Effects of olomoucine, a selective inhibitor of cyclin-dependent kinases, on cell cycle progression in human cancer cell lines

We have studied the effects of olomoucine, a selective inhibitor of cdk2, cdc2 and MAP kinase, on the rate of proliferation and the cell cycle progression in human cancer cells in culture. Olomoucine inhibited the growth of the KB 3-1, MDA-MB-231 and Evsa-T cell lines in a concentration-dependent manner, with EC50 values of 45, 75 and 85 mu M, respectively. Incubation of exponentially growing KB 3-1 cells in the presence of olomoucine led to an increased proportion of cells in G(1) phase after 24 h or more of incubation. Olomoucine failed to rapidly affect the phosphorylation of the Rb tumor-supressor gene product. However, [H-3]thymidine incorporation into the cell DNA was rapidly inhibited. We show that this inhibition is due, at least in part, to the diminution of thymidine entry into the cells. Suprisingly, all these cell lines, when synchronized at the G(1)/S interface and relaxed in the presence of olomoucine, progressed unhindered through the S phase. Under these conditions, the G(2) phase transit was markedly retarded but not prevented. Insufficient permeability of the cell membrane to olomoucine may explain the low activity of the drug.