Methotrexate kinetics in myeloid bone marrow cells and peripheral neutrophils

The pharmacokinetics of methotrexate (MTX) in the proliferating and the maturing myeloid compartments of the bone marrow and in the neutrophils of the peripheral blood was investigated in four patients with malignant non-Hodgkin's lymphoma after treatment with 24-h MTX infusions (500-790 mg/m2) followed by leukovorin rescue. The myeloid bone marrow cells were separated into two fractions, using a two-step discontinuous Percoll gradient with densities of 1.076 and 1.095 g/ml. The upper fraction consisted predominantly of immature bone marrow cells plus lymphocytes, and the lower fraction contained the mature myeloid bone marrow cells. Cells in the proliferating (immature) myeloid compartment took up and retained MTX to a much greater extent than the mature myeloid cells and the neutrophils. Two days after the MTX infusion no MTX was detected in the neutrophils in spite of a general rise in the total neutrophil count. MTX reappeared in the neutrophils on day 8 in concentrations not related to the concentrations in the proliferating myeloid cells during the MTX infusion. The time of reappearance of MTX in the neutrophils was in accordance with the time it takes for cells in the proliferating pool of the bone marrow to mature and be released into the circulation. No neutropenia was seen after the MTX infusions.