Aromatase

Definition

Aromatase is a cytochrome P450, hemoprotein-containing enzyme, located in the endoplasmic reticulum, which catalyzes the rate-limiting step in the conversion of androgens (androstenedione and testosterone), to estrogens (estrone and estradiol). Agents that inhibit aromatase (aromatase inhibitors) are widely used to prevent the development and progression of estrogen dependent breast cancers.

Basic Characteristics

Estrogen biosynthesis is mediated by the aromatase enzyme, which is a product of the CYP19 gene. The aromatase enzyme is a complex that consists of cyctochrome P450 hemoprotein and a flavoprotein, NADPH-cytochrome P450 reductase. This complex is responsible for catalyzing the conversion of steroidal C-19 androgens (androstenedione and testosterone) to C-18 estrogens (estrone and estradiol), which is the rate-limiting final step in the synthesis of estrogens (Fig. 1). This enzymatic reaction is comprised of three steps each of which requires 1M equiv. of NADPH and oxygen. The first step involves hydroxylation of the androgen substrate at C-19 to produce a 19-hydroxy intermediate. In the second step the 19-hydroxy intermediate is oxidized to produce a 19-oxo compound. The last step in the aromatization reaction is less well defined but is thought to involve the oxidative cleavage of the C10-19 bond to produce estrogens (estrone and estradiol) and formic acid.

Aromatase. Figure 1

Androgens converted to estrogens by aromatase.

Aromatase activity, and hence the capacity to synthesize estrogens, is found in a variety of tissues in the body. Gonadal sites include the ovaries in premenopausal women and the testes in men. Important extragonadal sites of aromatase activity include the placenta, chondrocytes and osteoblasts of bone, adipose tissue, muscle and brain. Aromatase plays an important role in a number of important biological processes including breast development during puberty and uterine growth and bone maturation during adolescence. In adults aromatase influences bone mineralization, lipid metabolism and cardiovascular risk. In pregnant women it protects against the virlizing effects of fetal androgens.

Drugs

Estrogen is known to be an important stimulus in the development and progression of some breast tumors. Thus targeting the disruption of either the synthesis (i.e., inhibiting aromatase enzyme) or the activity (i.e., blocking estrogen receptors) of estrogens are potential mechanisms for the prevention and treatment of hormone sensitive breast cancer. Since aromatization is a unique reaction and is the terminal step of the estrogen biosynthetic pathway, agents that block this reaction would not potentially affect the production of other steroids.

In premenopausal women the ovary is the richest source of aromatase and hence estrogen. Aromatase is confined to the granulosa cells and is produced under the influence of gonadotropins (FSH and LH). Despite being a rich source of aromatase, three separate studies have shown that aromatase inhibitors are unable to sufficiently suppress ovarian estrogen production to postmenopausal levels. One explanation for this phenomenon may be a compensatory rise in gonadotrophins which maintains adequate estrogen production, despite the presence of the inhibitor. As such aromatase inhibitors cannot be used in premenopausal breast cancer patients. After menopause, ovarian production of estrogen ceases. However estrogen production continues from peripheral sources of aromatase activity that convert adrenal androgens to estrogens. Aromatase inhibitors have been shown to adequately suppress estrogen production in postmenopausal women, and in this setting are used in the treatment of both early and advanced stage estrogen receptor positive breast cancer.

Over the last 30 years a number of aromatase inhibitors (Fig. 2) have been developed. The first (aminoglutethimide) and second (Fadrozole and Fromestane) generation aromatase inhibitors are not commonly used due to their lack of specificity in inhibiting the aromatase enzyme and associated significant side-effects. Two types of third generation aromatase inhibitors are commercially available and have been shown to be either equal to or superior to tamoxifen in the treatment of metastatic estrogen receptor positive breast cancer. Type I (suicidal, noncompetitive) inhibitors bind irreversible with the aromatase enzyme thereby permanently blocking its activity. Exemestane is an example of a type I inhibitor. Type II inhibitors bind reversible with the aromatase enzyme, examples of which include letrozole and anastrozole.

Aromatase. Figure 2

Aromatase Inhibitors.

