Recent Progress in JAK Inhibitors for the Treatment of Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a systemic autoimmune disease characterized by synovial inflammation and joint destruction. Considerable advance in the treatment of RA has been made following the advent of biological disease-modifying anti-rheumatic drugs (DMARDs). However, these biologics require intravenous or subcutaneous injection and some patients fail to respond to biological DMARDs or lose their primary response. Various cytokines and cell surface molecules bind to receptors on the cell surface, resulting in the activation of various cell signaling pathways, including phosphorylation of kinase proteins. Among these kinases, the non-receptor tyrosine kinase family Janus kinase (JAK) plays a pivotal role in the pathological processes of RA. Several JAK inhibitors have been developed as new therapies for patients with RA. These are oral synthetic DMARDs that inhibit JAK1, 2, and 3. One JAK inhibitor, tofacitinib, has already been approved in many countries. Results of phase III clinical trials using a JAK1/2 inhibitor, baricitinib, have shown feasible efficacy and tolerable safety. Both drugs are effective in patients who showed inadequate response to biological DMARDs as well as synthetic DMARDs. In addition, clinical phase III trials using filgotinib and ABT494, specific JAK1 inhibitors, are currently underway. JAK inhibitors are novel therapies for RA, but further studies are needed to determine their risk-benefit ratio and selection of the most appropriate patients for such therapy.