Rheumatoid Arthritis Preceded by Interstitial Lung Disease Controlled by Long‐Term Treatment With the Janus Kinase 1‐Selective Inhibitor, Filgotinib: A Case Report

We present a case of active interstitial lung disease (ILD) preceding the onset of rheumatoid arthritis (RA). A 68-year-old man had dyspnea, decreased forced vital capacity, and elevated Krebs von den Lungen-6 levels. Non-specific interstitial pneumonia-like patterns, including ground-glass opacities, fibrosis, and subpleural spared area, were observed on computed tomography. Both rheumatoid factor and anti-cyclic citrullinated peptide antibody were positive; however, he had no clinical evidence of arthritis. Two years later, he developed arthritis, which was diagnosed as RA with high disease activity. He was started on salazosulfapyridine, which had no effect on the arthritis. Filgotinib, a selective Janus kinase (JAK) 1 inhibitor, was added for arthritis treatment; thereafter, the ILD improved alongside the arthritis. He maintained remission for 4 years with no exacerbation, flare, or adverse events. JAK1 inhibitor may be safe and beneficial in the long-term control and treatment of arthritis and ILD.