The normalization of epigenetic regulation has emerged as a promising therapeutic strategy in oncology. In hormone receptor-positive (HR⁺) breast cancer, the highly selective HDAC inhibitor (HDACi) entinostat has shown the ability to induce cell cycle arrest and apoptosis in multidrug-resistant tumor cells, addressing resistance to endocrine and targeted therapies at the mechanistic level. Moreover, a large phase III clinical trial has demonstrated entinostat’s survival benefit and safety profile in patients with HR⁺ advanced breast cancer previously treated with CDK4/6 inhibitors. Its weekly dosing regimen also offers a clear advantage in treatment adherence. Dr. Wenming Cao of Zhejiang Cancer Hospital provides a mechanistic perspective on the therapeutic advantages of HDACi entinostat in HR⁺ breast cancer.HDACs: The “Conductor” of Epigenetic Regulation
Histone deacetylases (HDACs) are key enzymes involved in modifying the epigenetic landscape of tumor cells, acting much like a “conductor” orchestrating molecular biological processes. In the early 1960s, scientists first observed lysine acetylation on histones and discovered that this modification could profoundly alter protein function—including protein binding, stability, and catalytic activity. Shortly thereafter, enzymes responsible for catalyzing acetylation and deacetylation were identified and named histone acetyltransferases (HATs) and histone deacetylases (HDACs), respectively [¹⁻²].
HDACs function by removing acetyl groups from lysine residues on histones, thereby reducing the positive charge of histones. This leads to chromatin condensation and repression of gene expression. Such epigenetic changes—without any alteration to the DNA sequence—are now recognized as a key mechanism in tumor initiation and progression. By inhibiting HDAC activity, acetylation levels on histones increase, resulting in chromatin remodeling and gene reactivation, a process now viewed as a promising therapeutic strategy for a wide range of diseases, including cancer [¹⁻²].
A Novel Approach to Antitumor Therapy with HDAC Inhibitors
HDAC inhibitors (HDACis) are typically composed of three structural components: a cap group, a linker, and a ZBG (zinc-binding group). The cap and linker influence the drug’s selectivity and ability to penetrate cells, while the ZBG interacts with the zinc ion at the enzyme’s active site, which is essential for inhibitory activity [²]. One of the earliest compounds found to have HDAC-inhibitory effects was the anticonvulsant desicillin, whose activity primarily stemmed from its zinc-chelating group. Subsequent structural modifications led to the discovery that acetylated desicillin not only possessed antitumor properties but also demonstrated improvements in toxicity and metabolic stability.
HDAC inhibitors exert their antitumor effects through multiple mechanisms, including inhibition of angiogenesis, suppression of the cell cycle, induction of apoptosis, and disruption of DNA repair. HDACis can shorten the half-life of VEGFR-2, thereby reducing angiogenesis. By regulating the expression of cyclin E, they block the transition from the G1 to the S phase of the cell cycle. HDACi-induced hyperacetylation of key p53 residues enhances the expression of pro-apoptotic genes, promoting apoptosis. Furthermore, HDACis can lead to sustained DNA damage, improve immune responses, and modulate various immune cell populations [²].
New-Generation HDAC Inhibitors: Multifaceted Mechanisms Against a Stealthy Killer
Inhibiting HDACs has emerged as a promising strategy in anticancer therapy. However, traditional compounds that use hydroxamic acid as the zinc-binding group (ZBG) have long faced criticism due to their lack of selectivity and associated toxicity. In contrast, entinostat (ENT), a new-generation HDAC inhibitor, utilizes a complex cap structure that provides low toxicity and strong metabolic stability [³].
As a representative of this new class, entinostat has shown great promise in the treatment of breast cancer, a success largely attributed to its superior mechanistic profile. Firstly, entinostat can upregulate p21 expression by inhibiting HDACs, thereby enhancing the efficacy of CDK4/6 inhibitors and reversing resistance to CDK4/6i in breast cancer cells. Additionally, entinostat induces the expression of pro-apoptotic proteins, leading to apoptosis and acting synergistically with CDK4/6 inhibitors. On the immunomodulatory front, entinostat reduces levels of myeloid-derived suppressor cells (MDSCs) and the expression of CD40 on MDSCs, while increasing immune-active CD14⁺HLA-DR^hi monocytes, suggesting it could synergize with immune checkpoint inhibitors for enhanced antitumor activity [³].
