Cytotoxic action of bisabololoxide A of German chamomile on human leukemia K562 cells in combination with 5-fluorouracil

German chamomile (Matricaria recutita L.) is a popular ingredient in herbal teas. In previous study, micromolar bisabololoxide A, one of main constituents in German chamomile, exerted cytotoxic action on rat thymocyte, a normal non-proliferative cell. This result prompted us to study the effect of bisabololoxide A on proliferative cancer cells and to seek the possibility of its use with 5-fluorouracil, an anticancer agent. In this study, the effect of micromolar bisabololoxide A on human leukemia K562 cells was cytometrically examined. Although the incubation of K562 cells with 10 μM bisabololoxide A for 72 h did not significantly increase the percentage populations of dead cells and shrunken cells, the inhibitory action on the growth was obviously observed. It was not the case for the concentrations of less than 5 μM. The threshold concentration of bisabololoxide A to exert the cytotoxic action on K562 cells was ascertained to be 5–10 μM. Bisabololoxide A at 5–10 μM did not exert cytotoxic action on normal non-proliferative cells (rat thymocytes) in our previous study. Since the antiproliferative action of micromolar bisabololoxide A on cancerous cells was expected to be beneficial to cancer treatment, the modification of antiproliferative action of 5-fluorouracil (3–30 μM) by bisabololoxide A was studied. The combination of 5-fluorouracil and bisabololoxide further inhibited the growth of K562 cells although the additive inhibition of growth by bisabololoxide A became smaller as the concentration of 5-fluorouracil increased. Therefore, it is suggested that the simultaneous application of German chamomile containing bisabololoxide A may reduce the dose of 5-fluorouracil.