Comparative Effectiveness of Aromatase Inhibitors in Postmenopausal Breast Cancer: Anastrozole and Letrozole vs Exemestane

Aromatase inhibitors (AIs) such as anastrozole, letrozole, and exemestane form the cornerstone of treatment for postmenopausal women diagnosed with hormone receptor–positive early-stage breast cancer. While these agents are commonly prescribed, the comparative effectiveness of each AI in real-world clinical practice has remained underexplored—until recent comprehensive research provided valuable insights.

Understanding Aromatase Inhibitors and Their Role in Breast Cancer

Aromatase inhibitors work by blocking the aromatase enzyme, which converts androgens into estrogen, thereby reducing estrogen levels in postmenopausal women. Since estrogen can promote the growth of hormone receptor–positive breast cancer cells, suppressing its production is a critical therapeutic strategy following breast surgery.

Nationwide Real-World Study Overview

A landmark study analyzed data from the French Early Breast Cancer Cohort encompassing nearly 150,000 postmenopausal women aged 50 to 75. These women, diagnosed between 2011 and 2020, initiated AI therapy after breast surgery and were observed for up to eight years. This extensive dataset provided an unprecedented opportunity to compare long-term outcomes associated with anastrozole, letrozole, and exemestane.

Key Findings: Survival and Medication Persistence

• Disease-Free Survival (DFS): Eight-year DFS was approximately 81% for women on anastrozole and letrozole, compared to 79% for those on exemestane.
• Overall Survival (OS): At eight years, OS rates were 90.5% with anastrozole, 89.9% with letrozole, and 88.8% with exemestane.
• Therapy Discontinuation: Women treated with exemestane were more likely to discontinue therapy early than those on anastrozole or letrozole.
• Impact of Adherence: Even when assuming ideal therapy adherence over five years, exemestane showed slightly lower survival outcomes.

Why Do These Differences Occur?

The differences in efficacy may stem from variations in pharmacologic properties among these agents. Letrozole typically achieves the most profound estrogen suppression, which might explain its marginally superior clinical results. In contrast, exemestane’s unique steroidal structure may result in less complete estrogen suppression and potential activation of certain hormone receptors that could facilitate tumor progression in some patients.

Safety and Side Effect Profile

• Bone fracture risks were comparable across all three AIs.
• Exemestane users tended to have a slightly better cholesterol profile but showed a higher risk of developing diabetes.
• High rates of therapy switching and discontinuation were observed, especially with exemestane.

Clinical Implications

This extensive real-world evidence suggests that anastrozole and letrozole might provide a small but clinically meaningful advantage over exemestane in treating postmenopausal women with early-stage hormone receptor–positive breast cancer. These findings highlight the importance of considering drug efficacy, adherence potential, and side effect profiles when selecting initial AI therapy.

However, treatment decisions should remain individualized, incorporating patient preferences, tolerability, comorbidities, and other factors impacting adherence and quality of life.

Summary of Key Points

• This is the largest real-world comparison of anastrozole, letrozole, and exemestane to date.
• Anastrozole and letrozole are associated with slightly better 8-year disease-free and overall survival compared to exemestane.
• Higher discontinuation rates were noted with exemestane therapy.
• Differences in outcomes remained evident even with full adherence to treatment.
• The results support preferring anastrozole or letrozole as initial AI therapy in most postmenopausal breast cancer patients.

Reference

Dumas E, Hamy A-S, Wanis KN, et al. Outcomes of Anastrozole, Letrozole, and Exemestane in Patients With Postmenopausal Breast Cancer. JAMA Network Open. 2025;8(12):e2550842. doi:10.1001/jamanetworkopen.2025.50842

Author: Dr. Pooja N., MBBS, DGO, Private Practitioner