Inhibition of p-glycoprotein does not increase the efficacy of proteasome inhibitors in multiple myeloma cells

The aim of this study is to determine whether manipulation of the drug transporter P-glycoprotein improves the efficacy of proteasome inhibitors in multiple myeloma cells. P-glycoprotein is a well-known drug transporter that is associated with chemotherapy resistance in a number of cancers but its role in modulating the efficacy of proteasome inhibitors in multiple myeloma is not well understood. Research has shown that the second generation proteasome inhibitor carfilzomib is a substrate of P-glycoprotein and as such its efficacy may correlate with P-glycoprotein activity. In contrast to carfilzomib, research concerning the first-in-class proteasome inhibitor bortezomib is inconsistent with some reports suggesting that inhibition of P-glycoprotein increases bortezomib cytotoxicity in multiple myeloma cells whereas others have shown no effect. Through the mining of publicly available gene expression microarrays of patient bone marrow, we show that P-glycoprotein gene expression increases with the disease stages leading to multiple myeloma. However, RNA-seq on LP-1 cells treated with bortezomib or carfilzomib demonstrated minimal basal P-glycoprotein expression which did not increase with treatment. Moreover, only one (KMS-18) of nine multiple myeloma cell lines expressed P-glycoprotein, including RPMI-8226 cells that are resistant to bortezomib or carfilzomib. We hypothesised that by inhibiting P-glycoprotein, multiple myeloma cell sensitivity to proteasome inhibitors would increase, thus providing a potential approach to improving responses and reversing resistance to these agents. However, the sensitivity of multiple myeloma cells lines to proteasome inhibition was not enhanced by inhibition of P-glycoprotein with the specific inhibitor tariquidar. In addition, targeting glucosylceramide synthase with eliglustat did not inhibit P-glycoprotein activity and also did not improve proteasome inhibitor efficacy except at a high concentration. We conclude that P-glycoprotein is poorly expressed in multiple myeloma cells, its inhibition does not enhance the efficacy of proteasome inhibitors, and it is unlikely to be a useful avenue for further translational research.