Cyclohexyl-linked tricyclic isoxazoles are potent and selective modulators of the multidrug resistance protein (MRP1)

Structure–activity relationship (SAR) studies on the tricyclic isoxazole series of MRP1 modulators have resulted in the identification of potent and selective inhibitors containing cyclohexyl-based linkers. These studies ultimately identified compound 21b, which reverses drug resistance to MRP1 substrates, such as doxorubicin, in HeLa-T5 cells (EC₅₀ = 0.093 μM), while showing no inherent cytotoxicity. Additionally, 21b inhibits ATP-dependent, MRP1-mediated LTC₄ uptake into membrane vesicles prepared from the MRP1-overexpressing HeLa-T5 cells (EC₅₀ = 0.064 μM) and shows selectivity (1115-fold) against the related transporter, P-glycoprotein, in HL60/Adr and HL60/Vinc cells. Finally, when dosed in combination with the oncolytic MRP1 substrate vincristine, 21b showed tumor regression and growth delay in MRP1-overexpressing tumors in vivo.