Current Topics in Medicinal Chemistry - Volume 9, Issue 8, 2009

A variety of chronic inflammatory and immune-mediated diseases cause undue patient pain and suffering, and represent significant areas of unmet medical need. Rheumatoid arthritis is a chronic systemic inflammatory disorder which affects almost one percent of the world's population, and its long-term treatment is underserved by existing disease-modifying antirheumatic drugs. Chronic obstructive pulmonary disease, characterized by the dyad of chronic bronchitis and emphysema, is treated symptomatically with bronchodilators as well as corticosteroids. Asthma is a chronic, prevalent, respiratory condition manifesting bronchospasm with an element of underlying inflammation. Like COPD, there are a paucity of disease-modifying treatments for asthma. Multiple sclerosis is an autoimmune disease that attacks the nervous system, resulting in demyelination, and is frequently progressive and debilitating. For these conditions, symptomatic relief and disease modification with biologic agents are the current standard of care, but inhibition of a number of different kinases offers the opportunity to intervene in inflammatory and immune-mediated disease in a new way. Almost all aspects of cellular function are controlled, at least in part, by protein phosphorylation. Approximately 518 tyrosine and serine/threonine kinases are known to be encoded by the human genome, and about a third of proteins contain covalently bound phosphate. As key components of signaling pathways regulating immunoreceptors, kinases serve to control downstream targets with respect to proliferation, differentiation and survival of immune cells, both within the innate and adaptive immune systems. Kinase drug discovery has witnessed remarkable advances over the last decade. Inhibitors of thirty kinase targets have progressed as new molecular entities into Phase 1 clinical trials, while several kinase inhibitors have gained approval as drugs. Although the majority of kinase inhibitors that have reached the clinic are targeted toward the treatment of cancer, inhibitors for the dysregulated kinases implicated in many immune-mediated and inflammatory disorders are advancing as well. Small molecule kinase inhibitors may have inherent advantages over the biologic agents currently used to treat rheumatoid arthritis and multiple sclerosis, with respect to cost and ease of administration. Fears of off-target activity of ATP-competitive kinase inhibitors plagued early programs, and while some chemical scaffolds have been found to be relatively promiscuous, several factors have enabled selective compounds to be identified. Structural knowledge, largely in the form of x-ray cocrystal structures which have been obtained with many kinases, as well as the commercial availability of enzymatic or binding assays for over four hundred separate kinases, have enhanced the ability of drug discovery researchers to develop very selective drug candidates.

Activation of the extracellular signal-regulated kinase (ERK) signaling pathway has been implicated in mediating a diverse array of cellular functions including cell differentiation, proliferation, and inflammatory responses. In this review, we will discuss approaches to identify inhibitors of ERK proteins through targeting ATP-dependent and ATPindependent mechanisms. Given the diversity of ERK substrates and the importance of ERK signaling in normal cell functions, emphasis will be placed on the methods for identifying small molecular weight compounds that are substrate selective through ATP-independent interactions and potentially relevant to inflammatory processes. The approach for selective targeting of ERK substrates takes advantage of the basic understanding of unique ERK docking domains that are thought to interact with specific amino acid sequences on substrate proteins. Computer aided drug design (CADD) can facilitate the high throughput screening of millions of compounds with the potential for selective interactions with ERK docking domains and disruption of substrate interactions. As such, the CADD approach significantly reduces the number of compounds that will be evaluated in subsequent biological assays and greatly increases the hit rate of biologically active compounds. The potentially active compounds are evaluated for ERK protein binding using spectroscopic and structural biology methods. Compounds that show ERK interactions are then tested for their ability to inhibit substrate interactions and phosphorylation as well as ERK-dependent functions in whole organism or cell-based assays. Finally, the relevance of substrate-selective ERK inhibitors in the context of inflammatory disease will be discussed.

