Endocrine Therapy

Many breast cancers respond to hormones present in the body, such as oestrogen and progesterone. 

If a cancer is oestrogen receptor positive then it grows faster when oestrogen is present.  If a cancer is progesterone receptor positive, then it grows faster when progesterone is present.

Endocrine therapy (sometimes known as hormone therapy) works by blocking the effects of these hormones so that they are unable to stimulate the cancer cells to grow.

Your pathology report will tell you whether your breast cancer is responsive to oestrogen or progesterone or both. If your breast cancer is hormone receptor positive you may benefit from endocrine therapy.

Endocrine therapy is used to reduce the risk of hormone positive breast cancer from coming back following surgery, radiotherapy and/or chemotherapy. It can be used in both pre- and post-menopausal women, but your menopausal status and whether your cancer was node-positive or node-negative will influence your doctor’s decision about which treatment option is right for you. Endocrine therapy is often taken for a long time. You may need to switch from one option to another as time goes by; your doctor will advise you on this.

There are three general categories of endocrine therapy drugs, which are all taken as tablets.

Aromatase inhibitors

These drugs work by blocking the production of oestrogen in the body and are generally only suitable for post menopausal women. If taken as recommended, aromatase inhibitors have been shown to significantly reduce the risk of breast cancer returning to the same breast or occurring in the opposite breast.

Drugs in this category include anastrozole, letrozole and exemestane. All drugs are taken orally over a long period of time (usually for at least five years).

Side effects of these medicines can include hot flushes, joint and muscle aches, mild nausea, vaginal dryness and reduced bone density, sometimes causing osteoporosis. The potential risk of osteoporosis means that bone density monitoring is recommended while taking aromatase inhibitors.  Most cancer centres in New Zealand now offer regular bone density scans for patients on aromatase inhibitors, but if yours doesn't ask for regular DEXA scans.  

SERMs (Selective Estrogen Receptor Modulators):  

This group of drugs prevents oestrogen from taking hold in cancer cells and so reduces the ability of these cells to grow.

The most commonly prescribed SERM in New Zealand is tamoxifen, a drug that can be used in both pre- and post-menopausal women. Again, this drug will need to be taken orally for up to five years. Recent data has shown benefit from switching post-menopausal women to an aromatase inhibitor following treatment with tamoxifen.

Research (2012) has shown that taking tamoxifen for ten years, rather than five helps to reduce the recurrence of breast cancer and results in better overall survival rates. Find out more here. The 2018 Amercian Society of Clinical Oncology (ASCO) guidelines list the different combinations of tamoxifen and aromatase inhibitor treatments that may be recommended by your doctor depending on your menopausal status, the node status of your original tumour, and other breast cancer recurrence risk factors.

Side effects of tamoxifen include menopausal type effects such as hot flushes, and vaginal dryness. More seriously, occasionally (in fact in only one per cent of cases over five years) some women can experience blood clots and very rarely tamoxifen has resulted in uterine cancer. Any vaginal bleeding should be immediately reported to your doctor.

Oestrogen Receptor Downregulators (ERDs)

Similar to SERMs, these drugs work by blocking the effects of oestrogen in the breast tissue so they starve the cancer cells of the oestrogen they need to grow.

At present there is only one ERD available in New Zealand – Faslodex® (generic name: fulvestrant). It is used to treat hormone-receptor-positive advanced breast cancer in post-menopausal women with disease progresssion following other anti-oestrogen therapy, such as tamoxifen. Fulvestrant may also be used in conjunction with Ibrance® (generic name: palbociclib).

Fulvestrant is given as an injection into a muscle once a month. Side effects are generally minimal, however, they can include discomfort at the injection site, hot flushes, vomiting, nausea, and diarrhoea.

Other endocrine therapy options

In pre-menopausal women most of the oestrogen and progesterone hormones are produced by the ovaries, and to a lesser degree also in the liver and in fat tissue. A different way of lowering the hormone levels in pre-menopausal women is to remove the ovaries with surgery or by chemically blocking the function of the ovaries with drugs known as lutenising hormone releasing hormone (LHRH) blockers, such as goserelin (Zoladex®) or leuproreline (Lucrin®).

Both drugs are given after chemotherapy as a monthly injection into the abdominal wall. They are fully funded. Both options have a similar effect, i.e. they induce a menopause, but the surgical option is irreversible whereas with the injection option, the pre-menopausal hormone levels will return once the injections are ceased. 

These injections may be recommended for pre-menopausal women with hormone receptor positive breast cancer when taking tamoxifen is not advised or could be used as alternative to tamoxifen.

For more information on LHRH medicines see our fertility section on our pages for young women.