What clinical trials have been conducted for Entinostat?

Introduction to Entinostat

Entinostat is a synthetic benzamide derivative that acts as a selective inhibitor of class I and IV histone deacetylases (HDACs). Its chemical and pharmacological profile reveals that it is an orally bioavailable, once‐weekly agent with a long half‐life, which enables continuous exposure and epigenetic modulation despite its intermittent dosing schedule. Entinostat’s favorable pharmacokinetic profile and tolerability have lent support to its investigation in various solid and hematologic malignancies.

Chemical and Pharmacological Profile

Chemically, entinostat belongs to the benzamide class of HDAC inhibitors and is tailored to selectively target class I HDAC enzymes. This selectivity is critical because class I HDACs are primarily overexpressed in many cancers, and their inhibition is associated with the reactivation of tumor suppressor genes and modulation of cell growth. Pharmacologically, entinostat has demonstrated dose-proportional pharmacokinetics over the studied dose ranges, with its serum concentrations and area under the curve (AUC) increasing proportionately with the dose. This attribute ensures that dosing regimens can be optimized both for maximum efficacy and minimal toxicity. In terms of safety, clinical studies have generally shown that entinostat is well tolerated; the adverse event profile includes manageable hematologic toxicities such as neutropenia and thrombocytopenia, along with nonhematologic effects like gastrointestinal discomfort and fatigue.

Mechanism of Action

Entinostat exerts its therapeutic effect primarily by inhibiting HDACs, which leads to an increase in the acetylation of histone proteins. This alteration in acetylation status opens the chromatin structure, thereby promoting the transcription of previously silenced genes involved in cell-cycle arrest, apoptosis, and differentiation. In addition, entinostat modulates the tumor microenvironment by affecting nonhistone proteins and immune regulatory cells. For example, preclinical studies have shown that it can downregulate regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs), thereby enhancing antitumor immune responses. Through such dual mechanisms—direct epigenetic reprogramming of cancer cells and the immunomodulation of the tumor microenvironment—entinostat holds promise for improving outcomes when used both as a monotherapy and in combination with other anticancer agents.

Overview of Clinical Trials

Clinical trials play a pivotal role in establishing the efficacy and safety of new therapeutic agents. With entinostat, clinical investigations have spanned multiple phases and indications, reflecting its potential as both a standalone treatment and as part of combination regimens. These studies have not only provided proof-of-concept data but also opened up avenues for improving standard treatments across various tumor types.

Phases of Clinical Trials

Clinical research with entinostat has been carefully designed to progress through the traditional phases:
- Phase I Trials: These early studies primarily focus on determining the safety profile, tolerability, pharmacokinetics, and dose-limiting toxicities of entinostat. For instance, in patients with metastatic castration-resistant prostate cancer, Phase I studies examined various dose levels in combination with enzalutamide to define the recommended phase 2 dose.
- Phase II Trials: Phase II trials extend these findings by evaluating the preliminary efficacy of entinostat in defined patient populations. A randomized controlled Phase II study in hormone receptor–positive advanced breast cancer showed that the addition of entinostat to endocrine therapy improved progression-free survival.
- Phase III Trials: The advanced-stage studies, such as the Phase III study in advanced breast cancer (E2112, NCT02115282), are designed to confirm the efficacy signals observed in earlier studies and to support regulatory submissions.

Importance in Drug Development

Entinostat’s development illustrates a modern approach in oncology drug development where epigenetic therapies are leveraged not only to reverse aberrant gene silencing but also to modulate the immune microenvironment. The initial clinical trials focusing on safety and tolerability allowed researchers to move forward with combination strategies. Such combination trials attempt to overcome resistance to conventional therapies, as well as to enhance the efficacy of immune checkpoint inhibitors and targeted agents. This integration of entinostat into multidrug regimens marks an important shift in drug development where epigenetic modifiers are being combined with standard-of-care and novel agents to improve patient outcomes.

