Modulation of daunorubicin cellular resistance by combination of P-glycoprotein blockers acting on drug efflux and intracellular drug sequestration in Golgi vesicles.

Background S9788 and PSC833 were developped as P-glycoprotein (Pgp) blockers and found to act additionally on daunorubicin subcellular distribution, involving different putative targets. On this basis, combinations of S9788 and PSC833 were evaluated in Pgp-expressing MCF7DXR cells in which we recently demonstrated that daunorubicin was sequestered in Golgi vesicles (Bour-Dill et al.: Cytometry, 39: 16–25, 2000). Methods Combinations of S9788 and PSC833 consisted in complementary fractions of iso-effective concentrations (IEC) leading to 90% (IEC90) and median (IEC50) reversion of daunorubicin resistance. Resistance modulation was assessed using cytotoxicity assays, flow cytometry determination of intracellular daunorubicin, and fluorescence microscopy analysis of daunorubicin subcellular distribution. Results Individually, both S9788 and PSC833 were found to be very potent with IEC90 of 5 and 15 μmol/l, and IEC50 of 0.1 and 0.2 μmol/l, respectively, for S9788 and PSC833. When combined, synergistic cytotoxicity was observed for both IEC90 and IEC50 combinations while intracellular daunorubicin fluorescence was only synergistically increased for IEC90 combinations. For IEC50 combinations, no increase in intracellular fluorescence was observed, and fluorescence microscopy examination of the cells suggested that daunorubicin sequestration in Golgi vesicles could be modulated at concentrations that do not significantly increase daunorubicin cellular concentration. Using immunofluorescence and reverse transcription-polymerase chain reaction analyses, multidrug resistance-associated protein, major vault lung-resistance protein, and anthracycline-resistance associated protein were not found to be implicated. Conclusions Synergistic combinations of S9788 and PSC833 might offer alternative ways to decrease the toxicity generated by high-dose Pgp-blockers without altering the efficacy of the resistance modulation. Cytometry 41:62–72, 2000 © 2000 Wiley-Liss, Inc.