Biologic treatment patterns and challenges in defining difficult-to-treat disease in children with polyarticular juvenile idiopathic arthritis: a real-world study

To evaluate the clinical characteristics, and treatment patterns of difficult-to-treat (D2T) disease in children with polyarticular juvenile idiopathic arthritis (pJIA) treated with biologic disease-modifying antirheumatic drugs (bDMARDs). This single-center, cross-sectional study included children with RF-positive or RF-negative pJIA who received bDMARD. D2T defined according to EULAR criteria, as inadequate response to at least two bDMARDs with different mechanisms of action. Disease activity was assessed using the JADAS-27. Treatment patterns, switches, and factors associated with D2T were analyzed. A total of 59 pJIA patients (79.9% female) were included, with a median follow-up of 60 months. Ten patients (16.9%) fulfilled the criteria for D2T disease after a follow-up duration of 36 months. At diagnosis, D2T patients had significantly higher inflammatory markers than non-D2T patients, with higher median CRP (45 vs. 12 mg/L, p = 0.002) and ESR (77 vs. 38 mm/h, p = 0.003) values. In univariate analyses, higher CRP, ESR, temporomandibular joint involvement, and higher JADAS-27 at bDMARD initiation were associated with D2T status (p < 0.05). D2T course was associated with temporomandibular joint involvement, elevated inflammatory markers, and higher disease activity at the initiation of the first biologic therapy.