Effects of the Epipodophyllotoxin VP-16-213 on Cell Cycle Traverse, DMA Synthesis, and DMA Strand Size in Cultures of Human Leukemic Lymphoblasts1

Serial studies of human leukemic lymphoblasts (CCRF-CEM line) cultured with 0.25 to 2.5 »MVP-16-213 for 0 to 6 hr indicated that the mechanism of cytotoxicity of this compound involves a primary effect on DMA. The most striking early change shown by flow cytometry in VP-16-213-treated cells was a delay in Sphase transit before arrest of cells in G2. Coinciding with this Sphase delay was a selective inhibition of thymidine incorporation into DNA as well as concentration-dependent scission of DMA strands. Using alkaline elution methods, we were able to detect DNA breakage at concentrations of VP-16-213 well below the level required to demonstrate kinetic effects or inhibition of DNA synthesis. These data suggest that DNA strand scission is the initial event in the sequence of kinetic and biosynthetic changes leading to growth inhibition and death of VP-16-213-treated cells. Inhibition of replicÃ3n initiation due to strand scission is a plausible explanation for the cytotoxic action of this podophyllotoxin deriv ative.