from binding to the serm-er complex. third-generation chemical structure of nafoxidine with the dihydronapthalene group in red. third-generation compounds display either no uterine stimulation, improved potency, no significant increases in hot flushes or even a combination of these positive attributes. the first dihydronapthalene serm, nafoxidine, was a clinical candidate for the treatment of breast cancer but had side effects including severe phototoxicity. nafoxidine has all three phenyls constrained in a coplanar arrangement like tamoxifen. but with hydrogenation, the double bond of nafoxidene were reduced, and both phenyls are cis-oriented. the amine-bearing side chain can then adopt an axial conformation and locate this group orthogonally to the plane of the core, like ralofoxifene and other less uterotropic serms. chemical structure of lasofoxifene shows cis-oriented phenyls. modifications of nafoxidine resulted in lasofoxifene. lasofoxifene is among the most potent serms reported in protection against bone loss and cholesterol reduction. the excellent oral potency of lasofoxifene has been attributed to reduced intestinal glucuronidation of the phenol. unlike raloxifene, lasofoxifene satisfies the requirement of a pharmacophore model that predicts resistance to gut wall glucuronidation. the structural requirement is a non-planar topology with the steric bulk close to the plane of a fused bicyclic aromatic system. the interactions between the er and lasofoxifene are consistent with the general features of serm-er recognition. lasofoxifene's large flexible side chain terminates in a pyrrolidine head group and threads its way out toward the surface of the protein, where it interferes directly with the positioning of the af-2 helix. a salt bridge forms between lasofoxifene and asp-351. the charge neutralization in this region er may explain some antiestrogenic effects exerted by lasofoxifene. bazedoxifene includes an indole system (red) which is connected to an amine through a benzyloxyethyl chain (green). the indole system has served as a core unit in serms, and when an amine is attached to the indole with a benzyloxyethyl, the resultant compounds were shown to have no preclinical uterine activity while sparing rat bone with full efficacy at low doses. bazedoxifene is one of those compounds. the core binding domain consists of a 2-phenyl-3-methyl indole and a hexamethylenamine ring at the side chain affecter region. it is metabolized by glucuronidation, with the absolute bioavailability of 6.2%, 3-fold higher than that of raloxifene. it has agonistic effects on bone and lipid metabolism but not on breast and uterine endometrium. it is well tolerated and displays no increase in hot flush incidences, uterine hypertrophy or breast tenderness. chemical structure of ospemifene. ethoxy side chain ends with a hydroxy group (red) instead of a dimethylamino group as with first-generation serms. ospemifene is a triphenylethylene and a known metabolite of toremifene. it's structurally very similar to tamoxifen and toremifene. ospemifene does not have 2-(dimethylamino)ethoxy group as tamoxifen. structure–activity relationship studies showed that by removing that group of tamoxifen agonistic activity in the uterus was significantly reduced, but not in bone and cardiovascular system. preclinical and clinical data show that ospemifene is well tolerated with no major side effects. benefits that ospemifene may have over other serms is its neutral effect on hot flushes and er-agonist effect on the vagina, improving the symptoms of vaginal dryness. binding modes the abcd steroid ring system in 17β-estradiol. the serms are known to feature four distinctive modes of binding to er. one of those features are strong hydrogen bonds between the ligand and erα's arg-394 and glu-353 that line the "a-ring pocket" and help the ligand to stay in er's binding pocket. this is unlike 17β-estradiol which is hydrogen bonded to his-524 in the "d-ring pocket". other distinctive bindings to the ligand-binding pocket are with a nearly planar "core" structure typically composed of a biaryl heterocycle, equivalent to the a-ring and b-ring of 17β-estradiol, to the corresponding binding site; a bulky side chain from the biaryl structure, analogous to the b-ring of 17β-estradiol and finally a second side group that is the c- and d-ring equivalent and usually aromatic, fills the remainder volume of the ligand-binding pocket. the small differences between the two subtypes of er have been used to develop subtype-selective er modulators, but the high similarity between the two receptors make the development very challenging. amino acids in the ligand-binding domains differ at two positions, leu-384 and met-421 in erα and met-336 and ile-373 in erβ, but they have similar hydrophobicity and occupying volumes. however, the shapes and the rotational barrier of the amino acid residues are not the same, leading to distinguish α- and β-face of the binding cavity between erα and erβ. this causes erα-preferential-binding of ligand substituents that are aligned downwards facing met-336 while ligand substituents aligned upwards facing met-336 are more likely to bind to erβ. another difference is in val-392 in erα, which is replaced by met-344 in erβ. erβ's binding pocket volume is slightly smaller and the shape a bit different from erα's. many erβ-selective ligands have a largely planar arrangement as the binding cavity of erβ is slightly narrower than that of erα, however, this by itself leads to modest selectivity. to attain strong selectivity, the ligand must place substituents very close to one or more of the amino acid differences between erα and erβ in order to create a strong repulsive force towards the other subtype receptor. in addition, the structure of the ligand must be