**1. Introduction**

Estrogens play a key role in multiple physiological functions in women. They have important actions on bone and lipid metabolism, cardiovascular function, and diffuse effects on other target organs. Estrogens have important roles in cognitive function and influence psychological well-being in women, in development and maintenance of the reproductive system in the female **(Gustafsson, 2003).** The most potent estrogens are 17β-estradiol **1,**  estrone **2** and estriol **3** may have tissue-specific roles (**Gruber & Huber, 1999**).

In female body estrogens are formed in ovaries, and as result the menopause is associated with an increased risk of the development of cardiovascular diseases, osteoporosis and many other diseases. In postmenopausal women, many of these functions (positive effect on prevention of osteoporosis and improved serum profile enhanced by the use of estrogen replacement therapy) are achievement. The positive action of estrogens on prophylactic and treatment of osteoporosis has been proved undoubtedly, however other clinical data are considered as contradictory (**Davison & Davis, 2003**). Moreover, during the use of estrogens for HRT the number of strokes is increasing (**Bushnell, 2005**). It was absolutely unexpected because the adverse effect has been observed in experiments on animals. It was also found that during long-term using of estrogens the risk of some oncological diseases is increasing (**Beral, 2003; Beral et al., 2005**). Other side effects, which we discuss later, also indicate the necessity for the search of new safe estrogen analogues with the improved biological properties.

Firstly we consider the results in these directions, and then we discuss the main advantages of the creation of agents with directed biological action, which are perspective for HRT, for

Approaches for Searching

enhances the binding.

**OH**

**O**

**<sup>4</sup>**

starting from 2000 (**Lu et al., 2000).**

**N**

of Modified Steroid Estrogen Analogues with Improved Biological Properties 173

Ligand-independent transactivational function AF1 is in zone А/В of both sub-types of receptors, zone E/F of -receptor of estrogens has additional ligand-independent activation function (**Tora et al., 1989**). ER has ligand-dependent trans-activation function AF-2. Without ligand estrogen receptors are in complexes with heat shock proteins (Hsp90, Hsp70, Hsp56, Hsp60, Hsp48, Hsp23), this binding takes place in domain С. Probably, being in complexes with heat shock proteins estrogens are protected against the action of proteases. After binding its ligand conformational rearrangement of LBD of nuclear receptor and dissociation of complexes ER – heat shock proteins takes place. Except of the potential possibility to activate transcription, this rearrangement causes the change of topography of receptor regions, sensitive to proteolysis (**Ramsey & Klinge, 2001).** Phosphorylation of estrogen receptors is observed after their binding with hormone (**Kato et al., 1995)** that

Transformed receptor forms dimers and in this form binds with DNA and may activate the transcription. Effective dimerization of ER requires a weak constitutive activity of sequences in domain C (**Kumar & Chambon, 1988).** It is necessary to note, that AF1 and AF2 exhibit relatively weak activity, whereas the maximum of transcription induction is observed when they act together (**Tora et al., 1989; Pham et al., 1992)**. From other side, in some cases the activation of AF-1 is enough to activate the transcription. Thus, 4 hydroxytamoxifen **4** is unable to induce AF-2 activity, but it is a strong agonist in cellular and promoter context where AF-1 is effective transcriptional activator (**Berry et al., 1990**).

**O**

Transcriptional process is modulated by receptors' ligands and various co-regulators **(Cheskis et al., 1997; An et al., 1999; 2001; Tcherepanova et al., 2000; Wong et al., 2001; Liu, J. et al., 2003; Bai & Gugière, 2003; Xu & Li, 2003)**. Conformation of ER changes upon interaction with coactivator proteins **(Tamrazi et al., 2005)**, as result the activity of one ligand may change in depending on cell nature. Thus, tamoxifen **5** and raloxifene **6** are agonists of estradiol in cardiovascular system, whereas they show antagonistic properties in breast and endometrium **(Jordan, 2007)**. Many others SERMs have similar properties. Nowadays more 10 estrogen receptor-β isoforms are known, which have been identified

To study the roles of each receptor *in vivo*, a series of the mice were generated lacking either a functional ER and ERβ or both (ERKO, βERKO, βERKO) (**Emmel & Korach, 2001).** ERβ may modulate the functions of ER, if these receptors are in the same cells (**Matthews et al., 2006**). It became crucial during the diagnostics and the treatment of oncological

 4-hydroxytamoxifen tamoxifen raloxifene  **4 5 6** 

**N**

**OH <sup>S</sup>**

**O**

**N**

**O**

**OH**

the treatment of oncological estrogen-sensitive diseases, cardio-vascular system, osteoporosis, neuroendocrinal diseases. The division of agents on such groups has relative character, because the activity of steroid estrogens has multifunctional character and one compound may be effectively used in various fields. This is a main reason why modified steroid estrogens have the advantage in comparison with huge number of heterocyclic compounds, having more selective action.

Obviously that most perspective search of such compounds is done on the basis of knowledge about estrogen action mechanism, particularity of its structure and metabolism. Several authors consider that main fast non genomic effects of estrogens are mediated by their membrane receptors (**Levin, 2002; Sak & Evaraus, 2004).** Experimental data about the identity of nuclear and membrane ERα in MCF7 cell are presented in the publication (**Pedram et al., 2009**). Obviously, ligand specificity may be significantly different. Mechanisms of membrane receptors action are still unclear, very often it is not proved that one or another effect is mediated by namely this group of receptors (**Warner & Gustafsson, 2006**). The absence of data about ligand specificity to membrane receptors does not allow to plan the synthesis of modified steroids with selective action, especially taking into account that their activity is realizing on many ways. During the consideration of osteoprotective action of estrogens and its influence on processes of cardio-vascular system we restrict ourselves to state the facts, because the decision concerning the synthesis of potential agents is usually reached on the analogy with known compounds.
