Breast cancer cells are cells that have lost control of cell division. They also become unstable, often over-producing some proteins. Breast cancers often over-express receptors for the sex hormones oestrogen and progesterone.

These receptors, present on the surface of the cancer cell, bind hormones that circulate in the blood. This then fuels the cancer cells to grow faster and makes it more likely they will start to spread.

Breast cancer cells can over-produce three key receptors:

  • 80% of breast tumours over-express oestrogen receptors.
  • Almost 65% over-express both oestrogen and progesterone receptors.
  • Overall, 20% of breast cancers over-produce the HER2 receptor, which picks up growth signals from the blood and pushes the cancer cells to divide even more quickly.
  • Only about 15 out of 100 women with breast cancer develop triple negative breast tumours – they don’t produce oestrogen or progesterone receptors and don’t have the HER2 receptor either.

Why are hormone receptors important for breast cancer treatment?

If a cancer cell divides more rapidly when its oestrogen receptors bind oestrogen, it follows that starving the tumour of oestrogen could slow down its growth. This is now an established mechanism and has been used to develop new treatments for breast cancer:

  • Tamoxifen was the first anti-oestrogen treatment to be approved for use in breast cancer in the 1970s. Tamoxifen binds to oestrogen receptors in the breast, including those on breast cancer cells, blocking up the receptor so that circulating oestrogen cannot attach. Tamoxifen is one of the selective oestrogen receptor modulators (SERMs) that can be used to prevent breast cancer recurrence in pre-menopausal women. Other SERMs such as raloxifene (Evista®) are sometimes considered for use off-label but are not licensed for this indication in the UK.
  • Aromatase inhibitors work differently; they block the synthesis of oestrogen in the body. This reduces the amount of oestrogen circulating in the blood to very low, almost negligible levels. Aromatase inhibitors, which include anastrozole (Arimidex®), exemestane (Aromasin®) and letrozole (Femara®), are effective anti-oestrogen treatments suitable for women after the menopause.
  • Oestrogen receptor downregulators such as fulvestrant (Faslodex®) are used in women after the menopause whose breast cancer has become advanced and no longer responds to other hormonal treatments. Faslodex® blocks oestrogen receptors on breast cancer cells and also knocks out many of the receptors so the cancer cell has less of them.
  • Luteinising hormone releasing hormone blockers – shortened to LHRH blockers – stop the pituitary gland releasing luteinising hormones. This hormone normally acts on the ovaries of women before the menopause, stimulating the production of oestrogen. LHRH blockers prevent this signal getting through to the ovaries so that blood levels of oestrogen remain lower than normal. Goserelin (Zoladex®) is approved for use in the UK as a treatment for breast cancer and prostate cancer.

The more we know about the molecular characteristics of each breast tumour, the better we can target the tumour with specific drugs.

Trade names quoted are given as examples only of the drug types described, alternatives may be available.