Entinostat Added to Nivolumab Plus Ipilimumab Shows Early Efficacy/Safety in Advanced BC

Evanthia Roussos Torres, MD, PhD

In a phase 1 study of entinostat (MS-275) combined with nivolumab (Opdivo) and ipilimumab (Yervoy), treatment lead to an expected number of immune-related adverse events (irAEs) along with a 30% objective response rate in patients with advanced HER2-negative breast cancer.

Eighteen patients received the recommended phase 2 dose of the combination, which was entinostat 3 mg weekly with nivolumab 3mg/kg twice per week, and ipilimumab 1 mg/kg twice weekly. The cohort received a median of 2 cycles of therapy (range, 1-17).

At 24 weeks, the clinical benefit rate was 33% in the overall population. Notably, benefit in patients with triple-negative breast cancer continued beyond the data cutoff point and response continued after treatment progression.

The safety analysis showed grade 3 and 4 AEs anemia, decreased neutrophil count and increased lipase occurred in 17%, 13%, and 8% of patients, respectively. The most common irAEs were rash (29%), hypothyroidism (21%), and pneumonitis (8%).

In an interview with Targeted Oncology™, Evanthia Roussos Torres, MD, PhD, assistant professor of Medicine at the Keck School of Medicine University of South California (USC), discussed the phase 1 investigation of the safety and efficacy of entinostat administered in combination with nivolumab/ipilimumab for the treatment of advanced HER2-negative breast cancer.

TARGETED ONCOLOGY™: Can you discuss the efficacy historically observed with the HDAC inhibitor, entinostat?

Torres: The efficacy of entinostat monotherapy has been primarily studied in some of the liquid tumors like the leukemia and myeloma. I don't know for sure what the extent of all of those studies is, but as monotherapy and specifically in breast cancer, it was being studied in combination with anti-endocrine therapies, anti-estrogen therapies to try and improve the response to endocrine therapies for patients who were no longer responsive.

The idea was that entinostat would re-sensitize the tumor microenvironment to estrogen blockade for patients with estrogen receptor and progesterone receptor-positive breast cancer. Then, multiple other epigenetic modulating drugs, as well as the DNA methyltransferase inhibitors, have been studied in combination with checkpoint inhibition, not just in breast cancer, but in numerous different types of cancer as part of a way to sensitize the tumor microenvironment.

Those data were from preclinical models, primarily by a group at Johns Hopkins University Oncology Center under the leadership of Bret Vogelstein, MD as well as Roberto Pili, MD, who has done some of those studies. Essentially, what they were studying was what are the different changes to the immune cells within the tumor microenvironment that are initiated by entinostat that could potentially improve checkpoint response.

Those preclinical models and contributed to understanding how intense that alters the tumor microenvironment by increasing the number of CD8 infiltrating T cells as well as altering the suppression of myeloid-derived suppressor cells, which are known and not just in breast cancer, but also in pancreas cancer and other cancers to provide an immunosuppressed microenvironment. This is thought to be 1 mechanism by which there is intrinsic suppression and lack of response to checkpoint inhibition

Can you explain the ETCTN-9844 study design and methods used to conduct the study?

This was a phase 1 trial. The first part of the phase 1 trial was recently published in Clinical Cancer Research as a trial to test the safety of his novel combination in patients with advanced solid tumors. Our recent publication of the results reported that this treatment combination is associated with expected adverse events for these agents and a recommended phase 2 dose was identified.

An overall response rate of 16% was observed in this initial cohort, including responses in triple-negative breast cancer and hormone receptor-positive breast cancer, and correlative studies from the initial phase. One shows that an increase in CD8 T regulatory ratio was noted after combination therapy.

The second part of the study was presented at ESMO had a similar design similarly, in which the cohort was specifically breast cancer. So, the cohort was advanced HER2-negative breast cancers with no prior and checkpoint inhibition. Then patients were enrolled and were given a run what we call the running entinostat, which they got for 2 weeks, followed by the addition of nivolumab and ipilimumab. The combination of entinostat, nivolumab, and ipilimumab again was administered in the recommended phase 2 dose that was determined from the first dose-escalation cohort. We had 3 biopsies and blood draws. One was done prior to starting in entinostat, 1 was after the 2-week run-in entinostat, and 1 set was at the end of the 8-week treatment with the combination of therapies. After that, we have the ability to do some of the correlative analyses.

