Aminoglutethimide as second-line hormonal therapy in advanced breast cancer: response and toxicity

Recent Progress in the Discovery of Next Generation Inhibitors of Aromatase from the Structure-Function Perspective

Human aromatase catalyzes the synthesis of estrogen from androgen with high substrate specificity. For the past 40 years, aromatase has been a target of intense inhibitor discovery research for the prevention and treatment of estrogen-dependent breast cancer. The so-called third generation aromatase inhibitors (AIs) letrozole, anastrozole, and the steroidal exemestane were approved in the U.S. in the late 1990s for estrogen-dependent postmenopausal breast cancer. Efforts to develop better AIs with higher selectivity and lower side effects were handicapped by the lack of an experimental structure of this unique P450. The year 2009 marked the publication of the crystal structure of aromatase purified from human placenta, revealing an androgen-specific active site. The structure has reinvigorated research activities on this fascinating enzyme and served as the catalyst for next generation AI discovery research. Here, we present an account of recent developments in the AI field from the perspective of the enzyme's structure-function relationships.

4-hydroxyandrostenedione--further clinical and extended endocrine observations

4-Hydroxyandrostenedione (4-OHA) was administered (250 mg intramuscularly 2-weekly) in a phase 2 clinical trial to 20 postmenopausal patients with advanced breast cancer, who had failed other endocrine therapy. Seven out of 18 assessable patients (39%) responded with minimal toxicity. Endocrine studies demonstrated that the drug produced significant initial falls in oestradiol and oestrone levels, but that these levels rose toward pretreatment levels as the study progressed. Sex hormone binding globulin (SHBG) levels gradually fell during the study suggesting that the drug has a minor degree of androgenic activity albeit of no clinical significance. There was a transient reduction of adrenal steroid levels, which remained however within the normal range. There were no symptoms of adrenal insufficiency.

Development of tolerance to the CNS effects of aminoglutethimide in mice

Initially, there is a high incidence of CNS-depressant side-effects when the aromatase inhibitor, aminoglutethimide, is used in the treatment of patients with advanced breast cancer. Tolerance to these effects develops with continued dosing. This study examines the development of tolerance to various indices of CNS depression with the drug in mice. Single doses of aminoglutethimide induced a dose-dependent depression of spontaneous locomotor activity, rotarod performance, righting reflex and body temperature and a dose-related antileptazol activity. On repeated dosing with the drug, tolerance to these various activities occurred. The tolerance was found to be dose-dependent in the rotarod and righting reflex tests and time-dependent in the locomotor and body temperature tests. Although the results do not allow a determination of whether this clearly demonstrated phenomenon in the mouse is primarily functional or dispositional, the slow onset (14 days) for complete tolerance may be indicative of a functional mechanism.

Aminoglutethimide and ketoconazole: historical perspectives and future prospects

Aminoglutethimide and ketoconazole, although originally developed as an anticonvulsant and antifungal agent respectively, have both been used to suppress steroid biosynthesis in patients with hormone-sensitive cancer. Aminoglutethimide inhibits several enzymes involved in the synthesis of corticosteroids as well as the aromatase enzyme which converts androgens to oestrogens. About one third of patients with breast cancer show objective improvement with aminoglutethimide, and it may also be of use in the treatment of adrenal carcinoma. However, its toxicity, and the need for concomitant cortisol replacement, severely limit its usefulness. Ketoconazole also inhibits several steroidogenic enzymes, notably C17,20-lyase, and has been used to treat carcinoma of the prostate. Again however, its toxicity and limited efficacy limit its value, although it may be useful in the treatment of certain endocrine conditions such as precocious puberty. Several aromatase inhibitors similar in structure to aminoglutethimide have been developed in an attempt to create more selective and efficient inhibitors. Some of these compounds have been tested in animals but none have as yet been subjected to clinical trials. Attempts to produce imidazole inhibitors of steroidogenesis are less advanced, although one compound (CGS 16949A) has been reported to be a more selective and potent aromatase inhibitor than aminoglutethimide. Selective and effective compounds could be of great value in the treatment of hormone-sensitive carcinoma.