Perturbation of Cell Cycle of Human Leukaemic Myeloblasts in vivo by Cytosine Arabinoside

The effect of a single intravenous flash injection of cytosine‐arabinoside (ARA‐C) (2.0–2.5 mg/kg BW) on the cell cycle of leukaemic myeloblasts in vivo was studied in 7 patients with acute myeloid leukaemia. It was found that the 3 H‐thymidine labelling index, the mitotic index and the mean grain count of the cells showed a transitory decrease but within the observation period (36 h) returned to pretreatment values. Microspectrophotometric measurements of single cell DNA content suggested an undisturbed flux of cells into DNA synthesis from G 1 phase. The findings are in accordance with the assumption that ARA‐C arrests cells in DNA synthesis without directly interfering with the progression of cells through G 1 , G 2 , or mitosis. Problems of cell synchronization and recruitment due to cytotoxic therapy are discussed.