Reversal of multidrug resistance by valinomycin is overcome by CCCP

Reversal of P-glycoprotein-mediated multidrug resistance by valinomycin is overcome by the proton ionophore, CCCP. This effect, a complete suppression of the 5- to 10-fold valinomycin-induced reversal ("re-reversal"), exhibits a sharp extracellular potassium concentration ([K+(0)]) dependence. It is observed at [K+(0)] > 2-4 mM and not at [K+(0)] greater than or equal to 2 mM, in the case of the fluorescent substrates rhodamine 123 and daunorubicin. The fact that "re-reversal" is detected only for the combination of CCCP with valinomycin raises the possibility that a direct interaction between these ionophores may explain the phenomenon. We show spectroscopic evidence of such an interaction, with a [K+(0)]-dependence similar to that of the "re-reversal." These data suggest that the reversal of P-glycoprotein activity by valinomycin can be compromised by anionic compounds such as CCCP due to complex formation. More generally, molecular interactions involving P-glycoprotein substrates or reversing agents may significantly affect drug accumulation in multidrug resistant cells.