Aminoglutethimide

Aminooglutethimide was the first aromatase inhibitor to be used in patients with metastatic breast cancer, where response rates of up to 30% have been reported. Unfortunately, due to its lack of selectivity for aromatase, it induced a medical adrenelectomy that resulted in suppression of aldosterone and cortisol. With the development of more selective aromatase inhibitors, aminoglutethimide is now rarely used for the treatment of breast cancer. It is occasionally used for the treatment of medical conditions involving excess hormone production such as Cushing’s syndrome.

Anastrazole

Anastrazole is a nonsteroidal, type II, aromatase inhibitor that is 200 times more potent than aminoglutethimide. It is eliminated primarily via hepatic metabolism, has a terminal half life of 50 h with steady state concentrations achieved approximately 10 days with once daily dosing regimens. It is administered orally at a dose of 1 mg/day that achieves near maximal aromatase inhibition and hence estrogen suppression in breast cancer patients. No effect on adrenal steroidogenesis has been observed at up to ten times the daily recommended dose. When used in the metastatic setting, anastrozole has been shown to increase time to progression when compared to tamoxifen. In the ATAC (Arimidex, Tamoxifen, Alone or in Combination) trial over 9,000 patients with early stage breast cancer were randomized to 5 years of anastrozole 1 mg/day or 5 years of tamoxifen 20 mg/day or a combination of both. At a median follow up of 33 months, 47 months, and 68 months, compared to tamoxifen, anastrozole upfront significantly increased disease free survival and time to recurrence and reduced the risk of contra lateral breast cancer. The combination arm of the trial was closed as it was no more effacious than tamoxifen alone. Currently trials are ongoing evaluating anastrozole as chemopreventive agent for breast cancer.

Letrozole

Like anastrozole, letrozole is a third generation, type II nonsteroidal aromatase inhibitor. Renal excretion of its inactive glucuronide metabolite represents its main pathway of clearance. It has a half-life of 2 days and at the recommended daily dose of 2.5 mg steady-state plasma levels is reached in 2–6 weeks. Letrozole has proved effective when used either sequentially after tamoxifen or upfront in the treatment of patients with early stage breast cancer. The MA-17 trial randomized approximately 5,000 postmenopausal breast cancer patients who had received 5 years of tamoxifen to either placebo or 5 years of letrozole. The Breast International Group (BIG) 1–98 trial randomized postmenopausal breast cancer patients to 5 years of tamoxifen, 5 years of letrozole, or 2 years of either agent (i.e., tamoxifen or letrozole) followed by three years of the other agent (i.e., tamoxifen or letrozole). In terms of disease free survival the MA-17 trial showed an advantage to switching to letrozole and the BIG 1–98 trial showed an advantage to up front letrozole with results awaited for the switching group.

Exemestane

Examestane is a type II, steroidal aromatase inhibitor with an androgen structure. It is metabolized by CYP3A4 enzyme and has a half-life of 27 h. At the recommended once daily dose of 25 mg no effect is seen on adrenal steroid production and maximal estrogen suppression is achieved in 7 days. In early breast cancer treatment it has been studied as a sequential agent after several years of tamoxifen. The Intergroup Exemestane Study (IES) randomized over 4000 patients to either 5 years of tamoxifen 20 mg/day or 2–3 years of tamoxifen followed by exemastane 25 mg/day. A significant reduction in disease free survival (hazard ratio, 0.76, p = 0.0001) and risk of contralateral breast cancer favoring the group switching to exemestane was observed.

Side Effects

Aromatase inhibitors are relatively well tolerated; however have a number of distinct side effects are observed that stem from the state of estrogen deprivation induced by aromatase inhibitors. Side effects include hot flashes, joint and muscle aches, vasomotor symptoms and vaginal dryness. Variable effects of aromatase inhibitors on lipid levels have been observed. Trials comparing third generation aromatase inhibitors to tamoxifen have also reported an increased risk of cardiovascular events in the group receiving aromatase inhibitors.

Other Uses

Aromatase inhibitors have also been used in premenopausal women for the treatment of endometriosis and to induce ovarian folliculogenesis as part of the treatment for infertility.

Sex Steroid Receptors