Effects of CP-336,156, a new, nonsteroidal estrogen agonist/antagonist, on bone, serum cholesterol, uterus and body composition in rat models

The selective estrogen receptor modulator acolbifene reduces cholesterolemia independently of its anorectic action in control and cholesterol-fed rats

The cancer-preventing selective estrogen receptor modulator (SERM) acolbifene (ACOL) exerts a potent and pure antiestrogenic action in the mammary gland and uterus, yet it displays beneficial, estrogen-like actions on energy and lipid metabolism in rodents. The compound reduces food intake and strongly decreases cholesterolemia in rats fed a cholesterol-free diet. This study was designed to establish whether the anorectic effect of ACOL is involved in its cholesterol-lowering action, and whether the compound retains its ability to lower cholesterol concentrations in rats with diet-induced hypercholesterolemia. Female rats were fed a purified diet devoid of cholesterol (reference diet) or containing 2% cholesterol (C-diet); they were either not treated or treated daily with ACOL or not treated and pair-fed to the ACOL-treated rats. The C-diet did not affect food intake or weight and fat gains. ACOL reduced food intake (16%) and weight gain (45%, mainly fat) similarly in both dietary cohorts. ACOL, but not pair feeding, reduced cholesterolemia by 33% in rats fed the reference diet. As expected, the C-diet raised serum total cholesterol almost 3-fold and this increase was largely prevented by ACOL but not by pair feeding. Cholesterol was reduced by ACOL, mainly in the HDL fraction, in rats fed the reference diet, but only in the non-HDL fraction in those fed the C-diet. In livers of rats fed the reference diet, ACOL, but not pair feeding, increased protein abundance of the scavenger receptor, class B, type 1, and the LDL receptor, thought to be involved in ACOL-mediated cholesterol lowering. These findings demonstrate that the potent hypocholesterolemic action of ACOL is independent of the concomitant reduction in food intake and fat accretion, and that such action occurs in rats with overt diet-induced hypercholesterolemia.

Bazedoxifene acetate: a selective estrogen receptor modulator with improved selectivity

We assessed the preclinical characteristics of a novel, stringently screened selective estrogen receptor modulator, bazedoxifene acetate, including its ability to bind to and activate estrogen receptors and promote increased bone mineral density and bone strength in rats, and the effects impacting the uterine endometrium, breast cancer cell proliferation, and central nervous system-associated vasomotor responses in an animal model. Bazedoxifene bound to estrogen receptor-alpha with an IC50 of 26 nm, an affinity similar to that of raloxifene. Bazedoxifene did not stimulate proliferation of MCF-7 cells but did inhibit 17beta-estradiol-induced proliferation with an IC50 of 0.19 nm. In an immature rat uterine model, bazedoxifene (0.5 and 5.0 mg/kg) was associated with less increase in uterine wet weight than either ethinyl estradiol (10 microg/kg) or raloxifene (0.5 and 5.0 mg/kg). Histological analysis revealed that coadministration of bazedoxifene also appeared to reduce raloxifene-stimulated endometrial luminal epithelial cell and myometrial cell hypertrophy. In ovariectomized rats, bazedoxifene was associated with significant increases in bone mineral density at 6 wk, compared with control, and better compressive strength of bone samples from the L4 vertebrae, compared with samples from ovariectomized animals. In the morphine-addicted rat model of vasomotor activity, bone-sparing doses of bazedoxifene alone were not associated with 17beta-estradiol inhibition of increased vasomotor activity. Bazedoxifene acetate represents a promising new treatment for osteoporosis, with a potential for less uterine and vasomotor effects than selective estrogen receptor modulators currently used in clinical practice. Controlled clinical trial data will be needed to confirm these effects.

Hypolipidemic action of the SERM acolbifene is associated with decreased liver MTP and increased SR-BI and LDL receptors

This study aimed to identify the mechanisms of the hypolipidemic action of the selective estrogen receptor modulator (SERM) acolbifene (ACOL). Four weeks of treatment with ACOL reduced fasting and postprandial plasma triglycerides (TGs), an effect associated with lower VLDL-TG secretion rate (-25%), and decreased mRNA of microsomal triglyceride transfer protein (MTP; -29%). ACOL increased liver TG concentration (+100%) and amplified the feeding-induced increase in the master lipogenic regulators sterol-regulatory element binding protein-1a (SREBP-1a) and SREBP-1c. ACOL decreased total, HDL, and non-HDL cholesterol (CHOL) by 50%. SREBP-2 mRNA and HMG-CoA reductase activity were minimally affected by ACOL. However, in the fasted state, liver concentration of scavenger receptor class B type I (SR-BI) protein, but not mRNA, was 3-fold higher in ACOL-treated than in control animals and correlated with plasma HDL-CHOL levels (r = 0.80, P < 0.002). Liver LDL receptor (LDLR) protein, but not mRNA, was increased 2-fold by ACOL, independently of the nutritional status. This study demonstrates that ACOL possesses the unique ability among SERMs to reduce VLDL-TG secretion, likely by reducing MTP expression, and strongly suggests that the robust hypocholesterolemic action of ACOL is related to increased removal of CHOL from the circulation as a consequence of enhanced liver SR-BI and LDLR abundance.

Estrogen receptor ligands. Part 5: The SAR of dihydrobenzoxathiins containing modified basic side chains

Dihydrobenzoxathiin analogs (1-11) with modifications on the basic side chain region were prepared and evaluated for estrogen/anti-estrogen activity in both in vitro and in vivo models. The compounds generally maintained a high degree of selectivity for ERalpha over ERbeta, similar to the original lead compound I. Many of the compounds also maintained high potency in the inhibition of human carcinoma MCF-7 cell growth. However, all were less potent in the inhibition of estradiol-triggered uterine growth. This work demonstrates the sensitive nature of modification to the antagonist basic side chain region.

Estrogen receptor ligands. Part 6: Synthesis and binding affinity of dihydrobenzodithiins

Dihydrobenzodithiin compounds (1-6) were prepared to explore the expansion of the dihydrobenzoxathiin lead compounds I-III as SERAMs (Selective Estrogen Receptor Alpha Modulators). The dihydrobenzodithiin compounds generally maintained a high degree of selectivity for ERalpha over ERbeta, however, they lacked the in vivo antagonism/agonism activity exhibited by the lead class in an immature rat uterine growth model.

