Hormone receptor-positive (HR+) breast cancer accounts for approximately 70% of all breast cancer cases and remains the most prevalent subtype. While endocrine therapy continues to serve as the cornerstone of HR+ breast cancer treatment, and CDK4/6 inhibitors (CDK4/6i) have become standard in both early and advanced settings, resistance to therapy remains a formidable clinical challenge. With growing insights into resistance mechanisms, histone deacetylase inhibitors (HDACi) have emerged as a promising strategy for reversing endocrine resistance through epigenetic modulation. In this article, Dr. Peifen Fu of The First Affiliated Hospital, Zhejiang University School of Medicine, discusses the underlying biology of endocrine resistance in advanced HR+ breast cancer and highlights the therapeutic potential of the novel HDACi, entinostat.Endocrine Resistance in HR⁺ ABC: A Critical Barrier
HR⁺ breast cancer, the most common molecular subtype, accounts for approximately 70% of all breast cancer diagnoses. It represents a patient population with a significant need for effective long-term disease management. Endocrine therapy is the treatment foundation for HR⁺ breast cancer, spanning the entire course of care. In recent years, the combination of endocrine therapy and CDK4/6 inhibitors (CDK4/6i + ET) has become standard for high-risk early-stage disease as well as for first-line treatment of advanced disease. Nonetheless, resistance to endocrine therapy and reduced sensitivity to CDK4/6i continue to compromise patient outcomes.
Retrospective data indicate that approximately 35% of patients with endocrine-resistant advanced HR⁺ breast cancer experience primary resistance, with a median overall survival (OS) of only 20.1 months—significantly shorter than the 33.4 months observed in those with acquired resistance. While CDK4/6 inhibitors have demonstrated substantial clinical benefit, about 10% of patients exhibit primary resistance, and the majority will develop acquired resistance after roughly two years of treatment in the first-line setting.
The mechanisms behind resistance to both ET and CDK4/6i are complex. Endocrine therapy efficacy hinges on the activation of estrogen receptor (ER)-driven pathways that regulate cellular proliferation and differentiation. Loss of ER expression is a primary driver of ET resistance. This can occur through multiple mechanisms, including aberrations in the ER/PgR pathway, dysregulation of co-activators and co-repressors, genomic and epigenetic alterations in ESR1, expression of truncated ER isoforms, post-translational modifications, increased receptor tyrosine kinase signaling, disrupted cell cycle regulation, and genetic or epigenetic abnormalities that affect drug absorption, metabolism, and response.
Among these pathways, epigenetic regulation—particularly through histone deacetylase (HDAC)—plays a pivotal role. For instance, in breast cancers treated with tamoxifen (TAM), HDAC activity reduces acetylation at the ESR1 gene promoter while suppressing the activity of myocyte enhancer factor 2 (MEF2), contributing to endocrine resistance. Furthermore, in CDK4/6i-treated tumors, the cyclin-dependent kinase inhibitor p21 is essential for treatment response. HDAC-mediated deacetylation suppresses p21 expression, which has been identified as a key mechanism driving CDK4/6i resistance.
Given the complexity of resistance mechanisms to endocrine therapy and CDK4/6 inhibitors, along with the current body of clinical evidence, both domestic and international guidelines recommend switching to alternative treatments in HR⁺ advanced breast cancer (ABC) once resistance emerges. However, traditional chemotherapy offers limited benefit and is associated with considerable toxicity. Precision therapies such as PI3K-AKT-mTOR pathway inhibitors (targeting PIK3CA/AKT1/PTEN mutations), PARP inhibitors (for BRCA-mutated patients), novel selective estrogen receptor degraders (SERDs, for ESR1 mutations), and immune checkpoint inhibitors (ICIs, for tumors with PD-L1 expression) are generally limited to patients with specific biomarkers. In contrast, histone deacetylase inhibitors (HDACi), as emerging epigenetic therapies, are recommended by clinical guidelines as a viable option that is not constrained by biomarker status.
Unlocking Endocrine Resistance with HDAC Inhibition
Histone deacetylases (HDACs) are a group of enzymes that play a central role in epigenetic regulation within tumors. By removing acetyl groups from histones, HDACs reduce the positive charge on histone proteins, which in turn alters chromatin structure and gene expression. HDAC inhibitors (HDACi) exert anti-tumor effects through multiple mechanisms including inhibition of angiogenesis, suppression of the cell cycle, promotion of apoptosis, and interference with DNA repair. These mechanisms position HDACi as a representative strategy in epigenetic anti-cancer therapy.
Structurally, HDAC inhibitors typically consist of three key components: a cap group, a linker, and a zinc-binding group (ZBG). First-generation HDAC inhibitors, often containing hydroxamic acid as the ZBG, have been criticized for their poor selectivity and significant toxicity. Entinostat (ENT), a novel HDACi, incorporates a more complex cap structure, resulting in lower toxicity and improved metabolic stability.
Beyond structural optimization, a growing body of research supports entinostat’s ability to overcome resistance to endocrine therapy and CDK4/6 inhibitors. Mechanistically, entinostat inhibits HDAC activity, leading to increased expression of p21—a crucial cyclin-dependent kinase inhibitor—thereby enhancing the effectiveness of CDK4/6 inhibitors and reversing resistance. It also promotes the expression of pro-apoptotic proteins, facilitating tumor cell death and working synergistically with CDK4/6 blockade.
