Endocrine therapy

Around 70% of breast cancers are hormone receptor-positive, relying on oestrogen (ER+) and/or progesterone (PR+) to grow. Endocrine therapy is a systemic treatment that blocks the production of oestrogen and/or prevents it stimulating breast cancer cells, minimising the cells’ exposure to oestrogen. Endocrine therapy is also used to slow or shrink the growth of breast cancer when surgery is not appropriate. This treatment is only effective for hormone receptor-positive breast cancer.

Endocrine therapy is taken for five to 10 years, depending on risk of recurrence. Depending on the therapy prescribed, patients may experience menopausal side effects and joint pain that, if not addressed, can reduce treatment adherence. The NZ Breast Special Interest Group (SIG) recommends that all patients have a consultation with their SMO (or GP if discharged from surgical follow up) after five years of endocrine treatment to determine whether treatment should be extended.

There are three types of endocrine therapy. Treatment decisions are determined by menopausal status, risk of recurrence, and the patient’s general health and individual risk of certain side effects, e.g. deep vein thrombosis (tamoxifen) or osteoporosis (aromatase inhibitors).

Tamoxifen is part of the SERMS (Selective [o]Estrogen Receptor Modulators) drug group. It blocks the effect of oestrogen on breast cancer cells, while allowing a small amount of oestrogen to supply the bones and uterus. It has been shown to reduce the risk of recurrence, prevent breast cancer-related deaths and reduce the risk of breast cancer developing in women who are high risk. It is an oral medication. Women who become post-menopausal during their five-year tamoxifen treatment may be switched to an aromatase inhibitor.

Aromatase inhibitors block oestrogen production, thereby reducing the risk of recurrence, preventing breast-cancer related deaths and reducing the risk of a second breast cancer developing. This drug is prescribed to post-menopausal women, but may also be prescribed alongside ovarian suppression to pre-menopausal women who are at high risk of recurrence. Bone health must be monitored during aromatase inhibitor treatment and the addition of oral or intravenous bisphosphonates may be considered.

Ovarian suppression can reduce the risk of recurrence in pre-menopausal women with high-risk ER+ cancer. Ovarian suppression is achieved with drug therapies or surgery:

1. Goserelin or leuprorelin are luteinising hormone blockers (GnRH/LHRH agonists) that suppress production of LH. They are delivered via injection every 28 days (or alternatively, at three-monthly intervals) to shut down oestrogen production. These medications can be used alone or in combination with tamoxifen or aromatase inhibitors.
2. Oopherectomy results in permanent and immediate suppression of ovarian function, and the onset of a surgically induced menopause.

With the exception of oophorectomy, ovarian function should recover for pre-menopausal women once treatment ends. However, in patients near the age of natural menopause onset, treatment may induce early menopause.