Selective estrogen receptor modulators protect hippocampal neurons from kainic acid excitotoxicity: differences with the effect of estradiol

Neuroprotective effects of estradiol are well characterized in animal experimental models. However, in humans, the outcome of estrogen treatment for cognitive function and neurological diseases is very controversial. Selective estrogen receptor modulators (SERMs) may represent an alternative to estrogen for the treatment or the prevention of neurodegenerative disorders. SERMs interact with the estrogen receptors and have tissue-specific effects distinct from those of estradiol, acting as estrogen agonists in some tissues and as antagonists in others. In this study we have assessed the effect of tamoxifen, raloxifene, lasofoxifene (CP-336,156), bazedoxifene (TSE-424), and 17beta-estradiol on the hippocampus of adult ovariectomized rats, after the administration of the excitotoxin kainic acid. Administration of kainic acid induced the expression of vimentin in reactive astroglia and a significant neuronal loss in the hilus. SERMs did not affect vimentin immunoreactivity in the hilus, while 17beta-estradiol significantly reduced the surface density of vimentin immunoreactive profiles. Estradiol, tamoxifen (0.4-2 mg/kg), raloxifene (0.4-2 mg/kg), and bazedoxifene (2 mg/kg) prevented neuronal loss in the hilus after the administration of kainic acid. Lasofoxifene (0.4-2 mg/kg) was not neuroprotective. These findings indicate that SERMs present different dose-dependent neuroprotective effects. Furthermore, the mechanisms of neuroprotection by SERMs and estradiol are not identical, because SERMs do not significantly affect reactive gliosis while neuroprotection by estradiol is associated with a strong down-regulation of reactive astroglia.

Pre- and postnatal development studies of lasofoxifene, a selective estrogen receptor modulator (SERM), in Sprague-Dawley rats

BACKGROUND

Lasofoxifene is a nonsteroidal selective estrogen receptor modulator (SERM) developed for the treatment of postmenopausal osteoporosis. The purpose of these studies was to evaluate the effects of lasofoxifene on the postnatal development, behavior, and reproductive performance of offspring of female rats given lasofoxifene during organogenesis and lactation.

METHODS

Two range-finding studies were conducted to determine the effects of lasofoxifene at doses from 0.01-10 mg/kg on parturition and lactation in pregnant rats and on the early postnatal development of the offspring, and to optimize the dosing regimen. Maternal milk and plasma were sampled for concentrations of lasofoxifene on Lactation Days 4, 7, and 14. In the pre- and postnatal development study, lasofoxifene was administered to pregnant and lactating rats by oral gavage at dose levels of 0.01, 0.03, and 0.1 mg/kg on Gestation Days 6-17 and Lactation Days 1-20. Maternal body weight and food consumption were measured throughout pregnancy, and body weight was measured throughout lactation. Parturition was monitored closely. The F1 offspring were measured for viability, body weight, anogenital distance, the appearance of postnatal developmental indices and reflex behaviors, sensory function, in an age-appropriate functional observational battery, motor activity, auditory startle, passive avoidance, and the Cincinnati Water Maze. The F1 generation was assessed for reproductive function, and the F2 offspring were measured for body weight and viability throughout the lactation period.

RESULTS

In the range-finding studies, indications of maternal toxicity included decreased body weight and food consumption, increased length of gestation, prolonged parturition, dystocia, and increased offspring mortality at birth. Concentrations of lasofoxifene in maternal plasma were similar to those in milk, increased with increasing dose, and remained consistent over a 10-day period. In the pre- and postnatal development study, maternal body weights and food consumption were decreased in all treated groups during gestation. Length of gestation was increased, parturition was prolonged, and dystocia was noted in the dams in the 0.1 mg/kg group. There was increased pup mortality in the F1 litters in the 0.1 mg/kg group and all treated groups had decreased offspring body weights beginning at 1 week of age, continuing into the postweaning period and, for the F1 males, into adulthood. Female F1 offspring in the 0.03 and 0.1 mg/kg groups had increased body weights as adults. There were delays in the age of appearance of preputial separation in the males in the 0.1 mg/kg group and vaginal opening in the females in all treated groups. Body temperature was decreased by <0.5 degrees C after weaning for male and female offspring in the 0.1 mg/kg group. The sensory, behavioral, and functional measures, including the tests of learning and memory, were unaffected by treatment. Mating success was lower for the F1 animals in the 0.1 mg/kg group, but there were no effects on the reproductive parameters. Mating, reproduction, and maternal behavior of the F1 animals in the 0.01 and 0.03 mg/kg groups and the survival and body weights of the F2 offspring in all treated groups through Postnatal Day 21 were unaffected by treatment.

CONCLUSION

The maternal findings in this study were related to the pharmacologic activity of lasofoxifene. Inhibition of growth of the F1 offspring after perinatal exposure to lasofoxifene was observed, but there were no significant effects on the sensory, behavioral, or functional measures, including learning and memory. There were no effects on the F2 generation. The findings are consistent with those reported for at least one other SERM. The findings of this study do not suggest increased risk for the primary indication of use in postmenopausal women.

Reproductive toxicity assessment of lasofoxifene, a selective estrogen receptor modulator (SERM), in male rats

BACKGROUND

Lasofoxifene is a nonsteroidal selective estrogen receptor modulator (SERM) with greater than 100-fold selectivity against all other steroid receptors and is a potentially superior treatment for postmenopausal osteoporosis. The purpose of this study was to evaluate the effects of lasofoxifene on male reproduction in rats in light of the known effects of estrogen modulating compounds on male reproductive ability.

METHODS

Lasofoxifene was administered to adult male rats at doses of 0.1, 1, 10, and 100 mg/kg for 66-70 consecutive days. After 28 days of dosing, male rats were cohabited with untreated female rats. Female rats were euthanized on gestation day 14 and a uterine examination was carried out for evaluation of reproductive parameters and embryo viability. Male rats were euthanized after 66-70 days of dosing and epididymal sperm motility and concentration were assayed. The testes, epididymides, prostate, and seminal vesicles were weighed and microscopically examined.

RESULTS

The duration of cohabitation was increased for 100 mg/kg males by 0.7 days. The number of males copulating and the number of implantation sites produced per copulation were reduced in the 10 and 100 mg/kg groups. Weights of the seminal vesicles and epididymides were reduced for all groups, although the testes weight and epididymal sperm motility and concentration were not affected by treatment. There were no microscopic findings in the male reproductive tissues.

CONCLUSION

The changes in male fertility and reproductive tissue weights after exposure to lasofoxifene are consistent with those previously described for estrogen receptor-modulating compounds.

