Clinical use of selective estrogen receptor modulators and down regulators with the main focus on breast cancer

Selective estrogen receptor modulators (SERMs) and selective estrogen down regulators (SERDs) act as estrogen receptor (ER) agonists or antagonist depending on the targeted tissue and the specific configuration of the used SERM or SERD. Effects on bone, endometrium and breast cancer are of interest. Endocrine treatments have been used in breast cancer since the end of the 19th century. In the second part of the last century different compound of SERMs and SERDs have been developed and we will discuss them mainly as used in the treatment and prevention of breast cancer. Tamoxifen is the widely investigated and most used representative of these drugs and has been introduced in the advanced disease, in the neoadjuvant and adjuvant setting and for prevention of the disease. Its role has been challenged in the last years by the introduction of third generation aromatase inhibitors that have proven a higher activity than tamoxifen and a different toxicity pattern. Several other SERMs have been investigated, but none of them was clearly superior to tamoxifen. The main interest in different SERMs has to be seen in the slightly different safety profile between the different compounds. SERDs act as pure estrogen antagonist. They have been used in the treatment of advanced breast cancers and their role in other settings still needs further investigation. The increased use of aromatase inhibitors as first line endocrine therapy raises new questions on the role that tamoxifen and other SERMs or SERDs may play in breast cancer. The sequencing of endocrine therapies and the combination of endocrine therapies with new targeted therapies in hormone sensitive breast cancer remains a very important research issue. Polymorphisms in genes coding for tamoxifen metabolizing enzymes, as for instance, the CYP2D6 genotype, have the potential of becoming clinically useful predictive marker for tamoxifen response. With this meaningful newer knowledge it is possible that the place of tamoxifen in the treatment of breast cancer will be redefined in the future.