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Updated: Jun 20, 2026

Implementation of In Vitro Drug Resistance Assays: Maximizing the Potential for Uncovering Clinically Relevant Resistance Mechanisms
08:46

Implementation of In Vitro Drug Resistance Assays: Maximizing the Potential for Uncovering Clinically Relevant Resistance Mechanisms

Published on: December 9, 2015

Aromatase resistance mechanisms in model systems in vivo

Angela Brodie1, Luciana Macedo, Gauri Sabnis

  • 1Department of Pharmacology and Experimental Therapeutics, School of Medicine, University of Maryland Baltimore, Baltimore, MD 21201, USA. abrodie@umaryland.edu

The Journal of Steroid Biochemistry and Molecular Biology
|September 26, 2009
PubMed

Insights

Aromatase inhibitors (AIs) can lead to Her-2 upregulation and resistance in estrogen receptor-positive breast cancer. Combining Her-2 and AI therapy restores ER levels and inhibits tumor growth, offering new treatment strategies.

Area of Science:

  • Oncology
  • Endocrinology
  • Molecular Biology

Background:

  • Aromatase inhibitors (AIs) are effective for estrogen receptor-positive (ER+) breast cancer but can lead to treatment resistance.
  • Mechanisms of AI resistance, including alterations in signaling pathways, require further investigation.

Purpose of the Study:

  • To investigate the mechanisms of resistance to Aromatase Inhibitors (AIs) in ER+ breast cancer.
  • To explore therapeutic strategies combining AIs with Her-2 inhibition.

Main Methods:

  • Utilized MCF-7Ca human breast cancer cell line xenografts in athymic mice.
  • Administered letrozole (AI) and trastuzumab (anti-Her-2 antibody) alone and in combination.
  • Monitored tumor growth, Her-2 and ERalpha protein levels, and cell growth characteristics.

Main Results:

  • Letrozole treatment upregulated Her-2 and led to estradiol-independent growth with decreased ERalpha and aromatase expression.
  • Her-2 inhibition with trastuzumab restored ERalpha levels, indicating Her-2 as a negative regulator of ERalpha.
  • Combined letrozole and trastuzumab treatment restored ER levels and markedly inhibited tumor growth compared to monotherapy.
  • Discontinuation of letrozole decreased Her-2 and restored ER levels, enabling re-sensitization to AI therapy.

Conclusions:

  • Tumor cells adapt to AI treatment by upregulating Her-2 and reducing ERalpha expression, leading to resistance.
  • Combined inhibition of estrogen receptor and Her-2 signaling pathways is crucial for overcoming AI resistance.
  • Therapeutic strategies involving sequential or combined AI and Her-2 inhibition hold promise for improving patient outcomes in ER+ breast cancer.

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