Aromatase Inhibitors

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The previously mentioned enzyme, aromatase, is partially responsible for lowered levels of testosterone in men; it achieves this by converting the testosterone molecule into the closely related but vastly different estradiol molecule. Aromatase is a cytochrome P-450 enzyme that catalyzes the rate-limiting step in estrogen synthesis, from the conversion of androgens (an-drostenedione and testosterone) to estrogens. (Aromatase is also known as estrogen synthetase.) By inhibiting this enzyme, the transformation of testosterone into estradiol (and resultant decreased levels of testosterone) can be slowed. Aromatase inhibitors are useful for both men and women; in women, aromatase transforms stored androgens into estrogens. For this purpose, aromatase inhibitors are now used as anti-cancer agents for treating estrogen-dependent cancers. Currently, three aromatase inhibitors are approved by the U.S. Food and Drug Administration: anastrazole (Arimidex ),exemestane(Aromasin ), and letrozole (Femara ) mainly for treating estrogen-dependent breast cancers when they first arise and when they recur.


Perhaps the most powerful of the naturally derived aromatase inhibitors in vitro, chrysin is thought to be one of the most potent inhibitors of human estrogen aromatase. Chrysin belongs to the flavone class of flavonoids and is derived from several plant species, the primary of which is Passiflora coerulea. Other sources include geranium species, such as lemon geranium (Pelargonium crispum), honey and bee propolis, and the Pinaceae species, which include pine trees. The ability of chrysin to inhibit aromatization of androstenedione and testosterone has been demonstrated in vitro; however, in vivo studies are necessary. Other investigators have noted a phytoestrogenic effect29 (binds weakly to alpha and beta estrogen receptors), and antioxidant30 (inhibits xanthine oxidase and the consequent formation of uric acid and related reactive oxygen species) and anxiolytic31 actions (binds to the "benzodiazepine receptor'' portion of gamma-aminobutyric acid [a] receptors. Much of the research on chrysin has been performed in vitro; this shows the potential for chrysin to inhibit the aromatase enzyme,32 but human research has not shown that chrysin increases testosterone levels when used with androgen precursor molecules, such as androstenedione and DHEA.33 Chrysin is suspected of having low oral bioavailability. In fact, this flavone is thought to induce the very enzyme (UGT1A1) that hastens its own elimination in the intestine and liver.34 Nonetheless, due to presumptive marketing, chrysin is widely used by athletes who tend to welcome any product with promise to enhance physical performance. The end result of using aromatase inhibitors is to preserve testosterone by preventing its transformation into estrogens.

Lignans and Flavonoids

Various lignans and flavonoids have been shown to inhibit the aromatase enzyme. Lignans are phytoestrogens with weak estrogenic effects and possibly antiestrogenic effects. Numerous flavonoids and lignans such as apigenin, quercetin, genistein, biochanin A, daidzein, and zearalenone have shown the ability to inhibit aromatase in vitro.35 The isoflavone genistein, found in soy and red clover, and the lignans enterolactone and enterodiol found in flaxseeds, have been shown to inhibit aromatase in breast cancer cells in vitro, resulting in decreased production of estradiol and estrone.36 Due to the increased absorption of flaxseed and soy, among other sources of lignans and flavonoids, these are areas of increased research for aromatase inhibition and hormone-dependent cancer prevention.

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