Uterotropic Activity Research Articles

New treatment paradigm of combined raloxifene and conjugated estrogen for postmenopausal symptoms in VCD-induced menopausal rats

IntroductionThe decreased ovarian estrogen production that occurs at menopause, results in osteoporosis and climacteric manifestations, and decreases women’s quality of life. The hormone replacement therapy (HRT) is the primary treatment options but has been associated with increased oncogenic potential. The tissue selective estrogen complex (TSEC) is a novel therapy, partnering a selective estrogen receptor modulator (SERM) with one or more estrogens. AimOur study was done to evaluate the potential relative estrogenic agonist activities of a SERM, raloxifene (RLX), when dosed alone and its antagonist activities when paired with conjugated estrogen (CE), as a TSEC and its potential use for the postmenopausal osteoporosis, vulvar/vaginal atrophy (VVA) in VCD induced menopausal rat model. Material and methodsFemale rats were dosed daily with 4-vinylcyclohexene diepoxide (VCD) (80mg/kg/d, IP) for 15days to induce ovarian failure, followed by one month free drug. VCD injected rats received 12weeks of RLX, CE, or combined RLX/CE with 17β-estradiol (E2), vehicle treated groups used as positive and negative controls, respectively. The bone turnover markers (BTM) were measured. The uterotropic activity was assessed by the uterine index and peroxidase assay. Vaginal wet weight (wt.) and glycogen were measured to evaluate the vaginotropic effects. Uterine and vaginal (ER) protein levels were assayed. ResultsOur findings showed that the appropriate RLX/CE dose combination exhibits significant bone sparing with minimal vaginal stimulation and neutral uterine effect. ConclusionWe can conclude that appropriate RLX/CE combination could effectively be a promising alternative for the prevention of postmenopausal osteoporosis and VVA with no oncogenic risk.

Serum Estrogens and Estrogen Metabolites and Endometrial Cancer Risk among Postmenopausal Women.

Although endometrial cancer is clearly influenced by hormonal factors, few epidemiologic studies have investigated the role of endogenous estrogens or especially estrogen metabolites. We conducted a nested case-control study within the Women's Health Initiative Observational Study (WHI-OS), a cohort of 93,676 postmenopausal women recruited between 1993 and 1998. Using baseline serum samples from women who were non-current hormone users with intact uteri, we measured 15 estrogens/estrogen metabolites via HPLC/MS-MS among 313 incident endometrial cancer cases (271 type I, 42 type II) and 354 matched controls, deriving adjusted ORs and 95% confidence intervals (CI) for overall and subtype-specific endometrial cancer risk. Parent estrogens (estrone and estradiol) were positively related to endometrial cancer risk, with the highest risk observed for unconjugated estradiol (OR 5th vs. 1st quintile = 6.19; 95% CI, 2.95-13.03, Ptrend = 0.0001). Nearly all metabolites were significantly associated with elevated risks, with some attenuation after adjustment for unconjugated estradiol (residual risks of 2- to 3-fold). Body mass index (kg/m(2), BMI) relations were somewhat reduced after adjustment for estrogen levels. The association with unconjugated estradiol was stronger for type I than type II tumors (Phet = 0.01). Parent estrogens as well as individual metabolites appeared to exert generalized uterotropic activity, particularly for type I tumors. The effects of obesity on risk were only partially explained by estrogens. These findings enhance our understanding of estrogen mechanisms involved in endometrial carcinogenesis but also highlight the need for studying additional markers that may underlie the effects on risk of certain risk factors, for example, obesity. Cancer Epidemiol Biomarkers Prev; 25(7); 1081-9. ©2016 AACR.

Synthesis and studies of structure and biological properties of D-homoanalogs of steroid estrogens

In order to estimate the potential of steroid estrogens modification, three D-homoanalogs of estrogens have been prepared; their structures and biological properties have been studied. The expansion of D-ring in such compounds has lead to strong decrease if the uterotropic action, however, the unfavorable hypertriglyceridemic effect has been retained. The latter has been eliminated by combined action of the studied steroids and ursolic acid; therewith the hypocholesterolemic activity has been retained.