Interleukin-2-inducible T cell kinase (ITK) is a non-receptor tyrosine kinase expressed in T cells, NKT cells and mast cells which plays a crucial role in regulating the T cell receptor (TCR), CD28, CD2, chemokine receptor CXCR4, and FcηR-mediated signaling pathways. In T cells, ITK is an important mediator for actin reorganization, activation of PLCγ, mobilization of calcium, and activation of the NFAT transcription factor. ITK plays an important role in the secretion of IL-2, but more critically, also has a pivotal role in the secretion of Th2 cytokines, IL-4, IL-5 and IL-13. As such, ITK has been shown to regulate the development of effective Th2 response during allergic asthma as well as infections by parasitic worms. This ability of ITK to regulate Th2 responses, along with its pattern of expression, has led to the proposal that it would represent an excellent target for Th2-mediated inflammation. We discuss here the possibilities and pitfalls of targeting ITK for inflammatory disorders.

Inhibitors of Rho kinase (ROCK) have many potential therapeutic applications. The biological focus of this overview will be on the role of ROCK in inflammatory disorders such as multiple sclerosis and asthma. Many ROCK inhibitors have been described in the primary and patent literature, and a diverse range of chemical scaffolds is beginning to emerge. Some of the structural classes that will be highlighted in this review include: isoquinolines, indazoles, aminofurazans, dihydropyrimidines, aminopyrimidines, benzodioxane carboxamides, benzimidazoles, and chroman amides.

Interleukin-1 receptor-associated kinases (IRAKs) are key components in the signal transduction pathways utilized by interleukin-1 receptor (IL-1R), interleukin-18 receptor (IL-18R), and Toll-like receptors (TLRs). Out of four members in the mammalian IRAK family, IRAK-4 is considered to be the “master IRAK”, the only family member indispensable for IL-1R/TLR signaling. In humans, mutations resulting in IRAK-4 deficiency have been linked to susceptibility to bacterial infections, especially recurrent pyogenic bacterial infections. Furthermore, knock-in experiments by several groups have clearly demonstrated that IRAK-4 requires its kinase activity for its function. Given the critical role of IRAK-4 in inflammatory processes, modulation of IRAK-4 kinase activity presents an attractive therapeutic approach for the treatment of immune and inflammatory diseases. The recent success in the determination of the 3-dimensional structure of the IRAK-4 kinase domain in complex with inhibitors has facilitated the understanding of the mechanistic role of IRAK-4 in immunity and inflammation as well as the development of specific IRAK-4 kinase inhibitors. In this article, we review the biological function of IRAK-4, the structural characteristics of the kinase domain, and the development of small molecule inhibitors targeting the kinase activity. We also review the key pharmacophores required for several classes of inhibitors as well as important features for optimal protein/inhibitor interactions. Lastly, we summarize how these insights can be translated into strategies to develop potent IRAK-4 inhibitors with desired properties as new anti-inflammatory therapeutic agents.

In recent years, pharmaceutical companies have increasingly focused on phosphoinositide 3-kinases delta (PI3Kδ) and gamma (PI3Kγ) as therapeutic targets for the treatment of inflammatory and autoimmune diseases. All class 1 PI3-kinases (α/β/γ/δ) generate phospholipid second messengers that help govern cellular processes such as migration, proliferation, and apoptosis. PI3K δ/γ lipid kinases are mainly restricted to the hematopoetic system whereas PI3Kα/β are ubiquitously expressed, thus raising potential toxicity concerns for chronic indications such as asthma and rheumatoid arthritis. Therefore, the challenge in developing a small molecule inhibitor of PI3K is to define and attain the appropriate isoform selectivity profile. Significant advances in the design of such compounds have been achieved by utilizing x-ray crystal structures of various inhibitors bound to PI3Kγ in conjunction with pharmacophore modeling and high-throughput screening. Herein, we review the history and challenges involved with the discovery of small molecule isoform-specific PI3K inhibitors. Recent progress in the design of selective PI3Kδ, PI3Kγ, and PI3Kδ/γ dual inhibitors will be presented.