Procollagen I N-terminal peptide correlates with inflammation on sacroiliac joint magnetic resonance imaging in ankylosing spondylitis but not in non-radiographic axial spondyloarthritis: A cross-sectional study

Objectives To identify disease activity scores and biomarkers that reflect magnetic resonance imaging (MRI)-determined sacroiliac joint (SIJ) inflammation in ankylosing spondylitis (AS) and non-radiographic axial spondyloarthritis (nr-axSpA). Methods Patients who had AS and nr-axSpA were enrolled. All the patients underwent SIJ MRI. SpondyloArthritis Research Consortium of Canada (SPARCC) method was used to score bone marrow edema in the inflammatory lesions on MRI. Radiographic assessment of the spine was performed using modified Stoke Ankylosing Spondylitis Spine Score. Clinical variables, inflammatory markers, serum alkaline phosphatase, osteocalcin (OC), C-terminal telopeptide of type I collagen (CTX-I), and procollagen I N-terminal peptide (PINP) were measured. Correlation analysis between MRI-determined SIJ inflammation scores and disease activity scores and laboratory variables was performed. Results Thirty-five patients had AS and 36had nr-axSpA. Significant differences were noted between the AS group and the nr-axSpA group in terms of erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), Ankylosing Spondylitis Disease Activity Score (ASDAS)-ESR, ASDAS-CRP, PINP, and SPARCC (p < .001, p = .004, p < .001, p < .001, p = .030, p < .001, respectively). MRI-determined SIJ inflammatory scores correlated with Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), Bath Ankylosing Spondylitis Functional Index (BASFI), OC, CTX-I, and PINP in AS (p = .036, p = .023, p = .002, p = .041, p = .004, respectively) and correlated with ESR, CRP, ASDAS-ESR, ASDAS-CRP, BASDAI, and BASFI in nr-axSpA (p = .003, p = .002, p < .001, p < .001, p = .010, p = .007, respectively). Multivariate analysis showed that PINP exhibited a positive correlation independent of the MRI inflammatory score and that age exhibited a negative correlation independent of the MRI inflammatory score. Conclusions In AS, PINP and age independently correlated with active inflammation on SIJ MRI. PINP may be useful as a marker of objective inflammation in AS.