Tetrahydroquinoline/4,5-dihydroisoxazoline derivatives counteract multidrug resistance in cancer cells by inhibiting P-glycoprotein (ABCB1)-mediated transport.

Cancer treatment is challenged by the emergence of multidrug resistance (MDR). MDR is often caused by the overexpression of certain ABC transporters, such as P-glycoprotein (P-gp, ABCB1) in the plasma membrane of tumor cells and tumor stem cells. Inhibition of ABC transporter-mediated efflux of anticancer drugs might be a plausible approach to overcome MDR. Here, we studied the interaction of 16 tetrahydroquinoline/4,5-dihydroisoxazole derivatives (A1 - D4) with human P-gp to identify and characterize new P-gp inhibitors. We found that compounds C1 and D1 inhibited the P-gp-mediated efflux of rhodamine 123 (R123), with IC50 values of 41.5 and 6.6 μM, respectively. Both compounds showed low cytotoxicity on NIH3T3 and NIH3T3-ABCB1 cells over a broad concentration range. Interestingly, C1 and D1 increased the ATPase activity of P-gp at sub-micromolar concentrations, showing EC50 values of 0.17 and 0.62 μM, respectively. However, thermal inactivation and UIC2 reactivity assays supported that, similar to potent P-gp inhibitors, C1 and D1 can hinder the dimerization of the nucleotide binding domains (NBDs), when applied at higher concentrations (≥10 μM). In addition, docking studies showed that D1 preferentially interacts with the central substrate binding cavity of P-gp. Finally, D1 chemosensitized drug-resistant KB-V1 cells overexpressing P-gp. In view of our previous findings that C1 and D1 also inhibit ABCG2 and MRP1, they can be considered as novel pan-ABC transporter inhibitors offering potential for treating chemotherapy-resistant tumors.