PCI29732, a Bruton’s Tyrosine Kinase Inhibitor, Enhanced the Efficacy of Conventional Chemotherapeutic Agents in ABCG2-Overexpressing Cancer Cells

Background/Aims: Multidrug resistance (MDR) induced by the ABC transporter subfamily B member 1 (ABCB1) and subfamilyG member 2 (ABCG2) limits successful cancer chemotherapy and no commercially available MDR modulator is used in the clinic. In the current study, we aimed to investigate the effects of PCI29732 on the enhancement of chemotherapeutic agents. Methods: Cell cytotoxicity and reversal effect were measured with MTT assay. Additionally, flow cytometry was employed to detect the accumulation and efflux of the drugs. We investigated the interaction of PCI29732 and the substrate binding sites of ABCG2 was investigated via the photo-labeling of ABCG2 with the [ 125 I] iodoarylazidoprazosin. The vanadate-sensitive ATPase activity of ABCG2 was measured to identify whether the drug affected the ATPase activity. RT-PCR and Western blot were utilized to analyze mRNA and protein expression respectively. Results: Here, we found that PCI29732 significantly enhanced the efficacy of substrate chemotherapeutic agents in ABCG2-overexpressing cells and also in xenografts harboring the H460/MX20 cell that overexpress ABCG2, but not in their parental sensitive cells and ABCB1-overexpressing cells. Mechanistically, the intracellular accumulations of doxorubicin and Rhodamine 123 were increased in ABCG2-overexpressing S1-MI-80 cells with the presence of PCI29732. PCI29732 stimulated the ATPase activity of ABCG2 at low concentrations. However at the high concentrations, PCI29732 inhibited the ATPase activity, and competed with [ 125 I]-iodoarylazidoprazosin for photo-affinity labeling of ABCG2. PCI29732 did neither alter the mRNA or protein expression levels of ABCG2 nor the phosphorylation levels of AKT and ERK1/2. Conclusion: Our study demonstrates that PCI29732 inhibits the function of ABCG2 by competitively binding to the ATP-binding site of ABCG2 and enhances the anti-tumor efficacy of substrate chemotherapeutic agents, This findings encourages the development of combinational chemotherapy for the treatment of ABCG2- overexpressing cancer patients.