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MONALEESA-2: What the Breast Cancer Trial Found
MONALEESA-2 reported early, before the ribociclib group had reached a median. Following the numbers across three publications shows how a trial result fills in over six years.
Original commentary from the Cancer Explained editorial team.
Please note: this page is educational only — it is not medical advice, and it does not speculate about anyone’s health beyond reliable public reporting. For questions about your own health, talk with your healthcare team.
A result reported before it was finished
Most trial coverage treats a result as a single fixed number. MONALEESA-2 is a good place to see why that is misleading. Its first report came out while the key number was still missing, and the full picture took another six years to arrive.
Who took part
668 postmenopausal women joined. All had hormone-receptor-positive, HER2-negative breast cancer that had come back or spread, and none had yet had any drug treatment for the advanced disease. Half received ribociclib plus letrozole. Half received a placebo plus letrozole.
Letrozole is an aromatase inhibitor, a hormone treatment. Ribociclib blocks CDK4 and CDK6, two proteins that drive cell division.
The first report, and the number that was missing
The first analysis was planned, and it came early. Median follow-up was 15.3 months.
Adding ribociclib clearly delayed growth: hazard ratio 0.56 (95% CI 0.43 to 0.72; P=3.29 x 10 to the minus 6). That is roughly a 44% lower risk of the cancer growing or the woman dying at any moment.
But more than half the ribociclib group had not yet had their cancer grow, so there was no median to report for them. The trial published landmark figures instead. At 18 months, 63.0% of the ribociclib group were alive without progression (95% CI 54.6 to 70.3), against 42.2% on placebo (95% CI 34.8 to 49.5). Among women with measurable tumors, 52.7% responded against 37.1% (P<0.001).
What longer follow-up filled in
At a median follow-up of 26.4 months, the missing median appeared: 25.3 months with ribociclib against 16.0 months with letrozole alone, hazard ratio 0.568 (95% CI 0.457 to 0.704).
Survival took longest. After a median follow-up of 6.6 years, median overall survival was 63.9 months with ribociclib and 51.4 months without, hazard ratio 0.76 (95% CI 0.63 to 0.93; P=0.008). More than a year of difference, reported six years after the first headline.
The blood-count problem
Ribociclib suppresses white cells. Severe neutropenia occurred in 59.3% of the ribociclib group against 0.9% on placebo. Severe leukopenia occurred in 21.0% against 0.6%. That means regular blood tests, and sometimes dose changes.
7.5% of women stopped ribociclib because of side effects, against 2.1% on placebo.
What this study cannot tell you
- The first report was an interim analysis at 15.3 months. Everything about the size of the delay came later.
- Growth was judged by the treating investigator in the first analysis, not by blinded independent reviewers.
- Only postmenopausal women were enrolled. It does not directly answer the question for premenopausal patients.
- CDK4/6 inhibitors differ from one another in side effects and in what their own trials have shown. This is evidence about ribociclib.
Questions about CDK4/6 inhibitors
- Which drug in this class is being suggested for me, and why that one?
- How often would my blood counts be checked, especially at the start?
- What would happen to the dose if my counts drop?
- Would this be combined with letrozole or another hormone drug?
Sources
This article was written from the sources below, which were checked on the source-check date shown above.
- NEJM: Ribociclib as First-Line Therapy for HR-Positive, Advanced Breast Cancer (MONALEESA-2) (primary)
- Annals of Oncology: Updated results from MONALEESA-2 (primary)
- NEJM: Overall Survival with Ribociclib plus Letrozole in Advanced Breast Cancer (primary)
How this article was prepared
Prepared by Cancer Explained's AI-assisted editorial system and checked against the sources listed below. This article has not been reviewed by a healthcare professional unless a named reviewer is specifically shown.
Cancer Explained is published by the National Cancer Information Foundation as a nonprofit-oriented public-interest education project. It is not a diagnostic service, does not recommend treatments, and is not for emergencies.
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Prevention, possible warning signs, screening, and diagnosis
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Not every cancer can be prevented. Avoiding tobacco, protecting skin from ultraviolet radiation, limiting alcohol, staying active, and receiving recommended HPV or hepatitis B vaccination can lower the risk of certain cancers. A risk factor is not a prediction or a cause in one individual.
Symptoms and possible early signs
Possible signs vary and are often caused by conditions other than cancer. Changes worth discussing include a new lump, unexplained bleeding or weight loss, a persistent cough, lasting bowel or bladder changes, a changing skin spot, or symptoms that persist or worsen. Some early cancers cause no symptoms.
Screening and early detection
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