Tetrahydroquinoline/4,5-dihydroisoxazoline hybrids counteracts multidrug resistance by inhibiting MRP1-mediated drug efflux

Current cancer treatments are challenged by multidrug resistance (MDR). MDR can be a result of overexpression of ABC transporters, such as the multidrug resistance-associated protein 1 (MRP1). They act as efflux pumps expelling chemotherapy outside of the cells, and ABC transport inhibitors can overcome MDR. To identify and characterize new MRP1 inhibitors, 16 tetrahydroquinoline/4,5-dihydroisoxazole derivatives (A1 - D4) were tested in stably transfected cells overexpressing MRP1 (BHK21-MRP1). Compounds A1 and D1 showed the best results (IC₅₀ of 0.58 and 2.74 μM, respectively). They showed low cytotoxicity and were not transported by MRP1. Compound A1 showed a therapeutic ratio (TR) higher than 170. Our structural model's analysis suggests that A1 fluctuates between the P and H-pockets in the MRP1. A1 interacts with Gln377 and Lys332 and is further supported by π-contacts with Trp1246 and Phe594. The different binding site of A1 and the substrate calcein-AM, as revealed by the molecular modelling analysis, was confirmed by the non-competitive inhibition observed by flow cytometry. The major significance lies in the confirmation of the A1 and D1 inhibition, as they improved the effect of chemotherapeutics (etoposide and daunorubicin) in cells overexpressing MRP1. Together, these results demonstrated the potential of A1 to follow in pre-clinical studies.