Abstract:
Overexpression of P-gp efflux pumps is believed to be responsible for multidrug resistance of mammalian cancer cells. Inhibition of these pumps is expected to increase the efficacy and decrease the toxicity of chemotherapeutic agents. The non-ionic triblock copolymer Pluronic P85 is reported to inhibit mammalian P-gp efflux pumps, leading to higher intracellular drug concentrations. An analogous, well-characterized efflux transporter, Pdr5p, has been identified in the yeast Saccharomyces cerevisiae, but the effect of Pluronic copolymers on this transporter has not been studied. We have examined the inhibitory effects of P85 on three strains of S. cerevisiae: a pdr5 deletion strain, a PDR5 over-expressing strain, and the PDR5 wild-type strain using the hydrophilic Pdr5p substrate cycloheximide (CHX) as a model antifungal “drug”. Yeast cells were grown in the presence of a range of CHX (0-0.3 mcg/ml) and P85 (0-10 mg/ml). After incubation for 24hrs at 30°C, the ability of P85 to inhibit Pdr5 efflux activity was examined as a function of cellular growth (as determined by absorbance at 600 nm). Our results indicate that P85 lowers the minimal inhibitory concentration of cycloheximide in both the PDR5 wild-type and PDR5 over-expressing strains, while having no effect on the pdr5 deletion strain. This shows that P85 can increase intracellular concentrations of CHX by inhibiting the Pdr5 transporter. Our results in S. cerevisiae are consistent with reported observations in mammalian cells. Further investigations are being performed to (1) see if pluronics F127, L64, and L81 also inhibit Pdr5 using substrates CHX and itraconazole (ITZ), a hydrophobic antifungal drug, and (2) determine whether solubilization of ITZ in P85 micelles influences its antifungal activity.
