Recent Advances in the Discovery of Small Molecule Inhibitors of Interleukin-1 Receptor-Associated Kinase 4 (IRAK4) as a Therapeutic Target for Inflammation and Oncology Disorders

Small Molecule Mimetics of α-Helical Domain of IRAK2 Attenuate the Proinflammatory Effects of IL-33 in Asthma-like Mouse Models

IL-33 and its receptor ST2 play important roles in airway inflammation and contribute to asthma onset and exacerbation. The IL-33/ST2 signaling pathway recruits adapter protein myeloid differentiation primary response 88 (MyD88) to transduce intracellular signaling. MyD88 forms a complex with IL-R-associated kinases (IRAKs), IRAK4 and IRAK2, called the Myddosome (MyD88-IRAK4-IRAK2). The myddosome subsequently activates downstream NF-κB and MAPKs p38 and JNK. We established an asthma-like mouse model by intratracheal administration of IL-33. The IL-33 model has a very similar phenotype compared with the OVA-induced mouse asthma model. The importance of MyD88 in the IL-33/ST2 signaling transduction was demonstrated by the MyD88 knockout mice, which were protected from the IL-33-induced asthma. We synthesized small molecule mimetics of the α-helical domain of IRAK2 with drug-like characteristics based on the recent advances in the designing of α-helix compounds. The mimetics can competitively interfere in the protein-protein interaction between IRAK2 and IRAK4, leading to disruption of Myddosome formation. A series of small molecules were screened using an NF-κB promoter assay in vitro. The lead compound, 7004, was further studied in the IL-33-induced and OVA-induced asthma mouse models in vivo. Compound 7004 can inhibit the IL-33-induced NF-κB activity, disrupt Myddosome formation, and attenuate the proinflammatory effects in asthma-like models. Our data indicate that the Myddosome may represent a novel intracellular therapeutic target for diseases in which IL-33/ST2 plays important roles, such as asthma and other inflammatory diseases.

TLR4 Signaling Pathway Modulators as Potential Therapeutics in Inflammation and Sepsis

Toll-Like Receptor 4 (TLR4) signal pathway plays an important role in initiating the innate immune response and its activation by bacterial endotoxin is responsible for chronic and acute inflammatory disorders that are becoming more and more frequent in developed countries. Modulation of the TLR4 pathway is a potential strategy to specifically target these pathologies. Among the diseases caused by TLR4 abnormal activation by bacterial endotoxin, sepsis is the most dangerous one because it is a life-threatening acute system inflammatory condition that still lacks specific pharmacological treatment. Here, we review molecules at a preclinical or clinical phase of development, that are active in inhibiting the TLR4-MyD88 and TLR4-TRIF pathways in animal models. These are low-molecular weight compounds of natural and synthetic origin that can be considered leads for drug development. The results of in vivo studies in the sepsis model and the mechanisms of action of drug leads are presented and critically discussed, evidencing the differences in treatment results from rodents to humans.

Discovery and structure-based design of 4,6-diaminonicotinamides as potent and selective IRAK4 inhibitors

The identification of small molecule inhibitors of IRAK4 for the treatment of autoimmune diseases has been an area of intense research. We discovered novel 4,6-diaminonicotinamides which potently inhibit IRAK4. Optimization efforts were aided by X-ray crystal structures of inhibitors bound to IRAK4. Structure activity relationship (SAR) studies led to the identification of compound 29 which exhibited sub-micromolar potency in a LTA stimulated cellular assay.

Discovery of Clinical Candidate 1-{[(2S,3S,4S)-3-Ethyl-4-fluoro-5-oxopyrrolidin-2-yl]methoxy}-7-methoxyisoquinoline-6-carboxamide (PF-06650833), a Potent, Selective Inhibitor of Interleukin-1 Receptor Associated Kinase 4 (IRAK4), by Fragment-Based Drug Design

Through fragment-based drug design focused on engaging the active site of IRAK4 and leveraging three-dimensional topology in a ligand-efficient manner, a micromolar hit identified from a screen of a Pfizer fragment library was optimized to afford IRAK4 inhibitors with nanomolar potency in cellular assays. The medicinal chemistry effort featured the judicious placement of lipophilicity, informed by co-crystal structures with IRAK4 and optimization of ADME properties to deliver clinical candidate PF-06650833 (compound 40). This compound displays a 5-unit increase in lipophilic efficiency from the fragment hit, excellent kinase selectivity, and pharmacokinetic properties suitable for oral administration.

