JAK1 and rheumatoid arthritis: Upadacitinib (UPA), an oral selective JAK1 inhibitor, exhibits higher specificity for JAK1 than JAK2, JAK3, or tyrosine kinase 2.[7] Initially approved in 2019 for moderate-to-severe rheumatoid arthritis in patients with inadequate responses to methotrexate, UPA received Food and Drug Administration approval in 2022 for the treatment of moderate-to-severe AD in adolescents and adults.