Decernotinib (VX-509) is an investigational selective inhibitor of Janus kinase 3 (JAK3), primarily developed for the treatment of autoimmune diseases, particularly rheumatoid arthritis. It targets JAK3, which plays a crucial role in the signaling pathways of immune cells. By inhibiting JAK3, it was intended to reduce inflammation and slow the progression of joint damage in patients with rheumatoid arthritis. Although clinical development was discontinued after Phase II trials due to efficacy concerns, it remains a valuable compound for research into JAK inhibitors and disease-modifying antirheumatic drugs (DMARDs). Its targeted mechanism offers potential insights into therapies with fewer side effects compared to broader immunosuppressants.