Clinical Trials of Entinostat

The clinical investigation of entinostat has been extensive and diverse, encompassing multiple cancer types and a range of combination strategies. Below is an in-depth exploration of the completed and ongoing clinical trials conducted for entinostat, along with key findings from these studies.

Completed Trials

Several clinical trials using entinostat have reached completion over the past years, and their outcomes have significantly contributed to the understanding of its antitumor activity and safety profile.

- Breast Cancer Trials: One of the most notable completed studies in breast cancer is a Phase III randomized trial evaluating entinostat versus placebo in combination with endocrine therapy for patients with hormone receptor–positive advanced breast cancer. In this trial, entinostat plus an aromatase inhibitor (exemestane) was compared to exemestane alone, based on earlier promising Phase II data from the ENCORE 301 trial that demonstrated an overall survival benefit. Additional Phase II studies in Japanese and Chinese patient populations further confirmed that the combination improved progression-free survival and overall outcomes, despite some hematologic toxicity. These trials provided robust evidence that epigenetic modulation with entinostat could reverse endocrine resistance in advanced breast cancer, making it a promising candidate for further drug development in this indication.

- Lung Cancer Trials: An important Phase I study evaluated the combination of entinostat with a standard chemotherapy and immunotherapy regimen (atezolizumab, carboplatin, and etoposide) in previously untreated extensive-stage small-cell lung cancer. The trial aimed to determine whether adding entinostat could improve the efficacy of the standard-of-care treatment. Although the primary focus was on safety and tolerability, early findings suggested that epigenetic modulation might enhance responses in lung cancer patients. Another completed Phase I/II trial combined entinostat with other targeted agents in advanced solid tumors, providing safety data and preliminary evidence of antitumor activity in multiple lung cancer subtypes.

- Melanoma Trial: In melanoma, an exploratory study combined entinostat with the immune checkpoint inhibitor pembrolizumab in non-inflamed stage III/IV melanoma patients. This study was designed based on preclinical evidence that entinostat could enhance antigen presentation and reverse immune resistance. Early results indicated that the combination was tolerable and showed signs of clinical benefit, particularly in patients with an immune “cold” tumor microenvironment.

- Renal Cell Carcinoma (RCC) Study: A Phase II study assessed entinostat in combination with nivolumab plus ipilimumab in patients with renal cell carcinoma who had previously been treated with this immunotherapy combination. The trial focused on evaluating whether entinostat could further modulate immune responses in RCC, and initial data revealed promising safety profiles along with biomarker changes indicative of reduced MDSC-mediated immunosuppression.

- Advanced Solid Tumor and Lymphoma Trials: Multiple Phase I studies have explored entinostat as part of combination regimens in advanced solid tumors and refractory lymphomas. One such study combined entinostat with ZEN003694 (a novel anti-cancer drug) in advanced and refractory solid tumors and lymphomas. Another study tested the sequential combination of BN-Brachyury, entinostat, ado-trastuzumab emtansine, and M7824 in advanced-stage breast cancer. These trials have been invaluable in determining dose levels and combinations that maximize antitumor efficacy while maintaining an acceptable safety profile.

- Prostate Cancer: A Phase I trial evaluated the combination of entinostat with enzalutamide in metastatic castration-resistant prostate cancer (mCRPC). This study showed promising safety data—with no significant pharmacokinetic interactions—and observed increased acetylation in peripheral blood mononuclear cells, suggesting biological activity at the molecular level.

- Pancreatic Cancer: In pancreatic adenocarcinoma, a Phase Ib study assessed the combination of entinostat with the standard FOLFOX regimen (a combination of chemotherapeutic agents including folinic acid, fluorouracil, and oxaliplatin). The results indicated that the combination was feasible and produced a synergistic effect in inhibiting tumor growth, even though hematologic toxicity (with a high incidence of leukocyte and platelet declines) was observed.

