Current Topics in Medicinal Chemistry - Volume 5, Issue 10, 2005

The last several years have seen an explosion of interest in developing inhibitors to kinases found critical to the pathogenesis of disease. Improvements in screening techniques, surveying kinase selectivity and understanding the side-effects of kinase inhibition have made targeting kinases for therapeutic intervention more attainable. Development of p38α MAP kinase inhibitors for the treatment of inflammatory disease has historically been elusive for several reasons. Different structural classes appeared to have different toxicity profiles both in animal studies and in the clinic. Furthermore, several of the compounds with reported data were determined not to be highly selective for p38α further compounding the issue of whether the toxicities observed were p38α related. Recent advances in our understanding of p38-related mechanisms as well as significant improvements in the ability to screen compounds for kinase selectivity suggest there may be opportunities to overcome these issues. To fully explore recent advances in the p38 field, Current Topics in Medicinal Chemistry has devoted this entire issue to a review of the current chemistry and biology of p38 inhibition. The initial article authored by Gary Schieven from Bristol-Myers Squibb provides a biological background for why p38α kinase activation leads to inflammation at the cellular level and the mechanisms by which p38α regulates gene expression. Donna Dambach from Bristol-Myers Squibb describes potential adverse effects of p38 inhibition as well as what is known though KO mouse technology and published disclosures from clinical trials. Ioana Papa-Burke and her colleagues from Amphora describe the use of microfluidic technology to rapidly screen compounds against a collection of more than 60 kinases enabling a very rapid and accurate identification of multiple scaffolds. David Diller and his colleagues from Pharmacopeia further describe advances in selectivity screening for kinases. John Hynes and K. Leftheris from Bristol-Myers Squibb provide a survey of the p38 kinase inhibitor patent and published literature over the last several years and describe new chemotypes that have been disclosed. Ravi Natarajan and James Doherty from Merck describe the development of novel quinazolinone and associated chemotypes as p38 inhibitors. Steve Wrobleski and Arthur Doweyko from Bristol-Myers Squibb co-authored a review of known X-ray structures of p38 inhibitor-enzyme complexes. They describe the current understanding of structural elements and interactions that may play a role in how p38 inhibitors bind to the enzyme. Finally, David Goldstein and Tobias Gabriel from Roche review ten chemotypes that were selected for development and describe data obtained from clinical trials. With a number of companies in clinical trials with p38 inhibitors, it should become clear in the near future whether this attractive target for suppressing inflammation can lead to a marketed drug for p38- mediated diseases. I would like to express my gratitude the following individuals who served as manuscript reviewers for this issue: J. Das, M. Dhar, A. Dyckman and S. Wrobleski. Finally, I am grateful to all the authors for their timely and well written contributions in what we hope will be an informative contribution to the field.

The p38 kinase plays a central role in inflammation, and it has been the subject of extensive efforts in both basic research and drug discovery. This review summarizes the biology of the p38 kinase with a focus on its role in inflammation. The p38 kinase regulates the production of key inflammatory mediators, including TNFα, IL-1β, and COX- 2. In addition, p38 also acts downstream of cytokines such as TNFα, mediating some of their effects. The potential efficacy of p38 inhibitors may thus be greater than would be expected from the inhibition of the mediators alone. Inhibitors of p38 kinase are currently in development for the treatment of rheumatoid arthritis. The biological processes regulated by p38 kinase suggest a wide variety of additional potential indications.

Inhibitors of p38 MAP kinases show promise for the treatment of inflammatory and immunological disorders and some cancers. There is a substantial body of experimental evidence across several organ systems suggesting that p38 also mediates developmental, differentiation and proliferation processes. As a consequence of the wide-ranging regulatory role of p38 kinase in diverse cellular processes, the possibility of adverse events resulting from undesired pharmacological activity is a major concern for the p38 inhibitor drug class. Taking into consideration the limitations of experimental modeling systems, together the data may indicate that profound inhibition of p38 has the potential to impact these processes during fetal or neonatal development. The difficulty comes in extrapolating these findings to predict potential adverse effects under conditions of partial inhibition of p38 activity, and in an adult population in which these processes are typically only recapitulated during repair or adaptive responses. As such, the goal of this review of the targets of p38 activity is to bring an awareness of the those organ systems that should be monitored for potential toxicity, as well as to present a potential mechanistic basis for such monitoring or for investigation of adverse effects that may develop with administration of a p38 inhibitor.

The p38 MAP kinases are a family of serine/threonine protein kinases that play a key role in cellular pathways leading to pro-inflammatory responses. We have developed and implemented a method for rapidly identifying and optimizing potent and selective p38α inhibitors, which is amenable to other targets and target classes. A diverse library of druggable, purified and quantitated molecules was assembled and standardized enzymatic assays were performed in a microfluidic format that provided very accurate and precise inhibition data allowing for development of SAR directly from the primary HTS. All compounds were screened against a collection of more than 60 enzymes (kinases, proteases and phosphatases), allowing for removal of promiscuous and non-selective inhibitors very early in the discovery process. Follow-up enzymological studies included measurement of concentration of compound in buffer, yielding accurate determination of Ki and IC50 values, as well as mechanism of action. In addition, active compounds were screened against less desirable properties such as inhibition of the enzyme activity by aggregation, irreversible binding, and timedependence. Screening of an 88,634-compound library through the above-described process led to the rapid identification of multiple scaffolds (>5 active compounds per scaffold) of potential drug leads for p38α that are highly selective against all other enzymes tested, including the three other p38 isoforms. Potency and selectivity data allowed prioritization of the identified scaffolds for optimization. Herein we present results around our 3-thio-1,2,4-triazole lead series of p38α selective inhibitors, including identification, SAR, synthesis, selectivity profile, enzymatic and cellular data in their progression towards drug candidates.

