Description In November 2012, the US FDA approved tofacitinib (also referred to as CP-690550) for the treatment of adult patients with moderate to severely active rheumatoid arthritis (RA) who have had an inadequate response or intolerance to methotrexate. Tofacitinib is the first small molecule kinase inhibitor approved for the treatment of RA. Tofacitinib is an inhibitor of the four subtypes of Janus kinase (JAK): JAK1, JAK2, JAK3, and Tyk2. The JAKs are intracellular, nonreceptor tyrosine kinases that play important roles in the signal transduction pathway of many cytokines (e.g., interleukins 2, 4, 7, 9, 15, and 21) and are involved in the propagation of inflammation in RA. Tofacitinib acts by inhibiting the phosphorylation and activation of signal transducers and activators of transcription (STATs), thereby suppressing the production of inflammatory mediators in joint tissue. At the enzyme level, tofacitinib inhibits JAKs 1, 2, 3, and Tyk2 with IC50s of 3.2, 4.1, 1.6, and 34 nM, respectively. At the cellular level, tofacitinib inhibits the in vitro activities of JAK1/JAK2, JAK1/JAK3, and JAK2/JAK2 combinations with IC50s of 406, 56, and 1377 nM, respectively.