs) further suggests a role in osteoclastogenesis. Here we investigated effects of PCI-32765, an oral and selective Btk inhibitor, on osteoclastogenesis as well as on multiple myeloma (MM) growth within the BM microenvironment. PCI-32765 blocked RANKL/M-CSF-induced phosphorylation of Btk and downstream PLC-γ2 in OCs, resulting in diminished TRAP5b (ED50 = 17 nM) and bone resorption activity. PCI-32765 also inhibited secretion of multiple cytokines and chemokines from OC and BM stromal cell cultures from both normal donors (ED50 = 0.5 nM) and MM patients. It decreased SDF-1-induced migration of MM cells, and down-regulated MIP1-α/CCL3 in MM cells. It also blocked MM cell growth and survival triggered by IL-6 or coculture with BM stromal cells or OCs in vitro. Importantly, PCI-32765 treatment significantly inhibits in vivo MM cell growth (P < .03) and MM cell-induced osteolysis of implanted human bone chips in SCID mice. Moreover, PCI-32765 prevents in vitro colony formation by stem-like cells from MM patients. Together, these results delineate functional sequelae of Btk activation mediating osteolysis and growth of MM cells, supporting evaluation of PCI-32765 as a novel therapeutic in MM.

Pathways for bone loss in inflammatory disease

Chronic inflammation including autoimmune disease is an important risk factor for the development of osteoporosis. Receptor activator of nuclear factor-κB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF) play a central role in osteoclast differentiation and function, and the molecular pathways by which M-CSF and RANKL induce osteoclast differentiation have been analyzed in detail. Proinflammatory cytokines directly or indirectly regulate osteoclastogenesis and bone resorption providing a link between inflammation and osteoporosis. Tumor necrosis factor-α, interleukin (IL)-1, IL-6, and IL-17 are the most important proinflammatory cytokines triggering inflammatory bone loss. Inhibition of these cytokines has provided potent therapeutic effects in the treatment of diseases such as rheumatoid arthritis. Further investigation is needed to understand the pathophysiology and to develop new strategies to treat inflammatory bone loss. This review summarizes new data on inflammatory bone loss obtained in 2011.

Tyrosine kinase inhibitors as potential drugs for B-cell lymphoid malignancies and autoimmune disorders

INTRODUCTION

In the last few years, several tyrosine kinase inhibitors (TKIs) have been synthesized and become available for preclinical studies and clinical trials. This article summarizes recent achievements in the mechanism of action, pharmacological properties, and clinical activity and toxicity, as well as the emerging role of TKIs in lymphoid malignancies, allergic diseases, and autoimmune disorders.

AREAS COVERED

A literature review was conducted of the MEDLINE database PubMed for articles in English. Publications from 2000 through January 2012 were scrutinized. The search terms used were Bruton's tyrosine kinase (Btk) inhibitors, PCI-32765, GDC-0834, LFM-A13, AVL-101, AVL-292, spleen tyrosine kinase (Syk) inhibitors, R343, R406, R112, R788, fostamatinib, BAY-61-3606, C-61, piceatannol, Lyn, imatinib, nilotinib, bafetinib, dasatinib, GDC-0834, PP2, SU6656 in conjunction with lymphoid malignancy, NHL, CLL, autoimmune disease, allergic disease, asthma, and rheumatoid arthritis. Conference proceedings from the previous 5 years of the American Society of Hematology, European Hematology Association, American Society of Clinical Oncology, and ACR/ARHP Annual Scientific Meetings were searched manually. Additional relevant publications were obtained by reviewing the references from the chosen articles.

EXPERT OPINION

The use of TKIs, especially inhibitors of Btk, Syk, and Lyn, is a promising new strategy for targeted treatment of B-cell lymphoid malignancies, autoimmune disorders and allergic diseases. However, definitive data from ongoing and future clinical trials will aid in better defining the status of TKIs in the treatment of these disorders.

