key: cord-0036672-0lkslnd3
authors: Gore, Manish; Puranik, Amita; Indurkar, Abhishek; Sonowal, Bismita; Devarajan, Padma V.; Jain, Ratnesh; Dandekar, Prajakta
title: Cancer of Reproductive System: Receptors and Targeting Strategies
date: 2019-07-25
journal: Targeted Intracellular Drug Delivery by Receptor Mediated Endocytosis
DOI: 10.1007/978-3-030-29168-6_4
sha: 8e379df10bdc373c8494ce5656be5718127c60c9
doc_id: 36672
cord_uid: 0lkslnd3

Carcinogenesis in the different organs of the reproductive system, particularly, prostate, ovarian, and cervical tissues, involves aberrant expression of various physiological receptors belonging to different superfamilies. This chapter provides insights into the physiological receptors that are associated with the genesis, progression, metastasis, management, as well as the prognosis of the cancers of the male and female reproductive systems. It also highlights the structural and binding characteristics of the highly predominant receptors, namely, androgen, estrogen, progesterone, and gonadotropin-releasing hormone (GnRH) receptors, which are overexpressed in these cancers and discusses various strategies to target them.

Cervical and ovarian cancers are the fourth most and the seventh most common cancers in women, with a global prevalence of approximately 3.7 percent and 1.7 percent, respectively [1] [2] [3] . The prostate cancer is considered as the fifth leading cause of cancer-associated mortality in men, with a global prevalence of about 7.9 percent [3, 4] . The risk factors for cancers related to reproductive system include but not limited to endogenous factors such as genetic history, race, aging, hormonal imbalance, and exogenous factors such as inappropriate diet, unhealthy lifestyle, and environmental and occupational factors. Moreover, cervical cancer risk factors are extended to the infection of human papilloma virus (HPV), extended usage of contraceptives, age of menarche and menopause, unsafe sexual activities such as sexual intercourse at an early stage and multiple sexual partners [1, [5] [6] [7] [8] [9] [10] [11] [12] . This chapter provides a detailed overview of the structural, pharmaceutical, and clinical aspects of the agents discovered to target the dominant receptors involved in the development, treatment, and prognosis of the reproductive neoplasia. Table 4 .1 lists the receptors that are ubiquitous in various cancers of the reproductive system, specifically during their prognosis, diagnosis, progression, and therapy.

The subsequent discussion is focused on four principal receptors that display a significant expression pattern during the genesis, diagnosis, treatment, and prognosis of the reproductive neoplasia. [13] [14] [15] P2 receptors: P2X7 [13, 14] P2 receptors: P2X4, P2X5, P2X7 [13, 14] NMDA (N-methyl-Daspartate) receptor [16] GABAa receptor [17] G-protein-coupled receptors (GPCRs) (metabotropic) Endothelin-1: (ET A R) [18] [19] [20] Protease-activated receptor-2 (PAR-2) [21] Gastrin-releasing peptide receptor (GRPR) [22, 23] G-protein-coupled estrogen receptor-1 (GPER-1) -prognostic maker for early-stage cancer [7] Folate receptor subtype alpha (FRα) [24] Endothelin-1: (ET A R) [19, 25, 26] Protease-activated receptor-1 and 2 (PAR 1 and 2) [27, 28] β-Adrenergic receptor [29] Gastrin-releasing peptide receptor (GRPR) [30, 31] G-protein-coupled estrogen receptor-1 (GPER-1) [32, 33] Folate receptor -FRα and Reduced Folate Carrier (RFC) [34] Follicle-stimulating hormone receptor (FSHR) Luteinizing hormone receptor (LHR) Gonadotropinreleasing hormone receptor (GnRHR) Thyroid-stimulating hormone receptor (TSHR) Kisspeptin receptor Angiotensin II type 1 receptor [2] Serotonin (5-HT) receptors -5-HTR1A, 5-HTR1RB, 5-HTR2B, 5-HTR4 [35] Endothelin-1: (ET A R) [19, 36, 37] Protease-activated receptor-1, 2, and 4 (PAR-1, 2, and 4) [38] [39] [40] β-Adrenergic receptor [29, 41] Gastrin-releasing peptide receptor (GRPR) [31, 42] G-protein-coupled estrogen receptor-1 (GPER-1) [33] Prostate-specific G-protein-coupled receptor (PSGR)-PSGR2 [43, 44] G-protein-coupled receptor-158 (GPR158) [45] Lysophosphatidic acid (LPA)-1 receptor [46] Gonadotropin-releasing hormone receptor (GnRHR) [47] Serotonin (5-HT) receptors -5-HTR1A, 5-HTR1RB, 5-HTR2B, 5-HTR4 [48, 49] (continued) M. Gore et al.

