Abstract PD5-09: PD5-09 Oncogenic Pathways of Tumor Cells Signal to Stem-ness in TNBC: Relationship between Stem Cell Markers and PI3K-DDR Pathway Targeted Drugs
Authors: Dey Nandini , Aske Jennifer , Lin Xiaoqian , Dale Adam , Sun Yuliang , De Pradip 
Published: March 2023

Abstract
Background: The interest in cancer stem cells in TNBC is encouraged by the finding that CD44+/CD24- and ALDH1+ breast cancer stem cells (CSC) are enriched in TNBC and contribute to chemotherapy resistance and aggressive tumor progression (O’Conor et al., 2018; Ma et al. 2014). CD44+ CD24- phenotype can be utilized to determine TNBC patients’ prognosis (Shadbad et al., 2021; Wang et al., 2017). CD44 has also been known for its tumor-intrinsic function in TNBC (Kong et al., Cancer Res. 2020). We have reported a functional correlation between CD44+/CD24- cancer stem cells and 3D clonogenic growth in testing rational combinations of PI3K pathway inhibitors and Wnt-beta-catenin pathway inhibitors in TNBC (Aske et al., 2016; 2017). We also reported that doubling down on the PI3K pathway enhances the anti-tumor efficacy of PARP inhibitor in TNBC (De et al., 2014). Hypothesis: We hypothesized that the anti-tumor effect of PI3K pathway targeted drugs in combination with PARP inhibitor will regulate the expression of CSC in TNBC. Methods: To test our hypothesis, we examined the expression of stem cell markers CD44, CD44v6, and CD24 in MDA-MB-231, SUM149, and MDA-MB-468 TNBC cells in vitro and in vivo. IHC for CD44 and CD44v6 was standardized using tumor controls and identified in (1) BC TMAs, (2) TNBC cell lines, and (3) patients from our Avera cohort. FFPE sections of tumors from athymic mice (bearing xenograft tumors) treated with a combination of Apitolisib with Veliparib plus carboplatin were used to test stem cell markers by IHC. Staining was evaluated independently by a pathologist who was unaware of the study design. Mechanism-based in vitro studies were conducted to understand the mode of action of the drugs using flow cytometry and qRT-PCR. Results: CD44 was identified primarily in the membrane and to a lesser extent in the cytosol of tumor cells of BC patients (TMA and Avera cohort), TNBC cell lines, and xenograft-tumors. Apitolisib, in combination with Veliparib plus carboplatin, blocked the growth of established xenograft tumors by 80-90%, with a concomitant decrease in Ki67 levels. Membrane expression of CD44 inversely correlated to tumor sizes which significantly reduced in response to drug combinations. The relationship between the expression of CD44 and CD44v6 is being worked out, which will be presented at the meeting. Significance: Our findings showed that the anti-tumor effect of Veliparib plus carboplatin and Apitolisib effectively blocked the specific CD44v6 CSC population in TNBC xenograft tumors. Our results demonstrate the integral role of the PI3K-mTOR and the DDR pathways in the control of stem-ness orchestrating anti-tumor actions of PARP inhibitor and mTOR-PI3K dual inhibitors in TNBC.
Citation Format: Nandini Dey, Jennifer Aske, Xiaoqian Lin, Adam Dale, Yuliang Sun, Pradip De. PD5-09 Oncogenic Pathways of Tumor Cells Signal to Stem-ness in TNBC: Relationship between Stem Cell Markers and PI3K-DDR Pathway Targeted Drugs [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr PD5-09.