monarchE: What the Breast Cancer Trial Found

The problem monarchE was built around

Most hormone receptor-positive, HER2-negative breast cancer does well. Surgery, then years of hormone-blocking tablets, and the cancer does not return.

The trial's authors put a number on the exception. Up to 30% of patients with high-risk features go on to develop distant recurrence, and many of those recurrences happen in the first few years.

Hormone therapy alone was not enough for that group. monarchE tested whether adding a second kind of drug would help.

What abemaciclib blocks

Cells divide on a schedule controlled by proteins called cyclin-dependent kinases. Two of them, CDK4 and CDK6, act as the gate that lets a cell move from resting into copying its DNA.

In hormone-driven breast cancer that gate is pushed open too often. Abemaciclib is a CDK4/6 inhibitor. It holds the gate shut.

The drug was already approved for advanced hormone receptor-positive breast cancer. monarchE asked whether it also works earlier, in people whose cancer has been removed but might come back. That use is called adjuvant treatment.

Who was eligible

This is the part that matters most for anyone reading the headline.

All 5,637 participants had hormone receptor-positive, HER2-negative early breast cancer, and all had already had surgery, plus radiation and chemotherapy where indicated.

They also had to be high risk, defined by the pathology report:

• Four or more lymph nodes containing cancer, or
• One to three positive nodes plus one of: a tumor 5 cm or larger, histologic grade 3, or a Ki-67 score of 20% or higher.

Ki-67 is a marker of how many cells in a tumor are actively dividing. A high score means a faster-growing tumor.

Anyone outside those criteria was not in this trial, and the result does not extend to them.

What the trial did and found

Participants were randomly assigned 1:1 to standard endocrine therapy alone, or to endocrine therapy plus abemaciclib at 150 mg twice a day for two years. It was open-label, meaning everyone knew which arm they were in. Two years at that strength was the trial plan. Your own course and amount come from your breast team.

That was the study schedule. People prescribed abemaciclib today should take the amount their own oncologist set, which is often reduced when side effects appear.

The main measure was invasive disease-free survival. That counts invasive cancer returning, a new invasive cancer appearing, or death from any cause.

At a preplanned interim analysis, 323 such events had occurred. Adding abemaciclib was better, with a hazard ratio of 0.75 and a 95% confidence interval of 0.60 to 0.93.

A hazard ratio of 0.75 means the rate of those events was about 25% lower in the abemaciclib group at any given point.

In absolute terms, 92.2% of the abemaciclib group were free of invasive disease at two years, against 88.7% on hormone therapy alone. That is a gap of 3.5 percentage points.

Safety matched what was already known about the drug. Abemaciclib commonly causes diarrhea, low white cell counts and fatigue, and it requires regular blood monitoring.

Two ways to read the same number

A 25% reduction in risk sounds larger than a 3.5 percentage point difference. Both describe the same result.

The first is a relative figure. The second is absolute, and it is the one that answers a patient's actual question: out of 100 people like me, how many are better off?

At two years, roughly 4 in 100 avoided an event they would otherwise have had. The other 96 either would not have had one anyway, or had one regardless. All 100 took the drug and were exposed to its side effects.

Neither framing is dishonest. Reading only the relative one is how a modest benefit starts sounding like a transformation.

When to get checked

Screening and symptoms work differently, and confusing them causes delay.

The U.S. Preventive Services Task Force recommends screening mammography every two years for women aged 40 to 74. It finds the evidence insufficient for women 75 and older.

Anything you can feel or see is not a screening question. See a clinician about a new lump in the breast or armpit, one breast changing shape, skin dimpling or an orange-peel texture, a nipple newly turning inward, discharge from one nipple, or a rash on the nipple that will not clear.

If you already have a diagnosis, three things on your pathology report decide whether this trial is even about you: hormone receptor status, HER2 status, and the number of involved lymph nodes. Ask for those in plain terms. Our page on breast cancer explains what each one changes.

What this trial cannot tell you

• It applies only to hormone receptor-positive, HER2-negative early breast cancer meeting specific high-risk criteria.
• The main measure was invasive disease-free survival at an interim analysis, not overall survival.
• Two years is a short window for a cancer that can return a decade later.
• Side effects are real and continuous. Two years of twice-daily tablets is a substantial commitment while feeling well.
• Trial participants meet strict entry rules, so results may not transfer to everyone. Clinical trial phases covers what an interim phase 3 analysis can and cannot establish.

The wider picture

The American Cancer Society projects about 321,910 new female breast cancer diagnoses in the United States in 2026 and about 42,140 deaths, and SEER publishes that pair. NCI's own SEER measurements put five-year relative survival across all stages at 91.9% for 2016 to 2022, with 64% of cases found while still confined to the breast.

Those are group figures drawn from past years of treatment. They describe a population, not a person, and they say nothing about how any individual will do.

Sources

• Johnston SRD, Harbeck N, Hegg R, et al. Abemaciclib Combined With Endocrine Therapy for the Adjuvant Treatment of HR+, HER2-, Node-Positive, High-Risk, Early Breast Cancer (monarchE). J Clin Oncol 2020;38:3987-3998
• ClinicalTrials.gov record for NCT03155997
• NCI PDQ: Breast Cancer Treatment (Health Professional Version)
• SEER Cancer Stat Facts: Female Breast Cancer

How this page was made

An AI-assisted editorial system helped prepare this page. No named medical reviewer has reviewed it unless one is listed.

Cancer Explained is published by the National Cancer Information Foundation. It is not medical advice and does not suggest a test or treatment.

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