Lethality, DNA Alkylation, and Cell Cycle Effects of Adozelesin (U-73975) on Rodent and Human Cells

Adozelesin (U-73975) is an extremely potent cytotoxic agent which causes 90% lethality, after 2 h exposure in vitro , of Chinese hamster ovary and lung (CHO and V79), mouse melanoma (B16), and human ovarian carcinoma (A2780) cells at 0.33, 0.19, 0.2, and 0.025 ng/ml, respectively. Under similar conditions, Adriamycin and cisplatin had 90% lethality values in CHO cells of 150 ng/ml (=249 nm) and 6800 ng/ml (=2266 nm), respectively. The relative drug sensitivity of the cell lines (A2780 > V79, B16, CHO) was correlated to the relative amounts of [3H]adozelesin alkylated to DNA. The greater sensitivity of A2780 was due to ( a ) greater DNA alkylation at different drug doses and ( b ) greater intrinsic sensitivity of A2780 which resulted in greater cell kill at comparable DNA alkylation. Phase specific toxicity studies show that adozelesin was least lethal to CHO cells in mitosis and very early G1. Lethality increased as cells progressed through G1 and was maximal in late G1 and early S. Mitotic cells had lower drug uptake and correspondingly less drug binding to DNA than G1 or S-phase cells. However, based on the amount of drug alkylated per µg of DNA, cells in M, G1, and S were equally sensitive. Therefore, the lower sensitivity of M-phase cells was due to lower drug uptake. Adozelesin had three different effects on progression of CHO, V79, B16, and A2780 through the cell cycle: ( a ) slowed progression through S which resulted in significantly increasing the percentage of S-phase cells. This effect was transient; ( b ) cell progression was blocked in G2 for a long time period; ( c ) the response of the cell lines to the G2 block differed. CHO and V79 cells escaped G2 block by dividing and entered the diploid DNA cycle or did not undergo cytokinesis and became tetraploid. On the contrary, B16 and A2780 cells remained blocked in G2 and did not become tetraploid. Cell progression was inhibited in a similar manner when a synchronized population of M, G1, or S-phase cells were exposed to adozelesin.