Kinases as Novel Therapeutic Targets in Asthma and Chronic Obstructive Pulmonary Disease

Multiple kinases play a critical role in orchestrating the chronic inflammation and structural changes in the respiratory tract of patients with asthma and chronic obstructive pulmonary disease (COPD). Kinases activate signaling pathways that lead to contraction of airway smooth muscle and release of inflammatory mediators (such as cytokines, chemokines, growth factors) as well as cell migration, activation, and proliferation. For this reason there has been great interest in the development of kinase inhibitors as anti-inflammatory therapies, particular where corticosteroids are less effective, as in severe asthma and COPD. However, it has proven difficult to develop selective kinase inhibitors that are both effective and safe after oral administration and this has led to a search for inhaled kinase inhibitors, which would reduce systemic exposure. Although many kinases have been implicated in inflammation and remodeling of airway disease, very few classes of drug have reached the stage of clinical studies in these diseases. The most promising drugs are p38 MAP kinases, isoenzyme-selective PI3-kinases, Janus-activated kinases, and Syk-kinases, and inhaled formulations of these drugs are now in development. There has also been interest in developing inhibitors that block more than one kinase, because these drugs may be more effective and with less risk of losing efficacy with time. No kinase inhibitors are yet on the market for the treatment of airway diseases, but as kinase inhibitors are improved from other therapeutic areas there is hope that these drugs may eventually prove useful in treating refractory asthma and COPD.

Small Molecule Inhibition of Interleukin-1 Receptor-Associated Kinase 4 (IRAK4)

In recent years, interleukin-1 receptor-associated kinase 4, IRAK4, has become an attractive target for many medicinal chemistry programmes. Target inhibition is of potential therapeutic value in areas including autoimmune disorders, cancer, inflammatory diseases, and possibly neurodegenerative diseases. Results from high-throughput screening efforts have led, in conjunction with structure-based drug design, to the identification of highly potent and selective small molecule IRAK4 inhibitors from many diverse chemical series. In vitro and in vivo studies with entities from distinct structural classes have helped elucidate the downstream pharmacological responses associated with IRAK4 inhibition as a proof of concept in disease models, leading to the recent initiation of human clinical trials. Within this review, we will highlight the considerable effort by numerous groups dedicated to the development of small molecule IRAK4 inhibitors for the treatment of human disease.

Lipopolysaccharide induced vascular smooth muscle cells proliferation: A new potential therapeutic target for proliferative vascular diseases

Vascular smooth muscle cells (VSMCs) proliferation is involved in vascular atherosclerosis and restenosis. Recent studies have demonstrated that lipopolysaccharide (LPS) promotes VSMCs proliferation, but the signalling pathways which are involved are not completely understood. The purpose of this review was to summarize the existing knowledge of the role and molecular mechanisms involved in controlling VSMCs proliferation stimulated by LPS and mediated by toll-like receptor 4 (TLR4) signalling pathways. Moreover, the potential inhibitors of TLR4 signalling for VSMCs proliferation in proliferative vascular diseases are discussed.

Review: biologically active pyrazole derivatives

Nitrogen-containing heterocyclic compounds and their derivatives have historically been invaluable as a source of therapeutic agents.