New analogs of steroid estrogens

Aiming at the investigation of the mechanism of functioning of steroid estrogens a series of compounds with unnatural rings junction was synthesized. All investigated compounds exhibit a reduced uterotropic activity. It was established applying the NMR spectroscopy that 7α-methyl-3-methoxy-D-homo-6-oxa-8α,14β-estra-1,3,5(10)-trien-17a-one existed in solution in two conformations distinguished by the structure of the rings B, C, and D simultaneously. The reaction of 17-methylidene-3-methoxy-6-oxa-8α-estra-1,3,5(10)-triene with hydrobromic acid in acetic acid promotes a rearrangement with the migration of a methyl group into the position 17 resulting in the formation of 17,17-dimethyl-6-oxa-8α-gona-1,3,5(10),13(14)-tetraene derivatives.

The EDKB: an established knowledge base for endocrine disrupting chemicals

BackgroundEndocrine disruptors (EDs) and their broad range of potential adverse effects in humans and other animals have been a concern for nearly two decades. Many putative EDs are widely used in commercial products regulated by the Food and Drug Administration (FDA) such as food packaging materials, ingredients of cosmetics, medical and dental devices, and drugs. The Endocrine Disruptor Knowledge Base (EDKB) project was initiated in the mid 1990’s by the FDA as a resource for the study of EDs. The EDKB database, a component of the project, contains data across multiple assay types for chemicals across a broad structural diversity. This paper demonstrates the utility of EDKB database, an integral part of the EDKB project, for understanding and prioritizing EDs for testing.ResultsThe EDKB database currently contains 3,257 records of over 1,800 EDs from different assays including estrogen receptor binding, androgen receptor binding, uterotropic activity, cell proliferation, and reporter gene assays. Information for each compound such as chemical structure, assay type, potency, etc. is organized to enable efficient searching. A user-friendly interface provides rapid navigation, Boolean searches on EDs, and both spreadsheet and graphical displays for viewing results. The search engine implemented in the EDKB database enables searching by one or more of the following fields: chemical structure (including exact search and similarity search), name, molecular formula, CAS registration number, experiment source, molecular weight, etc. The data can be cross-linked to other publicly available and related databases including TOXNET, Cactus, ChemIDplus, ChemACX, Chem Finder, and NCI DTP. ConclusionThe EDKB database enables scientists and regulatory reviewers to quickly access ED data from multiple assays for specific or similar compounds. The data have been used to categorize chemicals according to potential risks for endocrine activity, thus providing a basis for prioritizing chemicals for more definitive but expensive testing. The EDKB database is publicly available and can be found online at http://edkb.fda.gov/webstart/edkb/index.html.Disclaimer:The views presented in this article do not necessarily reflect those of the US Food and Drug Administration.

Supplemental Dietary Racemic Equol Has Modest Benefits to Bone but Has Mild Uterotropic Activity in Ovariectomized Rats , ,

Soy isoflavones and their metabolites, with estrogenic activity, have been considered candidates for reducing postmenopausal bone loss. In this study, we examined the effect of dietary equol, a bioactive metabolite of the soy isoflavone daidzein, on equol tissue distribution, bone parameters, and reproductive tissue activity using an adult ovariectomized (OVX) rat model. An 8-wk feeding study was conducted to compare 4 dietary treatments of equol (0, 50, 100, 200 mg/kg diet) in 6-mo-old OVX female Sprague-Dawley rats. A dose response increase in tissue equol concentrations was observed for serum, liver, kidney, and heart, and a plateau occurred at 100 mg equol/kg diet for intestine. In OVX rats receiving 200 mg equol/kg diet, femoral calcium concentration was greater than those receiving lower doses but was still less than SHAM (P < 0.05), and other bone measures were not improved. Tibia calcium concentrations were lower in OVX rats receiving 100 and 200 mg equol/kg diet compared with the OVX control rats. Trabecular bone mineral density of tibia was also lower in equol-fed OVX rats. At this dietary equol intake, uterine weight was higher (P < 0.05) than in other OVX groups but lower than the SHAM-operated intact rats. The 200 mg/kg diet dose of dietary equol significantly increased proliferative index in the uterine epithelium. Dietary equol had no stimulatory effect on mammary gland epithelium. We conclude that in OVX rats, a dietary equol dose that had modest effect on bone also exerts mild uterotropic effects.