- Pediatric Malignancies: A unique trial known as the INFORM2 study evaluated the use of entinostat in combination with nivolumab in children and adolescents with high-risk refractory malignancies. Although this trial included a small cohort, it was significant as it demonstrated that entinostat might be safely incorporated into combination regimens for pediatric populations, expanding the scope of its clinical applicability.

- Additional Hematologic and Solid Tumor Studies: Other completed studies have explored the role of entinostat in combination with capecitabine for advanced triple-negative breast cancer and as part of combination immunotherapy regimens in advanced cancers with various histologies. These studies further underscore the versatility of entinostat in modulating both cancer cell biology and the host immune response.

Each of these completed trials has contributed uniquely to our collective understanding of entinostat’s clinical activity. They have provided critical insights into its single-agent efficacy and, more importantly, its potential as a synergistic partner in combination regimens aimed at overcoming resistance mechanisms in various cancers.

Ongoing Trials

Several clinical trials investigating entinostat are ongoing, reflecting the continued interest in its potential to enhance current therapeutic strategies:

- Esophageal Cancer Combination Study: A Phase I/II evaluation is currently being conducted on a cancer lysate vaccine and Montanide ISA-51 VG in combination with entinostat and nivolumab as adjuvant therapy following chemoradiation—with or without surgery—in patients with locally advanced esophageal cancer. This study aims to determine the optimal dosing, safety, and early efficacy signals of a combined vaccine-immunotherapy strategy in a highly aggressive tumor type.

- Advanced Breast Cancer (Phase III): Building on previous Phase II success, a large Phase III clinical trial is being conducted to compare entinostat in combination with endocrine therapy against placebo with endocrine therapy in patients with hormone receptor–positive advanced breast cancer. The E2112 trial is of particular significance because it aims to confirm the overall survival benefit that was observed in previous smaller studies, potentially paving the way for regulatory approval.

- Window of Opportunity Study in Bladder Cancer: A study is ongoing that uses a "window of opportunity" design to evaluate the immunogenomic effects of pembrolizumab, alone and in combination with entinostat, in muscle-invasive bladder cancer. This trial is designed to provide key insights into the molecular and cellular alterations induced by the combination therapy, which can guide subsequent larger-scale studies.

- Combination with Immune Checkpoint Inhibitors in Solid Tumors: There are additional ongoing trials assessing the combination of entinostat with various immune checkpoint inhibitors. For example, studies combining entinostat with nivolumab and ipilimumab in advanced solid tumors are being expanded to better understand the immunomodulatory mechanisms and to identify optimal patient populations. These trials are particularly focused on determining changes in tumor-infiltrating lymphocyte subsets and immune checkpoints in response to treatment.

- Other Ongoing Multimodality Studies: Ongoing studies are also exploring the integration of entinostat into broader combination regimens, such as its simultaneous administration with targeted therapies like EGFR inhibitors in lung cancer and with PARP inhibitors in ovarian cancer. Such trials are designed to assess whether entinostat’s epigenetic modulation can potentiate the effects of conventional therapies and reverse mechanisms of resistance observed in heavily pretreated populations.

The continued enrollment and data collection in these ongoing trials highlight the dynamic nature of clinical research on entinostat. They are expected to provide further clarity regarding its efficacy and safety in broader patient populations and to refine its use in combination with other therapies to potentially improve long-term survival outcomes.

Key Findings and Results

Key findings from the various clinical trials conducted with entinostat have emerged from both early and later-phase studies:

- Enhanced Efficacy in Combination Regimens: One of the pivotal outcomes from the ENCORE 301 trial and subsequent studies was the demonstration that combining entinostat with endocrine therapy—specifically in hormone receptor–positive advanced breast cancer—led to improved progression-free survival and overall survival compared to endocrine therapy alone. This finding supports the concept that epigenetic modulation can reverse endocrine resistance, a major challenge in breast cancer treatment.

