In vitro activity: Etoposide inhibits DNA synthesis by forming a complex with topoisomerase II and DNA, which induces breaks in double stranded DNA and prevents repair by topoisomerase II binding. Accumulated breaks in DNA prevent entry into the mitotic phase of cell division, and lead to cell death. Etoposide acts primarily in the G2 and S phases of the cell cycle. Etoposide inhibits the growth of murine angiosarcoma cell line (ISOS-1) in a 5 days-period with IC50 of 0.25 μg/mL. Cell growth of normal murine microvascular endothelial cells (mECs) is less sensitive to Etoposide with IC50 of 10 μg/mL). Etoposide treated for 6 hr inhibits colonies of tetraploid variant of the human leukemic lymphoblast line CCRF-CEM with IC50 of 0.6 μM. Etoposide treated for 2 hr inhibits growth of human pancreatic cancer cell line Y1, Y3, Y5, Y19, YM. YS, and YT with IC50s of 300 μg/mL, 300 μg/mL, 300 μg/mL, 91 μg/mL, 0.68 μg/mL, 300 μg/mL, 300 μg/mL, and 260 μg/mL, respectively. Etoposide exposed for 1 hr inhibits growth of human glioma cell lines CL5, G142, G152, G111, and G5 with IC50 of 8, 9, 9.8, 10, and 15.8 μg/mL respectively for 12 days. Under same condition, the IC90 value is attained in cell lines CL5, G152, G142, and G111 at 26, 27, 32, and 33 μg/mL. Etoposide inhibition of topoisomerase II is homogeneous for each cell. The average inhibition rates are 15%, 21.8%, 31.8%, 41.5%, and 49.5% for 1, 2, 4, 8, and 16 μg Etoposide, respectively.

Kinase Assay: Nuclear extracts are prepared, and nuclei are isolated. The activity of topoisomerase II is calculated from the percentage of decatenation obtained. Tritiated kinoplast DNA (KDNA 0.22 μg) is used as a substrate. Etoposide and topoisomerase II are incubated for 30 min at 37 ℃ and are stopped with 1% sodium dodecyl sulfate (SDS) and proteinase K (100 μg/mL). The percentages of decatenation and inhibition of topoisomerase II by Etoposide are obtained.

Cell Assay: After the Etoposide treatment, cells are removed from the dish with phosphate-buffered saline (PBS) containing 0.03% trypsin and 0.27 mM ethylenediaminetetraacetic acid (EDTA) and are diluted into culture dishes in appropriate numbers to yield between 20 and 200 colonies. After 12 days, cultures are fixed with methanol-acetic acid, stained with crystal violet, and scored for colonies containing more than 50 cells. The standard errors are typically less than 15% of the mean value unless otherwise stated.

J Dermatol Sci. 2000 Nov;24(2):126-33; J Natl Cancer Inst. 1988 Dec 7;80(19):1526-33.