PCI-32765: a novel Bruton's tyrosine kinase inhibitor for the treatment of lymphoid malignancies

INTRODUCTION

There has been a significant paradigm shift in the manner in which lymphoid malignancies are treated and managed. Treatment has been moving away from conventional chemotherapy and towards targeted therapy. The success of new classes of agents such as monoclonal antibodies, proteasome inhibitors and immunomodulatory derivatives has sparked further searches for novel pathways to inhibit. The Bruton's tyrosine kinase (Btk) pathway is a downstream mediator of the B-cell receptor (BCR) pathway, which is crucial in B-cell production and maintenance, and a potential therapeutic target.

AREAS COVERED

This review will summarize the current knowledge of the Btk pathway and its role in lymphoid malignancies. It will also discuss the present data about PCI-32765 in both the preclinical and clinical setting.

EXPERT OPINION

PCI-32765 is an oral irreversible Btk inhibitor with high potency and both preclinical and clinical activity in chronic lymphocytic leukemia (CLL) and non-Hodgkin's lymphoma (NHL). Phase I studies have demonstrated that it is well tolerated and has an excellent safety profile. Further studies are ongoing as a single agent and in combination with other targeted and conventional therapies. PCI-32765 is a very promising targeted therapy, and the data from these trials will ultimately decide its future role and success.

TEC and MAPK Kinase Signalling Pathways in T helper (TH) cell Development, TH2 Differentiation and Allergic Asthma

Significant advances in our understanding of the signalling events during T cell development and differentiation have been made in the past few decades. It is clear that ligation of the T cell receptor (TCR) triggers a series of proximal signalling cascades regulated by an array of protein kinases. These orchestrated and highly regulated series of events, with differential requirements of particular kinases, highlight the disparities between αβ+CD4+ T cells. Throughout this review we summarise both new and old studies, highlighting the role of Tec and MAPK in T cell development and differentiation with particular focus on T helper 2 (TH2) cells. Finally, as the allergy epidemic continues, we feature the role played by TH2 cells in the development of allergy and provide a brief update on promising kinase inhibitors that have been tested in vitro, in pre-clinical disease models in vivo and into clinical studies.

What is the best strategy for incorporating new agents into the current treatment of follicular lymphoma?

Although there is increasing knowledge about the pathobiology of follicular lymphoma (FL), the incorporation of new agents is challenged by the long clinical course and inherent heterogeneity of the disease. Furthermore, a longstanding concept in FL is that although most patients have an indolent initial phase of disease, this is typically followed by sequentially shorter remission durations and justifies the continued intense search for new rationally designed agents. Ideally, there would be personalized prognostic tools, preemptive target identification, and means to predict response in individual patients. Short of having these tools, one conceptual approach is to consider FL as a series of clinical disease states divided between treatment-naïve (low tumor burden and high tumor burden), relapsed (typically still chemoimmunotherapy-sensitive), and multiply relapsed (usually chemoimmunotherapy-resistant) disease. By applying what is known about the biology of FL along with the available agents, new treatment options can be better defined and tested within these clinical contexts. During the last few years, novel chemotherapeutics, biologic agents, monoclonal antibodies, antibody drug conjugates, and maintenance strategies are all either replacing or adding onto existing strategies. These new agents and approaches challenge the notion of inevitably shorter response durations, and offer hope of improved clinical outcomes compared with traditional sequential cytotoxic therapy.

More than just B-cell inhibition

Despite tremendous advances in the therapy of rheumatoid arthritis (RA), there remains interest in oral agents that may offer benefits that are similar to, or better than, those of biologic therapies. In their paper, Chang and colleagues demonstrate the effectiveness of a Bruton tyrosine kinase (Btk) inhibitor in two models of RA. Btk inhibition impacts several pathways affecting both B-cell and macrophage activation, making it a promising target in RA. However, other kinase inhibitors have failed to transition from animal models to human therapy, so it remains to be seen whether a Btk inhibitor will have a role in the RA treatment armamentarium.