With rapid advancements in medical science and technology, the field of oncology is undergoing an unprecedented transformation. In particular, the emergence of novel therapeutic agents in breast cancer is offering patients new hope for improved survival. Among these, histone deacetylase inhibitors (HDAC inhibitors or HDACi)—a class of epigenetic modulators—have demonstrated promising advantages in the treatment of hormone receptor-positive, HER2-negative (HR+/HER2-) advanced breast cancer. In this article, we invited Dr. Lu Gan from The First Affiliated Hospital of Chongqing Medical University to explore the mechanisms of HDAC inhibitors and their strategic application in HR+/HER2- advanced breast cancer. The discussion focuses on clinical research data for entinostat and chidamide, while highlighting the therapeutic efficacy, safety, and convenience of HDAC inhibitors.

The Evolution of Epigenetics and the Mechanism and Role of HDAC Inhibitors

1. The Evolution of Epigenetics

Epigenetics is a subfield of genetics that studies heritable changes in gene expression that do not involve alterations to the underlying DNA sequence. The term “epigenetics” was first introduced by British scientist Conrad Waddington in 1942, who proposed the concept of an interplay between genetics and epigenetics—specifically the relationship between genotype and phenotype. Since then, the field has rapidly evolved[¹]. Key areas of epigenetic research include DNA/RNA methylation, histone modifications, nucleosome positioning, non-coding RNAs, and the three-dimensional architecture of chromatin. These epigenetic modifications shape an organism’s unique epigenome and offer a dynamic mechanism for regulating cellular diversity[²]. Traditionally, cancer has been regarded as a genetic disease driven by mutations in oncogenes and tumor suppressor genes. However, over the past three decades, the advent of technologies such as high-throughput sequencing has led to growing recognition that epigenetic alterations are also key drivers of tumorigenesis. A growing body of research has demonstrated that epigenetic modifications are closely associated with cancer development and progression[²]. Epigenetic dysregulation in cancer typically involves mutations or aberrant expression of epigenetic enzymes and/or alterations in related cofactors. These changes influence chromatin structure and dynamics, leading to abnormal gene expression and ultimately contributing to cancer initiation and evolution[³].

2. Mechanism and Role of HDAC Inhibitors

Histone deacetylases (HDACs) are critical enzymes involved in epigenetic modifications associated with cancer. They function by removing acetyl groups from N-acetyl lysine residues on histones. HDACs are involved in regulating various processes, including tumor cell growth and gene expression, making them important therapeutic targets in the field of epigenetics[⁴]. Decreased histone acetylation due to HDAC activity is a common epigenetic abnormality in tumors. Targeted inhibition of HDACs can reverse such acetylation imbalances[⁵]. HDAC inhibitors exert their anticancer effects by modulating histone acetylation levels, thereby altering chromatin structure and regulating gene transcription, which ultimately suppresses tumor cell proliferation. These agents have shown potential in overcoming endocrine resistance mechanisms that arise due to epigenetic abnormalities in breast cancer, particularly in the treatment of HR+/HER2- subtypes[⁶].

In recent years, with the continuous advancement of genomic research, increasing attention has been directed toward the mutational landscape of breast cancer. Understanding the heterogeneity of gene mutations is essential for unraveling the mechanisms underlying tumorigenesis and developing more effective, personalized treatment strategies. HDAC inhibitors (HDACi), with their distinct mechanisms of action, have shown promising potential in the treatment of breast cancer. Basic research on entinostat and chidamide has demonstrated that HDACi can enhance the efficacy of CDK4/6 inhibitors by suppressing HDAC activity and subsequently upregulating p21 expression[⁷]. Preclinical studies have further revealed that HDAC inhibitors can induce the expression of pro-apoptotic proteins, leading to tumor cell apoptosis, and thereby act synergistically with CDK4/6 inhibitors. In addition, HDACi may overcome resistance to CDK4/6 inhibitors by reducing the transcriptional activity of the activator protein-1 (AP-1) complex[⁸]. Given these multifaceted mechanisms, HDAC inhibitors occupy a unique position in oncology. They are effective as monotherapy agents and can also be combined with other anticancer therapies to achieve synergistic effects. HDACi and immune checkpoint inhibitors target different immune cell subpopulations through distinct immunomodulatory mechanisms. Therefore, combining HDAC inhibitors with immune checkpoint blockade may produce a complementary and synergistic antitumor response[⁹].

Advances in Clinical Research of HDAC Inhibitors in Breast Cancer

Among the various HDAC inhibitor-based combination therapies under investigation, both entinostat and chidamide have shown favorable outcomes when paired with endocrine therapy in hormone receptor-positive breast cancer[¹⁰]. Entinostat is a novel oral HDAC inhibitor that specifically targets class I HDACs. In the phase III EOC103A3101 clinical trial[¹¹], the combination of entinostat with exemestane significantly prolonged median progression-free survival (PFS) compared to exemestane alone. This randomized, double-blind, placebo-controlled trial enrolled 354 patients with HR+/HER2- locally advanced or metastatic breast cancer who had relapsed or progressed following prior endocrine therapy.