Embryo/fetal toxicity assessment of lasofoxifene, a selective estrogen receptor modulator (SERM), in rats and rabbits

BACKGROUND

The purpose of this study was to evaluate the effects of lasofoxifene, a selective estrogen receptor modulator (SERM), on rat and rabbit fetal development.

METHODS

Lasofoxifene was administered orally to rats (1, 10, 100 mg/kg) between gestation days (GD) 6-17, and in rabbits (0.1, 1, 3 mg/kg) between GD 6-18. Maternal body weight and food consumption were monitored throughout pregnancy. Fetuses were delivered by Cesarean section on GD 21 in rats, and GD 28 in rabbits, to evaluate fetal viability, weight, and morphology. Drug concentrations in maternal plasma were measured in a separate cohort of animals at several time points commencing on GD 17 (rats) and 18 (rabbits). On GD 18 (rat) and GD 19 (rabbit) drug concentrations were measured in maternal plasma and in fetal tissue 2 hr post dosing to determine the fetal to maternal drug ratio.

RESULTS

In rats, there were dose-related declines in maternal weight gain and food consumption. Post implantation loss was significantly increased at dosages of 10 and 100 mg/kg, and the number of viable fetuses was decreased at 100 mg/kg. The placental weights increased, whereas fetal weights decreased in a dose-dependent manner. Lasofoxifene-related teratologic findings were noted at 10 and 100 mg/kg and included imperforate anus with hypoplastic tails, dilatation of the ureters and renal pelvis, misaligned sternebrae, hypoflexion of hindpaw, wavy ribs, and absent ossification of sternebrae. In rabbits, neither maternal weight gain nor food consumption were affected during treatment. Between GD 26-28, there was a dose-dependent increased incidence of red discharge beneath the cages. At 1 and 3 mg/kg, resorptions and post-implantation loss increased. There were no significant external or visceral effects, but 3 mg/kg there was an increased incidence of supernumerary ribs. Although the maternal plasma Cmax and AUC(0-24) were dose-dependent, the exposures in the rat were many orders of magnitude greater than in the rabbit even for the same 1 mg/kg dose. The single time point fetal/maternal drug ratio was higher in the rat (1.3-0.78) than in the rabbit (0.21-0.16).

CONCLUSION

In general, both maternal and fetal effects of lasofoxifene were similar to those reported with other SERMs. Although the incidence or severity of these effects was, in some instances, greater in the rat than in the rabbit, the doses and the resultant maternal and fetal exposures were many orders of magnitude higher in the rat, suggesting the rabbit to be more sensitive to the toxicological effects of lasofoxifene.

Reproductive toxicity assessment of lasofoxifene, a selective estrogen receptor modulator (SERM), in female rats

BACKGROUND

Lasofoxifene is a nonsteroidal selective estrogen receptor modulator (SERM). With high affinity to the alpha and beta human estrogen receptors and greater potency than other SERMs, lasofoxifene is potentially a superior treatment for postmenopausal osteoporosis. In light of the known effects of estrogen-modulating compounds on female reproductive indices, two studies were conducted to evaluate the effects of lasofoxifene on female rat cyclicity, reproduction, and parturition.

METHODS

One study evaluated effects of lasofoxifene on estrous cyclicity, and the second study assessed effects on implantation and parturition. In the cyclicity study, lasofoxifene was administered to female rats at doses of 0.1, 0.3, and 1.0 mg/kg/day for 14 consecutive days. After treatment, there was a 3-week reversibility phase followed by a mating phase. In the implantation study, lasofoxifene was administered to pregnant female rats at doses of 0.01, 0.03, and 0.1 mg/kg/day for 7 consecutive days (gestation day [GD] 0-6). Some animals were euthanized on GD 21, and the remainder of the group was allowed to deliver the F1 generation. Several developmental indices were evaluated in the F1 pups through post-natal day (PND) 21.

RESULTS

In the cyclicity study, all lasofoxifene-treated females were anestrous by Study Day 7 (1.0 mg/kg) or 9 (0.3 and 0.1 mg/kg). The reversibility phase resulted in restoration of normal estrous cycles by the end of 1 (0.1 mg/kg) or 2 weeks (0.3 and 1.0 mg/kg). During the mating phase, no adverse effects occurred in pregnancy success or reproductive parameters. In the implantation study, all doses of lasofoxifene increased pre- and post-implantation losses, increased gestation length, and reduced litter size. None of the developmental parameters measured on the F1 generation was adversely affected.

CONCLUSION

Lasofoxifene reversibly altered the estrous cycle and inhibited implantation, consistent with what would be expected from a member of the SERM class.

ER-alpha variants and the cardiovascular effects of hormone replacement therapy

There is an accumulating body of evidence linking estrogen receptor-alpha (ER-alpha) gene polymorphisms with variation in cardiovascular risk factors or disease. The ER-alpha IVS1-397 T/C (PvuII), IVS1-351 A/C (XbaI) and the promoter region TA repeat polymorphisms have received the most attention. If ER-alpha genetic variants identify women at high risk for cardiovascular events, this information could be used to improve clinical decisions regarding the use of hormone replacement therapy (HRT). More detailed haplotype and genome-wide studies of many of the major HRT clinical trial cohorts are currently underway and these efforts promise to clarify many outstanding issues concerning estrogen action and cardiovascular disease.

Novel therapies for osteoporosis

Osteoporosis remains a significant clinical problem despite effective therapies. Many patients cannot or will not take currently available therapies. For this reason research continues in search of more effective and more tolerable agents. Anabolic agents offer a unique mechanism of action. The anabolic agents parathyroid hormone and strontium will be discussed. The investigational bisphosphonates ibandronate, minodronate and zoledronic acid may offer the advantage of less frequent dosing. Arzoxifene, bazedoxifene, lasofoxifene, MDL-103,323 and ospemifene are investigational selective oestrogen receptor modulators shown to be effective in animal studies and are now in clinical studies. Tibolone is a tissue-specific steroid that is currently used in Europe for prevention and treatment of osteoporosis. Multiple studies have shown efficacy in improving bone mineral density, but no fracture studies have been conducted to date. While studies of the effect of isoflavones on bone mineral density have been encouraging, a large, multi-centre study in Europe showed no effect of isoflavones on fractures. The newly described agent osteoprotegerin has been shown in early studies to inhibit bone turnover. Other agents with unique mechanisms of action in early development include cathepsin K inhibitors, integrin receptor inhibitors, nitrosylated non-steroidal anti-inflammatory agents and Src inhibitors. The efficacy of statins in bone continues to be debated with no prospective, randomised studies yet to confirm the suggestion of benefit seen in epidemiological studies.