rigid. repulsive interactions may otherwise lead to conformational change of the ligand and, therefore, creating alternative binding modes. first-generation triphenylethylenes tamoxifen is converted by the liver cytochrome p450 into the 4-hydroxytamoxifen and is a more selective antagonist of the erα subtype than erβ. 4-hydroxytamoxifen binds to ers within the same binding pocket that recognizes 17β-estradiol. the receptor recognition of 4-hydroxytamoxifen appears to be controlled by two structural features of 4-hydroxytamoxifen, the phenolic a ring, and the bulky side chain. the phenolic a ring forms hydrogen bonds to the side groups of er's arg-394, glu-354 and to structurally conserved water. the bulky side chain, protruding from the binding cavity, displaces helix 12 from ligand-binding pocket to cover part of the coactivator binding pocket. the er-4-hydroxytamoxifen complex formation recruits corepressors proteins. this leads to decreased dna synthesis and inhibition of estrogen activity. clomifene and torimefene produce binding affinities similar to that of tamoxifen. thus, these two drugs are more selective antagonists of the erα subtype than erβ. second-generation benzothiophenes "a ring" (a) and "d ring" (d) marked in raloxifene. raloxifene, like 4-hydroxytamoxifen, binds to erα with the hydroxyl group of its phenolic "a ring" through hydrogen bonds with arg-394 and glu-353. in addition to these bonds, raloxifene forms a second hydrogen bond to er through the side group of his-524 because of the presence of a second hydroxyl group in the "d ring". this hydrogen bond is also unlike that between 17β-estradiol and his-524, as the imidazole ring of his-524 is rotated to counteract the difference of the oxygen position in raloxifene and in 17β-estradiol. just like in 4-hydroxytamoxifen, the bulky side chain of raloxifene displaces helix 12. third-generation lasofoxifene interaction with erα is typical of those between serm-erα such as a nearly planar topology (the tetrahydronapthalene carbocycle), hydrogen bonding with arg-394 and glu-353 and the phenyl side chains of lasofoxifene filling the c-ring and d-ring volume of the ligand-binding pocket. lasofoxifene diverts helix 12 and prevents the binding of coactivator proteins with lxxll motives. this is achieved by lasofoxifene occupying the space normally filled by leu-540's side group and modulating the conformation of residues of helix 11 (his-524, leu-525). furthermore, lasofoxifene also directly interferes with helix 12 positioning by the drug's ethyl pyrrolidine group. in vitro studies indicate that bazedoxifene competitively blocks 17β-estradiol by high and similar binding to both erα and erβ. bazedoxifenes main binding domain consists of the 2-phenyl-3-methylindole and a hexamethylenamine ring at the side chain affected region. ospemifene is an oxidative deaminated metabolite of toremifene as has a similar binding to er as toremifene and tamoxifen. the competitive binding to erα and erβ of the three metabolites 4-hydroxy ospemifene, 4'-hydroxy ospemifene and the 4-hydroxy-, side chain carboxylic acid ospemifene is at least as high as the parent compound. history timeline of when serms came on the market. the discovery of serms resulted from attempts to develop new contraceptives. clomifene and tamoxifen prevented conception in rats but did the opposite in humans. clomifene successfully induced ovulation in subfertile women and on february 1, 1967, it was approved in the us for the treatment of ovulation dysfunction in women who were trying to conceive. toxicological issues prevented long term use of clomifene and further drug development for other potential applications such as breast cancer treatment and prevention. it was another ten years before tamoxifen was approved in december 1977, not as a contraceptive but as a hormonal treatment to treat and prevent breast cancer. the discovery in 1987 that the serms tamoxifen and raloxifene, then thought to be antiestrogens because of antagonist effects in breast tissue, showed estrogenic effects in preventing bone loss in ovariectomized rats had a great effect on our understanding of the function of estrogen receptors and nuclear receptors in general. the term serm was introduced to describe these compounds that have a combination of estrogen agonist, partial agonist, or antagonist activities depending on the tissue. toremifene has been shown to be compatible with tamoxifen, and in 1996 it was approved for use in the treatment of breast cancer in postmenopausal women. raloxifene originally failed as a breast cancer drug due to its poor performance in comparison to tamoxifen in the laboratory but the estrogenic effects of raloxifene on bone led to its rediscovery and approval in 1997. it was approved for prevention and treatment of osteoporosis and was the first clinically available serm to prevent both osteoporosis and breast cancer. ospemifene was approved on february 26, 2013, for the treatment of moderate to severe dyspareunia, which is a symptom, due to menopause, of vulvar and vaginal atrophy. combined therapy with conjugated estrogens and the serm bazedoxifene, was approved on october 3, 2013, for the treatment of vasomotor symptoms linked with menopause. bazedoxifene is also used in the prevention of postmenopausal osteoporosis. the search for a potent serm with bone efficacy and better bioavailability than raloxifene led to the discovery of lasofoxifene. although lasofoxifene was approved in 2009, it was not marketed for three years following the approval, so the marketing authorization for it has expired. in europe, bazedoxifene is indicated for the treatment of osteoporosis in postmenopausal women at increased risk of fracture while in india ormeloxifene has been used for dysfunctional uterine bleeding and birth control.