postMONARCH
Trial question
What is the role of abemaciclib plus fulvestrant in patients with HR+/HER2- advanced breast cancer?
Study design
Multi-center
Double blinded
RCT
Population
Characteristics of study participants
99.0% female
1.0% male
N = 368
368 patients (365 female, 3 male).
Inclusion criteria: patients with HR+/HER2- advanced breast cancer who were on previous CDK4/6 inhibitor plus aromatase inhibitor as initial therapy.
Key exclusion criteria: visceral crisis, lymphangitic spread, or leptomeningeal carcinomatosis; symptomatic or untreated CNS metastasis; receipt of any systemic therapy between disease recurrence/progression; receipt of > 1 line of therapy for advanced or metastatic disease; receipt of prior chemotherapy for metastatic breast cancer.
Interventions
N=182 abemaciclib (abemaciclib 150 mg PO BID plus fulvestrant 500 mg IM once daily on days 1 and 15 of cycle 1, then once every 4 weeks).
N=186 placebo (matching placebo BID plus fulvestrant 500 mg IM once daily on days 1 and 15 of cycle 1, then once every 4 weeks).
Primary outcome
Median progression-free survival (investigator-assessed)
6 months
5.3 months
6.0 months
4.5 months
3.0 months
1.5 months
0.0 months
Abemaciclib
Placebo
Significant
increase ▲
Significant increase in median progression-free survival (investigator-assessed) (6 months vs. 5.3 months; HR 1.37, 95% CI 1.05 to 1.75).
Secondary outcomes
Significant increase in median progression-free survival (blinded independent central review-assessed) (12.9 months vs. 5.6 months; HR 1.81, 95% CI 1.3 to 2.56).
Significant increase in objective response rate (16.8% vs. 7.1%; AD 9.7%, 95% CI 1.88 to 17.52).
Significant increase in clinical benefit rate (29% vs. 18.1%; AD 10.9%, 95% CI 0.69 to 21.11).
Safety outcomes
Significant difference in ≥ 3 grade adverse events (55.2% vs. 20.0%).
Conclusion
In patients with HR+/HER2- advanced breast cancer who were on previous CDK4/6 inhibitor plus aromatase inhibitor as initial therapy, abemaciclib was superior to placebo with respect to median progression-free survival (investigator-assessed).
Reference
Kevin Kalinsky, Giampaolo Bianchini, Erika Hamilton et al. Abemaciclib Plus Fulvestrant in Advanced Breast Cancer After Progression on CDK4 / 6 Inhibition: Results From the Phase III postMONARCH Trial. J Clin Oncol. 2025 Mar 20;43(9):1101-1112.
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