Aromatase and its inhibitors: Significance for breast cancer therapy

Endocrine adjuvant therapy for breast cancer in recent years has focussed primarily on the use of tamoxifen to inhibit the action of estrogen in the breast. The use of aromatase inhibitors has found much less favor due to poor efficacy and unsustainable side effects. Now, however, the situation is changing rapidly with the introduction of the so-called phase III inhibitors, which display high affinity and specificity towards aromatase. These compounds have been tested in a number of clinical settings and, almost without exception, are proving to be more effective than tamoxifen. They are being approved as first-line therapy for elderly women with advanced disease. In the future, they may well be used not only to treat young, postmenopausal women with early-onset disease but also in the chemoprevention setting. However, since these compounds inhibit the catalytic activity of aromatase, in principle, they will inhibit estrogen biosynthesis in every tissue location of aromatase, leading to fears of bone loss and possibly loss of cognitive function in these younger women. The concept of tissue-specific inhibition of aromatase expression is made possible by the fact that, in postmenopausal women when the ovaries cease to produce estrogen, estrogen functions primarily as a local paracrine and intracrine factor. Furthermore, due to the unique organization of tissue-specific promoters, regulation in each tissue site of expression is controlled by a unique set of regulatory factors. These factors are potential targets for the design of selective aromatase modulators, which could selectively inhibit aromatase expression in breast with the same efficacy as the phase III inhibitors of activity but leave expression in other local sites such as bone and brain untouched.