Chronic effects of dehydroepiandrosterone on rat adipose tissue metabolism

The goal of the present study was to examine cellular mechanisms that regulate adipose cell metabolism in ovariectomized (OVX) and intact rats that were subjected to long-term (27 weeks) treatment with dehydroepiandrosterone (DHEA). Forty-eight 16-month-old female rats were divided into 4 groups of 9 to 11 animals (intact, intact-DHEA, OVX, OVX-DHEA). Adipose tissue lipoprotein lipase (LPL), hormone-sensitive lipase (HSL), and cyclic adenosine monophosphate (cAMP)-dependent phosphodiesterase (cAMP-PDE) activities were determined, and alpha2-, beta1/beta2-, and beta3-adrenoceptors (ARs) were quantified. DHEA did not affect body weight, fat, or muscle mass in intact rats. The similar retroperitoneal fat pad weight of intact-DHEA rats compared to intact animals was in agreement with the lack of difference in the enzyme activities and AR densities. The increased body weight of OVX rat was paralleled by a greater retroperitoneal adipose tissue mass (P <.01), which was in turn associated with a marked rise in LPL activity (P <.005) and a slight decrease in HSL activity (P <.05) compared to intact animals. OVX-DHEA rats, compared to untreated OVX animals, had a smaller retroperitoneal fat depot, which correlated with a decrease in LPL activity (P <.005) and moderate increase in both HSL activity and beta3-AR density (P <.05). DHEA-treatment lowered fasting insulin and triglyceride levels in both intact and OVX rats (P <.05). Plasma testosterone, androsterone, androstenedione, and androstenediol levels were also significantly increased in both intact-DHEA and OVX-DHEA rats compared to untreated animals (P <.0001). These findings suggest that the antiobesity action of DHEA may be related in part to changes in lipase activities and in beta3-AR density, and that it is dependent on the ovarian status of the animal.

The estrogen antagonist EM-652 and dehydroepiandrosterone prevent diet- and ovariectomy-induced obesity

OBJECTIVE

EM-652 is a pure antiestrogen in human breast and uterine cancer cells that also reduces bone loss and plasma lipid levels in the rat. This study aimed to assess the ability of EM-652, alone or with dehydroepiandrosterone (DHEA), to prevent obesity and related metabolic abnormalities induced by an obesity-promoting diet and ovariectomy.

RESEARCH METHODS AND PROCEDURES

Female rats were fed a high-sucrose, high-fat (HSHF) diet, were left intact or ovariectomized (OVX), and were treated with EM-652, DHEA, or both for 20 days. Variables of energy balance and determinants of lipid metabolism and insulin sensitivity were assessed.

RESULTS

The HSHF diet (vs. chow) and OVX both increased energy intake and gain, as well as energetic efficiency. Both EM-652 and DHEA prevented diet- and OVX-induced energy gain mainly by decreasing fat deposition, without being additive. The modest EM-652-induced increase in liver triglycerides of intact rats was prevented by its combination with DHEA. EM-652, but not DHEA, decreased cholesterolemia. The HSHF diet and OVX reduced insulin sensitivity, an effect that was attenuated by EM-652 and abrogated by DHEA and EM-652+DHEA. Treatment with EM-652, DHEA, or their combination abolished the diet- and OVX-induced increase in adipose lipoprotein lipase activity that accompanied fat gain.

DISCUSSION

EM-652 is an effective agent to prevent diet- and OVX-induced obesity and its associated cardiovascular risk factors such as insulin resistance. The addition of DHEA prevents hepatic lipid accumulation and further ameliorates insulin sensitivity. The beneficial metabolic effects of such combined steroid therapy may, therefore, eventually prove to be clinically relevant.

Long-term dosing of arzoxifene lowers cholesterol, reduces bone turnover, and preserves bone quality in ovariectomized rats

Long-term effects of a new selective estrogen receptor modulator (SERM) arzoxifene were examined in ovariectomized (OVX) rats. Arzoxifene was administered postoperatively (po) at 0.1 mg/kg per day or 0.5 mg/kg per day to 4-month-old rats, starting 1 week after OVX for 12 months. At study termination, body weights for arzoxifene groups were 16-17% lower than OVX control, which was caused by mainly reduced gain of fat mass. Longitudinal analysis of the proximal tibial metaphysis (PTM) by computed tomography (CT) at 0, 2, 4, 6,9, and 12 months showed that OVX induced a 22% reduction in bone mineral density (BMD) at 2 months, which narrowed to a 12% difference between sham-operated (sham) and OVX rats by 12 months. Both doses of arzoxifene prevented the OVX-induced decline in BMD. Histomorphometry of the PTM showed that arzoxifene prevented bone loss by reducing osteoclast number in OVX rats. Arzoxifene maintained bone formation indices at sham levels and preserved trabecular number above OVX controls. Micro-CT analysis of lumbar vertebrae showed similar preservation of BMD compared with OVX, which were not different from sham. Compression testing of the vertebra and three-point bending testing of femoral shaft showed that strength and toughness were higher for arzoxifene-treated animals compared with OVX animals. Arzoxifene reduced serum cholesterol by 44-59% compared with OVX. Uteri wet weight from arzoxifene animals was 38-40% of sham compared with OVX rats, which were 29% of sham. Histology of the uterine endometrium showed that cell heights from both doses of arzoxifene were not significantly different from OVX controls. In summary, treatment of OVX rats with arzoxifene for nearly one-half of a lifetime maintained beneficial effects on cholesterol and the skeleton. These data suggest that arzoxifene may be a useful therapeutic agent for osteoporosis in postmenopausal women.

Kinetic analysis of estrogen receptor/ligand interactions

Surface plasmon resonance biosensor technology was used to directly measure the binding interactions of small molecules to the ligand-binding domain of human estrogen receptor. In a screening mode, specific ligands of the receptor were easily discerned from nonligands. In a high-resolution mode, the association and dissociation phase binding responses were shown to be reproducible and could be fit globally to a simple interaction model to extract reaction rate constants. On average, antagonist ligands (such as tamoxifen and nafoxidine) were observed to bind to the receptor with association rates that were 500-fold slower than agonists (such as estriol and beta-estradiol). This finding is consistent with these antagonists binding to an altered conformation of the receptor. The biosensor assay also could identify subtle differences in how the same ligand interacted with two different isoforms of the receptor (alpha and beta). The biosensor's ability to determine kinetic rate constants for small molecule/protein interactions provides unique opportunities to understand the mechanisms associated with complex formation as well as new information to drive the optimization of drug candidates.

