Imlunestrant Shows PFS Benefit in Advanced Breast Cancer

SAN ANTONIO — Second-line treatment with imlunestrant improved progression-free survival compared with standard endocrine monotherapy in patients with advanced estrogen receptor (ER)–positive, human epidermal growth factor receptor 2 (HER2)–negative breast cancer who had ESR1 mutations, according to recent findings from the EMBER-3 trial.

This progression-free survival benefit with imlunestrant did not extend to the overall population, but a combination of imlunestrant plus abemaciclib did lead to a significant improvement in progression-free survival compared with imlunestrant alone, regardless of patients’ ESR1 mutation status.

Lead author Komal Jhaveri, MD, a breast oncologist at Memorial Sloan Kettering Cancer Center in New York City, called the findings “encouraging.”

The phase 3 results raise the possibility of a second-line all-oral targeted therapy option for patients with ER–positive, HER2–negative advanced breast cancer, said Jhaveri, who presented the findings at the San Antonio Breast Cancer Symposium (SABCS) 2024, which were published simultaneously in NEJM.

However, outside experts provided a note of caution that the trial design may limit how relevant the findings are to clinical practice.

First-line treatment for advanced ER–positive, HER2–negative breast cancer includes an aromatase inhibitor, such as exemestane, and a CDK4/6 inhibitor, such as abemaciclib. However, an ESR1 mutation may develop, which can undermine the effectiveness of the aromatase inhibitor. These patients may swap in a selective estrogen receptor degrader (SERD) — typically, the injectable fulvestrant — in place of the aromatase inhibitor.

Over the past several years, researchers have searched for a better agent than fulvestrant because this injectable drug has limited efficacy in patients with ESR1 mutations, and the monthly intramuscular shots are painful and inconvenient for patients.

The oral SERD imlunestrant is one such candidate.

The EMBER-3 trial initially randomized 661 patients after progression/recurrence evenly to either imlunestrant monotherapy (400 mg once daily) or a standard treatment arm that included either exemestane or fulvestrant, with 90% of these patients receiving fulvestrant. Investigators added a third combination arm shortly after the trial started, which included 213 patients who received imlunestrant plus abemaciclib.

About 60% of the overall population had received prior CDK4/6 inhibitors, primarily palbociclib and ribociclib. About 37% of the study population had ESR1 mutations.

Among patients with ESR1 mutations, imlunestrant monotherapy led to a significant improvement in median progression-free survival of 5.5 months vs 3.8 months in the standard care arm (P 

When comparing the two treatment arms in the overall population, the median progression-free survival was significantly better in patients who received imlunestrant plus abemaciclib — 9.4 months vs 5.5 months in the imlunestrant group (hazard ratio [HR] for progression or death, 0.57; P ESR1 mutation status, as well as among patients who had received a CDK4/6 inhibitor previously.

Data from other studies presented at SABCS indicate that another SERD, elacestrant, in combination with abemaciclib, may provide a similar progression-free survival benefit in this patient population. Elacestrant was approved by the US Food and Drug Administration (FDA) in January 2023 for second-line treatment of patients with advanced breast cancer and ESR1 mutations.

The EMBER trial also reported early overall survival findings. Although immature, overall survival trends favored imlunestrant over the standard treatment. The estimated overall survival at 18 months was 77% in the imlunestrant group and 58.6% in the standard therapy group among patients with ESR1 mutations (HR, 0.55), and 78.6% in the imlunestrant group vs 71.8% in the standard-therapy group for all patients (HR, 0.69).

Common all-grade adverse events with imlunestrant vs standard therapy included fatigue (22.6% vs 13.3%), diarrhea (21.4% vs 11.7%), and nausea (17% vs 13%). Grade 3 or higher anemia and neutropenia were low and similar in both arms.

All-grade diarrhea (86%) and nausea (49%) were more common with the combination of imlunestrant and abemaciclib.

The incidence of grade 3 or higher events was 17% with imlunestrant monotherapy, 21% for standard treatment, and 49% for imlunestrant plus abemaciclib.

EMBER-3 discussant Harold Burstein, MD, PhD, a breast oncologist at the Dana-Farber Cancer Institute, Boston, said that, overall, oral SERDs are starting to “break out from the ESR1 mutation box,” perhaps reflecting the idea that an agent more active than fulvestrant in combination with a non-cross–resistant CDK4/6 inhibitor like abemaciclib might lead to a better long-term outcome, regardless of ESR1 status.

A major limit of EMBER-3, however, is that it did not compare imlunestrant/abemaciclib with fulvestrant/abemaciclib, which would have been a true standard-of-care control, said Burstein.

Kathy Miller, MD, a breast oncologist at Indiana University, Indianapolis, agreed.

She was also concerned about the use of monotherapy in the standard care arm.

“Monotherapy hormone therapy is not what people would be treated with,” Miller said. Patients would typically get fulvestrant with either a targeted therapy or everolimus.

Without appropriate controls, “the data are impossible to interpret” in the context of current practice, Miller told Medscape Medical News.

Eli Lilly and Company, maker of imlunestrant, funded, designed, and largely conducted the trial. Jhaveri is a consultant and researcher for the company. Burstein and Miller didn’t have any disclosures. Miller is an editorial advisor for Medscape Oncology.