Adding Entinostat to Exemestane Fails to Overcome Aromatase Inhibitor Resistance in Advanced Breast Cancer

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The histone deacetylase inhibitor (HDAC) entinostat, added to exemestane, failed to overcome endocrine therapy resistance in advanced breast cancer, according to the E2112 phase III trial conducted by the ECOG-ACRIN Cancer Research Group. Results showed that overall survival was not improved by the combination of the two drugs in aromatase inhibitor–resistant advanced hormone receptor–positive, HER2-negative breast cancer.1 These disappointing results occurred in the context of pharmacodynamic analysis confirming that entinostat had hit its intended target.

“The combination of exemestane and entinostat did not improve survival in aromatase inhibitor–resistant advanced hormone receptor–positive, HER2-negative breast cancer. A low overall response rate and short progression-free survival of around 3 months were observed. These results differ from those of other trials suggesting an improvement with a HDAC inhibitor. We need improved decision-making tools to determine which patients could benefit from this strategy,” said lead author Roisin M. Connolly, MD, of University College Cork, Ireland, who presented the final analysis of the study.

“Our results highlight the importance of phase III confirmation of phase II data.”
— Roisin M. Connolly, MD

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“This was an important research question, and collaboration was essential for the phase III trial. Our results highlight the importance of phase III confirmation of phase II data. Clearly, we were very disappointed with these results after so many years of work,” Dr. Connolly said after her presentation at the 2020 San Antonio Breast Cancer Symposium. “Despite robust preclinical and phase II data to support the development of entinostat, HDAC inhibitors do not currently have a role in the treatment of aromatase inhibitor–resistant advanced breast cancer unless additional analyses identify predictive biomarkers in a subgroup of patients.”


Endocrine therapy is a mainstay in the treatment of patients with hormone receptor–positive/HER2-negative breast cancer. However, resistance to endocrine therapy remains a significant problem. Entinostat, a selective oral class I HDAC inhibitor, has overcome resistance to endocrine therapy in letrozole-resistant mouse models, and preclinical studies have suggested HDAC inhibitors may normalize endocrine therapy resistance.

Based on preclinical data and preliminary study, the phase II ENCORE 301 trial showed improvement in progression-free survival and an absolute improvement of 8 months in overall survival with the addition of entinostat to exemestane vs placebo in patients with hormone receptor–positive/HER2-negative breast cancer. In that study, the protein lysine acetylation in entinostat was associated with prolonged progression-free survival.2

“Based on these promising phase II results, we hypothesized that the combination of entinostat and exemestane would improve survival in patients with advanced breast cancer after disease progression on an aromatase inhibitor,” explained Dr. Connolly.

Study Details

The randomized, multicenter, double-blind, placebo-controlled, phase III E2112 trial enrolled 608 participants (women and men of all ages) with advanced hormone receptor–positive/HER2-negative breast cancer that progressed on prior nonsteroidal aromatase inhibitors. All patients could receive up to one prior chemotherapy for metastatic disease, and prior treatment with fulvestrant and a cyclin-dependent kinase 4/6 (CDK4/6) inhibitor was allowed.

Between March 2014 and October 2018, a total of 608 participants were randomly assigned 1:1 to receive exemestane at 25 mg orally daily plus entinostat at 5 mg weekly vs exemestane at 25 mg orally daily plus placebo at 5 mg weekly. Patients were stratified according to prior receipt of an aromatase inhibitor for adjuvant or metastatic use, presence or absence of visceral disease, use or nonuse of prior fulvestrant, and United States vs elsewhere.

Blood sampling was done for lysine acetylation evaluation at baseline and day 15. The primary composite endpoint was progression-free survival (according to central review) and overall survival (by local assessment). Secondary endpoints included safety, objective response rate, and changes in protein lysine acetylation status.