Nuclear receptors comprise a family of transcription factors that get activated due to the binding of the lipophilic ligands, to carry out reproduction, development, homeostasis, and metabolism. They act by responding to the signals generated by the steroid hormones and regulate the expression of the target genes [75] [76] [77] .

The organs of the reproductive system serve as the primary sites of action of sex steroid hormones, such as the estrogen, progesterone, and androgen. These hormones are responsible for mediating the developmental activities and physiological Human epidermal growth factor receptor (HER-2)/neu [50] Epidermal growth factor receptor (EGFR) [51] Insulin-like growth factor-I receptor (IGF-IR) [52] Vascular endothelial growth factor (VEGF) receptor [53] Prolactin receptor (PRLR) [54] Hepatocyte growth factor/(cMET) [55] Fibroblast growth factor receptor (FGFR) [56] HER-2/neu receptor [57] Epidermal growth factor receptor (EGFR) [58] Insulin-like growth factor-I receptor (IGF-IR) [59] Vascular endothelial growth factor (VEGF) receptor [53] Hepatocyte growth factor/(cMET) [60] Fibroblast growth factor receptor (FGFR) [61] HER-2/neu receptor [62] Epidermal growth factor receptor (EGFR) [63] Insulin-like growth factor-I receptor (IGF-IR) [64] Vascular endothelial growth factor (VEGF) receptor [65] Hepatocyte growth factor/(cMET) [60] Fibroblast growth factor receptor (FGFR) [ [67] Retinoic acid receptor [70] Androgen receptor (AR) [71] Estrogen receptor (ERβ) [72] Progesterone receptor (PR) [73] Peroxisome proliferatoractivated receptor-Gamma (PPARγ) [74] 114 functions of the male and female reproductive systems. They exert their functions through the action of steroid hormone receptors, namely, the estrogen (ER), progesterone (PR), and androgen (AR) receptors, respectively. Aberrations in their expression and/or in the factors regulating them, termed as coregulators, lead to either activation or suppression of their transcription machinery, eventually impacting their physiological functions. These abnormalities trigger a cascade of pathological changes in vivo, thereby resulting into carcinogenesis [78, 79] . ER-subtype α (ERα) and PR receptors play a pivotal role in the pathophysiology of cervical cancer. The PRs were found to exhibit tumor-suppressive properties in cervical cancer [1, 7] . In the case of ovarian malignancy, ERα expression provided a better prognosis, while the role of ERβ was insignificant. On the other hand, an elevated PR expression was observed to improve the survival rate in patients with endometrial ovarian cancer (EOC) [80] . The activity of AR has been closely linked to the prostatic carcinogenesis. The biochemical pathway of AR, the principal regulator of prostatic cancer, is perturbed during the carcinogenesis. Castration-resistant prostate cancer (CRPC), an advanced stage of the disease which is nonresponsive to hormone deprivation therapy, occurs due to increase in sensitivity of the AR to the agonists, mutations in the receptor, ligand-independent activation of the ARs, etc. [71] 

GnRH-I, produced in vivo, by the GnRHR (located in the hypothalamus) stimulates the secretion of the gonadotropins, namely the luteinizing hormone (LH) and the follicle-stimulating hormone (FSH), which further regulate the in vivo levels of sex hormones [81] . The GnRHR is also expressed in the CRPC [82] . Primary cultures of ovarian carcinomas and biopsy specimens of malignant ovarian tissue have revealed the predominant expression of the GnRHR receptor (>80%), signifying its role in the genesis of malignancy and metastasis. The GnRHR has also been speculated to be associated with the early phases of ovarian carcinogenesis, including cell migration and invasion [83] . It was observed that administration of GnRH-1 agonists and antagonists lead to the downregulation and inactivation of GnRHR, respectively. As a result, the GnRH agonists cause inhibition of cell proliferation, metastasis, and angiogenesis. Moreover, the GnRH antagonists also possess antineoplastic activity [81, 82] . Thus, exploration of the structure and regulation of the GnRHRs in cancers of the reproductive system may enhance their applicability, as a target receptor, for the discovery of new-age anticancer therapeutics.

Understanding the structure and the binding chemistries of these receptors and reviewing the potential of targeting it may pave the way to the discovery of the breakthrough anticancer therapies in the near future.

