Multidrug resistance protein 1 (MRP1) is frequently upregulated in cancer and has an established role in chemoresistance. MRP1 inhibitors would have substantial clinical potential, allowing improved cancer control or reduction in the dose of chemotherapeutics used, however there are no available agents that are both selective for MRP1 and suitable for in vivo use. We have developed a series of promising new MRP1 inhibitors and are characterizing their selectivity and preclinical activity with a view to identifying a candidate molecule suitable for clinical trials. We are particularly focused on neuroblastoma, a malignancy where high MRP1 is strongly associated with poor outcome.

Methods: In vitro activity and selectivity were determined using cytotoxicity assays in cells overexpressing MRP1, P-glycoprotein and other clinically relevant drug transporters in combination with established drug substrates. Levels of the endogenous MRP1 substrate glutathione (GSH) were assessed by GSH recycling assay. In vivo activity was assessed in the TH-MYCN mouse neuroblastoma model in combination with the MRP1 substrate etoposide.

Results: The inhibitors demonstrated unprecedented selectivity for MRP1 over P-glycoprotein and showed very promising preclinical activity in the TH-MYCN mouse model, doubling median survival over etoposide alone. Surprisingly, the inhibitors also strongly depleted intracellular GSH in an MRP1-dependent manner. This effect was synergistic with the GSH synthesis inhibitor buthionine sulfoximine (BSO) and conferred a collateral sensitivity to certain cytotoxic agents in MRP1 overexpressing cells.

Conclusion: The selectivity of our inhibitors for MRP1 over P-glycoprotein and their demonstrated preclinical activity is a major advance over previously developed compounds. MRP1-dependent depletion of GSH may provide an additional therapeutic window in MRP1 overexpressing tumours.