Rheumatoid Factor Levels Are Associated With Arthritis Exacerbation After Bucillamine Discontinuation

ABSTRACT Aim Bucillamine is a conventional synthetic disease‐modifying antirheumatic drug that can be used as a methotrexate alternative. However, which patients with rheumatoid arthritis can benefit from bucillamine treatment has not been established. Methods Patients who had taken bucillamine and discontinued it in 2021 due to a nationwide supply shortage were included in this study. We followed up these patients regarding arthritis exacerbation after bucillamine discontinuation until March 2025. Baseline clinical patient characteristics were compared between the exacerbation and non‐exacerbation groups. Results Thirteen of the 18 patients experienced arthritis exacerbation, whereas five remained exacerbation‐free for up to 48 months. The baseline rheumatoid factor levels were significantly higher in the exacerbation group (49 U/mL) than in the non‐exacerbation group (6 U/mL). Receiver operating characteristic and Kaplan–Meier analyses showed that baseline RF levels ≥ 28 U/mL predicted arthritis exacerbation after bucillamine discontinuation (sensitivity 76.2% and specificity 80.0%). Conclusion Following bucillamine discontinuation, arthritis exacerbations occurred more frequently in patients with rheumatoid factor levels of 28 U/mL or higher, alongside an increase in these levels. Rheumatoid factor levels may serve as a useful biomarker for identifying patients who would benefit from continued bucillamine therapy.