Yan Lou

Selective p38α Inhibitors Clinically Evaluated for the Treatment of Chronic Inflammatory Disorders

p38R is a member of the well characterized mitogen activatedprotein (MAP) kinase familyof serine/threonineprotein kinases. p38R is widely expressed in endothelial, immune, and inflammatory cells and plays a central role in the regulation of proinflammatory cytokine production including TNF-R, IL-1β, and IL-6. Selective blockade of any one of these cytokineswith biologic agents has proven efficacious for inflammatory diseases including rheumatoid arthritis (RA), psoriasis, and inflammatory bowel disease. The p38 subfamily of MAP kinases includes four isoforms (p38R, p38β, p38γ, and p38δ) that are encoded by separate genes. Analysis of differential tissue expression and activation of these isoforms in synovial tissue extracted fromRApatients has suggested that the p38R isoform is overactivated within inflamed tissue andmay be a preferred target for intervention in the disease. This expectation has prompted a huge investment by thepharmaceutical industry in thedevelopment of p38R inhibitors as potential oral disease modifying antirheumatic drugs (DMARDs). This Perspective will summarize key learnings from over 15 years of industrial experience with p38R as a drug target, with a focus on the rational design of highly selective small molecule inhibitors, followed by a discussion of data for examples 1-11 that have entered into or are recruiting for phase 2 clinical studies (summarized in Table 1). To date, these results have been disappointing.We conclude that p38R inhibition alone is unlikely to be a successful strategy toward treating chronic inflammatory disorders. Others have also concluded that “the era of optimism surrounding the use of p38 MAPK inhibition for the treatment of RA is over”.

Bruton’s Tyrosine Kinase Inhibitors: Approaches to Potent and Selective Inhibition, Preclinical and Clinical Evaluation for Inflammatory Diseases and B Cell Malignancies

■ INTRODUCTION Bruton’s tyrosine kinase (BTK) is a member of the Tec tyrosine kinase family. BTK is expressed in most hematopoietic cells such as B cells, mast cells, and macrophages but not in T cells, natural killer cells, and plasma cells. BTK plays key roles in multiple cell signaling pathways including BCR and FcR signaling cascades. Mutations in the human BTK gene cause the inherited disease X-linked agammaglobulinemia (XLA), with lack of peripheral B cells and low levels of serum Ig. In XLA, the primary immune deficit is B cell specific. In fact Rituxan, a CD20 antibody, has impacted B cells on the pathogenesis of many autoimmune diseases, such as RA, SLE, and MS. This has fueled interest by multiple pharmaceutical companies in pursuing small molecule BTK inhibitors in the treatment of autoimmune diseases. Likewise, there is also interest in the development of BTK inhibitors for the treatment of hematological malignancies, as aberrant activating BTK has been implicated in the pathogenesis of B cell lymphoma. Detailed reviews and articles on BTK biology and its therapeutic potentials have been reported. Kinase selectivity is a central issue in discovering efficacious and safe small molecule inhibitors for kinase targets, especially for non-life-threatening diseases such as RA. To prevent adverse toxicological events caused by immunological responses, the pharmaceutical industry has largely focused its small molecule drug discovery efforts on agents that interact noncovalently with their target proteins. This strategy has been followed despite numerous examples of marketed drugs with target-specific covalent mode-of-action. In the case of protein kinase targets for which selectivity and efficacy pose major challenges for noncovalent inhibitors, targeted covalent inhibition has provided an attractive alternative. Both approaches, noncovalent and covalent inhibition of protein kinases, benefit tremendously from high resolution structural information from protein crystal structures. This article will review BTK structural biology with a focus on design features for selective BTK inhibitors. We first summarize the publically available structural information on the BTK kinase domain. Then we will provide a brief summary and analysis with key SAR information for the most potent inhibitors reported for the chemical classes that have been disclosed in patents and publications. When available, preclinical and clinical data for advanced compounds will be summarized.

Finding the perfect spot for fluorine: improving potency up to 40-fold during a rational fluorine scan of a Bruton's Tyrosine Kinase (BTK) inhibitor scaffold.

A rational fluorine scan based on co-crystal structures was explored to increase the potency of a series of selective BTK inhibitors. While fluorine substitution on a saturated bicyclic ring system yields no apparent benefit, the same operation on an unsaturated bicyclic ring can increase HWB activity by up to 40-fold. Comparison of co-crystal structures of parent molecules and fluorinated counterparts revealed the importance of placing fluorine at the optimal position to achieve favorable interactions with protein side chains.

Mitigation of reactive metabolite formation for a series of 3-amino-2-pyridone inhibitors of Bruton’s tyrosine kinase (BTK)

Reactive metabolites have been putatively linked to many adverse drug reactions including idiosyncratic toxicities for a number of drugs with black box warnings or withdrawn from the market. Therefore, it is desirable to minimize the risk of reactive metabolite formation for lead molecules in optimization, in particular for non-life threatening chronic disease, to maximize benefit to risk ratio. This article describes our effort in addressing reactive metabolite issues for a series of 3-amino-2-pyridone inhibitors of BTK, e.g. compound 1 has a value of 459pmol/mg protein in the microsomal covalent binding assay. Parallel approaches were taken to successfully resolve the issues: establishment of a predictive screening assay with correlation association of covalent binding assay, identification of the origin of reactive metabolite formation using MS/MS analysis of HLM as well as isolation and characterization of GSH adducts. This ultimately led to the discovery of compound 7 (RN941) with significantly reduced covalent binding of 26pmol/mg protein.