At baseline, demographic and disease characteristics were well balanced. The median age of patients was about 63 years; the majority were postmenopausal; almost 60% had an Eastern Cooperative Oncology Group performance status of 0; two-thirds had visceral disease. A total of 25% had received prior chemotherapy in the advanced setting, with a median of one line of therapy (range, 0–6 lines of therapy). About 30% had received prior fulvestrant; 3%, prior everolimus; and about 35%, a prior CDK4/6 inhibitor. The majority (about 85%) of participants had developed aromatase inhibitor resistance in the metastatic setting prior to study entry.

“The combination of exemestane and entinostat did not improve survival in aromatase inhibitor–resistant advanced hormone receptor–positive, HER2-negative breast cancer.”
— Roisin M. Connolly, MD

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The median number of treatment cycles was three. Dose reductions were needed in 30% of the entinostat-treated patients and 0% of the placebo group. There were four treatment-related deaths: three in the entinostat group (heart failure, pneumonitis, and hepatic failure) and one in the placebo group (myocardial infarction).

Overall, entinostat was reported to be well tolerated. The most common adverse events of all grades in the entinostat arm were neutropenia, anemia, and hypophosphatemia. Grade 3 side effects in the entinostat arm included neutropenia (19%), hypophosphatemia (13%), and anemia (7%). Other grade 3 side effects occurred much less frequently.

Efficacy Results

Progression-free survival was assessed by central review in the first 360 patients: median of 3.3 months in the entinostat arm and 3.1 months in the placebo arm, with no significant difference between the two arms. Objective response rates were low and comparable in both treatment arms: 4.6% with entinostat and 4.3% with placebo. In an analysis of the entire study population, no significant difference in overall survival was observed between the two treatment arms: median of 23.4 months with entinostat and 21.7 months with placebo.

A subgroup analysis for both progression-free survival and overall survival showed a similar pattern in all subgroups.

Lysine acetylation change, an integrated correlative objective of the study, was assessed in 397 paired available samples by multiparameter flow cytometry. A significantly higher increase in lysine acetylation by CD15 was observed with entinostat vs placebo (P < .001), showing that the drug did hit its target.

“Despite extensive study of HDAC inhibitors, they have not broken into the clinical space for the treatment of advanced breast cancer.”
— Roisin M. Connolly, MD

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“In contrast to results observed in ENCORE 301, we saw no significant correlation between fold change in lysine acetylation and progression-free survival,” Dr. Connolly said. The median fold change was 1.5 in the entinostat group and 1 in the placebo group.

Dr. Connolly said that correlative and patient-reported outcome analyses of the study are ongoing.

“Despite extensive study of HDAC inhibitors, they have not broken into the clinical space for the treatment of advanced breast cancer,” she noted.

During the question-and-answer session following her presentation, Dr. Connolly told listeners that the investigators have not yet studied the types of mutations involved in aromatase inhibitor resistance in this study, nor did she expect results would have been different with another backbone than exemestane or a different dose of entinostat than the one used in this trial. 


DISCLOSURE: Dr. Connolly has received institutional research funding from Genentech/Roche, MacroGenics, Merck, Novartis, and Puma Biotechnology; has been reimbursed for travel, accommodations, or other expenses by Genentech/Roche; and has held other institutional relationships with Pfizer.


1. Connolly RM, Zhao F, Miller KD, et al: E2112: Randomized phase 3 trial of endocrine therapy plus entinostat/placebo in patients with hormone receptor-positive advanced breast cancer. A trial of the ECOG-ACRIN cancer research group. 2020 San Antonio Breast Cancer Symposium. Abstract GS4-02. Presented December 11, 2020.

2. Yardley DA, Ismail-Khan R, Klein P: Results of ENCORE 301, a randomized, phase II, double-blind, placebo-controlled study of exemestane with or without entinostat in postmenopausal women with locally recurrent or metastatic estrogen receptor-positive breast cancer progressing on a nonsteroidal aromatase inhibitor. J Clin Oncol 29(27 suppl):268, 2011.

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