The addition of abemaciclib to fulvestrant significantly improved progression-free survival for patients with hormone receptor positive, HER2 negative advanced breast cancer who had been previously treated with cyclin-dependent kinase 4 and 6 inhibitor plus endocrine therapy, in a new study.
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) 2024 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=.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.
Data Support Switching CDK Inhibitors in Absence of Mutations
Switching CDK inhibitors to abemaciclib plus endocrine therapy significantly prolonged progression-free survival compared with endocrine therapy alone, with especially pronounced improvement in those without visceral metastases and those with longer durations of first-line CKD4/6 inhibitor therapy, said Ruth O’Regan, MD, of the University of Rochester, New York, who served as the discussant for the new research.
Dr Regan referenced the improvement with abemaciclib in the BICR, a technique used to identify potential bias introduced by the assessment of local investigators. This can result in more favorable PFS on a treatment arm as seen in this study, but its use generally does not impact overall trial results, she said.
In the context of other studies involving switching CDK 4/6 inhibitors post-progression, the difference of 0.7 months in PFS between the abemaciclib and placebo groups was less than the 2.5 months difference seen in the MAINTAIN trial and the 1.3 months difference seen in the PALMIRA trial, Dr O’Regan said in her presentation. Conversely, in the PACE trial, the intervention group did worse (4.6 months) than the control group in terms of the PFS (4.8 months), she said. Overall, the results of the postMONARCH trial support the use abemaciclib in patients with no actionable genetic mutation, she said.
In a question-and-answer session, Dr Kalinsky was asked whether clinicians should still bother with genetic testing, since patients in the current study showed benefits regardless of the presence or absence of a mutation.
“I would still recommend that we check for mutations,” he emphasized. The current study “is one chapter in a much larger book,” and the field continues to evolve, he said.
A Clinician’s Take
“Currently, no standard second-line treatment after progression on first line CDK4/6 inhibitor plus endocrine therapy exists,” Malinda T. West, MD, of the University of Wisconsin, Madison, said in an interview. “Using a different CDK4/6 inhibitor after progression on a first CDK4/6 inhibitor has mixed data,” she said.
“If benefit with a second CDK4/6 inhibitor is confirmed, it may represent an additional low toxicity, chemotherapy-sparing regimen,” she noted.
Earlier data from the MAINTAIN trial had shown benefit with using ribociclib after progression on a primarily first line palbociclib, though other trials looking at use of palbociclib after progression on CDK 4/6 inhibitor [including the PACE and PALMIRA trials] had not, she said.
Overall, the results from postMONARCH support that switching the CDK4/6 inhibitor at progression to ribociclib or abemaciclib may be another treatment option, and reasonable for patients who don’t have other actionable mutations, Dr West told this news organization.
The study was supported by Eli Lilly. Dr Kalinsky disclosed that immediate family members are employed by EQRx and GRAIL, with stock or other ownership interests in these companies. He disclosed consulting or advisory roles with 4D Pharma; AstraZeneca; Cullinan Oncology; Daiichi Sankyo/AstraZeneca; eFFECTOR Therapeutics; Genentech/Roche; Immunomedics; Lilly; Menarini Silicon Biosystems; Merck; Mersana; Myovant Sciences; Novartis; Oncosec; Prelude Therapeutics; Puma Biotechnology; RayzeBio; Seagen; and Takeda. Dr Kalinsky further disclosed research funding to his institution from Ascentage Pharma; AstraZeneca; Daiichi Sankyo; Genentech/Roche; Lilly; Novartis; and Seagen, and relationships with Genentech and Immunomedics.
Dr O’Regan disclosed honoraria from AstraZeneca/MedImmune; bioTheranostics; Gilead Sciences; Novartis; Pfizer; Puma Biotechnology; and Seagen, serving as a consultant or adviser for AstraZeneca/MedImmune; bioTheranostics; Lilly; Novartis; Puma Biotechnology; and Seagen, and funding to her institution from Novartis and Puma Biotechnology.
Dr West, who was not involved in the new research or other studies mentioned in this article, had no financial conflicts to disclose.
This article originally appeared on MDedge.com, part of the Medscape Professional Network.
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