Cucurbitacin E increases the doxorubicin (DOX) level in M5076 ovarian sarcoma via suppressed DOX efflux in vitro. An increase in DOX induced antitumor activity by cucurbitacin E in vivo has been reported previously. This paper attempts to clarify the mechanism of cucurbitacin E induced increments in the antitumor activity of DOX. MK-571, a multidrug resistance associated protein (MRP) inhibitor, significantly suppressed DOX efflux from M5076 ovarian sarcoma cells. The combination of cucurbitacin E with MK-571 also inhibited DOX efflux, whereas the efficacy was the same in each treatment. Namely, the inhibition of DOX efflux by cucurbitacin E was expected to be related to MRP. In contrast, it appeared that the effect of cucurbitacin E on DOX permeability did not relate to P-gp. The cucurbitacin E co-treatment significantly increased DOX concentration in the tumor within a short time after DOX administration, whereas the same treatment decreased the DOX concentration in normal tissues. The different effects of cucurbitacin E between tumor and normal tissues was speculated to be related to differences in DOX transport system on cell membrane. In DOX therapy, cucurbitacin E co-treatment was expected to increase DOX induced antitumor activity without an increase in adverse reactions due to DOX.
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