Characterization of selective estrogen receptor modulator (SERM) activity in two triarylethylene oxybutyric acids

Previously we identified 4-[1-(4-hydroxyphenyl)-2-phenylbuten-1-yl]phenoxy-n-butyric acid (4HBA) and its des-hydroxy analog (BA) as potential selective estrogen receptor modulators (SERMs) in the ovariectomized (OVX) rat. The aim of the present study was to characterize comprehensively the effects of 4HBA and BA in both the OVX rat and in estrogen-responsive cells. Thus, 4HBA was found to be an estrogen antagonist with partial agonist efficacy in estrogen-responsive reporter gene and estrogen-dependent proliferation assays (MVLN cells and MCF-7 human breast cancer cells, respectively). In the OVX rat, 4HBA and BA were equally effective and comparable to other known SERMs regarding (a) serum cholesterol reduction and suppression of serum markers of excessive bone metabolism, and (b) partial agonist efficacy in reproductive tissue relative to steroidal estrogens. Like steroidal estrogens, both compounds increased serum triglyceride levels, with BA being more effective in this regard. The maximal effects of 4HBA on all of these parameters except cholesterol lowering were seen at oral doses of 0.4 micromol/kg/day; maximal cholesterol lowering required doses of 10 micromol/kg/day. In OVX rat liver 9S fraction, BA was found to be efficiently converted to a single hydroxylated metabolite, 4HBA. These results suggest that the effects of BA in the OVX rat might, in part, be a consequence of biotransformation to 4HBA, and that those of 4HBA and BA in the OVX rat and in estrogen-responsive cells are qualitatively similar to those of SERMs such as tamoxifen and raloxifene.

Selective estrogen receptor modulation: the search for an ideal hormonal therapy for breast cancer

Female hormones, especially estrogens, play an important role in the pathogenesis of breast neoplasms and are a principal determinant of their biological behavior. Endocrine manipulation through medical or surgical means can often lead to objective shrinkage of breast tumors. Tamoxifen, a triphenylethylene estrogen receptor modulator, is currently the most widely used hormonal treatment for breast cancer. It has been conclusively demonstrated to reduce the risk of relapse following definitive local therapy (and systemic chemotherapy, when indicated) of invasive or noninvasive breast cancer. Recently, it has also been shown to reduce the incidence of breast cancer in healthy women who are at high risk of developing the disease. In addition, it can prevent osteoporosis and reduce the risk of fractures in postmenopausal women. However, its use is also complicated by an increased incidence of endometrial hyperplasia/carcinoma, venous thromboembolism, cataracts, and in some cases, emergence of tamoxifen-dependent clones of breast cancer. These side effects (except cataracts) are believed to be related to estrogen-agonist effects of tamoxifen. Newer drugs, which are "pure antiestrogens" or inhibitors of estrogen biosynthesis, are devoid of such estrogen-agonist activity and may not have the liability of many of these side effects. However, these agents would also be expected to lack the potentially beneficial effects of tamoxifen on lipids and skeletal system. The ability of tamoxifen to act as an estrogen-agonist or estrogen-antagonist in a tissue-specific fashion has led to the concept of selective estrogen-receptor modulation. Selective estrogen receptor modulators (SERMs), which are devoid of estrogen-agonist effects on the uterus or breast cancer cells but retain potentially beneficial effects on bones and lipids, have been described as "ideal" SERMs. A number of such compounds are currently being tested. Raloxifene is already approved for prevention of osteoporosis and has potential efficacy for prevention and treatment of breast cancer. An analogue of raloxifene, LY353381, is currently in Phase II clinical trials for treatment of breast cancer, with promising early results. EM800 and CP336156 are other promising ideal SERMs in clinical trials. These compounds may provide better treatment and chemoprevention alternatives for breast cancer as compared to tamoxifen, aromatase inhibitors, and pure antiestrogens. In addition, they may also prove to be useful for the treatment and prevention of prostate cancer as well as for treating benign gynecological diseases such as fibroids and endometriosis. Future laboratory efforts should focus on further broadening the efficacy profile of SERMs (e.g., prevention of Alzheimer's disease and elevation of high-density lipoproteins to improve the likelihood of cardiovascular benefit) and narrowing their side-effect profile (e.g., risk of thromboembolism and hot flashes).

Levormeloxifene prevents increased bone turnover and vertebral bone loss following ovariectomy in cynomolgus monkeys

Levormeloxifene, a nonsteroidal selective estrogen receptor modulator (SERM), has been evaluated for its effects on bone in cynomolgus monkeys (Macaca fascicularis). Adult female monkeys were imported from Indonesia and randomized into six groups of 25-28 animals each (n = 158). Animals in one group were sham ovariectomized (sham) and received vehicle. Animals in the remaining five groups were ovariectomized and received either vehicle (ovx); 17beta-estradiol at 0.016 mg/kg (est); or levormeloxifene at 0.5 (L1), 1 (L2), or 5 (L3) mg/kg. Lumbar spine and whole body bone mass were measured by dual-energy X-ray absorptiometry (DXA) pretreatment and at 6 and 12 months following the initiation of treatment. Bone mass at the femoral neck was measured by peripheral quantitative computed tomography (pQCT) at 0 and 12 months. Serum markers of bone turnover, including bone-specific alkaline phosphatase (BSAP), osteocalcin (BGP), tartrate-resistant acid phosphatase (TRAP), and urinary collagen C-terminal extension peptides (CrossLaps), were measured at 0, 6, and 12 months. Ovariectomy resulted in an increase in these markers; the increase was prevented by estradiol or levormeloxifene. Estradiol or levormeloxifene inhibited loss of lumbar spine bone mineral density (BMD) following ovariectomy compared with untreated monkeys (ovx -5.0%; sham -0.4%; est +0.2%; L1 -3.6%, L2 -2.0%, L3 -2.5%). Estradiol, but not levormeloxifene, prevented loss of BMD at the femoral neck (ovx -7.4%; sham -3.1%; est -3.6%; L1 -8.0%, L2 -6.5%, L3 -7.8%), and whole body bone mineral content (BMC) (ovx -7.6%; sham -1.9%, est -2.9%; L1 -6.2%, L2 -6.1%, L3 -6.7%). Bone loss at each site was correlated with bone turnover as measured by serum and urine biomarkers. There was no dose effect of levormeloxifene.

