The significance of estrogen receptors in tamoxifen and toremifene therapy.

Estrogen receptor (ER) concentration of breast cancer tissue is important in predicting the response of each patient to hormonal, especially antiestrogen treatment. About half of the patients with ER rich tumours respond and only about 10% of the patients with ER poor tumours respond to antioestrogen treatment. Tamoxifen is a well known and widely used drug. Toremifene is a new antioestrogen, developed in Finland. At standard doses both compounds have comparable hormonal and antitumour effects, and there is no clear difference between the compounds in the affinity to ER. The value of ER in predicting the response to tamoxifen and toremifene therapy in ER positive breast cancer is significant. It is not known, however, if the role of ER remains the same with high dose toremifene. Although ERs are an important predictive factor, the antioestrogens evidently act through them only in part. As the prediction is correct in about half of the patients, other mechanisms must influence tumour growth regulation, such as the expression of oncogenes and the synthesis and activity of growth factors.