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INTRODUCTION. Triple negative breast cancer (TNBC) is an aggressive form of BC that lacks effective targeted therapy. It is a biologically heterogeneous disease with several molecular subtypes: basal-like 1 (BL1), basal-like 2 (BL2), mesenchymal (M), mesenchymal-stem-like (MSL), immune-modulatory (IM) and unclassified (UNC). The PI3K/AKT/mTOR pathway affects cell proliferation, survival, and apoptosis through growth receptor interaction with downstream targets such as AKT and mTOR. TNBC frequently has activation of this pathway by mutation and other means across several subtypes. Recently the PI3K inhibitor alpelisib (BYL719) has been found to have efficacy in a combination study in ER positive metastatic BC (MBC) with a Phase 3 trial in progress. Ribociclib (LEE011) is a CDK inhibitor targeting cyclin D1/CDK4 and cyclin D3/CDK6 that recently met its primary endpoint in a phase 3 trial in a combination study in hormone receptor positive BC. As with all BC, TNBC has intact or mutant RB which affects susceptibility to CDK inhibitor. We hypothesize that targeting both PI3K with alpelisib and CDK with ribociclib may provide enhanced suppression of TNBC. MATERIALS AND METHODS. HCC38 and MDA468 are TNBC subtype BL1; MDA468 is RB1 and PTEN mutant. HCC1187 is IM. BT549 is M, mutant RB1 and PTEN. Hs578T, MDA157, and MDA231 are MSL. Cells were treated with alpelisib and ribociclib alone with 2-fold increase in concentrations, or the combination, for 72h and assessed by MTT assay. Western blot was performed using probes for pAkt (T308 and S473), Akt, pS6K1, S6K1, pS6, S6, and β-actin. RESULTS. Synergy in growth inhibition was seen combining alpelisib and ribociclib as reflected by combination indices Conclusion. Our study demonstrates that alpelisib plus ribociclib can synergistically enhance suppression of BC across multiple subtypes of TNBC. Cancer cells within the same subtype may not demonstrate a synergistic response, depending on RB status. Nevertheless, if RB mutant cells are susceptible to ribociclib then synergy can be seen even in these cancer cells. Antagonism can be seen independent of RB status in the same subtype. These findings point to a potential role for combination therapy with alpelisib and ribociclib in the treatment of TNBC. Citation Format: Yuan Y, Mortimer J, Xing Q, Yan J, Wen W, Han E, Yim JH. Synergistic suppression of triple negative breast cancer with the combination of PI3K inhibitor (alpelisib, BYL719) and CDK inhibitor (ribociclib, LEE011) [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P3-03-15.