Analysis of intracellular methotrexate polyglutamates in patients with juvenile idiopathic arthritis: Effect of route of administration on variability in intracellular methotrexate polyglutamate concentrations

摘要

ObjectiveIntracellular methotrexate (MTX) polyglutamates (MTXGlu) have been shown to be potentially useful biomarkers of clinical response in adult patients with rheumatoid arthritis. The present study was undertaken to measure intracellular MTXGlu concentrations in a cohort of patients with juvenile idiopathic arthritis (JIA) to determine the predictors of MTXGlu variability in these patients.MethodsBlood samples were obtained from patients with JIA who were being treated with a stable dose of MTX for ≥3 months. Clinical data were collected by chart review. Concentrations of MTXGlu_1–7 in red blood cell lysates were quantitated using an innovative ion-pairing chromatography procedure, with detection by mass spectrometry.ResultsPatients with JIA from a single center (n = 99; mean ± SD age 117.8 ± 56.5 months, 69 female) were included in the analysis. The mean ± SD dose of MTX was 0.51 ± 0.25 mg/kg per week, with a median treatment duration of 18 months (interquartile range 3–156 months). MTX was administered subcutaneously in 66 patients (67%). Fifty-six patients (57%) had active arthritis at the time of the clinic visit. Total intracellular MTXGlu (MTXGlu_TOT) concentrations varied 40-fold, with a mean ± SD total concentration of 85.8 ± 48.4 nmoles/liter. Concentrations of each MTXGlu subtype (MTXGlu_1–7) were measured individually and as a percentage of MTXGlu_TOT in each patient. MTXGlu₃ was the most prominent subtype identified, comprising 42% of MTXGlu_TOT, and the interindividual variability in the concentration of MTXGlu₃ was the most highly correlated with that of MTXGlu_TOT (r = 0.96). The route of MTX administration was significantly associated with MTXGlu_1–5 subtypes; higher concentrations of MTXGlu_{1 + 2} were observed in patients receiving oral doses of MTX, whereas higher concentrations of MTXGlu_3–5 were observed in patients receiving subcutaneous doses of MTX (P 0.0001).ConclusionIn this cohort of patients with JIA, the MTXGlu_TOT concentration varied 40-fold. Individual MTXGlu metabolites (MTXGlu_1–7), which have, until now, not been previously reported in patients with JIA, were detected. The route of MTX administration contributed to the variability in concentrations of MTXGlu_1–5.