Aromatase activity may be detected using in vitro and in vivo techniques in most breast cancers and mammary adipose tissue. This activity makes a variable contribution to endogenous estrogens within the breast and in many cases represents the major source of these hormones. Such local biosynthesis may maintain the growth of some hormone-dependent tumors. The factors which regulate aromatase activity within the breast are not defined but are likely to include growth factors and cytokines which may be produced by breast tissues so that autocrine and paracrine loops may exist. Estrogen biosynthesis within the breast, like other peripheral systems, appears sensitive to classical aromatase inhibitors and the new generation of drugs are capable of profoundly blocking the activity and markedly reducing endogenous estrogen levels; in turn these endocrine effects are translated in dramatic anti-tumor influences in hormone-dependent breast cancer.
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