Tamoxifen, screening and new oestrogen receptor modulators

Tamoxifen is a selective oestrogen receptor modulator (SERM) with anti-oestrogenic properties in the breast and oestrogenic effects in tissues such as bone and the cardiovascular system. It is an excellent breast cancer drug for all stages of the disease. Its SERM profile makes it a valuable alternative to hormone replacement therapy, especially for women at high risk of breast cancer. Tamoxifen, however, increases the incidence of benign and malignant lesions of the uterus. Secondary prevention of these, early detection and treatment, is feasible but not cost-effective in breast cancer patients because potential endometrial risks do not outweigh beneficial effects in the breast. This may be different in healthy women with an as yet unknown benefit on breast cancer mortality. We review the literature on the importance of tamoxifen's endometrial lesions and balance available evidence on whether and how best to screen them. In a subset of tamoxifen users it seems advisable to assess the uterine cavity prior to intake with a yearly endometrial assessment as pointed out, starting 3 years after initiation of treatment. In most cases there is endometrial thickening on ultrasonographic assessment and additional tests such as hydrosonography or hysteroscopy are required to confirm an empty atrophic uterus as remains the case in most asymptomatic women on tamoxifen. Newer compounds, such as raloxifene, have a similar SERM profile to tamoxifen but are neutral on the uterus. This has recently been proven by 3 years of endometrial follow-up data. Longer endometrial safety will hopefully confirm these early findings. Whether other SERMs in development are better, and which of them is better for the breast, is to be demonstrated in ongoing studies.

Estrogens, progestins, selective estrogen receptor modulators, and the arterial tree

We reviewed studies of the effects of different estrogens, progestins, and selective estrogen receptor modulators at the coronary and carotid arterial sites to help determine their likely effects on cardiovascular morbidity and mortality. All English-language studies published between 1997 and 2000 on MEDLINE, Current Contents, and Best Evidence were reviewed, including in vitro, other animal, human physiologic, and clinical trial studies. We synthesize, assess limitations, and integrate across systems with the in vivo experience in humans to evaluate the clinical context. Estrogens have favorable direct effects in most circumstances, progestins oppose these effects, and early studies suggest that selective estrogen receptor modulators are protective. In some systems the dosage, route of delivery, and type of progestin may be important and risk factors may modulate hormone effects. The evaluation of endothelial dysfunction gives a unique in vivo opportunity to assess the vascular properties of hormones, although the relationship between the in vivo physiologic effects of hormones and clinical outcomes remains to be determined.

Lasofoxifene (CP-336,156) protects against the age-related changes in bone mass, bone strength, and total serum cholesterol in intact aged male rats

The purpose of this study was to evaluate if long-term (6 months) treatment with lasofoxifene (LAS), a new selective estrogen receptor modulator (SERM), can protect against age-related changes in bone mass and bone strength in intact aged male rats. Sprague-Dawley male rats at 15 months of age were treated (daily oral gavage) with either vehicle (n = 12) or LAS at 0.01 mg/kg per day (n = 12) or 0.1 mg/kg per day (n = 11) for 6 months. A group of 15 rats was necropsied at 15 months of age and served as basal controls. No significant change was found in body weight between basal and vehicle controls. However, an age-related increase in fat body mass (+42%) and decrease in lean body mass (-8.5%) was observed in controls. Compared with vehicle controls, LAS at both doses significantly decreased body weight and fat body mass but did not affect lean body mass. No significant difference was found in prostate wet weight among all groups. Total serum cholesterol was significantly decreased in all LAS-treated rats compared with both the basal and the vehicle controls. Both doses of LAS treatment completely prevented the age-related increase in serum osteocalcin. Peripheral quantitative computerized tomography (pQCT) analysis at the distal femoral metaphysis indicated that the age-related decrease in total density, trabecular density, and cortical thickness was completely prevented by treatment with LAS at 0.01 mg/kg per day or 0.1 mg/kg per day. Histomorphometric analysis of proximal tibial cancellous bone showed an age-related decrease in trabecular bone volume (TBV; -46%), trabecular number (Tb.N), wall thickness (W.Th), mineral apposition rate, and bone formation rate-tissue area referent. Moreover, an age-related increase in trabecular separation (Tb.Sp) and eroded surface was observed. LAS at 0.01 mg/kg per day or 0.1 mg/kg per day completely prevented these age-related changes in bone mass, bone structure, and bone turnover. Similarly, the age-related decrease in TBV and trabecular thickness (Tb.Th) and the age-related increase in osteoclast number (Oc.N) and osteoclast surface (Oc.S) in the third lumbar vertebral cancellous bone were completely prevented by treatment with LAS at both doses. Further, LAS at both doses completely prevented the age-related decrease in ultimate strength (-47%) and stiffness (-37%) of the fifth lumbar vertebral body. These results show that treatment with LAS for 6 months in male rats completely prevents the age-related decreases in bone mass and bone strength by inhibiting the increased bone resorption and bone turnover associated with aging. Further, LAS reduced total serum cholesterol and did not affect the prostate weight in these rats. Our data support the potential use of a SERM for protecting against the age-related changes in bone and serum cholesterol in elderly men.

Enzyme-catalyzed asymmetric deacylation for the preparation of lasofoxifene (CP-336156), a selective estrogen receptor modulator

[structure] selective estrogen receptor modulator (SERM), Lasofoxifene (CP-336156), was prepared by an enzyme-catalyzed asymmetric deacylation with high optical purity and excellent yield even though the hydrolytic site is remote from the chiral centers.

