• Kazuya Abe, Yuichi Ishikawa, Michio Fujiwara, Hiroko Yukawa, Takeshi Yanagihara, Saori Takei, Hitoshi Arioka, and Yasuhiko Kita.
• Department of Rheumatology, Yokohama Rosai Hospital, 3211 Kozukue-cho, Kohoku-ku, Yokohama, Kanagawa, Japan.
• Medicine (Baltimore). 2022 Jan 14; 101 (2): e28428e28428.

RationaleImmune checkpoint inhibitors (ICIs) have shown efficacy for the treatment of various kinds of malignant tumors. However, ICIs can cause immune-related adverse events, such as arthritis. Nevertheless, the treatment of ICI-induced arthritis has not been established yet. Here we report a case of ICI-induced polyarthritis successfully treated using sarilumab and monitored using joint ultrasonography.Patient ConcernsA 61-year-old man presented with polyarthritis. He had been treated with nivolumab for recurrent renal cell carcinoma 11 months before. He developed ICI-induced nephritis (proteinuria and elevated serum creatinine) 3 months before, which resolved after discontinuing nivolumab for 1 month. Two months after resuming nivolumab, he developed polyarthralgia and joint swelling, which were suspected to be associated with nivolumab administration, and hence we discontinued nivolumab again. Laboratory tests revealed elevated C-reactive protein level and erythrocyte sedimentation rate, but were negative for rheumatoid factor and anti-cyclic citrullinated peptide antibody. Joint ultrasonography revealed active synovitis in several joints, but a joint X-ray revealed no bone erosion.DiagnosesWe diagnosed polyarthritis as ICI-induced arthritis because the findings were not typical of rheumatoid arthritis (no bone erosion and seronegativity) and the patient had already developed other immune-related adverse events (ICI-induced nephritis).InterventionsAfter discontinuation of nivolumab, we started treatment with 15 mg daily prednisolone and 1000 mg daily sulfasalazine, although it was ineffective. Hence, we initiated 200 mg biweekly sarilumab.OutcomesFollowing sarilumab administration, polyarthritis improved rapidly, and joint ultrasonography confirmed the rapid improvement of synovitis. Hence, we tapered off the glucocorticoid treatment. No recurrence of renal cell carcinoma was noted for 2 years after the initiation of sarilumab despite no anti-tumor therapy.LessonsSarilumab may serve as a good treatment option for treating refractory ICI-induced polyarthritis. Joint ultrasonography may contribute to the evaluation of ICI-induced polyarthritis and monitoring the effects of treatments.Copyright © 2022 the Author(s). Published by Wolters Kluwer Health, Inc.

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