Filgotinib is an oral, small-molecule inhibitor of Janus kinase (JAK)–signal transducer and activator of transcription, JAK1, one of the several subtypes of the enzyme.1 Four JAKs exist in humans: JAK1, JAK2, JAK3, and nonreceptor tyrosine-protein kinase TYK2.1,2 These kinases bind to types I and II cytokine receptors and transmit extracellular cytokine signals to activate various signal transducers and activators of transcription, which drive the proinflammatory machinery of the cellular immune response.1 Two other JAK inhibitor drugs, tofacitinib3 and baricitinib,4 have been approved in the United States for the treatment of rheumatoid arthritis (RA). Tofacitinib preferentially inhibits JAK1, JAK3, and to a lesser extent JAK2.1,2 Baricitinib inhibits JAK1 and JAK2.
Singh JA. Filgotinib, a JAK1 Inhibitor, for Treatment-Resistant Rheumatoid Arthritis. JAMA. 2019;322(4):309–311. doi:10.1001/jama.2019.9056
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