Estrogen deficiency, obesity, and skeletal abnormalities in follicle-stimulating hormone receptor knockout (FORKO) female mice

Targeted disruption of the receptor for glycoprotein hormone, FSH (FSH-R) causes a gene dose-related endocrine and gametogenic abnormality in female mice. The resulting FSH-R knockout (FORKO) mutants have disordered estrous cycles, ovulatory defects, and atrophic uterus. The heterozygous animals that initially show reduced fertility undergo early reproductive senescence and stop breeding altogether. Lack of FSH-R signaling in females causes severe ovarian underdevelopment producing chronic estrogen deficiency. This was accompanied by increases in serum testosterone levels. Ovarian aromatase gene transcription and translation are unaltered in the mutants. Early loss of estrogen in the null mutants leads to obesity and skeletal abnormalities that intensify with age producing (kyphosis), a hunchback appearance. Both these changes also become apparent in older heterozygous mice coincident with early reproductive senescence. The expression of nuclear estrogen receptor(s) alpha and beta genes and the corresponding proteins in the ovary and uterus of FORKO mice appear to be intact. The loss of ovarian estrogen creates an imbalance in A and B forms of the progesterone receptor in the uterus of both heterozygotes and null mutants. Some of the changes we have documented here in FORKO mice are reminiscent of the ovarian dysfunction and other major symptoms that are usually associated with estrogen deficiency. In null mutants, estradiol-17beta administration promptly induced uterine growth and reversed the accumulation of adipose tissue indicating that estrogen receptors are functional. Thus, the phenotypes evident in these genetically altered FSH-R mutants may provide an experimental system to explore the effects of estrogenic compounds on different targets including the ovary in a nonsurgical setting.

Selective estrogen receptor modulators: structure, function, and clinical use

The sex hormone estrogen is important for many physiologic processes. Prolonged stimulation of breast ductal epithelium by estrogen, however, can contribute to the development and progression of breast cancer, and treatments designed to block estrogen's effects are important options in the clinic. Tamoxifen and other similar drugs are effective in breast cancer prevention and treatment by inhibiting the proliferative effects of estrogen that are mediated through the estrogen receptor (ER). However, these drugs also have many estrogenic effects depending on the tissue and gene, and they are more appropriately called selective estrogen receptor modulators (SERMs). SERMs bind ER, alter receptor conformation, and facilitate binding of coregulatory proteins that activate or repress transcriptional activation of estrogen target genes. Theoretically, SERMs could be synthesized that would exhibit nearly complete agonist activity on the one hand or pure antiestrogenic activity on the other. Depending on their functional activities, SERMs could then be developed for a variety of clinical uses, including prevention and treatment of osteoporosis, treatment and prevention of estrogen-regulated malignancies, and even for hormone replacement therapy. Tamoxifen is effective in patients with ER-positive metastatic breast cancer and in the adjuvant setting. The promising role for tamoxifen in ductal carcinoma-in-situ or for breast cancer prevention is evolving, and its use can be considered in certain patient groups. Other SERMs are in development, with the goal of reducing toxicity and/or improving efficacy, and future agents have the potential of providing a new paradigm for maintaining the health of women.

Effects of the estrogen antagonist EM-652.HCl on energy balance and lipid metabolism in ovariectomized rats

OBJECTIVE

The estrogen antagonist EM-652.HCl behaves as a highly potent and pure antiestrogen in human breast and uterine cancer cells. Because of its pure antiestrogenic activity in these cells, and because its prodrug, EM-800, reduces bone loss and decreases serum cholesterol and triglycerides in the rat, EM-652.HCl can be classified as a pure selective estrogen receptor modulator (SERM). This study was conducted to assess the ability of EM-652.HCl to prevent obesity and abnormalities of lipid metabolism induced by ovariectomy in a rat model.

DESIGN

Female rats were left intact or ovariectomized (OVX), and OVX rats were treated with placebo, estradiol (E2), or EM-652.HCl for 20 days. At the end of the treatment period, parameters of energy balance and determinants of lipid metabolism were assessed.

RESULTS

As expected, OVX increased energy intake, which in turn was accompanied by an increased energy, fat and protein gain and higher food efficiency. OVX also increased the triglyceride content of the liver and produced hypercholesterolemia and hyperinsulinemia. The weight of representative white adipose depots was higher in OVX than in intact rats. Lipoprotein lipase activity was higher in white adipose tissues of OVX rats than in those of intact animals, whereas its activity was lower in oxidative tissues (brown adipose and soleus muscle). Replacement therapy with a physiological dose of E2 prevented most of the abnormalities in energy and lipid metabolism brought about by OVX, although its orexigenic effect was only partially corrected. In contrast, treatment of OVX rats with EM-652. HCl completely abolished OVX-induced obesity and its related abnormalities in lipid metabolism and glucose/insulin homeostasis.

CONCLUSION

These findings demonstrate that EM-652.HCl can be considered as an effective agent to prevent OVX-induced obesity. The present study also shows that EM-652.HCl reduces cardiovascular risk factors associated with obesity such as hyperlipidemia and insulin resistance.

Lasofoxifene (CP-336,156), a selective estrogen receptor modulator, prevents bone loss induced by aging and orchidectomy in the adult rat

It has been well documented that selective estrogen receptor modulators (SERMs) can prevent bone loss in ovariectomized rats and postmenopausal women. The purposes of this study were to determine the effects of a potent and orally active SERM, lasofoxifene (CP-336,156), on bone mass, bone strength, total serum cholesterol, prostate weight, and histology in adult male orchidectomized (ORX) rats. Sprague Dawley male rats at 10 months of age were divided into 6 groups, with 10 rats/group. The first group was necropsied on day 0 and served as basal controls. The remaining rats were either sham operated (n = 10) and treated orally with vehicle, or ORX (n = 40) and treated with either vehicle or lasofoxifene at 1, 10, or 100 microg/kg x day for 60 days. Total serum cholesterol, prostate weight and histology, distal femoral bone mineral density (DFBMD) by dual energy x-ray absorptiometry, and static and dynamic bone histomorphometry of the third lumbar vertebral body were determined. Maximal load and stiffness of the fifth lumbar vertebral body were also determined by compression tests. Age-related decreases in DFBMD (-9%) and trabecular bone volume (TBV; -13%) of the third lumbar vertebral body were found in sham-operated rats compared with basal controls. ORX induced significant increases in total serum cholesterol (+31%), eroded surface (+48%), activation frequency of bone turnover (+103%) and significant decreases in prostate weight (-89%), DFBMD (-14%), TBV (-23%), and maximal load (-17%) compared with basal controls. Compared with sham controls, ORX induced significant increases in eroded perimeter and activation frequency. Lasofoxifene decreased body weight in all dose groups compared with both sham and ORX control values. Compared with ORX controls, ORX rats treated with lasofoxifene at 10 or 100 microg/kg x day had significantly lower percent eroded perimeter activation frequency and significantly higher DFBMD, TBV, and maximal load. Further, lasofoxifene at 10 and 100 microg/kg x day significantly decreased total serum cholesterol by 46% and 68% in ORX rats, whereas no effect was found in prostate weight and histology parameters compared with ORX control values. These data showed that lasofoxifene prevented bone loss by inhibiting bone turnover associated with aging and orchidectomy in 10-month-old male rats. Further, lasofoxifene decreased total serum cholesterol and did not affect the prostate in these rats. These results suggest that SERMs such as lasofoxifene may be useful therapeutic agents for preventing bone loss in elderly men with some degree of hypogonadism.

