Discovery of Selective Irreversible Inhibitors for Bruton’s Tyrosine Kinase

The importance of B cells in rheumatoid arthritis (RA) pathogenesis has been recently demonstrated in several clinical studies using the anti-CD20 antibody rituximab, which selectively depletes B cells. A recent phase III clinical trial led to the FDA approval of rituximab for a subset of RA patients. Bruton’s tyrosine kinase (Btk), a member of Tec family kinases, is a key component in the B-cell receptor signal pathway (BCR). Upon activation by upstream kinases (for example, Lyn and Syk), Btk phosphorylates and thereby activates phospholipase-Cg (PLCg), leading to several important downstream events including calcium ion transportation, NF-kB activation, and (auto)antibody generation. Previous biological studies (genetic loss of function and siRNA knockdown) strongly suggest that Btk is also a mediator of proinflammatory signals. Taken together, these studies indicate Btk may be a potential target for the treatment of RA. However, despite the previous discovery of LFM-A13 as a selective Btk inhibitor, there is no published study that has demonstrated that inhibition of Btk activity leads to in vivo efficacy in an animal model of rheumatoid arthritis. As ATP binding sites in kinases are highly conserved, it is a formidable task to develop selective ATP competitive kinase inhibitors. Among several approaches, the use of electrophilic inhibitors has been shown as a viable method to achieve selectivity. Considering the relative scarcity of knowledge on “chemical knockdown” of Btk activity, it is crucial to discover a potent and selective tool compound for this kinase. Herein, we describe the discovery of a selective, irreversible Btk inhibitor and its efficacy in a mouse RA model. An initial campaign to scan for scaffolds capable of inhibiting Btk’s kinase activity identified compound 1 as having