Immune checkpoint inhibitor-induced inflammatory arthritis: a novel clinical entity with striking similarities to seronegative rheumatoid arthritis.

OBJECTIVE To determine the clinical and serologic similarities and differences between inflammatory arthritis induced by immune checkpoint inhibitors (IA-irAE) and rheumatoid arthritis (RA). METHODS In this retrospective cross-sectional comparative study, 20 patients with IA-irAE were age and sex matched to 40 seropositive and 40 seronegative RA patients. Electronic medical records were reviewed from diagnosis of inflammatory arthritis through May 2019. Arthritis characteristics, treatment, and relevant laboratory and serologic studies were captured. RESULTS Clinically, IA-irAE differed from seropositive and seronegative RA with respect to disease duration (4.18 versus 11.59 and 13.3 months, respectively, p = 0.005 (IA-irAE vs seropositive RA), p = 0.002 (IA-irAE vs seronegative RA)), polyarticular joint involvement at presentation (75% versus 97.5% and 100%, p = 0.013, p = 0.003), absence of erosive changes (5.9% vs 43.6% and 53.8%, p = 0.005, p = 0.001), mean prednisone dose (24.7 mg versus 16.53 mg and 15.68 mg, p = 0.008, p = 0.005), and use of methotrexate (5.0% versus 85.0% and 70.0%, p < 0.0001, p < 0.0001). Serologically, IA-irAE closely resembled seronegative RA. ANA positivity was seen in a minority of patients and did not differ significantly between all groups; however, the ANA staining pattern (speckled) was similar between IA-irAE and seronegative RA (100% versus 75%, respectively) and was not commonly observed in seropositive RA (18.2%). CONCLUSION IA-irAE is a new subset of IA that resembles seronegative RA immunologically. Our findings suggest that further study of IA-irAE might provide a window into underlying pathogenic mechanisms of early-stage seronegative RA.Key Points• Comprehensive comparison of clinical features between inflammatory arthritis irAE (IA-irAE) and regular rheumatoid arthritis indicates IA-irAE as a new subset of inflammatory arthritis.• IA-irAE resembles seronegative RA immunologically, suggesting that study of IA-irAE may provide a window into underlying pathogenic mechanisms of early-stage seronegative RA.