ERA-45

ERA-45 is a synthetic estrogen and a selective agonist of the ERα.[1][2][3][4] It shows 286-fold selectivity for transactivation of the ERα over the ERβ, with EC₅₀ values of 0.37 nM for the ERα (7-fold weaker than estradiol) and 13 nM for the ERβ (20,000-fold weaker than estradiol).[1] However, another found only about 35-fold potency for transactivation of the ERα over the ERβ.[2] The drug has no antagonistic activity at either receptor.[1] ERA-45 induced prostate cancer development in preclinical models when it was given in combination with testosterone, whereas testosterone alone did not do so.[1][5] In contrast, the selective ERβ agonist ERB-26 was protective against the development of prostate cancer produced by these two drugs.[1][5] These findings suggest opposing roles of the ERα and ERβ in the prostate gland.[1] The chemical structure of ERa-45 does not appear to have been disclosed.