Androgen receptor (AR), a 110-kDa protein, is a ligand-activated transcription factor belonging to the family of steroid hormone nuclear receptors [71] . AR, expressed in prostate, is activated by binding of endogenous androgens, such as testosterone and 5α-dihydrotestosterone (5 α-DHT). Functional AR is responsible for in vivo male sexual differentiation and occurrence of pubertal changes [84] . This receptor mediates normal growth and development of the prostate gland and also plays a vital role in the prostatic carcinogenesis and its progression to an androgen-independent disease. Androgen-independent stage of prostate cancer (e.g., CRPC) has been observed due to the activation of the AR receptor by overexpression of gene and cofactors, gene mutations, splice variants, and intracrine synthesis of androgen [85, 86] . In addition, AR is also expressed in the different subtypes of ovarian and cervical cancers [87, 88] .

The AR modular protein consists of four distinct domains, namely, the ligandbinding domain (LBD), the hinge domain, the DNA-binding domain (DBD), and the N-terminal transcriptional regulation/amino-terminal domain (NTD). NTD is the most variable region, whereas DBD and LBD are highly conserved among different receptors of the steroid hormone nuclear receptor family. The LBD, the key recognition domain of AR, is arranged in three layers and comprises eleven α-helices, particularly, H1-H11, except H2, which results into formation of an antiparallel "α-helical sandwich." The ligand-binding pocket (LBP) is formed by H5, C-terminal of H10 and H11, and N-terminal of H3. The activation function (AF)-1, located at the N-terminal, is not conserved in the sequence and is ligand-independent, whereas AF-2, at the C-terminal, is conserved and ligand-dependent. The nuclear localization signal (NLS) is located between the DBD and the hinge region [71, 89, 90] .

The Genomic Pathway of AR Unbound AR exists in an inactive state in the cytoplasm, in complexation with the heat shock proteins (HSP) such as HSP90. Upon binding to a ligand/agonist, it gets activated and dissociates from the HSP and undergoes dimerization and phosphorylation. Binding of an agonist leads to the formation of the AF-2 region on the surface of LBD. Upon ligand binding, AF-2 interacts with the amino-terminal motifs of the receptor, which leads to an establishment of N/C intradomain crosstalk, thereby leading to receptor stabilization, enhanced DNA-binding affinity, and reduced ligand dissociation, a phenomenon exclusively observed in the AR, unlike other steroid receptors. AF-2 domain also recruits coregulatory proteins to an activated AR, thereby contributing to its overall function. H12, the core structure of AF-2, acts as a lid to close LBP, upon binding of the agonist. Further, the NLS gets exposed upon the ligand/agonist binding and interacts with importin-α. This leads to translocation of AR from cytoplasm to the nucleus. DBD facilitates interaction of the translocated receptor with the DNA at specific recognition sites. These sites, located in the promoter and enhancer regions of the AR target genes, consist of consensus sequences and are termed as androgen response elements (ARE). Access of AR to the target chromatin requires concerted action of certain transcription factors. AF-1 and AF-5 of NTD mediate the transcriptional activity by recruitment of coactivator complexes and transcription machinery, essential to regulate the expression of the target genes. Selective recognition of specific ARE sequences is regulated by the ligand-binding and/or presence of other transcription factors [84, [89] [90] [91] . Figure 4 .1a provides a schematic layout of the structure of the steroid receptor (SR) and its pathway of transactivation after binding of the ligand.

Ligands modulate their action by binding to the LBP of LBD. The AR is capable to accommodate a large variety of ligands by modifying the volume of LBP [71] . Testosterone ( Fig. 4 .2a) and DHT ( Fig. 4 .2b) are the physiological ligands of the AR [89] . Carbon atoms of the testosterone skeleton have been numbered in order to provide basis for the SARs with various ligands and their derivatives [71] . Synthetic derivatives of testosterone have been prepared to enhance oral bioavailability. It is essential for the ligand (natural/synthetic) to contain a steroidal skeleton for retaining the androgenic activity. Hydrophobic amino acid residues in the LBP interact with the steroid scaffold. A/B ring junction usually has "trans" stereochemistry. 17β-OH atom is essential for ligand-receptor interaction via formation of a hydrogen bond with the amino acid residues [71, 89] . 

Estrogens naturally occur in three different forms in females, namely, estradiol (E2) ( Fig. 4 .2d), estriol, and estrone. It exerts its action through the estrogen receptors (ERs), which belong to the steroid hormone superfamily of nuclear receptors. ERs, occurring in two forms, ER-α and ER-β, act as ligand-activated transcription factors, upon binding to the endogenous ligands [92] [93] [94] . Expression of estrogen and its receptors have been very well documented in prostate, ovarian, and cervical cancers. ER-α and ER-β have been reported to play oncogenic and antioncogenic roles in the pathogenesis of prostate cancer. The aberrant expression of the enzymes involved in steroid biosynthesis and metabolism, such as aromatase and 5-α reductase, has also been implicated in prostatic malignancy [72, 95] . This chapter will further provide insights into the structural aspects of the receptor and its SAR with the ligands.

