The transport and accumulation of methotrexate in human erythrocytes

The uptake of methotrexate (MTX) by human erythrocytes was studied in vivoand in vitro. Following a pulse intravenous injection of MTX in two subjects, MTX accumulated rapidly in erythrocytes and its intracellular concentration declined at a slower rate than the serum concentration. Twenty‐four hours later erythrocytes had no MTX but the drug reappeared 3–7 days later. In subjects receiving twice monthly MTX, the drug accumulated in the erythrocytes as protein‐bound (61%) and nonprotein bound (39%) forms. The latter is presumably MTX‐polyglutamate. In vitro, MTX uptake by erythrocytes was virtually absent at plasma concentrations of InM‐1μM, but uptake increased substantially above 10 μM by a mechanism which was not saturated even at 1mM. The major fraction of MTX which accumulated by erythrocytes in vitroeffluxed during the first minute and was not protein‐bound, unlike MTX which accumulated in vivo. This suggests that MTX binds to a protein in developing erythroblasts which contain dihydrofolate reductase and that the delayed appearance of MTX in circulating erythrocytes following in vivoadministration corresponds to bone marrow maturation time of erythroblasts.