These agents block more proximally in the signaling cascade, which may explain their clinical success. In contrast, p38 MAPK may be too distal in the signaling pathway to be a relevant target. The JAKs were initially discovered in the 1990s. The JAK family of tyrosine kinases consists of four members, JAK1, JAK2, JAK3 and tyrosine kinase-2 (TYK2). Although JAKs were initially coined ‘just another kinase’ due to their uncertain function, these molecules are now known to play a central role in cytokine signaling
when coupled with STAT molecules. The JAK/STAT pathway is responsible for signal transduction of the type I and type II cytokine receptor family, which act as receptors of interferons, interleukins and colony-stimulating factors. Erythropoietin, thrombopoeitin, growth hormone, prolactin and leptin also associate with these receptors and rely on JAK PF01367338 signaling. Upon receptor ligation, a single JAK or combination of JAKs selectively associate
with the receptor’s cytoplasmic domain, leading to phosphorylation and activation of STATs. STATs are DNA binding proteins that, once phosphorylated, dimerize and translocate PD0325901 into the nucleus where they regulate transcription of STAT-dependent genes. JAK1 and JAK3 are mostly aligned with inflammation activation, whereas JAK2 plays a large role in hematopoiesis (Table 1). TYK2 is associated with immune response and may play a role in allergic inflammation. Interestingly, JAK3 associates with the common gamma chain-containing receptor that shares IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21 as ligands. In the mid-1990s it was shown that mutations in JAK3 lead
to severe combined immune deficiency (SCID) due to failure of signaling of the aforementioned cytokines and the subsequent failure of development of functional B, T and natural killer (NK) cells. This discovery provided great insight into the potential role of JAKs as immunomodulators (Table 2). As shown through recent drug development and clinical trials, JAK inhibition is now poised to expand the treatment options for RA. Defective erythropoiesis Defective myelopoiesis Anemia Neutropenia Immunodeficiency 17-DMAG (Alvespimycin) HCl Increased allergy Defective Th1 differentiation Defective interferon signaling Tofacitinib is a small-molecule selective inhibitor of JAK1, JAK3 and to a lesser extent JAK2. Tofacitinib is the first kinase inhibitor to be approved for use in the United States for the treatment of moderately to severely active RA. However, in July 2013, the European Medicines Agency voted not to approve tofacitinib for use in RA. This decision stemmed largely from concerns that there was not a consistent enough reduction in disease activity and structural damage to outweigh the risks of serious infection, malignancy and laboratory abnormalities. Table 3 summarizes the phase 2 and phase 3 clinical trials of tofacitinib.