- Immunomodulatory Effects: Clinical investigations in solid tumors, including melanoma and RCC, have underscored the immunomodulatory properties of entinostat. Studies combining entinostat with immune checkpoint inhibitors have demonstrated not only tolerable safety profiles but also promising evidence of enhanced T-cell activation and reduced populations of immunosuppressive cells such as MDSCs. These immune-based studies suggest that entinostat can “prime” the tumor microenvironment, making it more receptive to immunotherapy.

- Safety and Tolerability: Across the clinical trials, entinostat has generally been well tolerated. The most common adverse events included grade 3/4 hematologic toxicities (neutropenia, thrombocytopenia) and gastrointestinal reactions. Importantly, dose modifications and supportive care measures have allowed most patients to continue therapy without significant interruptions. Furthermore, pharmacodynamic markers such as increased histone acetylation in peripheral blood cells have been consistently observed, confirming the biological activity of entinostat at the studied doses.

- Biomarker-Driven Outcomes: The clinical trials have also begun to shed light on potential prognostic and predictive biomarkers. For instance, studies have correlated increased acetylation levels in CD3+ cells with improved response rates, while decreases in MDSCs have been linked with favorable outcomes in combination immunotherapy trials. Such biomarker analyses are crucial for identifying patient subgroups most likely to benefit from entinostat-based therapies, which can lead to more personalized treatment approaches.

- Efficacy in Diverse Cancer Types: While much attention has been paid to breast cancer, the completed trials have also shown encouraging signals in lung cancer, melanoma, RCC, prostate, pancreatic, and pediatric cancers. These studies collectively indicate that entinostat’s mechanism of action is not limited by tumor histology and that its integration into combination regimens may offer benefits across a broad spectrum of malignancies.

Implications and Future Directions

The collective clinical data on entinostat have important implications for cancer treatment and future research.

Impact on Cancer Treatment

The clinical trials of entinostat, both completed and ongoing, have had several significant impacts on the field of oncology:

- Overcoming Therapeutic Resistance: One of the major hurdles in oncology is resistance to standard therapies. The ability of entinostat to reverse endocrine resistance in breast cancer is a key example, where epigenetic modulation restores sensitivity to hormonal therapies, thereby extending progression-free survival and overall survival. Similar principles are being explored in other cancers, where combining entinostat with chemotherapeutic agents or targeted therapies aims to overcome inherent or acquired resistance mechanisms.

- Enhancing Immunotherapy: The immunomodulatory properties of entinostat offer an additional avenue for enhancing the efficacy of immune checkpoint inhibitors. By increasing the expression of antigen-presenting molecules and decreasing populations of immunosuppressive cells like Tregs and MDSCs, entinostat can potentially convert “cold” tumors into “hot” tumors that are more responsive to immunotherapy. This approach is especially critical in cancers such as melanoma, RCC, and certain subtypes of lung cancer.

- Broadening the Therapeutic Portfolio: The versatility of entinostat in combination with a variety of agents—from hormonal therapies to cytotoxic chemotherapy and immunotherapy—illustrates its potential to become a staple component of combination regimens. This broad application across multiple tumor types suggests that entinostat may play a role in the standard of care for several malignancies in the future.

- Pharmacodynamic and Biomarker Insights: The use of pharmacodynamic biomarkers such as histone acetylation and the modulation of immune cell subsets in clinical trials has provided valuable insights into the molecular effects of entinostat. These biomarkers are helping to refine patient selection criteria and optimize dosing strategies, ensuring that the therapeutic benefits of entinostat can be maximized while minimizing adverse effects.

Future Research and Development

Looking ahead, the future research on entinostat is likely to focus on several key areas:

- Optimization of Combination Strategies: Further Phase III trials, such as the ongoing advanced breast cancer study (E2112), are critical to validating the efficacy signals seen in earlier phases. Additionally, trials exploring combinations with emerging agents (e.g., PARP inhibitors, novel immunotherapies) are essential to address resistance in refractory cancers and to broaden the indications for entinostat-based regimens.

