The transport and accumulation of methotrexate in human erythrocytes.

The uptake of methotrexate (MTX) by human erythrocytes was studied in vivo and 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 1 nM-1 micro M, but uptake increased substantially above 10 micro M by a mechanism which was not saturated even at 1 mM. The major fraction of MTX which accumulated by erythrocytes in vitro effluxed 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 vivo administration corresponds to bone marrow maturation time of erythroblasts.