**1. Introduction**

Estrogen plays an important role in mammary gland development and has been implicated in the initiation and progression of breast cancer [1]. There are two major receptors with which this sex hormone binds to mediate its biological activities. These receptors are estrogen receptors, alpha and beta (ERα and ERβ). ERα is present mainly in mammary gland, uterus, ovary (thecal cells), bone, male reproductive organs (testes and epididymis), prostate (stroma), liver, and adipose tissue. By contrast, ERβ is found mainly in the prostate (epithelium), bladder, ovary (granulosa cells), colon, adipose tissue, and immune system. Both subtypes are

markedly expressed in the cardiovascular and central nervous systems. The alpha subtype has a more prominent role on the mammary gland and uterus, as well as on the preservation of skeletal homeostasis and the regulation of metabolism. The beta subtype seems to have a more profound effect on the central nervous and immune systems [2]. In terms of sequence homology, the ERβ shows a high homology to ER<sup>α</sup> in the DBD (more than 95% amino acid identity) and in the LBD (~55% amino acid identity) [3, 4]. However, the NTD of ERβ is shorter than that of ERα with a very poor sequence homology of only *~*15% compared to that of ERα (**Figure 1A** and **B**).

The major ER subtype is the ERα which has been reported in about 70% of breast cancer cases [7]. In addition to the well-studied nuclear functions of ERα, it also participates in extranuclear signaling which involve growth factor signaling components, adaptor molecules and the stimulation of cytosolic kinases [8]. ERα extranuclear pathways have the potential to activate gene transcription, modulate cytoskeleton, and promote tumor cell proliferation, survival, and metastasis. Inhibition of ERα extranuclear actions is, thus, a promising strategy to curb breast tumor progression and may be useful in preventing ERα positive metastasis.

Commonly used endocrine therapies include: selective estrogen receptor modulators (such as tamoxifen, raloxifene, toremifene), aromatase inhibitors (such as anastrozole, letrozole and exemstane) and selective estrogen receptor downregulators (such as fulvestrant). Unfortunately, tumor cells readily develop resistance to these therapies in a progressive manner, a major obstacle limiting the success of breast cancer treatment. Resiatance may be de novo or acquired and has been shown to be influenced by complicated crosstalks. These resistance to available therapies combined with their undue toxicities provoke the search into small molecules from plant, deemed

#### **Figure 1.**

*Sequence organization of estrogen receptors, ERα and ERβ (A and B) and the 3D structures of studied ligands (C). (A) Shows different domains highlighted in different colors: NTD = amino terminal domain (in red); DBD = DNA binding domain (in green); hinge region in blue; LBD = ligand-binding domain (in yellow); F region located towards the C-terminal end (in gray). Amino acid sequence position is given for each domain. (B) Shows 3-dimensional structures of ERα (left) and ERβ (right) bound to estradiol (PDB structures 1A52 [5] and 3OLS [6]. (C) Shows 3D structures of all studied ligands.*

#### *Small Molecules Inhibit Extranuclear Signaling by Estrogen: A Promising Strategy to Halt Breast… DOI: http://dx.doi.org/10.5772/intechopen.94052*

to be less toxic [9] which can destabilize and/or downregulate the commonly implicated estrogen receptor (hERα) in a bid to intercept the complicated crosstalks [10].

There are critical steps in the development of effective pharmacotherapy, with many phases and stages within each of them. The first step which is discovery and development involve target discovery and validation, lead refinement as well as preclinical development. This is often followed by preclinical research, which tests the new drug on non-human subjects for efficacy, toxicity, and pharmacokinetic (PK) information with unrestricted dosages. Preclinical research involves *in vivo*, *in vitro*, *ex vivo* and *in silico* assays. The next step is the clinical development and involves clinical trials and volunteer studies to fine-tune the drug for human use before submitting for a holistic FDA review. The final critical step is the FDA post-market safety monitoring. It cost so much before a suitable drug candidate finally gets to the market and failure which frequently taunts the process can better be imagined than experienced.

Several promising drug candidates have failed to reach the market due to their poor pharmacokinetic properties. Many compounds with promising pre-clinical medicinal properties may not even stand a chance of being tried because of their non-drug-likeness except after rigorous improvement which may end up increasing toxicity. Today, with the advancement in medical and pharmaceutical sciences, computational techniques has proven useful for early prediction of the absorption, distribution, metabolism, excretion and toxicity (ADMET) profile of potential drug molecules before subjecting them to rigorous pre-clinical and clinical testings [11]. *In silico* approaches like molecular docking has been successfully applied in the screening and selection of potent drugs in the treatment of diseases [12]. These techniques are now extensively employed by pharmaceutical companies for screening for lead compounds to facilitate entrance of potential drug molecules with good drug-likeproperties into the market while eliminating molecules with poor profile [13]. The purpose of this study was to investigate the pharmacokinetic properties and druglikeness of the selected small molecules and investigate its inhibitory potential to the ERα, with the view of mitigating this sex hormone's receptor extranuclear signaling.
