Multidrug Resistance Protein 1 (MRP1) is frequently overexpressed in tumours where it effluxes chemotherapeutic agents, protecting tumour cells from chemotherapy. MRP1 also effluxes glutathione (GSH), a key antioxidant linked to tumour drug and radiotherapy resistance. However, no selective MRP1 inhibitor possessing suitable properties for clinical use has been described. We have developed CTX-0397592 (‘592’), a novel, highly selective MRP1 inhibitor with favourable pharmacological properties that blocks MRP1 drug efflux whilst simultaneously enhancing MRP1-mediated GSH efflux. We are investigating the utility of this dual-function inhibitor for the treatment of MRP1-overexpressing cancers.
Inhibitor selectivity was determined using cells overexpressing MRP1, P-glycoprotein, or ABCG2 in combination with established drug substrates. Cell lines representing cancers with reported high MRP1 expression (A549 lung cancer, SKOV3 ovarian cancer, and Kelly neuroblastoma) were treated with 592, alone or in combination with the GSH synthesis inhibitor buthionine sulfoximine (BSO) then exposed to chemotherapy or radiation. Viability was assessed by short-term cytotoxicity or clonogenic assays and GSH levels determined by glutathione recycling assay. Ovarian cancer tissue microarrays were scored for MRP1 staining intensity and localisation.
The inhibitor demonstrated unprecedented selectivity for MRP1 over P-glycoprotein and ABCG2, and increased sensitivity of tumour cells to MRP1-substrate drugs. Striking synergy was observed between 592 and BSO, driving near complete GSH depletion in all three cell lines; diminishing clonogenic capacity in A549 and Kelly cells; and radiosensitising and further sensitising A549 cells to vincristine and arsenic trioxide compared to 592 or BSO alone. MRP1 was frequently expressed in the ovarian cancer cohort, with high punctate cytoplasmic MRP1 staining observed in 34.1% and high plasma membrane staining in 8.7% of 264 patients. This study therefore provides preliminary evidence that 592 may provide an enhanced therapeutic window to treat MRP1-overexpressing cancers.