Comparison of the established ASAS definition and preliminary proposed data-driven cut-offs for inflammatory and structural MRI lesions in the sacroiliac joints for eligibility of patients with nonradiographic axial spondyloarthritis in clinical trials

OBJECTIVES: The purpose of this paper is to evaluate patient characteristics and treatment outcomes in patients with nonradiographic axial spondyloarthritis (nr-axSpA) eligible for a clinical trial based on the 2009 established Assessment of SpondyloArthritis International Society (ASAS) magnetic resonance imaging (MRI)-sacroiliac joint (SIJ) definition for sacroiliitis (established definition [ED]) in comparison to the 2021 preliminary proposed data-driven definitions (proposed definitions [PD]) for inflammatory and structural lesions on MRI-SIJ. METHODS: COAST-X is a phase 3, multicentre, randomised, double-blind, placebo-controlled nr-axSpA study on ixekizumab (IXE) or placebo up to 52 weeks. In this post hoc analysis, baseline MRI-SIJ scans of IXE and placebo arms were independently categorised using the ED (bone marrow oedema [BME]) and PD (PD-BME; PD-erosion/fat lesions [FAT]/deep FAT; PD Total) cut-offs for MRI-SIJ lesions into groups fulfilling ED, PD, ED but not PD, and vice versa. Patient demographics, baseline characteristics, and clinical outcomes at week 16, including ASAS40 and Bath Ankylosing Spondylitis Disease Activity Index 50% improvement (BASDAI50) response, mean BASDAI change from baseline, Axial Spondyloarthritis Disease Activity Score (ASDAS) clinically important improvement, and attainment of different ASDAS disease states, were reported descriptively for each definition. RESULTS: Patient demographics and baseline disease characteristics were balanced between all ED or PD MRI-SIJ+ cut-off subgroups. Within each treatment arm, the respective clinical outcomes showed similar results across all ED or PD MRI-SIJ+ cut-off subgroups, whereas outcomes in subgroups fulfilling ED but not PD and vice versa were slightly lower compared with ED or PD MRI‑SIJ+ cut-off subgroups. CONCLUSIONS: Patients fulfilling ED or PD of a positive MRI-SIJ showed similar baseline characteristics and clinical outcomes at week 16. In this analysis, PD did not outperform ED in the identification of patients for a clinical trial in nr-axSpA.