Reversal of multidrug resistance by Pluronic block copolymers correlates with inhibition of drug efflux transporters and depletion of intracellular ATP and GSH levels.

2138 Drug resistance (intrinsic or acquired) is a common problem for all chemotherapeutic agents which limits their efficacy in the treatment of numerous cancer types. Multiple drug resistance mechanisms exist which may act simultaneously and/or in concert. Recently, Pluronic block copolymers (PBC) were evaluated for use in drug formulations to treat resistant cancer. It was found that PBC dramatically increased the cytotoxicity of antineoplastic agents in resistant P-glycoprotein (Pgp)-overexpressing cancer cells. Mechanistic studies indicated that exposure of resistant cancer cells to PBC induced significant decreases in intracellular ATP levels along with membrane fluidization, which decreases the affinity of Pgp for the drug and ATP molecules, resulting in powerful sensitization of Pgp-overexpressing cells. To further elucidate the tumor spectrum effectively sensitized by PBC, the present study explored the effects of PBC in a panel of cancer cell lines representing different cell types and drug resistance mechanisms: Pgp-, MRP- (multidrug resistance protein), BCRP- (breast cancer resistance protein) and cisplatin-related resistance. Cytotoxicity studies (measured by MTT assay) were conducted with various antineoplastic agents formulated with PBC in parental/drug-resistant cancer cell line pairs. The results indicated that the trend for sensitization by PBC was Pgp- >> MRP- ≥ BCRP- >> cisplatin-resistance. PBC sensitized Pgp-resistant cells by 100- to 500-fold, MRP- or BCRP-resistant cells by 10- to 130-fold, and had almost no affect on cisplatin-resistant cells. Drug accumulation studies indicated that the extent of inhibition of the drug efflux transporter by PBC decreases in the same order: Pgp (complete) > MRP (partial) > BCRP (low). Thus, the ability of PBC to inhibit drug efflux in resistant cancer cells correlated with the potency of sensitization. A relationship was also found within each drug resistance group: the stronger the decline in intracellular ATP levels following PBC treatment, the higher the sensitization effect in these cells. A similar trend was found for intracellular glutathione levels, which were also depleted following treatment with PBC. Finally, we established that, with some exceptions, epithelial, leukemia, breast and lung cancer cells were the most responsive to PBC treatment and that pancreatic carcinoma cells were among the worst. Hence, reversal of multidrug resistance by PBC correlates with the extent of ATP and glutathione depletion, and inhibition of drug efflux. Overall, these results indicate that careful examination of the cancer type and drug resistance mechanism should be conducted to maximize clinical benefits of PBC formulations for chemotherapy. This study was supported by the National Institutes of Health grant CA89225.