Ac cordingly, further studies that are primarily designed to assess tofacitinib related adverse events and have a longer duration of therapy with a large number of patients are warranted. Background Janus kinases have broad roles in immune regula tion via their action in cytokine signalling. These non receptor tyrosine kinases phosphorylate receptor chains, which in turn recruit and phosphorylate mem bers of the Signal Transducer and Activator of Transcrip tion family. The Jak family comprises Jak1, Jak2, Jak3 and Tyk2. These enzymes have very similar do main structures, containing a FERM domain, an SH2 do main, a pseudokinase domain, and a catalytic tyrosine kinase domain. Jaks serve overlapping but distinct func tions in cytokine signaling, as demonstrated by knockout, mutation and other studies.

Because of their roles in the signaling of many import ant cytokines, hormones, and growth factors such as IL 2, IL 4, IL 6, IL 7, IL 12, IL 13, IFN , IFN , Epo, and GM CSF, Jak inhibitors might have wide application in the treatment of inflammatory, myelopro liferative and autoimmune diseases, and therefore the Jak enzymes are attractive targets for drug discovery. Ini tial studies with Jak3 inhibitors were aimed at preventing solid organ transplant rejection. More recent studies have explored the potential of such compounds in chronic autoimmune diseases such as rheumatoid arthritis and psoriasis. For example, tofacitinib, which inhibits Jak1, Jak2, and Jak3, has demonstrated efficacy in Phase II trials for rheumatoid arthritis.

Ruxolitinib, a dual Jak1 and Jak2 inhibitor, was recently approved for the treat ment of myelofibrosis, a disorder involving myeloproli Dacomitinib ferative neoplasm. The development of Tyk2 inhibitors is less advanced. Tyk2 functions together with Jak2 in the signaling of IL 12 and IL 23 via its interaction with the IL 12RB1 re ceptor chain, and in the coordinated phosphorylation of STAT3 STAT4. Human Tyk2 gene deficiency causes defects in signaling of multiple cytokines, including IL 6, IL 10, IL 12 and IL 23, and reduced production of IFN��. Furthermore, Tyk2 deficient mice are resistant to experimental autoimmune encephalomyelitis, a model for multiple sclerosis. Given the importance of Tyk2 dependent downstream cytokine signaling in this and other diseases such as rheumatoid arthritis and Crohns disease, Tyk2 inhibitors have the potential to be important therapeutics.

Because Jak family active sites exhibit high sequence identity, designing inhibitors selective within the family is challenging. One way to approach this challenge is to target active site regions that differ in conformation be tween homologs. To identify these hot spot regions, we set out to obtain multiple crystal structures of Tyk2 in complex with a variety of ligands representing diverse chemotypes. At the time of our initial work, only Jak2 and Jak3 crystal structures had been published.