Abstract 3803: Synergistic inhibition of thioridazine combined with carboplatin on triple negative breast cancer by targeting cancer stem cells

In the past decades, chemotherapy has shown encouraging but limited efficacy treating triple negative breast cancer (TNBC). Targeting cancer stem cells (CSCs) has been recognized as a promising method to inhibit tumor growth and dissemination. This study aimed to determine whether an anti-psychotic drug thioridazine (THZ) could sensitize metastatic TNBC cells to anti-cancer drug carboplatin (CBP). To this end, metastatic TNBC (4T1)-bearing animals were treated with THZ, CBP and their combination respectively. MTT, flow cytometry, electron microscope and western blot were performed to evaluate the treated 4T1 cells. Tumor size and lung metastasis were monitored. The percentage of residual 4T1 cancer stem cells and the tumorigenecity of residual tumor cells from each treated group were compared. We found that both mono- and combination treatments suppressed 4T1 cells and induced apoptosis in a dose-dependent manner. These effects involved mitochondrial dysfunction and mitochondria-mediated apoptosis. The structure of mitochondrial membrane was impaired as observed by electron microscope, and the mitochondrial membrane stability was interrupted as evident by down-regulated Bcl-2 & Bcl-xL expression, and up-regulated BAX expression. Both extrinsic and intrinsic apoptosis were demonstrated by enrichment of activated caspase 3, 8, 9. Tumor growth and lung metastasis in 4T1-bearing animals were markedly inhibited by CBP combined with THZ. We also found suppression of PI3k/AKT/mTOR pathway and activation of ER stress by CSP + THZ both in vitro and in vivo. Moreover, angiogenesis was inhibited, demonstrating an important mechanism for tumor metastasis inhibition. More importantly, THZ demonstrated the capability of destroying CSCs and this effect was amplified when combined with CBP. The tumorigenecity of CBP + THZ combination-treated residual tumor cells was significantly impaired. These results show that THZ could inhibit 4T1 tumor cell proliferation and induce apoptosis and significantly destroy the stability of mitochondrial membrane, causing extrinsic and intrinsic apoptosis when combined with CBP. A signature of dysangiogenesis indicated that angiogenesis might be involved in this process. These findings suggest that the anti-psychotic drug THZ is a promising chemosensitizer in TNBC tumor and TNBC patients may benefit from THZ + CBP administration treatment by targeting cancer stem cells. Keywords: Triple-negative breast cancer (TNBC); cancer stem cells (CSCs); thioridazine (THZ); carboplatin (CBP). Citation Format: Yi Wang, Leiming Xia, Li Gong, Yang Xia, Liu Liu, Yuanyuan Liu, Wicha Max, Alfred E. Chang, Qiao Li, Yangyi Bao. Synergistic inhibition of thioridazine combined with carboplatin on triple negative breast cancer by targeting cancer stem cells [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3803.