From Adult to Adolescent: Alignment in Clinical Trials and Outcomes in Axial Spondyloarthritis.

Spondyloarthritis (SpA) is a group of chronic inflammatory diseases encompassing axial and peripheral forms, with up to 20% of patients developing symptoms before age 16. Despite this substantial pediatric burden, treatment options for juvenile-onset SpA (JSpA), particularly those with axial disease (axJSpA), remain limited. This gap underscores the need for evidence-based approaches to extend effective therapies to children. This review highlights the strong parallels between adult axial SpA (axSpA) and axJSpA, supporting the appropriateness of extrapolating efficacy and target drug exposure data from adults to children. Adult- and axJSpA share key clinical and pathogenic features, including overlapping clinical symptoms, genetic susceptibility, and treatment approaches. Classification systems also align: adult axSpA is classified according to the Assessment of Spondyloarthritis International Society criteria, whereas children are classified using the recently validated axJSpA criteria, which closely mirrors the adult framework. Treatment paradigms further reinforce these similarities. Nonsteroidal anti-inflammatory drugs and tumor necrosis factor inhibitors are standard therapy in both adults and children, and emerging mechanisms, such as inhibition of interleukin-17A/F and JAK, are relevant across age groups. However, the absence of approved agents for axJSpA underscores the importance of leveraging data from adults. Given the shared biology, clinical phenotype, treatment response patterns, and the availability of shared outcome measures such as the Spondyloarthritis Research Consortium of Canada magnetic resonance imaging inflammation and structural scores, extrapolating efficacy and target drug exposure data from adults with axSpA to children with axJSpA is scientifically justified and critical for addressing the unmet therapeutic needs in affected youth.