CYCLOHEXIMIDE INHIBITS THE CYTOTOXICITY OF PACLITAXEL (TAXOL(R))

Treatment of human breast (MCF-7) and lung (A549) adenocarcinoma cell lines with 10 mug/ml cycloheximide provided substantial protection fro m paclitaxel-induced cytotoxicity. Addition of cycloheximide to cells at 0, 6, 12 or 18 h into a 24 h exposure to paclitaxel resulted in cyt otoxicity similar to that found in cells treated with paclitaxel alone for only 0, 6, 12 or 18 h, respectively. DNA flow cytometry showed th at paclitaxel blocked cells in G2/M. Mitotic index studies demonstrate d that paclitaxel arrested cells in mitosis and that prolonged exposur e to paclitaxel resulted in the development of multiple micronuclei. C oncurrent incubation of cells in cycloheximide prevented the developme nt of a G2/M block, mitotic arrest and micronuclei formation. The addi tion of cycloheximide to cells at 6 or 12 h into a 24 h exposure to pa clitaxel reduced the degree of G2/M block to that produced by incubati on of cells in paclitaxel alone for only 6 or 12 h. Mitotic index stud ies confirmed that cells treated with cycloheximide during paclitaxel exposure had a marked reduction in the percentage of cells in mitosis. However, the percentage of paclitaxel-treated cells which had multipl e micronuclei was increased in cells treated with cycloheximide. These results indicate that entry into mitosis is a prerequisite for paclit axel-induced cytotoxicity and that cycloheximide reduces cytotoxicity due to paclitaxel by preventing cells from entering mitosis. However, once cells have entered mitosis in the presence of paclitaxel, protein synthesis is not required for the development of multiple micronuclei and cytotoxicity.