Background
Phase III, double-blind, placebo-controlled RCT. 1918 postmenopausal women with HR+ early breast cancer who had completed 4.5–6 years of aromatase inhibitor therapy (following prior tamoxifen or as primary adjuvant AI). Tested extended AI therapy beyond 5 years of AI.
Interventions and follow up
Arm A: Letrozole 2.5mg daily × 5 additional year
Arm B: Placebo × 5 year
Primary endpoint: Disease-free survival (DFS)
mFollow up: 6.3 year
Arm B: Placebo × 5 year
Primary endpoint: Disease-free survival (DFS)
mFollow up: 6.3 year
Results
DFS: HR 0.66, 95% CI 0.48–0.91, P=.01
Annual BC rate: 1.37% vs 2.18% (letrozole vs placebo)
OS: HR 0.97, 95% CI 0.73–1.28 — not significant
Contralateral BC: HR 0.42, 95% CI 0.22–0.81, P=.01
Annual BC rate: 1.37% vs 2.18% (letrozole vs placebo)
OS: HR 0.97, 95% CI 0.73–1.28 — not significant
Contralateral BC: HR 0.42, 95% CI 0.22–0.81, P=.01
Adverse events
Musculoskeletal: bone fractures 14% vs 9% (P=.001); arthralgia/myalgia more common; new-onset osteoporosis higher with letrozole
Vasomotor/GU: hot flashes and vaginal symptoms more frequent
Cardiovascular: no excess cardiovascular events or mortality
Vasomotor/GU: hot flashes and vaginal symptoms more frequent
Cardiovascular: no excess cardiovascular events or mortality
Conclusions
Extended letrozole for 5 years after completing 5 years of aromatase inhibitor therapy significantly improved DFS and reduced contralateral breast cancer risk. OS was not improved. The trial supports extended AI therapy in selected patients at continued risk of late recurrence.
Key Limitations
Key Limitations: OS benefit not demonstrated; DFS is a surrogate endpoint with uncertain translation to survival in this late-recurrence setting. The trial enrolled a selected population (patients who tolerated ≥4.5 years of AI), potentially overrepresenting compliant patients with lower toxicity burden. Cumulative musculoskeletal toxicity and bone loss from prolonged AI exposure are clinically meaningful and require proactive bone health management. Optimal selection criteria for extended AI (e.g., high nodal burden, late recurrence risk indices) not prospectively defined.
Clinical Context
MA.17R provided the first RCT evidence for extending AI beyond 5 years in postmenopausal HR+ BC. Clinical guidelines (ASCO, ESMO) support extended AI (up to 10 years total) for patients with node-positive disease or high recurrence risk, with shared decision-making weighing DFS benefit against bone and quality-of-life toxicities. Contrasts with MA.17 (letrozole after 5 years tamoxifen) which also showed DFS benefit, collectively supporting the principle of extended endocrine therapy in higher-risk HR+ early BC.
References
References: Goss PE et al, NEJM 2016