Simotinib as a modulator of P-glycoprotein: substrate, inhibitor, or inducer?

As a new antitumor drug, simotinib hydrochloride is prescribed for prolonged periods, often to patients with comorbidities. Therefore, the risk for developing drug resistance and drug-drug interactions between simotinib and other agents has to be taken into consideration. As P-glycoprotein (P-gp) is an efflux transporter, which plays a significant role in drug resistance and influences the pharmacological properties and toxicities of the drugs it interacts with, the interactions between simotinib and P-gp were investigated. Cytotoxicity was measured using the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide assay. Intracellular drug concentrations were detected by high-performance liquid chromatography, fluorescence-activated cell sorting and using a fluorescence reader. P-gp ATPase activity was measured using the Pgp-Glo assay, and intracellular pH was assessed using the fluorescent probe 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein acetoxymethyl. The expression and transcription of P-gp were detected by western blotting and the luciferase assay. Simotinib has no cross-resistance to P-gp substrates, and its efflux rate was independent of either the P-gp expression or the coadministered P-gp substrate. Simotinib reversed chemotherapeutic agent resistance in a short time by increasing the intracellular concentration of the chemotherapeutic agent and blocked rhodamine 123 efflux. Further studies demonstrated that simotinib inhibited P-gp activity by modulating its ATPase activity and the intracellular pH. Although simotinib induced P-gp expression after extended treatment, the induced expression of P-gp had little impact on drug resistance. Simotinib is not a substrate of P-gp. As a modulator, it functions mainly as an inhibitor of P-gp by modulating the intracellular pH and ATPase activity, although it also induces P-gp expression after extended treatment.