Towards Selective Inhibitors of Janus Kinase 3: Identification of a Novel Structural Variation between Janus Kinases 2 and 3

Abstract

The mammalian Janus kinase (JAK) family of intracellular nonreceptor protein tyrosine kinases consists of four isoforms (JAK1, JAK2, JAK3 and TYK2) that play key roles in the cytokine-mediated JAK-STAT (signal transducer and activator of transcription) signaling pathway which controls survival, proliferation, and differentiation of a variety of cells.1,2 Due to this pivotal role in cytokine signaling, JAKs have emerged as potential therapeutic targets for cytokinerelated diseases. JAKs bind to different receptors. In particular, JAK3 interacts with cytokine receptors that contain the common gamma chain (γc).3 The γc-receptors are the most important chemokine receptors because they bind interleukins (IL) -2, -4, -7, -9, -15 and -21, which are critical for development and function of lymphocyte. As a result, mutation of the γc-JAK3 signaling results in severe combined immunodeficiency (SCID) phenotype.4,5 Likewise, humans or animals lacking JAK3 display severe combined immunodeficiency disease, indicating the crucial role of JAK3 in Tcell development and homeostasis of the immune system. 6Also noteworthy is that, unlike other JAKs experiencing significant side effects due to their ubiquitous expression, 7 JAK3 is specifically expressed in activated immune cells, which renders JAK3 as an even more interesting target for immune-regulation. Thus, development of a selective JAK3 inhibitor has been anticipated to provide a novel immunosuppressive agent with an optimal therapeutic window. Several synthetic JAK3 inhibitors identified by pharmaceutical companies are currently in the process of clinical evaluation. In particular, CP-690,550, a highly potent inhibitor against JAK3 (IC 50 = 1 nM), showed significant extension of life of the organ-transplanted animal 8,9 and demonstrated efficacy in phase III clinical trials for the treatment of rheumatoid arthritis 10,11 and rejection in kidney transplant patients. 12Nevertheless, the difficulty of developing CP-690,550 as an immunosuppressant is related with the low selectivity between JAK3 and JAK2 (IC 50 = 1 nM and 20 nM, respectively) 8 because concurrent inhibition of JAK2 would result in significant anemia particularly dangerous for patients under immunomodulating therapy.13 Several pharmaceutical companies such as Merck, 14 Aventis, 15 and Procter and Gamble 16have also reported potent JAK3 inhibitors but the selectivity between JAK3 and JAK2 has rarely been accomplished. Obviously, the lack of selectivity of CP-690,550 and other ATP-competitive inhibitors must be attributed to the shared sequence homology (62%) between JAK3 and JAK2, along with a very high homology in the ATP-binding domain. 17-19In this context, it is noteworthy that the recently published JAK3-selective inhibitor 20 include a unique phenyl-indolyl maleimide scaffold of which maleimide functionality is believed to play a key role in discriminating the ATP-binding site of JAK3 from that of JAK2. This result demonstrates that, in spite of high sequence homology between JAK3 and JAK2, it is still feasi...