Abstract PO5-13-10: Determining tumor-driving cell signaling pathways in breast cancer: Support for targeted therapy selection

Abstract Introduction: Success of therapeutic interventions largely rely on the pathological and biological characteristics of the tumor and varies due to the heterogeneous nature of breast cancers. Signal transduction pathways play an important role and are often used as target for specific therapies like hormone treatment, PI3K inhibitors etc. Immunohistochemistry (IHC) is currently the first assessment to divide tumors for subtyping and treatment and, especially in more advanced disease, mutation analyses are often used. Both IHC and mutation analyses only analyze a specific part of a signal transduction pathway and do not correlate well with outcome. Therefore, there is a need for new approaches to look at signaling pathways in cancer. We have developed a methodology that measures and quantifies signal transduction pathways (STPs) activities to reveal potential tumor-driving STPs to identify new options for targeted therapy in breast cancer. Method: Using the mRNA-based OncoSIGNal pathway activity profiling qPCR test (InnoSIGN), STP activities of 7 pathways (AR, ER, PI3K, MAPK, HH, TGFβ and Notch) were measured and quantified on a scale from 0-100. By using a set of non-tumor epithelial breast tissue samples, a reference range for normal physiological pathway activity was defined. A threshold was set on the 95th percentile of this range. A pathway activity score above this threshold was considered to be high (activated). Using this methodology, 201 ER positive (based on IHC) and 64 triple negative FFPE breast tumor samples obtained from different study sites and commercial biobanks were analyzed, including samples from primary and metastatic lesions, to determine for each individual tumor its pathway activity profile and identify activated tumor-driving pathways. Results: In the 195 primary ER positive tumors, the ER pathway was found to be activated in 78% of the samples, even though all samples were strongly ER immunohistochemistry stain positive. In addition to ER pathway activation, occasionally other pathways were activated: PI3K in 37%, HH in 25%, MAPK in 19%, AR in 20%, TGFβ in 6%, and Notch in 1%, of these samples. Analysis of 27 metastatic samples of ER positive tumors revealed in only 63% an active ER pathway. Additionally, more frequently PI3K pathway was observed in 70% of the samples. Other frequencies of pathway activation were: AR in 7% and MAPK in 4%, No Notch and TGFβ activity was found in this cohort. In 76 of the 78 triple negative tumors (TNBC) the ER pathway was inactive as expected. Remarkably, 2 samples appeared to have an activated ER pathway. An activated MAPK pathway was found in 50%, PI3K in 49%, HH in 18%, AR in 14%, TGFβ in 5% and Notch in 3% of these TNBC tumors. Conclusion: Compared to ER positive primary tumors we observed in metastatic ER positive tumors less frequent activation of the ER and more frequent activation of the PI3K pathway, suggesting that the latter pathway is associated with a more aggressive tumor phenotype. The triple negative subtype is characterized predominantly by activation of the PI3K, MAPK, AR and/or HH pathways, which may create new options for personalized targeted therapy of this subtype. OncoSIGNal can be used to determine the tumor driving signaling pathways in breast cancer patients, guiding selection of personalized targeted therapies. Citation Format: Paul van de Wiel, Yvonne Wesseling-Rozendaal, Anke Pierik, Dianne van Strijp, Saskia Vermeer, Fatema Tashrifwala, Josette Hartnett, Robert Babkowski, Suzanne Rose. Determining tumor-driving cell signaling pathways in breast cancer: Support for targeted therapy selection abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-13-10.