- Identification and Validation of Biomarkers: More comprehensive biomarker studies are needed to predict which patients will benefit the most from entinostat. Future research should focus on correlating histone acetylation levels, immune cell infiltration patterns, and specific gene expression signatures with clinical outcomes. Such studies will pave the way for personalized treatment approaches and may even help in identifying new therapeutic targets.

- Expansion to Rare and Pediatric Cancers: The encouraging safety profile observed in pediatric trials, along with early signals of efficacy, suggests that entinostat could be further evaluated in rare malignancies and in pediatric populations. Expanded trials with larger cohorts and longer follow-up periods will be critical to understanding its full potential in these groups.

- Exploration of Novel Dosing Schedules: The once-weekly dosing regimen has been beneficial for maintaining continuous exposure while reducing toxicity. However, ongoing research may explore alternative dosing schedules or combination strategies that further enhance the drug’s pharmacodynamic properties and clinical efficacy.

- Understanding Resistance Mechanisms to HDAC Inhibitors: As with many targeted therapies, resistance can eventually develop. Further investigation into the molecular mechanisms underlying resistance to entinostat will be key to developing second-generation HDAC inhibitors or combination regimens that can circumvent these challenges. This research will also benefit from ongoing translational studies that use both tissue and blood-based biomarkers to track resistance patterns.

- Regulatory and Clinical Practice Implications: Ultimately, if ongoing Phase III trials confirm the efficacy benefits of entinostat, it could become a standard component in treatment regimens for certain cancers, particularly hormone receptor–positive advanced breast cancer. Regulatory approvals based on robust clinical data will, in turn, drive further research and integration of epigenetic therapies in standard clinical practice.

Conclusion

In summary, the clinical trials conducted for entinostat have been extensive and multifaceted, spanning from early Phase I safety studies to large-scale Phase III efficacy trials in various malignancies. The detailed investigations have demonstrated that:

- Entinostat, a selective HDAC inhibitor with a favorable pharmacokinetic profile, functions by reversing epigenetic silencing and modulating the tumor immune microenvironment, leading to antitumor activity.

- Completed trials in breast cancer, lung cancer, melanoma, RCC, prostate, pancreatic, and pediatric malignancies have validated its safety and provided early evidence of therapeutic benefit. Notably, combination regimens—especially in hormone receptor–positive advanced breast cancer and in studies involving immune checkpoint inhibitors—have shown particularly encouraging results.

- Ongoing trials are focused on evaluating entinostat in combination with cancer lysate vaccines, immune checkpoint inhibitors, targeted therapies, and chemotherapy. The results from these trials will further refine the dosing strategies and patient selection criteria, utilizing pharmacodynamic biomarkers to optimize treatment outcomes.

- The implications of these clinical trials are far reaching, as the incorporation of epigenetic therapy with entinostat is not only poised to reverse therapeutic resistance but also to enhance immune responsiveness in various cancers. This multifaceted approach may set a new paradigm in combination cancer therapy, improving survival outcomes and quality of life for patients.

In conclusion, entinostat represents a promising agent in the evolving landscape of cancer treatment. Its successful integration into combination regimens has the potential to overcome significant resistance mechanisms and improve the efficacy of existing therapies. The breadth of clinical trials—from Phase I safety assessments to large Phase III efficacy studies—demonstrates robust engagement across multiple cancer types and patient populations. As ongoing and future trials continue to unravel the biomarker correlations and mechanistic nuances of entinostat’s action, the prospect for its regulatory approval and incorporation into standard treatment protocols becomes increasingly tangible. This journey not only exemplifies the translational potential of epigenetic modifiers in oncology but also heralds a future in which personalized, biomarker-driven therapeutic strategies lead to improved outcomes for a broad spectrum of cancer patients.