Drivers of Methotrexate Polyglutamate Concentration in Erythrocytes: Insights from Immune-Mediated Inflammatory Diseases and Pediatric Acute Lymphoblastic Leukemia

<b>Background/Objectives</b>: Methotrexate (MTX) is a cornerstone drug used to treat immune-mediated inflammatory diseases (IMIDs) in low doses (10-30 mg/week), and malignancies in high doses (5000 mg/m²/2 weeks). Its active metabolites, Methotrexate polyglutamates (MTX-PG_2-5), quantified in erythrocytes, are associated with efficacy. This study aimed to compare erythrocyte MTX-PG concentrations in patients with IMIDs and pediatric acute lymphoblastic leukemia (ped-ALL) treated with low-dose or high-dose MTX, respectively, and to identify clinical, demographic, and treatment-related factors influencing their concentration. <b>Methods</b>: A total of 567 patients with rheumatoid arthritis, juvenile idiopathic arthritis, Crohn's disease, sarcoidosis, and ped-ALL were included. Erythrocyte MTX-PG concentration data was collected after 3 months (2.5 months for ped-ALL patients) of MTX-use. Multivariate linear regressing modelling adjusting for age, sex, body mass index (BMI), smoking status, starting MTX dose, route of MTX administration, use of predniso(lo)ne, disease-modifying anti-rheumatic drugs (DMARDs), and folic (or folinic) acid was performed. <b>Results</b>: Intravenous high-dose MTX increased MTX-PG_4&5 accumulation. Despite 50-fold higher doses in ped-ALL, MTX-PG_2-5sum concentrations were similar to those seen with subcutaneous low-dose MTX used in IMIDs. Age positively influenced MTX-PG concentrations, while DMARD use reduced MTX-PG_2-3&5 concentrations. Interestingly, predniso(lo)ne use was associated with higher MTX-PG_4&5 concentrations and folic (or folinic) acid with higher MTX-PG_3-5 concentrations. <b>Conclusions</b>: This is the first study to compare erythrocyte MTX-PG concentration in low-dose and high-dose patients. Intravenous high-dose MTX administration increased long-chain MTX-PG_4&5 concentrations, with MTX-PG_2-5sum concentrations similar compared to low-dose subcutaneous MTX use. This study demonstrated that route of administration, age, and concomitant therapies such as DMARDs, predniso(lo)ne, and folic (or folinic) acid significantly influence MTX-PG concentrations.