Estrogenic activity of the dichloromethane extract from Pueraria mirifica

Pueraria mirifica and its extracts are widely used as the ingredient(s) in many rejuvenating products. Up to now, the extract of P. mirifica roots that has been used in most studies, is the alcoholic extract. In the present study, we investigated the estrogenic activity using uterotropic and MCF-7 cell proliferation models of the dichloromethane extract as well as the water extract which was obtained from partitioning the ethanolic extract. The results indicated that among the three extracts, i.e. the ethanolic extract (PM1), the water extract (PM2) and dichloromethane extract (PM3), PM3 exhibited the most potent estrogenic activity in both models, followed by PM1. The extracts produced uterotropic activity associated with the increase of water content while uterotropic activity of 17beta-estradiol was related to the increase of muscle mass. The two isoflavonoids, genistein and daidzein, were not the major active phytoestrogens involving the estrogenic activity of these extracts.

Effects of dietary equol administration on ovariectomy induced bone loss in Sprague–Dawley rats

Oestrogen deficiency leads to a considerable bone loss, thus, osteopenia and osteoporosis are serious complications after menopause. ObjectivesTo evaluate the effects of a daidzein metabolite equol on bone mass density (BMD) and markers of bone remodelling in an ovariectomized (ovx) rat model of postmenopausal bone loss and compare them with the effects of 17β-estradiol. MethodsTwenty-eight female Sprague–Dawley rats were ovx and fed soy-free chow only (control group, n=8), or with the addition of oestradiol-3 benzoate (E2B) (10mg/kg, n=10) or equol (400mg/kg, n=10). At baseline and after 6-week treatment period, proximal tibia and lumbar spine BMD were measured using computer tomography. Animals were then sacrificed, blood was collected and uteri were removed. ResultsSimilarly to E2B, dietary equol decreased weight gain and showed mild uterotropic activity. E2B attenuated ovx induced BMD loss at proximal tibia whereas equol had no effect. At lumbar spine, however, equol not only attenuated trabecular bone loss but also increased its density. This effect was also apparent in animals treated with E2B. Cortical BMD at proximal tibia and lumbar spine were not very much influenced by ovx and treatment with E2B or equol did not induce significant changes at these sites. Plasma osteocalcin and type I collagen fragments (cross-laps) in equol treated animals did not differ from the controls whereas in E2B treated animals they were both significantly decreased. ConclusionsIn spite of its mild uterotropic potential, dietary equol shows limited bone sparing effects in ovx rats.

Synthesis and biological properties of D-homo-6-oxa-8α-analogues of steroid estrogens

Modifications of 6-oxa-D-homo-8alpha-analogues of steroid estrogens were found to lead to a complete loss of the uterotropic and hypertriglyceridemic activities. These compounds may be promising for the design on their basis of inhibitors of the steroid hormone metabolism and transporters of other compounds to the estrogen target organs.

Synthesis, structure, and biological properties of some 8α analogues of steroid estrogens with fluorine in position 2

A total synthesis of 8alpha analogues of steroid estrogens with fluorine in position 2 was achieved. Structural features of these compounds were studied by the example of 17beta-acetoxy-2-fluoro-3-methoxy-8alpha-estra-1,3,5(10)-triene. It was shown that the 8alpha analogues of 2-fluorosubstituted steroid estrogens have a low uterotropic activity and retain the osteoprotective and hypocholesterolemic activities.

