We have previously shown that multidrug resistance protein 1 (MRP1) is highly prognostic of clinical outcome in neuroblastoma (1,2) and contributes to chemoresistance in a neuroblastoma mouse model (3). Although MRP1 inhibitors would have substantial clinical potential, allowing improved cancer control and reduction in the dose of chemotherapeutics used, there are no available agents that are both selective for MRP1 and suitable for in vivo use. Based on our previously identified MRP1 inhibitor Reversan (3), we have developed a series of highly promising new MRP1 inhibitors with a view to identifying a candidate molecule suitable for clinical use.
Methods: To determine in vitro activity and selectivity, cells overexpressing clinically relevant drug transporters (MRP1, P-glycoprotein or ABCG2) were treated with established drug substrates in combination with MRP1 inhibitors and viability assessed. 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 drug etoposide.
Results: Our inhibitors demonstrated unprecedented selectivity for MRP1 over P-glycoprotein and ABCG2, and sensitized neuroblastoma cell lines three-fold to the frontline cancer drugs etoposide and vincristine in vitro (P<0.001). The inhibitors showed very promising preclinical activity in the Th-MYCN mouse model, doubling median survival over etoposide alone (11 days to 21–23 days; P<0.001) without impacting on etoposide pharmacokinetics. Surprisingly, these inhibitors also strongly depleted intracellular GSH in an MRP1-dependent manner. This effect was synergistic with the GSH synthesis inhibitor buthionine sulfoximine.
Conclusion: The selectivity of our inhibitors for MRP1 over other clinically relevant drug transporters 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.