Iguratimod: Novel Molecular Insights and a New csDMARD for Rheumatoid Arthritis, from Japan to the World

Iguratimod (IGU) is a conventional synthetic disease-modifying anti-rheumatic drug (csDMARD) routinely prescribed in Japan since 2012 to patients with rheumatoid arthritis (RA). Iguratimod acts directly on B cells by inhibiting the production of inflammatory cytokines (tumor necrosis factor-α, interleukin (IL)-1β, IL-6, IL-8, IL-17), thereby suppressing the production of immunoglobulin and inhibiting the activity of nuclear factor kappa-light chain enhancer of activated B cells. In Japan, it is one of the most used csDMARDs in daily practice, but it is not recommended as a treatment for RA due to the lack of large-scale evidence established overseas. However, recent reports on the novel pharmacological effects of IGU on lymphocytes and synovial fibroblasts, as well as its efficacy in daily practice, have increased its importance as a drug for the treatment of RA. In this review, we highlighted the basic and clinical studies in IGU and discuss its potential as a new therapeutic agent for the treatment of RA.