Objective: We evaluated the utility of neopterin and quinolinic acid (QUIN) as surrogate measures of disease activity in juvenile idiopathic inflammatory myopathies (IIMs).
Methods: Plasma and first morning void urine samples were measured for neopterin and QUIN using commercial ELISA, HPLC, or gas chromatography-mass spectrometry in 45 juvenile IIM patients and 79 healthy controls. Myositis disease activity assessments were obtained.
Results: Plasma and urine neopterin and QUIN concentrations were increased in juvenile IIM patients compared with healthy controls (P <0.017). Urine neopterin and QUIN highly correlated with each other (r(s) = 0.73; P <0.0001). Urine neopterin and QUIN correlated moderately with myositis disease activity assessments, including physician and parent global activity assessments, muscle strength testing, functional assessments (Childhood Myositis Assessment Scale, Childhood Health Assessment Questionnaire), skin global activity, and edema on magnetic resonance imaging (r(s) = 0.42-0.62; P <0.05), but generally not with muscle-associated enzymes in serum. Urine neopterin or QUIN, in combination with either serum lactate dehydrogenase (LD) or aspartate aminotransferase (AST), significantly predicted global disease activity (R(2) =0.40-0.56; P <0.002), and both were more sensitive to change than these serum enzymes (standardized response means, -0.41 to -0.48).
Conclusions: Urinary neopterin and QUIN are candidate measures of disease activity in juvenile IIM patients and add significantly to the prediction of global disease activity in combination with serum LD or AST values. Measurement of these markers in first morning void urine specimens appears to be as good as, or possibly better than, measurements of their concentrations in plasma.