Recent Progress in the Molecular Recognition and Therapeutic Importance of Interleukin-1 Receptor-Associated Kinase 4

Toll-like receptors (TLRs) are the most upstream pattern recognition receptors in the cell, which detect pathogen associated molecular patterns and initiate signal transduction, culminating in the transcription of pro-inflammatory cytokines and antiviral interferon. Interleukin-1 receptor-associated kinase 4 (IRAK4) is a key mediator in TLR (except for TLR3) and interleukin-1 receptor signaling pathways. The loss of kinase function of IRAK4 is associated with increased susceptibility to various pathogens, while its over-activation causes autoimmune diseases such as rheumatoid arthritis, systemic lupus erythematosus, and cancer. The therapeutic importance of this master kinase has been advocated by a number of recent preclinical studies, where potent inhibitors have been administered to improve various TLR-mediated pathologies. Increasing studies of X-ray crystallographic structures with bound inhibitors have improved our knowledge on the molecular recognition of ligands by IRAK4, which will be crucial for the development of new inhibitors with improved potencies. In this review, we briefly discuss the structural aspect of ligand recognition by IRAK4 and highlight its therapeutic importance in the context of TLR-associated unmet medical needs.

Efforts towards the optimization of a bi-aryl class of potent IRAK4 inhibitors

IRAK4 has been identified as potential therapeutic target for inflammatory and autoimmune diseases. Herein we report the identification and initial SAR studies of a new class of pyrazole containing IRAK4 inhibitors designed to expand chemical diversity and improve off target activity of a previously identified series. These compounds maintain potent IRAK4 activity and desirable ligand efficiency. Rat clearance and a variety of off target activities were also examined, resulting in encouraging data with tractable SAR.

Selective interleukin-1 receptor-associated kinase 4 inhibitors for the treatment of autoimmune disorders and lymphoid malignancy

Kelly et al. report the development of two highly selective and bioavailable small molecule IRAK4 inhibitors and show for the first time their therapeutic efficacy in autoimmune disorders and in a specific subset of diffuse large B cell lymphomas in mice.

Pathological activation of the Toll-like receptor signaling adaptor protein MYD88 underlies many autoimmune and inflammatory disease states. In the activated B cell–like (ABC) subtype of diffuse large B cell lymphoma (DLBCL), the oncogenic MYD88 L265P mutation occurs in 29% of cases, making it the most prevalent activating mutation in this malignancy. IRAK4 kinase accounts for almost all of the biological functions of MYD88, highlighting IRAK4 as a therapeutic target for diseases driven by aberrant MYD88 signaling. Using innovative structure-based drug design methodologies, we report the development of highly selective and bioavailable small molecule IRAK4 inhibitors, ND-2158 and ND-2110. These small molecules suppressed LPS-induced TNF production, alleviated collagen-induced arthritis, and blocked gout formation in mouse models. IRAK4 inhibition promoted killing of ABC DLBCL lines harboring MYD88 L265P, by down-modulating survival signals, including NF-κB and autocrine IL-6/IL-10 engagement of the JAK–STAT3 pathway. In ABC DLBCL xenograft models, IRAK4 inhibition suppressed tumor growth as a single agent, and in combination with the Bruton’s tyrosine kinase (BTK) inhibitor ibrutinib or the Bcl-2 inhibitor ABT-199. Our findings support pharmacological inhibition of IRAK4 as a therapeutic strategy in autoimmune disorders, in a genetically defined population of ABC DLBCL, and possibly other malignancies dependent on aberrant MYD88 signaling.

Inhibiting the Inflammasome: A Chemical Perspective

Inflammasomes are high molecular weight complexes that sense and react to injury and infection. Their activation induces caspase-1 activation and release of interleukin-1β, a pro-inflammatory cytokine involved in both acute and chronic inflammatory responses. There is increasing evidence that inflammasomes, particularly the NLRP3 inflammasome, act as guardians against noninfectious material. Inappropriate activation of the NLRP3 inflammasome contributes to the progression of many noncommunicable diseases such as gout, type II diabetes, and Alzheimer's disease. Inhibiting the inflammasome may significantly reduce damaging inflammation and is therefore regarded as a therapeutic target. Currently approved inhibitors of interleukin-1β are rilonacept, canakinumab, and anakinra. However, these proteins do not possess ideal pharmacokinetic properties and are unlikely to easily cross the blood-brain barrier. Because inflammation can contribute to neurological disorders, this review focuses on the development of small-molecule inhibitors of the NLRP3 inflammasome.

