SAR Study on Estrogen Receptor α/β Activity of (Iso)flavonoids: Importance of Prenylation, C-Ring (Un)Saturation, and Hydroxyl Substituents

Many botanicals used for women's health contain estrogenic (iso)flavonoids. The literature suggests that estrogen receptor beta (ERβ) activity can counterbalance estrogen receptor alpha (ERα)-mediated proliferation, thus providing a better safety profile. A structure–activity relationship study of (iso)flavonoids was conducted to identify ERβ-preferential structures, overall estrogenic activity, and ER subtype estrogenic activity of botanicals containing these (iso)flavonoids. Results showed that flavonoids with prenylation on C8 position increased estrogenic activity. C8-prenylated flavonoids with C2–C3 unsaturation resulted in increased ERβ potency and selectivity [e.g., 8-prenylapigenin (8-PA), EC50 (ERβ): 0.0035 ± 0.00040 μM], whereas 4′-methoxy or C3 hydroxy groups reduced activity [e.g., icaritin, EC50 (ERβ): 1.7 ± 0.70 μM]. However, nonprenylated and C2–C3 unsaturated isoflavonoids showed increased ERβ estrogenic activity [e.g., genistein, EC50 (ERβ): 0.0022 ± 0.0004 μM]. Licorice (Glycyrrhiza inflata, [EC50 (ERα): 1.1 ± 0.20; (ERβ): 0.60 ± 0.20 μg/mL], containing 8-PA, and red clover [EC50 (ERα): 1.8 ± 0.20; (ERβ): 0.45 ± 0.10 μg/mL], with genistein, showed ERβ-preferential activity as opposed to hops [EC50 (ERα): 0.030 ± 0.010; (ERβ): 0.50 ± 0.050 μg/mL] and Epimedium sagittatum [EC50 (ERα): 3.2 ± 0.20; (ERβ): 2.5 ± 0.090 μg/mL], containing 8-prenylnaringenin and icaritin, respectively. Botanicals with ERβ-preferential flavonoids could plausibly contribute to ERβ-protective benefits in menopausal women.