Persisting production of out-of-cycle blasts during methotrexate therapy in acute leukaemia

Kinetics of the blast populations in two cases of acute lymphoblastic leukaemia during treatment with methotrexate have been studied in vivo. Beside a striking cytocidal effect on blasts arrested in the S phase and a synchronization and a slowing of those in the G2 phase, the most important finding was the persistence of a flux of drug-affected cells from the proliferation of the non-proliferating compartment. Some of these non-proliferating cells were able to re-enter subsequently the mitotic cycle. This fact can explain why a cell-cycle-specific drug fails to eradicate completely acute leukaemic cells in man.