Inhibition of aromatase as treatment of breast carcinoma in postmenopausal women

Recent treatment strategies have been directed toward blockade of estrogen action or inhibition of estrogen biosynthesis as a means of inducing regression of hormone-dependent breast cancer. The major source of estrogen in postmenopausal women is the peripheral conversion of androstenedione to estrone through the enzyme aromatase. It is known that aromatase activity increases proportionately with degree of obesity in women. To test the importance of this modulatory factor, we correlated body weight with estrogen excretion in our population of patients with breast cancer and found significant relationships. In situ production of estradiol from plasma precursors within breast cancer tissue may provide another source of estrogen. Major enzymes mediating estrogen biosynthesis were found to be present in tumor biopsy specimens. Aromatase activity was found to be present in 48 61 human tumors, sulfatase in 35 35, and 17β-hydroxysteroid dehydrogenase in 41 41. One inhibitor of aromatase, aminoglutethimide, has been extensively studied in patients with breast cancer. The additional effects of this drug on cholesterol side-chain cleavage and on 11-hydroxylase activity require coadministration of replacement glucocorticoid in treatment regimens. In pilot trials, 37% of patients experienced objective tumor regression with a combination of 1000 mg aminoglutethimide and 40 mg hydrocortisone daily. In randomized clinical trials with this regimen, aromatase inhibition with aminoglutethimide produced tumor regression with similar frequency as did surgical hypophysectomy, surgical adrenalectomy, or tamoxifen administration. The side effects of aminoglutethimide, including lethargy, skin rash, and ataxia complicate its use even though these problems are generally transient. Regimens of low-dose aminoglutethimide are being developed to reduce these side effects. Low-dose aminoglutethimide appears to block aromatase effectively and to have limited side effects, and is undergoing extensive clinical trial. A more specific aromatase inhibitor, 4-hydroxyandrostenedlone, is now also being tested clinically, whereas MDL 18962, another new selective inhibitor, is undergoing study in animals.