Using Methotrexate Metabolites to Make Clinical Decisions in JIA

Abstract

Determining how to optimally treat juvenile idiopathic arthritis (JIA) remains a challenge, although we are better equipped to do so now than ever before. Capitalizing on currently available therapies for rapid and sustained disease control remains incredibly important since long-term outcomes are shown to be impacted by the duration of active disease, and arthritis in childhood can influence growth and development with long-term consequences in adulthood. Although achieving remission remains difficult, we are now better able to achieve clinically inactive disease utilizing the therapeutic armamentarium that has become available over the last several decades. The current challenge is determining which medication to use and when, since significant variability is observed in treatment response and toxicity and there is a wide array of costs associated with currently available treatments. The search for viable biomarkers and prediction tools to assist with these clinical decisions remains underway. Methotrexate is an important therapeutic option either alone or in combination with other medications, and although it has a relatively long history of clinical use, there remains significant variability in clinical response and an incomplete understanding of its mechanism of action. The polyglutamated form of methotrexate (MTXGlun) is a more stable and reliable cellular marker of methotrexate (MTX) activity and has received attention in recent years as a potentially useful biomarker for MTX. Improvements in the sensitivity of MTXGlun quantitation via HPLC-tandem mass spectrometry (LC-MS/MS) and observed associations between MTXGlun and clinical outcomes keep the optimism high for this biomarker. However, utilizing MTXGlun “real time” for clinical decision making is neither a currently mainstream practice in pediatrics, nor is it ready to be. Available data in pediatrics have not identified or validated targetable MTXGlun concentrations associated with clinical improvement in JIA, and the currently available high-performance liquid chromatography (HPLC) clinical assays are not as sensitive as LC-MS/MS methods reported in current research studies in JIA. Additionally, factors such as time to transport for processing, need for sample refrigeration, and protection from light can affect sample stability as improper handling can result in continued action of cellular enzymes within the folate pathway resulting in continued deglutamation and falsely low MTXGlun concentrations. Along these same lines, pharmacogenomic studies searching for genetic factors that impact MTX outcomes have also been challenged with lack of validation. Based on the complex genetic architecture of the folate pathway (discussed below), inherent redundancy in the system likely requires changes in many genes, rather than a single gene variant, to result in a significant impact on drug outcomes. More robust and consistent evidence is needed on the association of MTXGlun concentrations with clinical response in JIA. Furthermore, since MTX dosing in children is higher per body surface area than in adults, meaningful MTXGlun levels in children also need to be identified and validated before utilizing this clinical assay in JIA. Despite the limitations of use in the clinical setting at the current moment, MTXGlun measurement may prove to be additionally useful in exploring the mechanisms of action of MTX and the effect of this drug on the complex endogenous folate pathway—all important in understanding the complete picture of MTX action in JIA.