Tamoxifen and raloxifene are selective oestrogen receptor modulators (SERMs) which have an anti-oestrogenic effect on the breast. Tamoxifen prevents oestrogen binding at oestrogen receptors, reducing but not eliminating breast cancer risk. The result is a decrease in the number of oestrogen receptor-positive (ER+) tumour cells, but no change in oestrogen receptor-negative tumours.^16,18

Fulvestrant is a competitive oestrogen receptor inhibitor, blocking oestrogen growth activity without any partial-agonist activity (which tamoxifen has). It can also reduce expression of progesterone receptors.¹⁶

Aromatase inhibitors block oestradiol release. Ovary function declines at menopause, so oestradiol production occurs primarily in peripheral tissues with aromatase enzymes converting androstenedione to oestrone, with subsequent conversion to oestradiol. Blocking the aromatase enzyme complex reduces or removes the oestrogen stimulus promoting breast tumour growth.^16,17

To limit circulating oestrogen, ovary activity can be stopped permanently by ablation with surgery or radiotherapy, inducing the menopause. However, temporary suppression is possible with goserelin, a luteinising hormone-releasing hormone agonist (LHRHa). Periods will cease while having monthly goserelin injections but should start again after treatment ends.⁵