Relapse in adult idiopathic inflammatory myopathies: clinical predictors and the potential role of anti-NXP2 in a single-center retrospective cohort study.

INTRODUCTION/OBJECTIVES: Relapse is a frequent challenge in idiopathic inflammatory myopathies (IIM); however, predictors in adults remain insufficiently defined. This study aimed to determine the relapse frequency and risk factors, with an emphasis on myositis-specific autoantibodies (MSAs), in a Turkish IIM cohort. METHODS: Patients diagnosed with IIM according to established classification criteria between 2017 and 2024 at Hacettepe University Hospital, a referral center were retrospectively analyzed in a single-center cohort. Relapse was defined as a ≥ twofold increase in creatine kinase and/or escalation of immunosuppressive therapy for active muscle or extramuscular disease. Predictors were identified using logistic regression analysis. RESULTS: A total of 136 patients were included (66.2% female; mean age, 47.6 ± 17.5 years). The mean follow-up period was 5.3 ± 5.9 years. During this period, 46 patients (33.8%) relapsed, with a median time to relapse of 2.86 years (95% CI: 0.73-4.99). In the multivariate analysis, younger age at diagnosis (OR 0.97, 95% CI 0.95-0.99, P = 0.009), anti-NXP2 positivity (OR 5.31, 95% CI 1.24-22.58, P = 0.02), and intravenous immunoglobulin use during induction (OR 2.69, 95% CI 1.16-6.26, P = 0.02) independently predicted relapse. Maintenance immunosuppressive therapy for ≥ 5 years was associated with a significantly delayed relapse (log-rank P = 0.01). CONCLUSIONS: Relapse occurred in one-third of adult patients with IIM. Younger age, anti-NXP2 positivity, and IVIG use independently increased relapse risk, whereas long-term maintenance therapy substantially delayed relapse. These findings highlight the need for long-term immunosuppression and close monitoring of high-risk subgroups.