Combined effects of buthionine sulfoximine and cepharanthine on cytotoxic activity of doxorubicin to multidrug-resistant cells.

We studied the potentiation of doxorubicin (DOX) activity in multidrug-resistant (MDR) cells by buthionine sulfoximine (BSO), a specific inhibitor of gamma-glutamylcysteine synthetase, and by cepharanthine (CE), which interacts with P-glycoprotein. The glutathione (GSH) of MDR cells was approximately 1.5-fold greater than that of the parental cell line. BSO reduced GSH content of MDR cells compared to that of the sensitive ones. The BSO treatment (50 microM) enhanced the effect of DOX by 1.8-fold, while CE caused a greater reversal of drug resistance. The combination of BSO with CE produced further potentiation of DOX activity in an antiproliferative effect. Pretreatment of cells with BSO did not alter the cellular accumulation of DOX in the absence or presence of CE. The addition of BSO (30 mM) to the drinking water of mice reduced the tissue levels of GSH in tumor cells, suggesting that the marked decrease in GSH might diminish the ability of that tumor to resist DOX. Combined administration of CE and DOX resulted in enhancement of DOX antitumor activity and prolongation of survival time. The survival of mice treated with BSO and CE as a supplement to DOX treatment was superior that of mice receiving DOX alone. These studies demonstrated that the combinations of BSO with CE may be useful for killing drug-resistant tumor cells.