ER-α (66-kDa protein) is predominantly expressed in the reproductive tract, whereas ER-β (54-kDa protein) primarily occurs in the vascular endothelial cells, bones, and male prostatic tissues. The ER receptor consists of an NTD, DBD, hinge region, and LBD. The NTD stimulates transcription from particular estrogen-responsive promoters via AF-1. DBD binds to estrogen response elements (ERE) in the target DNA, while the hinge region contains nuclear NLS. The LBD and AF-2 activate the gene expression in response to ligand binding. The classical genomic pathway results in the formation of estrogen-dependent, nuclear ER homo-or heterodimers, such as ER-α/ER-α, ER-β/ER-β, and ER-α/ER-β, respectively. Further, these dimers subsequently bind to the estrogen response element (ERE) sequences located in the promoter region of estrogen-responsive genes, resulting in the recruitment of coregulatory proteins (coactivators or corepressors) to the promoter. This leads to either an enhancement or reduction in the mRNA levels, further impacting the production of associated proteins and eventually the physiological response [89, 96] .

A vast array of compounds acting as ligands for the ER-α and ER-β receptors have been classified as endo-estrogens (E2), phytoestrogens (Resveratrol), xenoestrogens (Mestranol), metalloestrogens (copper (Cu 2+ ), etc. [89, 97, 98] .

Recognition of the binding of endo-estrogen (E2) to ER is achieved partly by intermolecular hydrogen binding and van der Waals interactions with the receptor [97] . The aromatic A ring, C-3 and 17β hydroxyl groups and the distance between them, and planar hydrophobic structure are essential for estrogenic activity. On the other hand, substitution at C-1, hydroxylation at C-6, 7, and 11, removal of oxygen from C-3 or C-17, and epimerization of 17-β-hydroxyl group of E2 to α-configuration lead to reduction in the activity [89, 99] .

Progesterone (P4; represented in Fig. 4 .2e) and progesterone receptors (PRs) are necessary for the development of hormone-responsive tissues such as breasts and other organs of the reproductive tract. It is responsible for ovulation, embryo implantation, pregnancy, development of the mammary gland, and sexual differentiation and behavior. The hormone inhibits the proliferative action of estrogen in the reproductive tissues, such as endometrium and ovary, thus preventing them from undergoing neoplastic transformation [100, 101] . PRs, existing as two isoforms, namely PR-A ((molecular weight of 94 kDa) and PR-B (molecular weight of 114 kDa), are ligand-activated transcription factors, belonging to the superfamily of steroid hormone nuclear receptors. PR-B is referred to as full length and dominant receptor, while PR-A is the N-terminal truncated version [102, 103] . Both the isoforms suppress proliferation of the prostate stromal and cervical cancer cells [1, 104] . PR overexpression is associated with favorable prognosis in women with ovarian malignancies [105] .

The PR receptors share common structural elements with other steroid receptors, namely, NTD, DBD, hinge region, and LBD. The NTD is responsible for ligandindependent transcriptional activation and harbors a highly variable AF-1 domain. The DBD binds to the progesterone response elements (PREs) located in the target DNA. The hinge region contains NLS, while the LBD and a highly conserved AF-2 domain are responsible for the ligand-mediated transactivation of the gene expression, via the genomic pathway. The LBD or the primary recognition domain comprises a hydrophobic LBP, to facilitate ligand binding. The genomic pathway operates on binding of P4, which causes conformational change in the PR, thereby transforming it into an active transcriptional factor. As a result, receptor phosphorylation occurs and the PR undergoes dimerization (homo/hetero) and nuclear translocation, to further interact with the PREs. This also leads to the recruitment of coactivators that mediate gene transcription. It has been reported that PR-A and PR-B possess opposite transcriptional activities and the overall response of P4 is dependent on the relative in vivo levels of PR-A and PR-B. 5α-reductase and 20α-hydroxysteroid dehydrogenase are responsible for the metabolic conversion of P4 to a more active or less active form, before interaction with the receptors in the target cells [89, 102, 106] .

