CP-690550, also known by its generic name Tofacitinib, is a small molecule drug substance with the chemical name (3R,4R)-4-Methyl-3-(methyl(7H-pyrrolo[2,3-d]pyrimidin-4-yl)amino)-β-oxo-1-piperidinepropanenitrile citrate. It is a white to off-white crystalline powder. Tofacitinib is a Janus kinase (JAK) inhibitor. It functions by selectively inhibiting JAK enzymes, particularly JAK1 and JAK3, which are involved in the signaling pathways of various cytokines and growth factors crucial for immune cell function and inflammatory responses. Its primary and well-established application is as an active pharmaceutical ingredient (API) for the treatment of autoimmune diseases. It is approved for conditions such as rheumatoid arthritis, psoriatic arthritis, and ulcerative colitis. As a JAK inhibitor, Tofacitinib represents a class of targeted synthetic disease-modifying antirheumatic drugs (tsDMARDs). It is administered orally, offering an alternative to biologic therapies. Its development and use are subject to strict pharmaceutical regulations governing API manufacturing, quality control, and clinical safety monitoring.