Breast cancer (BC) is the second leading cause of cancer associated deaths in women globally (1). Broadcategorization of BC is based on protein receptor expression: triple negative breast cancer (TNBC) lacksexpression of estrogen receptors, progesterone receptors, and exhibits non-amplification of HER2/Neu. Due tothe receptor status in TNBC, hormone-based targeted therapies are not effective; systemic chemotherapy is theprimary treatment which results in significant adverse effects for patients (2). TNBC would benefit significantlyfrom the development of novel targeted therapeutics and in this study, we describe a small molecule inhibitor-based approach to identify candidate targets.We screened TNBC cells using the Published Kinase Inhibitor Set (PKIS), a highly annotated set of kinaseinhibitors (6) and demonstrated the utility of screening chemogenomic sets for target vulnerability identification. Ahigher rate of metastasis is a defining characteristic of the TNBC subtype; a proposed mechanism for acquisitionof a metastatic phenotype is a cellular process known as epithelial-mesenchymal transition (EMT). In an initialadherent cell culture screen of the PKIS library using established TNBC cell lines (MDA-MB-231, BT-549) andprimary patient-derived TNBC cells (TU-BCX-4IC, TU-BCX-49S), we identified three compounds that had themost dramatic effects on reversing the mesenchymal cell phenotype such as an increase in cell size and cellularproliferation/death: GSK907232A, GSK1440913, and GW494601. Following cell culture studies, molecular andbiochemical assays were performed to analyze known EMT-related cancer pathways. Results showed an overalldecrease in mesenchymal gene expression (Vimentin, FOXC2, SNAIL, SMAD2, TGF- β) and an increase inepithelial expression (E-Cadherin) in all four tested cell lines. In addition to EMT analysis, we investigated therole of our three hits in breast cancer stem cell populations. These inhibitors showed a reduction in breast cancerstem cell marker ganglioside GD2, a population that is strongly associated with tumorigenesis, metastasis, anddrug resistance. These data suggest these inhibitors reverse EMT and inhibit the cancer stem cell effects,attributes linked to poor prognoses in cancer (3,4,7).Because these three PKIS compounds are non-selective and affect diverse kinases, we then examined kinomewide activity of the selected inhibitors using the KINOMEscanâ assay to discover candidate kinase targets topursue. Some kinases identified as targets of these inhibitors, such as NEK5 and MAP3K19 (5) are understudiedboth in and out of the field of breast oncology and warrant further exploration in TNBC. In conclusion, our primaryobjective was to demonstrate the utility of the PKIS library as a primary screening tool for target discoveryapplications. Ongoing work focuses on validation of the identified understudied kinases as druggable therapeutictargets for TNBC.

Citation Format: Andrew Rivera, Matthew Burow, Khoa Nguyen. Use of the published kinase inhibitor set to identify therapeutic targets in TNBC [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS18-47.