Abstract 7592: Combined inhibition of CDK4/6 and AKT is highly active against the luminal androgen receptor (LAR) subtype of triple negative breast cancer (TNBC)

Abstract The LAR subtype of TNBC is enriched for targetable biomarkers such as androgen receptor (AR) expression, PIK3CA mutations, and intact Rb. The purpose of this study was to investigate the most effective combination of CDK4/6, AR, and PI3K-AKT inhibitors in preclinical models of LAR TNBC. The MDAMB453 and MFM223 (AR positive/Rb intact/PIK3CAmutant) LAR TNBC cells were treated with each inhibitor alone or in combination. Drug sensitivity was assessed using colony formation, cell viability, and live-cell analysis. The Loewe additivity equation was used to measure the additive drug effects. Immunoblot analysis was used to evaluate cell cycle and PI3K-AKT signaling and ARE luciferase assay was used to assess AR transcriptional activity. MDAMB453 and MFM223 cells were sensitive to the single agents palbociclib, alpelisib, and capivasertib (IC50, ~500 nM), which inhibited CDK4/6, PI3Kα, and AKT, respectively. Enzalutamide, an AR antagonist, exerted no growth inhibitory activity (IC50, 15-25 μM). Treatment with palbociclib, alpelisib, or capivasertib did not alter the AR protein levels or transcriptional activity. The combination of palbociclib+capivasertib synergistically inhibited the proliferation of MDAMB453 and MFM223 cells (synergistic scores 7. 34, p=5. 81-11, and 4. 78, p=0. 012, respectively) better than palbociclib+alpelisib. Enzalutamide did not enhance either combination. Palbociclib+capivasertib inhibited PI3KCA-mutant and Rb-intact LAR TNBC PDX growth more potently than palbociclib+alpelisib (p=0. 046). Treatment of LAR TNBC cells with palbociclib suppressed Rb phosphorylation and resulted in adaptive activation of P-AKTS473/T308 and the AKT substrates GSK3β, PRAS40, and FoxO3a after 24 h. Capivasertib inhibited the phosphorylation of AKT and AKT substrates, following CDK4/6 blockade with palbociclib, more potently than alpelisib. PI3Kβ inhibitors (TGX221, GSK2636771) did not block AKT substrate upregulation, suggesting PI3Kβ does not mediate adaptive responses. Rictor knockdown failed to reduce AKT activation following palbociclib treatment, suggesting an mTORC2-independent mechanism. Human Phospho-kinase Array of MDA-MB-453 cells treated with palbociclib showed time dependent upregulation of GSK3βS9, STAT3Y727 and PDFGRβY751. PDGFRβ activation after palbociclib treatment was confirmed in MFM223 cells. PDGFRβ siRNAs in MDAMB453 cells blocked P-AKTS473 upregulation after palbociclib treatment. Treatment with palbociclib and the PDGFRβ small-molecule antagonist CP673451 arrested LAR TNBC growth similar to palbociclib+capivasertib, suggesting an adaptive response involving PDGFRβ signaling. In conclusion, this study supports the clinical testing of the palbociclib and capivasertib combination for LAR TNBC patients and reveals adaptive responses after palbociclib treatment through PDGFRβ signaling. Citation Format: Rosario Chica-Parrado, Gun Min Kim, Yasuaki Uemoto, Dan Ye, Fabiana Napolitano, Chang-Ching Lin, Emmanuel Bikorimana, Kyung-min Lee, Saurabh Mendiratta, Ariella B. Hanker, Carlos L. Arteaga. Combined inhibition of CDK4/6 and AKT is highly active against the luminal androgen receptor (LAR) subtype of triple negative breast cancer (TNBC) abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 7592.