Kinetics of methotrexate and its metabolites in red blood cells

It has been suggested that the intra-erythrocyte levels of methotrexate (MTX) and its polyglutamized derivatives could offer a method for evaluating intracellular MTX accumulation and its metabolism in children with acute lymphoblastic leukemia. We were interested in measuring the intra-erythrocyte levels of MTX in patients with solid tumors receiving high-dose MTX treatment. After a first incorporation stage occurring during infusion, MTX concentrations subsequently increased 9-12 days after the treatment as polyglutamized derivatives. Thirty days after the infusion, MTX and its polyglutamates were still measurable in erythrocytes. The percentage of polyglutamization varied on an individual basis, but two groups of patients could be separated according to their ability to form polyglutamates. We also noted the presence of 7-hydroxy-methotrexate (7-OH-MTX) appearing 48 hours after the beginning of the infusion which was still present in 17/20 samples 30 days after the treatment.