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by Dr. C.H. Weaver M.D. updated 2/2021

HER2 (human epidermal growth factor 2) is a receptor on the surface of breast cells. When HER2 is "turned on" it causes the cells to grow and reproduce. Normal breast cells have 2 copies of the gene that makes HER2. In contrast to normal breast cells some have more than two HER2 genes and/or produce too much HER2. This results in more HER2 receptors on breast cells - a condition doctors refer to as "over expression" Over expression of HER2 leads to increased breast cell production or cancer.

Doctors can test for HER2 with two different tests.

  • IHC - measures the number of HER2 receptors on a scale of 1-3. 3 + means there are too many.
  • ISH - counts the number of HER2 genes.

HER2 + "positive" breast cancers can be treated with precision cancer medicines that target the HER2 receptor. Approximately 20-25% of breast cancers. These cancers are referred to as HER2-positive.

There are several precision cancer medicines that target HER2 and the use of these medications improves the outcomes of women with both early-stage and advanced HER2-positive breast cancer.

Herceptin (trastuzumab)

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Next Generation anti HER2 Precision Cancer Medicines

  • Tykerb Tykerb in Advanced Breast Cancer
  • Perjeta - targets different part of HER2 than Herceptin
  • Nerlynx - TKI that targets HER2 at multiple levels.
  • Kadcyla (T-DM1)- combines Herceptin with chemotherapy.
  • Enhertu- antibody-drug conjugate FDA approved for advanced disease.
  • Tucatinib - FDA approved and highly effective
  • Margetuximab - in development.
  • Pyrotinib - in development

What are the side effects of HER2 therapies?

Should all HER2 + breast cancers be treated?

  • All invasive and metastatic HER 2 positive breast cancers benefit from HER2 directed therapy. Even very small cancers appear to benefit. A U.S. study evaluated the records of 965 women with small cancers that had not been treated with HER2-targeted therapy or chemotherapy. By the end of five years, 6% of women with HER2-negative breast cancer had a recurrence compared with 23% of women with HER2-positive breast cancer.

What is the role of HER 2 in DCIS?

Compared with invasive breast cancer, DCIS more often expresses HER2, however current research does not support the use of human HER2 - directed therapy in DCIS.


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  3. Gonzalez AM, Litton JK, Broglio KR et al. High risk of recurrence for patients with breast cancer who have human epidermal growth factor receptor 2-positive, node-negative tumors 1 cm or smaller. Journal of Clinical Oncology[early online publication]. November 2, 2009.
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