Expert Explores the Next Frontier in Endocrine Therapy for Breast Cancer

William J. Gradishar, MD

Endocrine therapy has come a long way as a treatment for breast cancer since tamoxifen ruled the adjuvant therapy landscape in 1989, but much remains unknown about newer therapies and combating endocrine resistant mutations, according to William J. Gradishar MD, at the 2016 Lynn Sage Breast Cancer Symposium.

Currently, NCCN guidelines call for continuation of endocrine therapy until disease progression or unacceptable toxicity levels, noted Gradishar, deputy director of the Robert H. Lurie Comprehensive Cancer Center at Northwestern University Feinberg School of Medicine. “We try to continue patients on endocrine therapy because of the quality of life issues and because it’s better tolerated than chemotherapy,” said Gradishar.

Sixty percent to 70% of breast cancers are hormone receptor-positive, and current treatments aim to decrease estrogen levels or block ER. While some cancer are initially resistant to such therapies (de novo resistance), others become resistant over time (acquired resistance), highlighting the need for novel therapeutics to combat endocrine therapy resistance.

In order to prolong effective treatment with these therapies, research should now focus on overcoming resistance mechanisms to endocrine therapy, he said, by exploring estrogen receptor (ER) mutations, CDK4/6 inhibitors, mTOR inhibitors, PI3K inhibitors, and HDAC inhibitors.

“Biologic information may influence the choice of therapy we offer the patient, but we just don’t know enough about yet,” he said. “When estrogen receptors stop working, it’s usually the factor that one or more molecular pathways becomes more important.”

To delay or prevent resistance, endocrine therapy is now being given in various combinations. In most instances, it is no longer considered a monotherapy, Gradishar noted.

Palbociclib, which targets CDK4/6, was first FDA approved in 2015 in combination with the endocrine therapy letrozole. It is now also approved for use with fulvestrant. Gradishar explained that in response to mitogenic signals, cyclin D1 complexes with CDK4/6, leading to the phosphorylation of RB protein, causing the protein to no longer serve as a tumor suppressor, which results in uncontrolled cell proliferation.

Two other CDK 4/6 inhibitors, ribociclib and abemaciclib, are also in development for patients with breast cancer and other malignancies. Gradishar expects these agents to be approved soon. Both abemaciclib and ribociclib have received breakthrough therapy designations from the FDA and are in phase III studies; however, abemaciclib appears unique, as it has single-agent activity.

In May, a phase III study exploring ribociclib plus letrozole was stopped, after demonstrating an improvement in progression-free survival. For abemaciclib, the agent is also being explored in phase III studies. The phase III MONARCH-2 study is looking at the agent with fulvestrant. As a single-agent, the phase II MONARCH-1 study showed a response rate of nearly 20% in heavily pretreated patients with refractory, hormone-receptor—positive, HER2-negative advanced breast cancer.

Patients given palbociclib plus letrozole showed significant improvement in progression-free survival. Unfortunately, neutropenia is one of the common side effects seen with this dual treatment, said Gradishar, adding that while it doesn’t necessarily cause a problem for the patient it can be a headache for the treating physician to monitor and manage.

Uniquely, most significant side effects of abemaciclib appear to be gastrointestinal issues, said Gradishar, with no neutropenia. “Abemaciclib seems to have some ability to benefit Cyclin D1, which may be important, but it may also have some toxicity,” said Gradishar.

Targeting mTOR

Another approach once resistance has developed is the addition of an mTOR inhibitor endocrine therapy. TOR regulates metabolism in times of plenty and not, explained Gradishar, which is why it’s so prevalent in mammals.

The phase III BOLERO-2 study was instrumental in the FDA approval of the mTOR inhibitor everolimus in combination with exemestane for patients with postmenopausal advanced breast cancer following resistance to a non-steroidal aromatase inhibitor. Blocking mTOR with everolimus improved progression-free survival by 55% compared with exemestane alone; however, survival was not significantly improved.

Studies are now assessing the addition of an mTOR inhibitor in the frontline setting. The phase III SWOG S1222 trial is looking at fulvestrant with or without everolimus or exemestane with or without everolimus for untreated patients with ER-positive metastatic breast cancer.

Novel Combination Approaches

Several other options are also under exploration, although these have been met with mixed results. A primary reason for the lack of conclusive findings for these therapies may be the lack of a biomarker to select patients. With many new therapies, it’s difficult to define a biomarker for who would benefit, argued Gradishar. With PIK3 inhibition, for example, not many studies have been successful, he said.

As an example, the phase III FERGI study exploring fulvestrant with or without the PI3K inhibitor pictilisib was met with uncertain results. Progression-free survival was not significantly improved with the addition of pictilisib, and results were similar in those with PIK3CA-mutated and wild-type tumors.

The phase III BELLE-2 study was able to show efficacy for the combination of fulvestrant and the PI3K inhibitor buparlisib; however, it was too small a benefit to warrant a regulatory submission. In this study, the PI3K inhibitor improved progression-free survival by just 1.9 months across the full population. Furthermore, the improvement was not statistically significant in a PI3K activated group.

There was hope for a biomarker from this study, however, as patients with PIK3CA mutations in circulating tumor DNA showed a 44% reduction in progression or death with buparlisib. This finding will still need to be confirmed in other studies.

HDAC inhibitors are also of interest as a combination approach with endocrine therapy. Chief among these therapies is entinostat, which has received a breakthrough designation from the FDA in September 2011. The designation was based on significant improvements in survival experienced by patients in the phase II ENCORE 301 study, in which entinostat was administered in combination with exemestane. A phase III study is exploring entinostat and exemestane versus exemestane alone for patients with resistance to non-steroidal aromatase inhibitors, Gradishar noted.

Looking across the numerous studies of these newer targeted therapies, it is easy to say they are superior to their monotherapy counterparts, he said. “But these therapies are increasingly complex, potentially toxic, and certainly more expensive, so it’s yet to be determined."