Calprotectin (S100A8/A9) is not associated with ultrasound-detected synovitis in a longitudinal study of patients with psoriatic arthritis treated with biological disease-modifying anti-rheumatic drugs.

Calprotectin (S100A8/A9) is an established inflammatory marker in rheumatoid arthritis (RA), but its role in psoriatic arthritis (PsA) is less studied. This study evaluated plasma calprotectin as a biomarker of inflammatory activity in PsA by assessing its association with ultrasound-detected synovitis before and during treatment with a biological disease-modifying anti-rheumatic drug (bDMARD). The potential of S100A12, vascular endothelial growth factor, interleukin-6 (IL-6), IL-17A, IL-23, and C-X-C motif chemokine ligand 10 (CXCL10) was also explored. Forty-three PsA patients initiating bDMARD therapy were assessed clinically and by ultrasound at baseline and after 3, 6, 9, and 12 months. Biomarkers were measured using enzyme-linked immunosorbent assays and Luminex assays. Changes were analysed using the Wilcoxon signed-rank test, and correlations with Spearman's rank analysis. Mean (± SD) age was 47.6 (± 12.9) years, 60.5% were women, and median disease duration was 10 years (interquartile range 4.2-21.9). Significant reductions were observed in joint counts and in the Disease Activity Index for Psoriatic Arthritis, Disease Activity Score for 28 joints including CRP, and Bath Ankylosing Spondylitis Disease Activity Index. Baseline levels of calprotectin, S100A12, IL-6, IL-17A, IL-23, and CXCL10 were higher in PsA than in controls (p < 0.05). Calprotectin, S100A12, and IL-6 levels decreased during follow-up (p < 0.05). No clinically relevant correlations between the ultrasound scores and inflammatory markers were observed. Calprotectin levels were elevated in PsA patients and decreased with treatment but showed no clinically significant correlation with ultrasound-detected synovitis. Further studies are needed, particularly in cohorts with higher levels of inflammation.