Identification of N-(1H-pyrazol-4-yl)carboxamide inhibitors of interleukin-1 receptor associated kinase 4: Bicyclic core modifications

IRAK4 plays a critical role in the IL-1R and TLR signalling, and selective inhibition of the kinase activity of the protein represents an attractive target for the treatment of inflammatory diseases. A series of permeable N-(1H-pyrazol-4-yl)carboxamides was developed by introducing lipophilic bicyclic cores in place of the polar pyrazolopyrimidine core of 5-amino-N-(1H-pyrazol-4-yl)pyrazolo[1,5-a]pyrimidine-3-carboxamides. Replacement of the pyrazolo[1,5-a]pyrimidine core with the pyrrolo[2,1-f][1,2,4]triazine, the pyrrolo[1,2-b]pyridazine, and thieno[2,3-b]pyrazine cores guided by cLogD led to the identification of highly permeable IRAK4 inhibitors with excellent potency and kinase selectivity.

Translating nucleic acid-sensing pathways into therapies

Nucleic acid sensing by innate receptors initiates immune defences against viruses and other pathogens. A hallmark of this response is the release of interferons (IFNs), which promote protective immunity by inducing IFN-stimulated genes (ISGs). A similar ISG signature is found in autoinflammatory and autoimmune conditions, indicating that chronic activation of nucleic acid-sensing pathways may contribute to these diseases. Here, we review how nucleic acid-sensing pathways are currently being targeted pharmacologically with both agonists and antagonists. We discuss how an improved understanding of the biology of these pathways is leading to novel therapies for infections, cancer, and autoimmune and autoinflammatory disorders, and how new therapeutics will, in turn, generate a deeper understanding of these complex diseases.

Discovery and Structure Enabled Synthesis of 2,6-Diaminopyrimidin-4-one IRAK4 Inhibitors

We report the identification and synthesis of a series of aminopyrimidin-4-one IRAK4 inhibitors. Through high throughput screening, an aminopyrimidine hit was identified and modified via structure enabled design to generate a new, potent, and kinase selective pyrimidin-4-one chemotype. This chemotype is exemplified by compound 16, which has potent IRAK4 inhibition activity (IC50 = 27 nM) and excellent kinase selectivity (>100-fold against 99% of 111 tested kinases), and compound 31, which displays potent IRAK4 activity (IC50 = 93 nM) and good rat bioavailability (F = 42%).

Deconstructing innate immune signaling in myelodysplastic syndromes

Overexpression of immune-related genes is widely reported in myelodysplastic syndromes (MDSs), and chronic immune stimulation increases the risk for developing MDS. Aberrant innate immune activation, such as that caused by increased toll-like receptor (TLR) signaling, in MDS can contribute to systemic effects on hematopoiesis, in addition to cell-intrinsic defects on hematopoietic stem/progenitor cell (HSPC) function. This review will deconstruct aberrant function of TLR signaling mediators within MDS HSPCs that may contribute to cell-intrinsic consequences on hematopoiesis and disease pathogenesis. We will discuss the contribution of chronic TLR signaling to the pathogenesis of MDS based on evidence from patients and mouse genetic models.

Discovery of 5-Amino-N-(1H-pyrazol-4-yl)pyrazolo[1,5-a]pyrimidine-3-carboxamide Inhibitors of IRAK4

Interleukin-1 receptor associated kinase 4 (IRAK4) is an essential signal transducer downstream of the IL-1R and TLR superfamily, and selective inhibition of the kinase activity of the protein represents an attractive target for the treatment of inflammatory diseases. A series of 5-amino-N-(1H-pyrazol-4-yl)pyrazolo[1,5-a]pyrimidine-3-carboxamides was developed via sequential modifications to the 5-position of the pyrazolopyrimidine ring and the 3-position of the pyrazole ring. Replacement of substituents responsible for poor permeability and improvement of physical properties guided by cLogD led to the identification of IRAK4 inhibitors with excellent potency, kinase selectivity, and pharmacokinetic properties suitable for oral dosing.

