A Subanalysis of the Phase 3 MONARCH 3 Study of Abemaciclib as Initial Therapy for HR-Positive, HER2-Negative Advanced Breast Cancer

2021 Year in Review - HER2-Negative Breast Cancer —January 21, 2022


Breast Cancer

A subpopulation analysis of MONARCH 3, where a placebo-controlled phase 3 study of abemaciclib plus nonsteroidal aromatase inhibitors for advanced breast cancer, was used to study the progression-free survival (PFS) of abemaciclib as initial therapy in advanced breast cancer.1

Postmenopausal women with hormone receptor (HR)-positive, HER2-negative advanced breast cancer who had not received previous systemic therapy in the advanced illness setting were eligible. Abemaciclib or placebo (150 mg) plus either 1 mg anastrozole or 2.5 mg letrozole (physician’s option) were given to patients (N = 493) in a 2:1 ratio. Secondary objectives were objective response rate, pharmacokinetics, safety, and health-related quality of life. The median PFS in the abemaciclib and placebo groups was 29.1 months and 14.9 months, respectively.1 Abemaciclib plus a nonsteroidal aromatase inhibitor was a successful initial treatment in the Japanese subpopulation, with a tolerable safety profile, consistent with findings in the whole population.1

Continuous abemaciclib dosage with an aromatase inhibitor provided significant clinical benefit to postmenopausal women with HR-positive, HER2-negative advanced breast cancer in the MONARCH 3 trial.2 The development and approval of cyclin-dependent kinase (CDK)4/6 inhibitors for HR-positive, HER2-negative, advanced breast cancer has changed the treatment paradigm by significantly extending PFS and overall survival.2 Abemaciclib is the only CDK4/6 inhibitor currently licensed for the treatment of patients with HR-positive, HER2-negative, advanced breast cancer on a continuous dosage regimen as monotherapy in the United States or in combination with fulvestrant or a nonsteroidal aromatase inhibitor or an aromatase inhibitor.2

Abemaciclib given as an oral continuous schedule showed efficacy and tolerability as monotherapy and in combination with fulvestrant in patients with HR-positive, HER2-negative metastatic breast cancer.3 Abemaciclib had an acceptable safety profile and was an effective first-line treatment for patients with HR-positive, HER2-negative advanced breast cancer, improving PFS and objective response rate substantially.3


  1. Takahashi M, Tokunaga E, Mori J, et al. Japanese subgroup analysis of the phase 3 MONARCH 3 study of abemaciclib as initial therapy for patients with hormone receptor-positive, human epidermal growth factor receptor 2-negative advanced breast cancer. Breast Cancer. 2021 Oct 18. Epub ahead of print.
  2. Johnston S, O’Shaughnessy J, Martin M, et al. Abemaciclib as initial therapy for advanced breast cancer: MONARCH 3 updated results in prognostic subgroups. NPJ Breast Cancer. 2021;7:80.
  3. Di Leo A, Toi M, Campone M, et al. MONARCH 3: abemaciclib as initial therapy for patients with HR+/HER2- advanced breast cancer. Ann Oncol. 2017;28(suppl 5):V609.
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