Developed under the direction and sponsorship of Lilly Medical Affairs and is intended for US healthcare professionals only.

May 11, 2026

Unmet Needs and Challenges in mBC: Endocrine Therapy, Resistance Mechanisms, and ESR1 Mutations

Aditya Bardia, MD, of University of California, Los Angeles explores the unmet needs and challenges in the treatment of ER+, HER2– metastatic breast cancer. His discussion of key clinical trial data will help inform evidence-based therapeutic strategies for patients with endocrine-resistant disease.

Highlights

Understand the current ET landscape and explore the evolving role of estrogen receptor antagonists.
Explain the mechanisms of resistance, ESR1 mutations, and testing.
Interpret key clinical trial data to inform evidence-based sequencing and treatment selection in endocrine-resistant disease.
Integrate emerging therapeutic strategies into clinical practice to optimize patient outcomes and personalize care decisions.

mBC Post-Webinar Poll:

Post-Webinar Poll

1. What is a limitation of longer AI exposure?

A. Partial ER agonism through coactivator proteins, which leads to drug resistance and inferior efficacy

B. Increased prevalence of ESR1 mutations, a resistance mechanism that constitutively activates ER signaling in the absence of estrogen

C. Inferior pharmacokinetics and need for in-person visits

2. Tumors with ESR1 mutations are:

A. Estrogen-dependent, ER-independent

B. Estrogen-independent, ER-independent

C. Estrogen-independent, ER-dependent

3. ESR1 mutation prevalence in endocrine therapy–naïve metastatic breast cancer is:

A. Up to 50%

B. Up to 28%

C. <1%