Moreover, HDAC inhibitors exert anticancer effects in breast cancer by modulating commonly mutated genes and signaling pathways associated with the disease. Studies have shown significant differences in the mutation profiles of Chinese versus Western patients with breast cancer, particularly within the HR⁺/HER2⁻ subtype. Among Chinese patients, the most frequently mutated gene is TP53, followed by PIK3CA and NF1 [⁴]. These mutations are closely linked to the mechanisms of HDACi action, implying that HDAC inhibitors may have enhanced efficacy in Chinese populations. Compared to Western cohorts, Chinese patients also show higher mutation frequencies in the p53, PI3K, RTK-RAS, and Notch pathways [⁵], which may further potentiate the activity of HDAC inhibitors like entinostat through modulation of these critical signaling cascades.
Entinostat Reinforces the Therapeutic Arsenal for HR⁺ Advanced Breast Cancer
For patients with HR⁺/HER2⁻ advanced breast cancer (ABC), the combination of CDK4/6 inhibitors and endocrine therapy has become the standard first-line treatment [⁶]. However, there is no established treatment after progression on CDK4/6i, leaving a substantial unmet clinical need. Chemotherapy or switching to another CDK4/6 inhibitor has shown limited benefit. Precision therapies—such as PI3K-AKT-mTOR pathway inhibitors, PARP inhibitors, novel SERDs, and immune checkpoint inhibitors—are only suitable for patients with specific biomarkers.
As a novel HDAC inhibitor, entinostat offers a new therapeutic option for patients with HR⁺/HER2⁻ ABC. It demonstrates favorable efficacy, good safety, and broad applicability across patient populations.
The phase III Chinese clinical trial (EOC103A3101) enrolled 354 previously treated HR⁺/HER2⁻ advanced breast cancer patients, many of whom had received prior chemotherapy, endocrine therapy, and CDK4/6i. The trial met its primary endpoint, showing that entinostat significantly improved progression-free survival (PFS) compared to placebo (6.32 vs. 3.72 months; HR 0.76; 95% CI: 0.58–0.98; P = 0.046), with consistent benefits observed across all subgroups, including patients previously treated with CDK4/6 inhibitors (HR 0.57). For the secondary endpoint, the median overall survival (OS) in the entinostat group reached 38.39 months, nearly 30 months longer than the 9-month OS observed in the placebo group (HR 0.83).
In terms of safety, the most common grade ≥3 hematologic adverse events associated with entinostat were neutropenia (43.8%), thrombocytopenia (8.5%), and leukopenia (6.4%). The most frequent grade ≥3 non-hematologic treatment-related adverse event (TRAE) was elevated aspartate aminotransferase (AST), occurring in 2.6% of patients.
In addition, entinostat has an average elimination half-life of 61.9 hours, allowing for weekly dosing, which greatly reduces treatment burden and improves patient adherence. As a result, in terms of efficacy, safety, and convenience, entinostat has demonstrated an outstanding clinical profile compared to other drugs in its class.
In conclusion, the development and progression of breast cancer are closely linked to epigenetic regulation. As a novel therapeutic strategy targeting epigenetic abnormalities in breast cancer, the new-generation HDAC inhibitor entinostat has demonstrated encouraging survival benefits and a favorable safety profile. Its convenient weekly dosing enhances treatment adherence and ease of use. With a unique mechanism of action, entinostat holds great promise as a targeted therapy option in the post-CDK4/6i era, offering new hope for a broader population of patients in China.
References
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Chief Physician, Master’s Supervisor Head of the Breast Medical Oncology Group, Zhejiang Cancer Hospital
Academic and Professional Affiliations:
- Standing Member, Committee on Hereditary Tumors, Chinese Anti-Cancer Association
- Young Expert, Breast Cancer Committee, Chinese Anti-Cancer Association
- Vice Chair, Medical Cell Biology Committee, Zhejiang Society for Cell Biology
- Vice Chair, Hereditary Tumors Committee, Zhejiang Anti-Cancer Association
- Standing Member, Tumor Metastasis Committee, Zhejiang Anti-Cancer Association
- Member and Secretary, Medical Oncology Committee, Zhejiang Anti-Cancer Association
- Member, Clinical Oncology Pharmacy Committee, Zhejiang Anti-Cancer Association
- Selected Talent in Zhejiang’s High-Level Health Talent Program (“Medical Rising Star”)
- Visiting Scholar, MD Anderson Cancer Center, USA
Academic Contributions: Dr. Cao has published more than 20 SCI-indexed papers as first author or corresponding author (including co-first/co-corresponding authorship) in leading journals such as Journal of Clinical Oncology and Nature Communications. He also serves on the editorial board of BMC Cancer.
