Disease progression is common in these patients, for whom first-line treatment is cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors plus endocrine therapy, Kevin Kalinsky, MD, of the Winship Cancer Institute of Emory University, Atlanta, said in a presentation at the American Society of Clinical Oncology (ASCO) annual meeting.
A need exists for additional targeted therapies for patients with advanced hormone receptor (HR)+, HER2- breast cancer whose tumors have progressed on endocrine therapy plus a CDK4/6 inhibitor, he said.
Data on the benefits of continuing CDK4/6 inhibitor therapy after progression have been mixed in phase 2 trials, Dr. Kalinsky noted in his presentation. Abemaciclib, an oral CDK4/6 inhibitor, has shown more selectivity for CDK 4 than CDK 6, and is approved in combination with fulvestrant or an aromatase inhibitor for advanced breast cancer, he said.
In a phase 3 study known as postMONARCH, the researchers randomized 182 patients to abemaciclib plus fulvestrant and 186 to placebo plus fulvestrant. The primary endpoint was progression-free survival (PFS) based on investigator assessment; secondary endpoints included PFS based on blinded independent central review (BICR), objective response rate (ORR), and safety.
The PFS rates at 6 months were 50% and 37% for the abemaciclib and placebo arms, respectively.
In the primary analysis, abemaciclib led to a 27% reduction in risk of investigator-assessed progression-free survival events compared with the placebo (117 vs. 141 events, hazard ratio 0.73, P = 0.02).
The study population included men and pre- and postmenopausal women with advanced HR+, HER2- breast cancer and progression after initial CDK4/6 plus endocrine therapy from 96 centers in 16 countries, enrolled between March 2022 and June 2023. The median age of the patients in the abemaciclib and placebo groups was 58 years and 61 years, respectively. Patients underwent scans every 8 weeks for the first 12 months, then every 12 weeks. Most of the patients were enrolled immediately after CDK4/6i + ET as initial therapy for advanced breast cancer. The most common previous CDK4/6 inhibitor therapy was palbociclib (59%), followed by ribociclib (33%) and abemaciclib (8%).
Secondary Endpoints Also Favor Abemaciclib
The effects in favor of abemaciclib were consistent across subgroups, regardless of the presence or absence of baseline genetic mutations (ESR1 or PIK3CA), Dr. Kalinsky said in his presentation.
Overall response rate was significantly improved in the abemaciclib group compared with the placebo group in patients with measurable disease (17% vs. 7%) and PFS according to BICR also significantly improved (HR 0.55).
The magnitude of benefit was less in the subgroup of patients with visceral metastases, Dr. Kalinsky noted.
“Safety was consistent with what is known about the abemaciclib profile,” he added. Six percent of abemaciclib patients discontinued treatment because of adverse events.
The study is the first phase 3 trial to show improvement with CDK4/6 inhibition therapy with a combination of abemaciclib and fulvestrant and offers a new option for patients with HR+, HER2- advanced breast cancer not selected for biomarker status, Dr. Kalinsky concluded.