The Saga of the Ring B Unsaturated Equine Estrogens*

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Abstract
THE RING B unsaturated estrogens, equilin, equilenin, 17α-dihydroequilin, 17α-dihydroequilenin, 17β-dihydroequilin, and 17β-dihydroequilenin, are present in the urine of pregnant mares. Along with the ring B unsaturated estrogens, pregnant mare's urine also contains the classical estrogens, estrone, 17α-estradiol, and 17β-estradiol. The ring B unsaturated estrogens, as the name indicates, differ structurally from the classical estrogens by the presence of one or two additional double bonds in the ring B of the steroid nucleus (Fig. 1). The ring B unsaturated estrogens were first isolated by Girard et al. in 1932 (1, 2); however, their biosynthetic origin has not yet been fully elucidated. In 1941, a urinary extract containing the sulfate esters of the above nine estrogens was prepared from pregnant mare's urine by Ayerst Laboratories (Montreal, Quebec, Canada) and was named Premarin. This preparation has been for the last four decades, the most commonly prescribed estrogen for replacement therapy in estrogendeficient women. Recent epidemiological evidence suggests an etiological role for these estrogens in the carcinoma of the human endometrium. This review will discuss, 1) the existence of two pathways of steroidogenesis in the pregnant mare: (a) the classical pathway via cholesterol leading to the formation of the classical estrogens, estrone and estradiol, and (b) an alternate pathway which bypasses cholesterol and leads to the biosynthesis of ring B unsaturated estrogens, equilin and equilenin; 2) the biological activity of the ring B unsaturated estrogens; 3) the pharmacokinetics of equilin sulfate in postmenopausal women; 4) comparison of the interaction of ring B unsaturated estrogens and the classical estrogens with human endometrial estrogen receptors; 5) the role (if any) of ring B unsaturated estrogens in the etiology of endometrial cancer.

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