Molecular Action and Clinical Relevance of Aromatase Inhibitors

BREAST CANCER: HIGH PREVALENCE AND RISING INCIDENCE: Breast cancer is the most common form of cancer among women in Europe, North and South America and Australasia; approximately 1 in 10 women in Western countries will develop breast cancer during their lifetime. It is estimated that the disease will affect five million women worldwide over the next decade, and the incidence of breast cancer is increasing on average by about 1% per year in industrialized countries and at a greater rate in developing countries. COMPLEX ETIOLOGY: Although the specific etiology of breast cancer remains unknown, a number of factors are recognized which increase a woman's risk of developing the disease. Genetic predisposition, or family history of breast cancer, is known to be responsible for 5% of all cases. However, the variation in incidence throughout populations, and changes relating to population migration and adoption of altered lifestyles, all point to the critical importance of nongenetic determinants. Such factors include early menarche, late menopause, late age at birth of first child or nulliparity, a history of benign breast disease, and diet. There is also evidence that hormones play a major role in the etiology of breast cancer, with the risk of developing malignancies related to the cumulative exposure of the breast to estrogen and progesterone, which stimulate the growth of tumor cells. TREATMENT FOR EARLY BREAST CANCER: SURGERY -/+ ADJUVANT THERAPY: At the time of diagnosis, approximately 50% of patients will be diagnosed with early breast cancer. This proportion is increasing as a consequence of the introduction of early detection programs. Surgery remains the primary treatment for early breast cancer, and the frequency of radical mastectomy has been replaced by breast conserving surgery. After surgery, other therapeutic modalities such as radiation, chemotherapy or endocrine therapy may be given in the adjuvant setting. Surgical cure rates vary for patients with early breast cancer; the US figure is approximately 40%, and there are no definitive means to predict those who will be cured and those who will have recurrent disease. As a result, following primary surgical treatment, adjuvant therapy is usually recommended to destroy any remaining cancer cells at the primary site, to control micrometastases and to prolong disease-free survival, with the ultimate aim of providing an overall survival benefit. Upon disease recurrence in the remaining 60% of patients, endocrine therapy and chemotherapy represent the two general classes of treatment. One of the principle decisions to be taken in advanced breast cancer is which therapy to select in order to maximize patient benefit. The choice is largely dependent upon prognostic factors and whether the patient is pre- or postmenopausal. ENDOCRINE THERAPY OR CHEMOTHERAPY IN ADVANCED BREAST CANCER: Unlike chemotherapy, endocrine therapy is not cytotoxic and is therefore better tolerated by the patient. A recent study comparing therapy for prognostically different groups showed that patients benefiting most from the use of sequential endocrine agents are those regarded as low risk. The preferred sequence of treatment has been suggested to be tamoxifen followed by selective aromatase inhibitor and then a progestin. ENDOCRINES AND ENZYMES OFFER NEW TREATMENTS FOR ADVANCED BREAST CANCER: ESTROGEN-DRIVEN BREAST CANCER: Since 1896, when Sir George Beatson demonstrated that ovariectomy induced regression of mammary tumors in women, the aim of endocrine breast cancer therapy has been to selectively deprive the body of estrogen. Ovariectomy accomplished this by removing the gland that is the predominant source of estrogens in premenopausal women. Since the avoidance of such surgery is preferable, emphasis is devoted to the pharmacological inhibitors of estrogen production. ENDOCRINE PATHWAY REVEALS "ACHILLES' HEEL": Like other steroid hormones, the two circulating estrogens-estrone and estradiol-are produced from cholesterol. Inhibiting the enzymes that are involved at earlier steps in the branching pathway of steroidogenesis could have an undesirable impact on the production of other physiologically important hormones such as aldosterone and cortisol. Since aromatase catalyzes the last step in estrogen production, it makes an ideal target for the development of selective and potent inhibitors (Fig. 1). STRUCTURE OF AROMATASE REVEALS SECRETS OF SELECTIVE INHIBITION: Aromatase is a cytochrome P450 enzyme, with both an iron-containing and a steroid-binding site. The substrate, androstenedione, sits in the enzyme's steroid-binding site, that site which otherwise catalyzes the formation of estrogen. From this structural relationship, there are, therefore, two reasonable ways to inhibit aromatase: * by occupying the steroid-binding site of the enzyme with a compound such as formestane (Lentaron®), or * by binding the iron with nitrogen-containing compounds such as aminoglutethimide (Orimeten®), the oldest aromatase inhibitor. AROMATASE INHIBITORS: STEROIDAL AND NON-STEROIDAL: Formestane (Lentaron®) is the only commercially available steroidal compound which inhibits aromatase and must be administered parenterally. Other new aromatase inhibitors such as fadrozole (Afema®) and letrozole (Femara®) are orally active nitrogen-containing compounds that bind the heme iron of aromatase. AMINOGLUTETHIMIDE VERSUS LETROZOLE: OLD VERSUS NEW: Although aminoglutethimide has long been used to treat advanced breast cancer, its aromatase inhibition is not selective. Consequently, aminoglutethimide also binds to and thereby inhibits several other cytochrome P450 enzymes in the steroidogenesis pathway. An ideal aromatase inhibitor would fit the catalytic site of aromatase optimally and would thus interact only with aromatase. The affinity of letrozole (Femara®) for the heme group of aromatase makes it a selective and potent inhibitor (Fig. 2). In fact, studies show that Femara® has little effect on the other adrenal steroids, and is the most selective aromatase inhibitor available today.