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Conjugated estrogens, or conjugated equine estrogens (CEEs), are blended equine estrogens, which may include estrone sulfate, equilin sulfate, and equilenin sulfate. CEEs are used clinically in hormone replacement therapy, with marketed products including both natural preparations isolated from the urine of pregnant mares (brand names Premarin) as well as fully synthetic replications of the natural preparations (brand names Cenestin, Enjuvia, Congest, and C.E.S.). Prempro and Premphase are combination formulations of natural CEEs with medroxyprogesterone acetate.
The exact composition of Premarin specifically is as follows: sodium estrone sulfate (49.3%), sodium equilin sulfate (22.4%), sodium 17α-dihydroequilin sulfate (13.8%), sodium 17α-estradiol sulfate (4.5%), sodium 8,9-dehydroestrone sulfate (3.5%), sodium equilenin sulfate (2.2%), sodium 17β-dihydroequilin sulfate (1.7%), sodium 17α-dihydroequilenin sulfate (1.2%), sodium 17β-estradiol sulfate (0.9%), sodium 17β-dihydroequilenin sulfate (0.5%), and sodium 8,9-dehydroestradiol sulfate (small amounts).
17β-Dihydroequilenin has unexpectedly shown a selective estrogen receptor modulator-like profile of estrogenic activity in studies with monkeys, in which beneficial effects on bone and the cardiovascular system were noted but proliferative responses in breast or endometrium were not observed.