Participants were randomly assigned in a 2:1 ratio to receive either entinostat plus exemestane (entinostat arm) or placebo plus exemestane (placebo arm). The primary endpoint was PFS as assessed by an independent review committee (IRC), while secondary endpoints included investigator-assessed PFS, overall survival (OS), objective response rate (ORR), clinical benefit rate (CBR), and safety. The study results demonstrated that entinostat significantly prolonged median progression-free survival (PFS) in patients. The median PFS in the entinostat group was 6.32 months, notably longer than the 3.72 months observed in the placebo group (HR 0.76; 95% CI: 0.58–0.98, P=0.046). Subgroup analyses further revealed that patients with resistance to CDK4/6 inhibitors and those who had previously received salvage chemotherapy also derived similar benefits from treatment with entinostat. In terms of overall survival (OS), the median OS in the entinostat group reached 38.39 months, representing an extension of more than 9 months compared to the placebo group (HR 0.83; 95% CI: 0.62–1.10, P=0.184). In terms of safety, the primary adverse events in the entinostat group were hematologic toxicities. The most common grade ≥3 hematologic adverse events included neutropenia (43.8%), thrombocytopenia (8.5%), leukopenia (6.4%), and anemia (3.4%).

Chidamide is a selective inhibitor targeting both class I and class II HDACs[¹⁰]. The phase III ACE study[¹²] confirmed the efficacy of chidamide combined with exemestane in postmenopausal patients with HR+/HER2- locally advanced or metastatic breast cancer who had relapsed or progressed after prior endocrine therapy. Among postmenopausal patients with endocrine-resistant HR+ advanced breast cancer, the combination of chidamide and exemestane significantly improved median PFS compared to exemestane alone—7.4 months vs. 3.8 months (HR=0.75; 95% CI: 0.58–0.98, P=0.033). However, overall survival showed no significant difference between the two groups, with both arms reporting a median OS of 30.3 months (HR=1.050; 95% CI: 0.798–1.383, P=0.7259).

Overall Survival Outcomes in the ACE Study

In terms of safety, the incidence of grade ≥3 treatment-related adverse events (TRAEs) with chidamide was 76%. The most common grade ≥3 hematologic toxicities included neutropenia (51%), thrombocytopenia (27%), leukopenia (18.9%), and anemia (4%). The EOC103A3101 and ACE studies share both similarities and differences. Both trials enrolled patients with endocrine-resistant disease. However, while the EOC103A3101 study included both premenopausal and postmenopausal patients, the ACE study focused solely on postmenopausal women. In terms of progression-free survival (PFS), both studies showed comparable outcomes, demonstrating that HDAC inhibitors contribute meaningfully to delaying disease progression. Regarding overall survival (OS), the EOC103A3101 trial showed that patients in the entinostat group had a median OS over nine months longer than those in the placebo group. Although the data are not yet fully mature, this suggests a favorable survival trend. In contrast, the ACE study showed no significant difference in OS between the chidamide and placebo groups, with both arms reaching a median OS of 30.3 months. As for safety, the numerical incidence of adverse events appeared lower in the entinostat group than in the chidamide group. However, because these two trials were not direct head-to-head comparisons, such findings should be interpreted with caution. Taken together, the results of both studies support the efficacy and safety of HDAC inhibitors in the treatment of HR+ advanced breast cancer.

HDAC Inhibitors Offer Convenient Dosing and Expanded Treatment Options

Beyond efficacy and safety, HDAC inhibitors also offer advantages in terms of dosing convenience. Chidamide is administered twice weekly, while entinostat, with a half-life of up to 61.9 hours, requires only once-weekly dosing. This infrequent dosing schedule can reduce the treatment burden on patients, improve medication adherence, and help ensure optimal therapeutic outcomes.

In conclusion, HDAC inhibitors, as epigenetic agents, possess a unique mechanism of action and have demonstrated clinical benefits in HR+/HER2− advanced breast cancer. They offer manageable safety profiles and highly convenient dosing regimens. Reflecting these advantages, the 2025 CBCS & CSOBO Breast Cancer Diagnosis and Treatment Guidelines have officially included HDAC inhibitors—entinostat or chidamide combined with endocrine therapy—as recommended options for second-line and later-line treatment in patients previously treated with CDK4/6 inhibitors. As research in epigenetics continues to advance and clinical development of HDAC inhibitors progresses, these agents are expected to bring further survival benefits and improved quality of life to more patients in the future.

References

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Professor Lu Gan

Professor Lu Gan First Affiliated Hospital of Chongqing Medical University

  • Standing Committee Member, Breast Cancer Professional Committee, Chinese Anti-Cancer Association (CACA)
  • Standing Committee Member, Committee on Clinical Research Management in Oncology, CACA
  • Member, Committee on Cancer Prevention and Education, CACA
  • Member, Committee on Clinical Chemotherapy in Oncology, CACA
  • Standing Committee Member, Breast Disease Branch, Chongqing Medical Association
  • Member, Breast Cancer Diagnosis and Treatment Expert Panel, Chongqing Oncology Quality Control Center
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Last updated on 2025.03.27