Flavopiridol (L86-8275) is a synthetic flavone currently undergoing Phase I clinical trials. It is active against a series of human cancer cell lines and has been shown to inhibit a broad range of protein kinases, including cyclin-dependent kinases and protein kinase C (PKC). Previous studies have shown that the PKC-specific inhibitor safingol significantly enhances the induction of apoptosis by mitomycin-C (MMC) in gastric cancer cells. Because flavopiridol can potentially inhibit PKC, we elected to determine the extent to which flavopiridol would promote MMC-induced apoptosis in both gastric and breast cancer cells. For these studies, MKN-74 gastric cancer cells and MDA-MB-468 breast cancer cells were exposed to either no drug, 1 microgram/ml MMC alone, 300 nM flavopiridol alone, or a combination of chemotherapy with flavopiridol for 24 h. Sequence specificity was also examined by first exposing cells to MMC for 24 h followed by flavopiridol for 24 h or to the same drugs in the reverse order. Apoptosis was measured by quantitative fluorescence microscopy of nuclear chromatin condensation in cells stained with the dye, bisbenzimide trihydrochloride. Exposure of MKN-74 cells to flavopiridol alone induced apoptosis in 12 +/- 1% of the cells, and exposure to MMC alone induced apoptosis in 10 +/- 1%. However, the combination of flavopiridol and MMC increased the induction of apoptosis to 55 +/- 3% of the cells (P < 0.005 for the drug combination versus flavopiridol alone). Pretreatment with the PKC activator 3-phorbol 12-myristate 13-acetate only partially reversed this effect (43 +/- 1%; P < 0.025). In MDA-MB-468 cells, flavopiridol alone induced apoptosis in 17 +/- 1% of the cells, and MMC alone induced apoptosis in 10 +/- 1% of the cells. The combination of flavopiridol and MMC increased the percentage of MDA-MB-468 cells undergoing apoptosis to 58 +/- 4% (P < 0.005 for the drug combination versus flavopiridol alone). Sequential treatment with MMC followed by flavopiridol induced apoptosis in 63 +/- 2% of the MKN-74 cells (P < 0.05 versus the concomitant drug combination) and in 76 +/- 2% of the MDA-MB-468 cells (P < 0.025 versus the concomitant drug combination), whereas flavopiridol followed by MMC did not increase the induction of apoptosis in either cell line. As determined by the terminal deoxynucleotidyl transferase labeling of the 3' ends of DNA fragments produced in apoptotic cells, the induction of apoptosis with the combination of flavopiridol and MMC occurred to MKN-74 cells in all phases of the cell cycle (i.e., G0-G1, S, and G2-M). These results indicate that flavopiridol potentiates the cytotoxic effect of the chemotherapeutic agent MMC by promoting drug-induced apoptosis in tumor cells. Sequencing studies suggest that MMC followed by flavopiridol or simultaneous treatment is superior to flavopiridol followed by MMC. The enhancement of MMC-induced apoptosis by flavopiridol may be partially PKC dependent and is not associated with one specific region of the cell cycle.