Use of Upadacitinib to Treat a Severe Flare-Up of Rheumatoid Arthritis During Anti-PD-1 Immune Checkpoint Inhibitor Therapy for Stage IV Squamous Cell Carcinoma of the Lung

<b>Background:</b> Immune checkpoint inhibitor (ICI) therapy is becoming the standard of care for the treatment of advanced non-small-cell lung cancer. However, T-cell activation by ICIs frequently induces a flare-up of preexisting autoimmune diseases such as rheumatoid arthritis (RA). Janus kinase (JAK) inhibitors are increasingly used in the treatment of RA, but they could interfere with the efficacy of ICIs by inhibiting interferon signaling. <b>Case Report:</b> Here, we describe a case in which upadacitinib, a JAK1-selective inhibitor, was used to manage a severe RA flare-up occurring during ICI therapy with pembrolizumab, an anti-programmed cell death protein-1 antibody. A 54-year-old man with RA was diagnosed with grade IV lung squamous cell carcinoma. The patient had maintained RA remission for 4 years at the time of lung cancer diagnosis. After seven cycles of pembrolizumab therapy, the size of the primary tumor was markedly reduced, but a severe RA flare-up and organizing pneumonia (OP)-like pulmonary lesions occurred. Considering the severity of the flare-up, pembrolizumab was discontinued. Upadacitinib induced swift recovery from the RA flare-up and OP. Eleven months after the last pembrolizumab use, almost all metastatic lesions in the body had disappeared. We did not observe recurrence of lung cancer for more than 1 year during upadacitinib therapy. <b>Conclusions:</b> Upadacitinib could be a safe and effective option to treat severe RA flare-ups occurring during anti-PD-1 ICI therapy.