Background: Patients with triple-negative breast cancers (TNBCs) develop chemotherapeutic resistance and have a high mortality rate. Classification of TNBCs based on molecular profiling has enabled the identification of one subtype that is particularly enriched for cancer stem cells (CSCs) and genes involved in epithelial-mesenchymal transition (EMT) pathways. The EMT pathways, as well as CSCs, have been independently associated with chemo-resistance. We have previously demonstrated that cancer cells can acquire CSC properties via the activation of EMT. The FOXC2 transcription factor is a critical mediator for EMT/CSC properties as well as metastatic competence. Activated p38 MAP kinase is also high in EMT/CSC-enriched TNBCs and is important for FOXC2 protein stability, CSC and EMT properties as well as metastasis. We hypothesize that the inhibition of p38 MAP kinase will increase the sensitivity of TNBCs to chemotherapeutic drugs and in combination, it will help reduce tumor recurrence.

Methods: Three clinical candidate p38 inhibitors (LY2228820, VX-702, and PH-797804) were tested next to SB203580, which has shown great efficacy in our pre-clinical models. Effects of p38 inhibition were measured using sphere assay, CD44hi/CD24lo profiles and their effect on FOXC2 protein levels using cells lines as well as cells isolated from PDX. To identify drugs that work in combination with p38 inhibitor, cancer cells were pre-treated with p38 inhibitors and then co-treated with p38 inhibitor as well as NCI Approved Oncology/Custom Clinical Collections of 256 approved drugs and the Broad Informer Collection of about 400 mechanistic compounds.

Results: Inhibition of p38 significantly impaired stem cell properties in a dose-dependent manner. P38 inhibitor worked in combination with Gemcitabine or Epirubicin (currently used for TNBC treatment) and reduced the TNBC cell proliferation. Among PDX samples capable of forming spheres, p38 inhibition reduced their stemness in a dose-dependent manner. P38 inhibition also decreased expression of the stemness marker FOXC2 and increased the expression of the epithelial marker E-cadherin.

Conclusion: P38 inhibition is an effective way to inhibit EMT/CSC properties and it can work in combination with other agents to increase sensitivity in TNBCs.

Acknowledgments: This work was supported by CPRIT-MIRA RP-160710 (SAM, WFS).

Citation Format: Petra den Hollander, Shruti Shah, Xinhui Zhou, Abena Redwood, Shi-Rong Cai, Mary Sobieski, David Brunell, Clifford Stephan, Peter Davies, Helen Piwnica-Worms, Stacy Moulder, W Fraser Symmans, Sendurai A. Mani. Overcoming therapy resistance in stem cell-rich triple negative breast cancer through p38 MAP kinase inhibition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4669.