Apoptosis induced by doxorubicin and cinchonine in P388 multidrug-resistant cells

Acquired drug resistance is a major factor in the failure of doxorubicin-ba sed cancer chemotherapy. We determined the ability of cinchonine to reverse doxorubicin drug resistance in a doxorubicin-resistant leukaemia cell line (mouse P388/DOX). A non-cytotoxic concentration of cinchonine (10 muM) incr eased the sensitivity to doxorubicin of multidrug-resistant P388/DOX cells and significantly enhanced the doxorubicin-induced apoptosis and DNA fragme ntation in resistant cells, but had no effect in parent cells. Time-course studies demonstrated that DNA fragmentation was present 24 h after incubati on with doxorubicin and cinchonine, indicating that DNA degradation was a p receding event. In cultured cells, cinchonine increased the intracellular a ccumulation of doxorubicin in the resistant cells in a dose-dependent manne r. Using flow cytometry to measure the inhibition of the P-glycoprotein (P- gp) dependent efflux of rhodamine 123, cinchonine was found to be considera bly more effective than quinine. The results with cinchonine suggest that t here may be quinine derivatives with a similar capacity to inhibit drug tra nsport by P-gp. Additionally, the G(2)/M phase cell population in resistant cells is increased by doxorubicin/cinchonine treatment. Exposure of resist ant cells to 1 muM doxorubicin and 10 muM cinchonine resulted in the expres sion of Fas (APO-1/CD95) in cells after 6 h. These studies demonstrate that the cell killing effects of doxorubicin and cinchonine in resistant cells are mediated, at least in part, by the induction of apoptosis.