Baseline clinical features predicting macrophage activation syndrome in patients with systemic adult‐onset Still’s disease receiving interleukin‐6 inhibitor treatment

Aim Macrophage activation syndrome (MAS), a severe complication of systemic adult‐onset Still’s disease (AOSD), has been reported to occur during interleukin‐6 (IL‐6) inhibitor treatment. However, predictors for MAS development are unknown. Therefore, this study investigated predictive features for MAS development after starting IL‐6 inhibitor treatment in systemic AOSD patients. Method In a single‐center retrospective study involving systemic AOSD patients who were refractory to high‐dose glucocorticoids with immunosuppressants and started IL‐6 inhibitor treatment between April 2008 and March 2020, we compared the baseline clinical features between patients who developed AOSD flare with MAS features (MAS group) and those who did not (non‐MAS group) during IL‐6 inhibitor treatment. Results Only tocilizumab was used as an IL‐6 inhibitor. Six of 14 refractory systemic AOSD patients developed AOSD flares with MAS features during tocilizumab treatment, including 4 who developed them shortly after initiation. The MAS group had significantly lower neutrophil counts, fibrinogen, and higher IL‐18/C‐reactive protein (CRP) ratio at starting tocilizumab (baseline) than the non‐MAS group. Before starting tocilizumab, neutrophil counts were trending downward and upward in the MAS and non‐MAS groups, respectively, with significant differences in changes. Receiver operating characteristic analysis showed that baseline neutrophil counts and fibrinogen and their changes before tocilizumab treatment and baseline IL‐18/CRP ratio had significant discriminatory abilities for subsequent MAS development. Conclusion We identified baseline laboratory features associated with MAS development after initiating an IL‐6 inhibitor in refractory systemic AOSD patients. These features may reflect the suppression of IL‐6 signaling, and further suppression of IL‐6 signaling might trigger early‐onset MAS.