Women’s Health: Mining for Glyceollins

In recent years, agricultural scientists have begun to explore novel strategies for isolating plant compounds of potential medicinal value. One such strategy involves exploiting the defense mechanisms of the plant in order to elicit the production of novel antimicrobial compounds called phytoalexins, which the plants synthesize in response to infection, freezing, and other forms of stress. A recent study spearheaded by researchers at the Tulane Cancer Center and Center for Bioenvironmental Research demonstrates that glyceollins, the main phytoalexins produced by soybeans, may prove to be highly effective in blocking the growth and spread of gynecologic cancers. Glyceollins are metabolites of the well-studied isoflavone daidzein, and they appear to have more potent bioactivity than the isoflavones in standard soy protein. In work published 1 December 2006 in Clinical Cancer Research, the researchers used a procedure developed at the USDA’s Southern Regional Research Center in New Orleans to isolate a mixture of glyceollins I, II, and III. The compounds were extracted from newly germinated soybeans that had been challenged with the food-safe fungus Aspergillus sojae. Previous research by the same team, published in the April 2001 issue of the Journal of Clinical Endocrinology & Metabolism, had shown that glyceollins had a marked antiestrogenic effect on estrogen receptor signaling. The new study focused on the effect of the glyceollin mixture on the growth of human estrogen-dependent MCF-7 breast cancer cells and BG-1 ovarian cancer cells. Malignant cells were grafted onto female ovariectomized mice, which were then divided into four treatment groups: control, estradiol only, glyceollins only, and estradiol plus glyceollins. All doses were 20 mg/kg/day. The researchers report that the glyceollins suppressed MCF-7 tumor growth by 53.4% and BG-1 tumor growth by 73.1%, compared to estradiol alone. In addition, the glyceollins completely suppressed estradiol-induced expression of progesterone receptors in MCF-7 cells and partially suppressed their expression in the BG-1 cells. Thus, glyceollins seem to exert their anticancer activity, in part, by interfering with the cancer cells’ ability to respond to estradiol, the most potent endogenous estrogen and a major growth stimulus for breast and ovarian cancers. Tulane cancer researcher and report coauthor Matthew Burow asserts that the compounds may eventually play a role in breast cancer prevention and treatment. “Of particular interest is the fact that glyceollins seem to be acting like pure antiestrogens,” he notes. “They show antiestrogen effects in the absence of significant estrogenic or uterotropic activity. This distinguishes them from other phytoestrogens and more importantly from the antiestrogen drug tamoxifen, which is widely used [for breast cancer control] but has been linked with increased risk of uterine cancers.” Burow adds that some women resist using tamoxifen and other antiestrogenic drugs because of the potential side effects, and for this reason the chemopreventive potential of glyceollin-enriched soy merits further study. He cites a study that he and colleagues from Wake Forest University published in volume 56, issue 1 (2006) of Nutrition and Cancer, which found that glyceollin-enriched soy protein had anti-estrogenic effects on normal breast tissue in postmenopausal female monkeys. Another medical application that may stand to benefit from glyceollin research is hormone replacement therapy, which remains the most effective means for staving off menopausal symptoms and chronic diseases such as osteoporosis. However, hormone replacement therapy carries with it a small increase in breast cancer risk. “The Tulane findings, coupled with the findings from our recent primate study, point to the possibility that the co-administration of soy enriched with glyceollins might diminish the breast cancer risk associated with hormone replacement therapy for peri- and postmenopausal women,” says Thomas B. Clarkson, a professor of comparative medicine at Wake Forest University. The Tulane research has illuminated new opportunities for both drug development and a greater understanding of environmental influences on biological systems. “This study is a fascinating example of the potential for mining the biochemistry of plants,” says J. Mark Cline, a pathology professor at Wake Forest University who collaborated with Burow and Clarkson on the primate study. “It tells us that we should be paying attention not only to diversity between plant species, but also to biochemical differences within [the same] type of plant under different conditions of culture or stress.”

Post‐Coital Antifertility Activity of Hibiscus rosa-sinensis Linn. Roots

Ancient literature mentions the use of a number of plants/preparations for fertility regulation. Some local contraceptive agents have also been described in Ayurvedic and Unani texts. Documented experiments or clinical data are, however, lacking. Therefore, the present study was undertaken to explore the antifertility and estrogenic activity of ethanolic extract of the roots of Hibiscus rosa-sinensis Linn. A strong anti-implantation (inhibition 100%) and uterotropic activity was observed at the dose level of 400 mg/kg body weight. Histological studies were carried out to confirm this effect.

Characterization of the estrogenic activities of zearalenone and zeranol in vivo and in vitro

In the present study, we compared the estrogenic activity of zearalenone (ZEN) and zeranol (ZOL) by determining their relative receptor binding affinities for human ERα and ERβ and also by determining their uterotropic activity in ovariectomized female mice. ZOL displayed a much higher binding affinity for human ERα and ERβ than ZEN did. The IC 50 values of ZEN and ZOL for binding to human ERα were 240.4 and 21.79 nM, respectively, and the IC 50 values for binding to ERβ were 165.7 and 42.76 nM, respectively. In ovariectomized female ICR mice, s.c. administration of ZEN at doses ≥2 mg/kg/day for 3 consecutive days significantly increased uterine wet weight compared with the control group, and administration of ZOL increased the uterine wet weight at lower doses (≥0.5 mg/kg/day for 3 days). Based on available X-ray crystal structures of human ERα and ERβ, we have also conducted molecular modeling studies to probe the binding characteristics of ZEN and ZOL for human ERα and ERβ. Our data revealed that ZEN and ZOL were able to occupy the active site of the human ERα and ERβ in a strikingly similar manner as 17β-estradiol, such that the phenolic rings of ZEN and ZOL occupied the same receptor region as occupied by the A-ring of 17β-estradiol. The primary reason that ZOL and ZEN is less potent than 17β-estradiol is likely because 17β-estradiol could bind to the receptor pocket without significantly changing its conformation, while ZOL or ZEN would require considerable conformational alterations upon binding to the estrogen receptors (ERs).