What results did you present during ESMO?

First, the patient characteristics are important to note, we had a median age of 55 years, with a total of 24 patients. Of note, the patient cohort that we're reporting on here is 18 patients from this dose-expansion cohort and that also includes 6 patients with breast cancer from the dose-escalation cohort. These patients all received the combination of entinostat plus nivolumab and ipilimumab. Half the cancers were hormone-receptor-positive, and half were triple-negative. Patients were heavily pretreated with a median of 6 and a half prior therapies.

Immune-related adverse events were similar to those reported in the dose-escalation cohort. And notably, there were only 2 patients with higher-grade adverse events, which were a grade 4 increase in lipase, and a grade 5 respiratory failure that was both presumed to be not immune-related.

We're also reported that the objective response rate was 30%, with responses observed in both hormone receptor-positive and triple-negative subtypes. The majority of these responses occurred in patients with triple-negative breast cancer, however, and there was 1 patient who showed a complete response at the 6-month time point. The duration of response ranged from less than 2 months to the longest response ongoing at greater than 24 months after initiation of treatment. The median progression-free survival for the overall cohort was short at 2 and a half months, and the median overall survival was 7.7 months in all patients, 24.4 months in patients with triple-negative breast cancer, and 5.9 months in patients with hormone receptor-positive disease.

Of note,m those driving improved overall survival are the patients who experienced response. We also discussed correlative studies that investigated immunohistochemical staining of T cells which demonstrates that the ratio of CD8 T effector cells over FOXP3 t regulatory cells increased after therapy from baseline to time point 1, which was the baseline compared to the intent to stop run-in and then to time point to after combination therapy. This is primarily because of the increase in CD8-positive T cells infiltration, and this is most notable in biopsies from patients with triple-negative breast cancer. There was also no obvious trend in abundance of tumor-infiltrating lymphocytes at baseline across the breast cancer subtypes or with treatment.

So, this indicates that tumor-infiltrating lymphocytes are not likely to be a reliable biomarker of response in this setting. So. in conclusion, we reported that a combination of entinostat in combination with nivolumab/ipilimumab was associated with expected immune-related adverse events in advanced HER-negative breast cancer. The objective response rate of 30% suggests further evaluation is warranted in this setting, and correlative results suggest an increase in CD8 to FOXP3 ratio after treatment.

What are the next steps with this research?

We have a number of correlative results that we just started analyzing. We're going to be looking at some of the bulk RNA sequencing and B-cell receptor and T-cell receptor sequencing as well as using imaging mass cytometry to investigate the myeloid cell populations in patient specimens after the intense data and after the addition of checkpoint inhibition. We feel that there are multiple other immune cell types that are affected by the therapy that is likely responsible for the responses that we're seeing.

In addition, we were looking forward to planning the next phase results. Given the promising results we've seen here, we think there are more patients with breast cancer who can benefit from this novel combination therapy.

I'd also like to mention that I recently moved my lab from Johns Hopkins to USC and my lab is focusing on understanding the mechanisms of response to entinostat plus checkpoint inhibition in breast cancer. So, we're doing a lot of the preclinical work that kind of goes along in parallel with what we're doing in the clinical trials. We aim to better understand specific molecular mechanisms that entinostat is imparting on some of these important immune cell types to potentially provide data that would support even more specific therapies or help with biomarkers to select patients who are most likely to respond.

With early research in mind, what other novel therapies are showing promise in breast cancer right now?

It's important to note that in breast cancer, especially, we've been trying to understand the tumor environment in more detail so that we can come up with more specific targets and biomarkers that will help improve response rates for this large population of patients. And what I think is interesting is the suppressor cells that are becoming more and more characterized within the different subtypes of breast cancer, specifically, myeloid-derived suppressor cells, as well as differences between MI and M2 macrophages.

I think we're starting to understand how these innate immune cells are contributing to the lack of response to checkpoint inhibition, and that more specific targeting of those cell types has a lot of potential to improve responses. A lot of the work is being done is in the field.

_Reference:

_{Rousse ET, Leatherman J, Rafie C, et al. Entinostat, nivolumab and ipilimumab in advanced HER2-negative breast cancer (ETCTN-9844). Ann Oncol. 2021; 32(suppl 5): S829-S866. doi: 10.1016/annonc/annonc705}