Synthesis and pharmacology of a novel pyrrolo[2,1,5-cd] indolizine (NNC 45-0095), a high affinity non-steroidal agonist for the estrogen receptor

1-Ethyl-2-(4-hydroxyphenyl)pyrrolo[2,1,5-cd]indolizine (NNC 45-0095) is a novel compound which represents the parent pharmacophore structure of a series of pyrrolo[2,1,5-cd]indolizine derivatives with mixed estrogen agonist/antagonist properties. NNC 45-0095 binds with high affinity to the estrogen receptor (IC50=9.5 nM) and exhibits full protection of bone loss in the ovariectomized mouse model for post-menopausal osteoporosis.

Antiestrogens--tamoxifen, SERMs and beyond

Estrogens play a central role in reproductive physiology. The cellular effects of estrogens are mediated by binding to nuclear receptors (ER) which activate transcription of genes involved in cellular growth control. At least two such receptors, designated ERalpha and ERbeta, mediate these effects in conjunction with a number of coactivators. These receptors can directly interact with other members of the steroid receptor superfamily. A complex cross-talk exists between the estrogen-signaling pathways and the downstream signaling events initiated by growth factors, such as epidermal growth factor and insulin-like growth factors. Estrogens are also a causative factor in the pathogenesis of a variety of neoplastic and non-neoplastic diseases, including breast cancer, endometrial cancer, endometriosis, and uterine fibroids, among others. Antiestrogens, such as tamoxifen, are widely used for the treatment of breast cancer. Tamoxifen produces objective tumor shrinkage in advanced breast cancer, reduces the risk of relapse in women treated for invasive breast cancer, and prevents breast cancer in high-risk women. Although, initially developed as an antiestrogen, tamoxifen can also prevent postmenopausal osteoporosis as well as reduce cholesterol, due to its estrogen-agonist effects. Its estrogen-agonist activity, however, can lead to significant side-effects such as endometrial cancer and thromboembolic phenomena. This has led to the concept of "ideal" selective estrogen receptor modulators (SERMs), drugs that would have the desired, tissue selective, estrogen-agonist or -antagonist effects. Raloxifene is a SERM which has the desirable mixed agonist/antagonist effects of tamoxifen but does not cause uterine stimulation. "Pure" antiestrogens may provide very potent estrogen-antagonist drugs, but are likely to be devoid of beneficial effects on bone and lipids. Future drug development efforts should focus on developing superior SERMs that have a greater efficacy against ER-positive tumors and do not cause hot flashes or thromboembolism, and explore combination strategies to simultaneously target hormone-dependent as well as hormone-independent breast cancer.

Tissue distribution and pharmacological potential of SM-16896, a novel oestrogen-bisphosphonate hybrid compound

Postmenopausal osteoporosis is caused mainly by a deficiency of oestrogen with rapid bone loss. To target oestrogen to the bone effectively, we have synthesized and evaluated the effects of a novel hybrid compound of oestrogen and bisphosphonate, SM-16896. The tissue distribution pattern and pharmacological potential are reported. Although the affinity for calf uterine oestrogen receptor was very low (IC50: 73.3 microM; 1/25000 of that of 17beta-oestradiol (2.84 nM)), SM-16896 showed oestrogenic activity. SM-16896 (1 microM) induced a 4.5-fold transcriptional activity in rat osteosarcoma UMR-106 cells compared with vehicle-treated control, when we used the expression vector for human oestrogen receptor and a CAT reporter plasmid containing an oestrogen-responsive element. The distribution of SM-16896 after a subcutaneous administration to 7-week-old female rats was examined by radioluminography using 3H-labelled SM-16896. At 30 min after the administration, significant radioactivity was detected in the bone. At 24 h after administration, a high level of radioactivity was detected in the bone, but in the uterus it was only at a background level. Daily subcutaneous administration of 0.5 mgkg(-1) SM-16896 for 12 weeks (five times per week) to 13-week-old ovariectomized rats suppressed the ovariectomized-induced reduction in bone mineral density. A bone mineral density ratio of 120% was maintained compared with sham-operated rats, whereas a relatively low suppression of uterine weight was observed (about 50% loss compared with sham-operated rats). In the same experiment, the implantation of a 17beta-oestradiol time-release pellet (0.25 mg/pellet/90 days) almost completely suppressed the reduction of both the bone mineral density and uterine tissue weight. It is likely that the effect of SM-16896 on bone was due to its oestrogenic activity, since 1.0 mgkg(-1) SM-18108, the bisphosphonate moiety of this compound, had no effect on bone in 7-week-old ovariectomized rats. The results suggest that SM-16896, a bisphosphonate-conjugated oestrogen, showed a preference profile in the uterus and bone due to its characteristic distribution pattern compared with the natural oestrogen analogue 17beta-oestradiol. Thus, bisphosphonate-conjugated oestrogens have the potential to improve patient compliance in oestrogen therapy by minimizing adverse effects and reducing the frequency of medication.

Selective estrogen receptor modulators: tissue selectivity and differential uterine effects

Selective estrogen receptor modulators (SERMs) are compounds that bind to estrogen receptors and produce estrogen-like (agonist) effects in some tissues and estrogen-blocking (antagonist) effects in other tissues. One of the goals of SERM research has been to develop compounds that provide the potential benefits of estrogen in the skeleton and cardiovascular system, but avoid the negative effects of estrogen in other tissues. Estrogen therapy has been consistently associated with endometrial stimulation, including glandular proliferation, hyperplasia and cancer. In contrast, the presence or degree of endometrial stimulation observed with SERMs varies by compound. The purpose of this review is to differentiate the endometrial effects of compounds that display a SERM-like profile. Molecular mechanisms involving SERM binding to estrogen receptors, preclinical uterine effects in both tissue culture and animal models, and endometrial findings in clinical experience are discussed. There are several SERMs commercially available or in development. The favorable safety profile of raloxifene in the uterus differentiates it from the others. Future SERM development will continue to focus on finding compounds that exhibit minimal endometrial stimulation.