In the late 1970s and the early 1980s the initial p38 chemotype, the triaryl imidazoles, was discovered as an off-target effect during the development of cyclooxygenase and 5-lipoxygenase inhibitors long before the identity of the p38 kinase was known. During the last 10 years a number of novel p38 chemotypes were discovered via high throughput screening. More recently, the first series of p38 inhibitors discovered by xray crystallographic and virtual screening was announced. Finally, throughout the life span of p38 drug discovery programs significant medicinal chemistry effort has continually been placed on the design of new inhibitors from known chemotypes using molecular modeling, protein crystallography, hybrid design and simply sound intuition. Indeed, the search for p38 kinase inhibitors offers an excellent historical perspective as to how technological changes that have taken place in the pharmaceutical industry over the last decade, have affected the ways in which new leads are discovered and advanced. It is the intent of this review to highlight the discoveries of novel p38 chemotypes, emphasizing where possible the key technologies used in the discoveries and the knowledge gained from each discovery.

The discovery and development of selective, efficacious, and safe small molecule p38 mitogen-activated protein kinase inhibitors for the treatment of inflammatory diseases remains the focus of many pharmaceutical research programs. Advances in small molecule p38 inhibitor design in potency and oral efficacy have been accelerated with the large number of available inhibitor-enzyme x-ray structures. These advances have allowed for the discovery of diverse sets of inhibitors with the opportunity to map inhibitor interactions and design selective inhibitors. This review covers recent compound disclosures in the patent and published literature over the last three years. Many disclosures represent new chemotypes as well as creative modifications of known structures.

The initial disclosure of tri-substituted imidazole-based drug molecules such as 1 for inhibition of p38 MAP kinase by SmithKline Beecham (SB) sparked an effort in this area at Merck and other pharmaceutical research establishments. Although analogs in this class have shown good inhibitory properties against p38 MAP kinase, their selectivity profile were modest and left much room for improvement. Attempts to discover newer compounds with improved selectivity over the prototypical SB compound 203580 (1), led to the discovery of a new sub-class of p38 inhibitors typified by compound 18 at Merck. Although this benchmark compound was potent, highly selective and orally efficacious it was burdened with compound related adverse effects in dogs that has delayed further development. In 1999, a new class of p38 inhibitors represented by clinical candidate VX-745 (26), was disclosed by Vertex Pharmaceuticals. This compound displayed unprecedented selectivity due to its unique mode of binding to the active site in p38 MAP kinase. Inspired by the exquisite selectivity profile of VX-745 [26] a scaffold re-design was initiated at Merck which resulted in the discovery of the quinazolinone, pyrimido-pyrimidone, pyrido-pyrimidone, quinolinone and naphthyridinone based p38 inhibitors.

Small molecule inhibition of protein kinases in the treatment of significant diseases such as cancer, Alzheimer's disease, diabetes, and rheumatoid arthritis has attracted significant attention over the past two decades and has clearly become one of the most significant challenges for drug discovery in the 21st century. While the recent identification of 518 different kinases in the human genome has offered a wealth of opportunities for drug intervention in the treatment of these diseases, it has also created a daunting challenge with respect to selective kinase inhibition as a viable strategy in target-based drug design. Over the past decade, the design and development of a small molecule that selectively inhibits the p38 mitogen activated protein (MAP) kinase has clearly emerged as one of these challenges within the industry. This review will focus on the comparison of the x-ray crystal structures and binding models of the most recent p38 inhibitor-enzyme complexes and the identification of the structural elements and interactions that may be important in providing inhibitor potency and selectivity toward the p38 MAP kinase.

p38 mitogen activated protein (MAP) kinase remains the most compelling therapeutic target for oral drug intervention for a wide range of autoimmune disorders based on the central role this enzyme plays in inflammatory cell signaling. Efforts to discover inhibitors of p38 suitable for clinical investigation have continued to escalate in part due to the incredible diversity of unique chemotypes reported to inhibit the enzyme. Since 1993, at least seventeen p38 inhibitors have been reported to have entered into clinical trials. Next generation inhibitors have been disclosed with improved potency for p38 and enhanced selectivity versus other protein kinases. Over the last three years, there have been multiple reports of cytokine suppression in humans following oral administration of p38 inhibitors. These results, in addition to proof of concept studies in rheumatoid patients, have established p38 inhibition as an avenue for the future management of pro-inflammatory cytokine based diseases. This review describes the discovery at Roche of novel p38 inhibitors which have advanced into clinical trials. The pharmacology of the Roche compounds is then compared with eight chemically distinct p38 inhibitors known to have entered clinical development.