Guest Editorial: Assessing Risks and Benefits of Genistein and Soy

Guest Editorial: Assessing Risks and Benefits of Genistein and Soy

In vitro and in vivo estrogenic activity of chlorinated derivatives of bisphenol A

The estrogenic activity of bisphenol A (BPA) and its chlorinated derivatives, 2-(3-chloro-4-hydroxyphenyl)-2-(4-hydroxyphenyl)propane (3-ClBPA) and 2,2-bis(3-chloro-4-hydroxyphenyl)propane (3,3'-diClBPA) was assessed by determining their relative binding affinity for the human estrogen receptor-alpha and -beta (ERalpha and ERbeta) and also their uterotrophic activity in ovariectomized female rats. BPA and its chlorinated derivatives were active in competing with [3H]17beta-estradiol for their binding to the human ERalpha and ERbeta proteins. While 3-ClBPA and 3,3'-diClBPA competed more effectively for ERalpha binding than BPA (IC50 values of 2.48x10(-5), 1.28x10(-5), and 1.08x10(-4)M, respectively), they had similar activity as BPA for competing the binding to ERbeta (IC50 values of 1.43x10(-5), 1.87x10(-5), and 2.59x10(-5)M, respectively). To determine the uterotropic activity, three doses (10, 50 and 100 mg/kg/day) of BPA and its derivatives were given to mature ovariectomized Sprague-Dawley rats for 3 consecutive days. Treatment of animals with 50 and 100 mg/kg/day of BPA or its chlorinated derivatives caused a significant increase in the uterine wet weight and the endometrial area. The results of our present study demonstrated that the affinities of 3-ClBPA and 3,3'-diClBPA for ERalpha were higher than the affinity of BPA, although the in vivo estrogenic activity of the two chlorinated BPAs in ovariectomized female Sprague-Dawley rats appeared to be comparable to that of BPA.

Mechanisms for altered reproductive function in female rats following neonatal administration of raloxifene.

Raloxifene is a non-steroidal selective estrogen receptor modulator (SERM) that mimics estrogenic activity on bone density and blood lipid concentration without uterotropic actions. Previous data from our laboratory indicated that, as is the case for estrogen, neonatal administration of raloxifene disturbed normal differentiation of the hypothalamic circuitries governing the gonadotropic axis. In contrast, raloxifene did not act in the same way as estrogen does on the neuronal systems controlling sexual receptivity in the female rat. At present, however, the mechanisms for these organizing effects of raloxifene are not completely elucidated. To analyze this phenomenon, female rats were injected daily with raloxifene (50, 100, 250 or 500 microg/rat per day) between days 1 and 5 of age. On day 23, hypothalamic gonadotropin-releasing hormone (LHRH) mRNA expression was assessed, and pituitary and plasma luteinizing hormone (LH) and follicle-stimulating hormone (FSH) levels were measured in basal and LHRH-stimulated conditions. In addition, LH and FSH responses to ovariectomy were evaluated in raloxifene-treated females. Finally, we monitored the ability of neonatal administration of a potent LHRH agonist ([d-Ala(6),d-Gly(10)]-LHRH ethylamide; 0.01 microg/kg per 12 h on days 1-5) to counteract the effects of raloxifene. Our analyses demonstrated that prepubertal rats (23-day-old females) treated neonatally with raloxifene showed decreased hypothalamic LHRH mRNA expression levels, reduced pituitary content of LH and FSH, reduced basal and LHRH-stimulated LH secretion in vivo and in vitro, and decreased response to ovariectomy. In addition, adult females treated neonatally with raloxifene showed anovulation and reduced serum LH levels; these effects were not prevented by the simultaneous administration of a LHRH agonist. In conclusion, our data demonstrate that neonatal administration of raloxifene can disrupt the programming of hypothalamic-pituitary-ovarian axis function. Reduced LH secretion, under basal and LHRH-stimulated conditions and after ovariectomy, is probably related to decreased LHRH expression, reduced pituitary LH content and/or decreased pituitary responsiveness to hypothalamic LHRH.

