Abstract PS4-04-25: Selective Cytotoxicity of Natural Products in Triple-Negative Breast Cancer Mediated by PKCβ Activation

Abstract Triple-negative breast cancer (TNBC) accounts for approximately 15–20% of all breast cancercases and disproportionately affects younger women. Characterized by the absence of estrogen(ER), progesterone (PR), and HER2 receptors, TNBC lacks the molecular targets utilized in mostcurrent therapies, resulting in limited treatment options and a heavy reliance on cytotoxicchemotherapy. TNBC is associated with a more aggressive clinical course, early metastasis, andpoor overall survival as compared to other types of breast cancer, highlighting the urgent needfor novel and effective therapeutic strategies. To identify new therapeutic targets, TNBC hasbeen classified into four molecular subtypes: basal-like 1 (BL1), basal-like 2 (BL2),mesenchymal (M), and luminal androgen receptor (LAR). Among these, the BL2 subtypeexhibits particularly poor prognosis and resistance to standard of care treatments. In our study,we identified yuanhuacine and other related diterpenoid natural products as compounds withpotent (0.07 – 7.7 nM) cytotoxicity against BL2 TNBC cells. These compounds are over one-thousand-fold more selective for the BL2 subtype of TNBC as compared to other TNBCsubtypes or immortalized normal breast lines supporting the identification of a novel targetablevulnerability in this breast cancer subtype. Most notably, we demonstrated this exquisite potencyand selectivity for BL2 TNBC is also a feature of compounds of this class that are undergoingclinical development for the topical treatment of skin cancers demonstrating an opportunity fordrug repurposing.Mechanistic studies revealed that the efficacy of these compounds is mediated by activation ofProtein Kinase C beta (PKCβ), which is elevated in BL2 TNBC cells. Pharmacologic inhibitionof PKCβ significantly reduced the potency of yuanhuacine and ingenol-3-angelate in BL2 TNBCcells, indicating that PKCβ activation is necessary for their selective activity. Geneticexperiments are being conducted to interrogate whether PKCβ expression is necessary andsufficient for sensitivity to yuanhuacine with reduced potency expected in in BL2 lines uponPKCβ knockdown and sensitization of non-BL2 cell lines when ectopically expressed. Cellbiological experiments demonstrated treatment of BL2 cells with yuanhuacine promotesphosphorylation of PKCβ at Ser660 and activation of PKC substrates within one hour oftreatment, suggesting rapid activation and engagement of downstream signaling. Additionalimmunofluorescence studies demonstrate the effect of this activation on the intracellularlocalization and degradation of PKCβ as well as induction of apoptosis.Our findings demonstrate that yuanhuacine and other ingenane diterpenoids, includingcompounds in clinical development for other indications, have the potential for the targetedtreatment of the BL2 subtype of TNBC. Additionally, the identification of PKCβ expression as abiomarker of sensitivity to this class of drugs opens the potential for efficacy in other cancerswhere PKCβ is expressed. Given the lack of targeted therapies for BL2 TNBC, the developmentof PKCβ-activating compounds like yuanhuacine represents a promising and urgently neededadvancement in personalized oncology. Citation Format: S. C. Stanfield, C. S. Fermaintt, A. Risinger. Selective Cytotoxicity of Natural Products in Triple-Negative Breast Cancer Mediated by PKCβ Activation abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS4-04-25.