Relationship between urinary pyridinium crosslinks, disease activity and disease subsets of ankylosing spondylitis

Objective. In this study, we aimed to determine the urinary levels of pyrid inium cross-links and urinary beta-isomerized fragments derived from the C- telopeptide of the alpha 1 chain of type I collagen (beta-CTX) as markers o f bone resorption in patients with ankylosing spondylitis (AS), and to stud y their relationship to markers of disease activity [erythrocyte sedimentat ion rate (ESR)] and to disease subsets of this condition. Methods. The serum calcium, osteocalcin (OC), parathormone (PTH), 25 OHD3 l evels, beta-CTX and the urinary combined free pyridinolines (f-Pyr + f-Dpyr ), urinary free deoxypyridinoline (f-Dpyr) and urinary free pyridinoline (f -Pyr) were evaluated and compared in 32 AS patients and 25 controls. Bone m ineral density (BMD) was evaluated at the lumbar spine and the femoral neck . Results. The serum markers of bone metabolism (serum calcium, PTH, 25 OHD, and OC) were in the normal range in the AS group. AS patients had a lowered lumbar spine BMD (P = 0.01) (corresponding T score: P = 0.03). but femoral neck BMD did not differ significantly between AS and controls (P = 0.08) ( corresponding T score: P = 0.11). There was no difference in the urinary le vels of pyridinium cross-links and beta-CTX between AS patients and control s. A positive correlation between ESR, (f-Pyr + f-Dpyr) (r = 0.42; P = 0.01 8) and f-Dpyr (r = 0.49; P = 0.005) was observed. In the different disease subsets of AS. we found that patients with peripheral involvement had highe r (f-Pyr + f-Dpyr) (P = 0.04) and f-Dpyr levels (P = 0.03), patients with e arly disease had elevated (f-Pyr + f-Dpyr) (P = 0.01), f-Dpyr (P = 0.02) an d f-Pyr (P = 0.01) levels, and that those with raised ESR had enhanced f-Dp yr (P = 0.009) excretion. Patients were then stratified according to diseas e duration, peripheral involvement and sex, and this allowed us to observe that only urinary f-Dpyr remained elevated in patients independently from t hese variables and that raised ESR is the more relevant parameter for expla ining this high level of excretion. Conclusion. We conclude that there was no difference in the levels of urina ry pyridinium cross-links and beta-CTX between AS and controls. However, ur inary excretion of some of these collagen compounds was enhanced in subgrou ps of AS, mainly in patients with raised ESR. Thus. AS patients with labora tory evidence of active disease could have a higher risk of bone loss.