Effects of defatted safflower seed extract and phenolic compounds in diet on plasma and liver lipid in ovariectomized rats fed high-cholesterol diets.

Six polyphenolic compounds were isolated from ethylacetate extract secondary to 80% ethanol extraction of defatted safflower seeds. They were categorized into three types: lignans, flavones and serotonin derivatives. Female Sprague-Dawley rats weighing 163.4 +/- 6.3 g were ovariectomized (Ovx) and fed either ethylacetate extract at a level of 1% (w/w) or three types of safflower polyphenolic compounds at a level of 200 mg/kg in a diet containing 0.5% (w/w) cholesterol for four wk. The sham and Ovx control groups were fed the same diet without safflower components. Plasma GOT and GPT levels did not differ among the six experimental groups. The plasma levels of total cholesterol were reduced in the four safflower groups by 20-30% as compared to the Ovx control. The plasma level of HDL-cholesterol was higher in the Ovx+ethylacetate extract group or appeared to be in the three Ovx+safflower polyphenolic groups than in the Ovx control. The level of plasma triglyceride was also significantly lower in the Ovx+lignan group than in the Ovx control. The liver level of cholesterol was significantly reduced in the Ovx+ethylacetate extract group. Fecal excretion of cholesterol increased by the safflower lignans and flavones, whereas that of bile acid was not significantly changed by the safflower polyphenols. Matairesinol and acacetin isolated from safflower seeds reduced the cholesterol content in cultured HepG2 cells at a concentration of 0.01-0.1 microM and all three safflower polyphenolics decreased triglyceride content at the concentration of 0.1 microM. These results suggest that safflower polyphenols have the effect of improving blood lipid status via increasing HDL-cholesterol formation and cholesterol excretion without significant uterotropic action in estrogen-deficient animals.

Effects of IconÒ, a pyrethroid insecticide on early pregnancy of rats

Icon is a water miscible type II synthetic pyrethroid insecticide based on active ingredient lambda cyhalothrin (10% w/w). It is used in Sri Lanka as an adulticidal indoor spray against malaria vector mosquitoes. The goal of this study was to assess the effects of Icon on pregnancy outcome of rats when exposed during early pregnancy (days 1-7). Icon was gavaged daily for 7 consecutive days in three different doses; 63, 83, or 125 mg/kg/day (active ingredient; lambda cyhalothrin; 6.3, 8.3, 12.5 mg/kg/day), respectively. Several parameters of reproduction and pre- and post-natal development were monitored. The results show that Icon is detrimental to pregnancy outcome (in terms of quantal pregnancy, number of uterine implants, implantation index and foetal deaths) but induced no detectable developmental defects. The anti-reproductive effects of Icon were mainly due to increased pre-implantation losses. Enhancement of post-implantation losses played a subsidiary role. These effects resulted from multiple mechanisms: maternal toxicity, stress, uterotropic activity and embryo-foetotoxicity. Further progesterone had a protective effect against Icon induced anti-reproductive actions. Overall, the results suggest that exposure to Icon during early gestation may result in a threat to pregnancy.

Preliminary study on antifertility activity of Calotropis procera roots in female rats

The effect of ethanolic extract of the roots of Calotropis procera has been studied in albino rats to explore its antifertility and hormonal activities. A strong antiimplantation (inhibition 100%) and uterotropic activity was observed at the dose level of 250 mg/kg (1/4 of LD 50). No antiestrogenic activity could be detected.

A Study of the Binding of Estradiol and 8-Isoestradiol to the Estrogen α-Receptor by Molecular Modeling

The complexes of the estrogen α-receptor with estradiol and 8-isoestradiol were comparatively analyzed. The computations of ligand–receptor complexes, carried out using the FLEXX program, allowed us to propose a model for the binding of the analogues of 8-isoestradiol. It was found that rings Cand D of estradiol and 8-isoestradiol are similarly arranged in the ligand-binding pocket and coincide upon the superposition of the corresponding ligand–receptor complexes, whereas rings A and B do not coincide. The oxygen functions in position 17 of the estradiol analogues of both series coincide upon superposition, whereas the phenol 3-hydroxyl groups are 0.05 A apart. A comparison of the predicted biological properties of modified estradiol analogues of the natural and 8-iso-series with the available experimental data revealed their similarity. Synthetic 2-acetyl analogues of 8-isoestrogens were found to have no uterotropic activity, which is also consistent with the proposed model.