Potent and Selective Amidopyrazole Inhibitors of IRAK4 That Are Efficacious in a Rodent Model of Inflammation

IRAK4 is a critical upstream kinase in the IL-1R/TLR signaling pathway. Inhibition of IRAK4 is hypothesized to be beneficial in the treatment of autoimmune related disorders. A screening campaign identified a pyrazole class of IRAK4 inhibitors that were determined by X-ray crystallography to exhibit an unusual binding mode. SAR efforts focused on the identification of a potent and selective inhibitor with good aqueous solubility and rodent pharmacokinetics. Pyrazole C-3 piperidines were well tolerated, with N-sulfonyl analogues generally having good rodent oral exposure but poor solubility. N-Alkyl piperidines exhibited excellent solubility and reduced exposure. Pyrazoles possessing N-1 pyridine and fluorophenyl substituents were among the most active. Piperazine 32 was a potent enzyme inhibitor with good cellular activity. Compound 32 reduced the in vivo production of proinflammatory cytokines and was orally efficacious in a mouse antibody induced arthritis disease model of inflammation.

Initial optimization and series evolution of diaminopyrimidine inhibitors of interleukin-1 receptor associated kinase 4

IRAK4 plays a key role in TLR/IL-1 signaling. Previous efforts identified a series of aminopyrimidine IRAK4 inhibitors that possess good potency, but modest kinase selectivity. Exploration of substituents at the C-2 and C-5 positions generated compounds that maintained IRAK4 potency and improved kinase selectivity. Additionally, it was found that the pyrimidine core could be replaced with a pyridine and still retain potency and kinase selectivity. The optimization efforts led to compound 26 which had an IRAK4 IC50 of 0.7 nM, an IC50 of 55 nM on THP-1 cells stimulated with LPS, a TLR4 agonist, and greater than 100-fold selectivity versus 96% of a panel of 306 kinases.

Toll-like receptors: Activation, signalling and transcriptional modulation

Families of innate immune receptors serve as the bodies primary defence system by recognising and rapidly responding to infection by microorganisms or to endogenous danger signals and initiating inflammatory processes. Whilst Toll-like receptors (TLRs) were the first family to be discovered, important and exciting discoveries continue to emerge into the molecular mechanisms that control their activation and regulation. Herein, I will provide an overview of TLR activation and their downstream signalling cascades, and discuss some of the recent findings concerning the assembly of a TLR oligomeric signalling platform, known as the Myddosome. Further, a brief examination of the importance of crosstalk between multiple TLRs or between TLRs and other innate immune receptors for appropriate and coordinated immune responses will be presented. Finally, I will discuss the importance of mechanisms that regulate TLRs with a focus on the role of activating transcription factor 3 (ATF3) in modulating transcriptional responses downstream of TLRs.

Discovery and hit-to-lead optimization of 2,6-diaminopyrimidine inhibitors of interleukin-1 receptor-associated kinase 4

Interleukin receptor-associated kinase 4 (IRAK4) is a critical element of the Toll-like/interleukin-1 receptor inflammation signaling pathway. A screening campaign identified a novel diaminopyrimidine hit that exhibits weak IRAK4 inhibitory activity and a ligand efficiency of 0.25. Hit-to-lead activities were conducted through independent SAR studies of each of the four pyrimidine substituents. Optimal activity was observed upon removal of the pyrimidine C-4 chloro substituent. The intact C-6 carboribose is required for IRAK4 inhibition. Numerous heteroaryls were tolerated at the C-5 position, with azabenzothiazoles conferring the best activities. Aminoheteroaryls were preferred at the C-2 position. These studies led to the discovery of inhibitors 35, 36, and 38 that exhibit nanomolar inhibition of IRAK4, improved ligand efficiencies, and modest kinase selectivities.