P-glycoprotein-expressing tumor cells are resistant to anticancer drugs inhuman gastrointestinal cancer

The resistance to doxorubicin (DOX) by some tumor cells is mainly due to th e effect of P-glycoprotein encoded by the multidrug resistance-1 (mdr1) gen e. We tried to prove the correlations between P-glycoprotein expression and the sensitivity for anticancer drugs including DOX and other cytotoxic dru gs that are currently used for gastrointestinal cancer patients, We quantif ied the P-glycoprotein expression by flow cytometry techniques, and the sen sitivity for anticancer drugs using a tetrazolium salt, 3-(4,5-di-methylfhi azol-20-yl)-2,5-diphenyl tetrazolium bromide (MTT), assay in highly purifie d fresh human tumor cells obtained from 25 cancer patients. The inhibition rates were the lowest in DOX and mitomycin C (MMC), compared with other dru gs, The most significant correlation between DOX and MMC was seen in the in hibition rates. A significant correlation was also seen between the inhibit ion rates for DOX and P-glycoprotein expression, whereas only a slight corr elation between the sensitivity for MMC and P-glycoprotein expression was o bserved. We should therefore pay close attention to the effect of P-glycopr otein when treating cancer patients, especially if both the inhibition rate s of DOX and MMC are low based on the findings of an MTT assay.