Plasma Distribution of Methotrexate and Its Polyglutamates in Pediatric Acute Lymphoblastic Leukemia: Preliminary Insights

Background and Objective: High-dose methotrexate (HD-MTX) is the mainstream therapy of current acute lymphoblastic leukemia (ALL) regimens, but frequent intra- and interindividual differences in the clinical response to HD-MTX lead to chemotherapeutic interruption or discontinuation. The exact mechanism of transport across the cell membrane and the disposition of active methotrexate metabolites—methotrexate polyglutamates (MTXPGs)—are not well described in the literature. The aim of this study was to gain more insight into the plasma distribution of methotrexate and MTXPGs in pediatric patients with ALL and to clarify the obscure pathways of MTXPGs. Methods: We prospectively measured the concentrations of MTXPG_1–7in plasma samples from three male pediatric patients treated with HD-MTX and leucovorin rescue according to the IC-BFM 2009 protocol using liquid chromatography–mass spectrometry (LC-MS). Blood samples were obtained at 24, 36, 42, and 48 h after the start of HD-MTX treatment. Results: Noticeable plasma concentrations of MTXPGs with a 2.2-fold interpatient variability were detected. The highest interindividual variability in total plasma MTXPG concentration was observed at 36 h, and ranged from 13.78 to 30.82 μmol/L. Among all patients, the predominant polyglutamate types in relation to the total plasma MTXPG concentration at each time point were MTXPG₃(16.71–30.02%) and MTXPG₅(26.23–38.60%), while MTXPG₇was the least abundant MTXPG (3.22–5.02%). Conclusion: The presence of MTXPGs in plasma of patients with ALL could be related to the action of ABC efflux transporters on blood cells and hepatocytes resulting from the administration of high doses of methotrexate. This study may not draw definitive conclusions, but it does reduce uncertainty about the dynamics of methotrexate and its active metabolites, which may be of vital importance for achieving a clinical response.