MONALEESA-2

Design, population and eligibility

Treatment and comparator

Ribociclib 600 mg orally once daily, days 1–21 of each 28-day cycle, plus letrozole 2.5 mg orally daily versus placebo plus letrozole 2.5 mg daily. Treatment continued until progression or unacceptable toxicity.

Primary and secondary endpoints

Primary endpoint: investigator-assessed PFS. Key secondary endpoint: OS, tested hierarchically. Other endpoints included response and safety.

Key efficacy results

Median PFS was 25.3 versus 16.0 months; HR 0.568 (95% CI 0.457–0.704; p<0.001). At final OS analysis, median OS was 63.9 months (95% CI 52.4–71.0) versus 51.4 months (95% CI 47.2–59.7); HR 0.76 (95% CI 0.63–0.93; p=0.008).

Five-year OS was 52.3% versus 43.9%; at 72 months, 44.2% versus 32.0%. The 60-month absolute OS difference was 8.4 percentage points.

Source-reported outcome visualization

Landmark overall survival estimates.

The final OS analysis showed a statistically significant survival benefit; medians are reported separately and are not used to calculate NNT.

Absolute effect

At 60 months, OS was 52.3% versus 43.9%, an absolute difference of 8.4 percentage points. Approximate landmark NNT 12 at 5 years for one additional patient alive. This NNT is calculated from the fixed-time Kaplan–Meier OS rates, not from the median OS difference.

Trial-specific toxicity and treatment burden

Grade 3–4 neutropenia was approximately 62% (52.4% grade 3 and 9.6% grade 4) with ribociclib versus 1.2% with placebo; grade 3–4 leukopenia approximately 21.3% versus 0.9%. Grade 3–4 ALT elevation 8.4% and AST elevation 5.1% were reported with ribociclib. QT prolongation and hepatotoxicity require ECG and liver-function monitoring.

Adverse-event dose interruption/reduction is common; discontinuation due to AEs is higher with ribociclib than placebo. Treatment is continuous until progression or intolerance.

Quality of life and patient-reported outcomes

Patient-reported outcomes generally showed maintained global health status and QoL during ribociclib treatment, with delayed deterioration in some symptoms; toxicity monitoring and dose modification remain necessary.

Subgroups and applicability

Benefit was generally consistent across clinically relevant subgroups. Exploratory analysis suggested a stronger OS HR in de novo metastatic disease than in recurrent disease, but subgroup results should not determine treatment alone.

Current-practice context · reviewed 2 November 2026

A CDK4/6 inhibitor plus endocrine therapy is a standard first-line approach for most HR-positive/HER2-negative metastatic breast cancer without visceral crisis. MONALEESA-2 provides mature OS evidence for ribociclib plus an aromatase inhibitor; selection among CDK4/6 inhibitors depends on patient factors, comorbidity, toxicity and access.

Limitations and critical interpretation

Trial enrolled postmenopausal women and compared ribociclib with placebo, not with other CDK4/6 inhibitors. The trial predates newer endocrine-targeted combinations and molecularly guided sequencing. Results should not be generalized to visceral crisis requiring rapid chemotherapy.

Bottom line: MONALEESA-2 is landmark first-line evidence: ribociclib added to letrozole improves both PFS and OS, with an 8.4-point 5-year OS gain, at the cost of substantial but monitorable neutropenia and laboratory toxicity.

Primary sources and follow-up publications

1. Final OS analysis, NEJM 2022
2. Original primary PFS report, NEJM 2016
3. MONALEESA-2 registry

All outcome values above are transcribed from the linked trial reports or their prespecified follow-up publications. Unreported values are not imputed. No NNT is derived from median survival.