On the immunological front, entinostat reduces levels of myeloid-derived suppressor cells (MDSCs) and downregulates CD40 expression on these cells, while simultaneously increasing levels of immunologically active CD14⁺ CHLA-DRhi monocytes. These changes suggest that entinostat could have a synergistic anti-tumor effect when used in combination with immune checkpoint blockade therapies.
On a broader scale, as HDACs function as acetylation-modifying enzymes, they are deeply involved in numerous molecular signaling pathways—including the Cyclin D/CDK4/6 axis—and play a regulatory role in the expression of both oncogenes and tumor suppressor genes. Beyond regulating p21, HDAC inhibitors can modulate a wide array of genes commonly altered in breast cancer and influence key cancer-related pathways to exert anti-tumor effects.
Notably, compared to Western populations, Chinese breast cancer cohorts show higher mutation frequencies in pathways such as p53, PI3K, RTK-RAS, and Notch. By inhibiting HDACs—crucial epigenetic regulators—entinostat may help to mitigate aberrations in these signaling networks. This suggests that entinostat holds particular promise for benefiting a broader group of Chinese patients with breast cancer.
Entinostat: Demonstrated Efficacy in Overcoming Endocrine Resistance in HR⁺ Breast Cancer
Entinostat, a novel histone deacetylase inhibitor (HDACi) developed independently in China, has been officially approved based on results from the Phase III EOC103A3101 trial. It is indicated in combination with an aromatase inhibitor for the treatment of hormone receptor-positive, HER2-negative (HR⁺/HER2⁻) locally advanced or metastatic breast cancer (MBC) in patients who have experienced relapse or progression following endocrine therapy—marking the first approved oral, once-weekly HDACi regimen in China.
The EOC103A3101 trial enrolled 354 pretreated HR⁺/HER2⁻ MBC patients, with prior therapies including chemotherapy, endocrine therapy, and CDK4/6 inhibitors. Notably, 32.3% of patients in the treatment arm had primary endocrine resistance. The study’s primary endpoint showed a significant improvement in progression-free survival (PFS) with entinostat compared to placebo (6.32 vs. 3.72 months; HR 0.76, 95% CI: 0.58–0.98; P=0.046). In terms of overall survival (OS), a key secondary endpoint, entinostat extended median OS to 38.39 months—nearly 30 months longer than the placebo group (HR 0.83).
In the subgroup analysis for PFS, a consistent trend of clinical benefit was observed across all subgroups. Notably, patients with primary endocrine resistance or prior exposure to CDK4/6 inhibitors experienced greater improvements, with a hazard ratio (HR) of approximately 0.57. This suggests that entinostat can reduce the risk of disease progression or death by nearly 50% in patients with primary endocrine therapy or CDK4/6i resistance, highlighting its strong potential in overcoming endocrine resistance.
In the EOC103A3101 study, entinostat demonstrated several other notable advantages, especially regarding safety. The hematologic toxicity of this regimen was lower than previously reported HDAC inhibitors. The incidence of grade ≥3 neutropenia, thrombocytopenia, and leukopenia was 43.8%, 8.5%, and 6.4%, respectively [¹⁴], whereas earlier HDACi reports documented rates of approximately 51%, 27%, and 18.9% for these adverse events [¹⁵]. In addition, entinostat’s long half-life of 61.9 hours allows for once-weekly oral administration, significantly improving patient adherence.
Conclusion
In summary, endocrine resistance remains a critical and unresolved challenge for patients with HR⁺ breast cancer, substantially affecting treatment outcomes and prognosis. The mechanisms driving endocrine resistance are complex, involving ER signaling disruptions and epigenetic alterations. HDAC inhibitors, as a new class of epigenetic anti-cancer agents, offer significant therapeutic potential. Entinostat, a next-generation HDACi, can upregulate p21 expression and modulate the immune microenvironment, thereby enhancing the efficacy of CDK4/6 inhibitors and reversing endocrine resistance. These effects have been confirmed in both preclinical studies and a large phase III clinical trial. With its favorable toxicity profile, metabolic stability, and convenient once-weekly dosing, entinostat presents a promising new treatment option for patients with HR⁺/HER2⁻ advanced breast cancer, with the potential to improve both prognosis and quality of life.
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Chief Physician, Doctor of Surgery Director of the Breast Disease Diagnosis and Treatment Center, The First Affiliated Hospital of Zhejiang University School of Medicine
- Member, Breast Surgery Group, Surgery Branch of the Chinese Medical Association
- Member, Breast Cancer Professional Committee, Chinese Anti-Cancer Association
- Member, Breast Cancer Expert Committee, Chinese Society of Clinical Oncology
- Member, Breast Oncology Group, Chinese Medical Doctor Association
- Standing Committee Member, Infectious Diseases Branch, Chinese Medical Education Association
- Standing Committee Member, Cancer Prevention and Control Promotion Committee, China Health Promotion Foundation
- Deputy Director, Breast Cancer Professional Committee, Zhejiang Anti-Cancer Association
- Deputy Director, Breast Disease Branch, Zhejiang Medical Association
- Deputy Director, Breast Tumor Expert Committee, Zhejiang Medical Doctor Association