P4 is an endogenous ligand of the PR receptors. The progestin activity is confined to the molecules having a steroid nucleus. The synthetic progestins have been categorized into two classes, namely, the androgens (19-norandrostane or estrane derivatives), and 17 α-hydroxyprogesterones. In case of the compounds belonging to the androgen category, 17α-substituents like ethyl, methyl, etc., lead to increase in the oral bioavailability (e.g., Ethisterone). Removal of the methyl group at C19 position and chlorination at C21 or methylation at C18 provided norethisterone, whose activity was further enhanced by chlorine substitution at C21 or by the addition of methyl group at C18 (e.g., Norgestrel). Acylation of 17β-hydroxyl group of Norethisterone extended the duration of its action. Synthetic progestins include medroxyprogesterone acetate and norethisterone (first generation), norgestrel and levonorgestrel (second generation), etonogestrel and nosgestimate (third generation), drospirenone and trimegestone (fourth generation), etc. [89] .

The GnRH (Fig.4.2f) is a decapeptide that plays a pivotal role in regulating the reproductive functions by functioning through the hypothalamic-pituitary gonadal (HPG) axis. The action of GnRH is mediated by the action of GnRHR, which belongs to the rhodopsin-like GPCR superfamily. GnRHR is expressed in various reproductive cancers, such as the prostate, ovarian, endometrial, and the breast cancer, as well as the nonreproductive cancer types. In these tumors, the GnRH functions as a paracrine-autocrine growth factor and displays a strong anticancer activity. The GnRHR (for 3D structure, refer to Flanagan C.A. et al.) consists of seven transmembrane (TM) domains, as well as an extracellular amino-terminal domain that contains 35 amino acids, along with two putative glycosylation sites. However, it lacks the carboxy-terminal cytoplasmic tail resulting in slow internalization and desensitization of the receptor. The membrane-spanning segments are highly conserved, while the loops and the termini constitute to be the variable regions [47, 82, 107, 108] .

The receptor (R) exists in an equilibrium between an inactive R, which does not activate G proteins and is stabilized by an antagonist, and an active R * conformation, which activates G proteins and is stabilized by agonists, depending on the presence or absence of the ligand. The ligands of the receptor interact with the variable, extracellular half of the receptor molecule. The membrane-spanning receptor domain transmits the signal generated upon ligand binding to the cytosolic receptor surface, which further interacts with the G protein. The GnRHR must exist in a silent state that does not activate the G protein, in order to transduce the signals mediated by agonists across the cell membrane. Binding of the agonist causes transition from the silent state and leads to the binding and activation of G proteins, situated on the opposite side of the cell membrane. Thus, agonists like GnRH act as allosteric activators of the receptor. The primary features of the inactive form of the receptor include closed G-protein-binding pocket, a hydrogen-bonding network, and a hydrophobic barrier. Binding of the ligand activates the receptor, causing rotation and change in the interfaces of specific TM segments, leading to opening of the hydrophobic barrier, movement of specific amino acid residues toward the interior of the TM bundle, and ultimately opening of the cytoplasmic surface cleft that allows contact and binding of the G-proteins [108] . The structure of the recognition domain or LBD of the receptor, involved in binding, depends on the type of the ligand such as a neurotransmitter, a glycoprotein hormone, and a peptide. In the case of a neurotransmitter, the TM domains (TMDs) themselves form the LBP to facilitate ligand binding. The amino-terminal domain of the receptor, encompassing the high-affinity ligand-binding site, has been reported to be recognition domain for glycoprotein hormones. A high-affinity binding site for the peptide-based ligands include both extracellular and TM residues [109] .

Binding of the hormone causes coupling of GnRHR to G αq/11 protein, which stimulates the phospholipase Cβ (PLCβ) activity. This leads to the enhancement of intracellular levels of inositol triphosphate (IP3) and diacylglycerol (DAG), further causing intracellular mobilization of Ca 2+ ions and activation of the protein kinase C (PKC). These downstream effects lead to the activation of various signaling pathways, which operate through the MAPK reaction cascade. Phospholipase D and phospholipase A2 are also activated in a sequential manner. These biochemical pathways are vital in eliciting the GnRH-mediated downstream effects, such as gonadotropin synthesis and secretion [47] . Figure 4 .1b provides a schematic representation of the GnRHR, a GPCR, and the intracellular pathway activated upon binding of GnRH to the receptor.

Amino-and carboxy-terminal residues are critical for receptor binding and activation. The presence of achiral glycine (Gly) or D-amino acids is essential at position 6, to facilitate active conformation in the folded state. His 2 , Trp 3 , as well as pGlu 1 of the GnRH decapeptide possess significant roles in receptor activation. Substitution of amino acid residues located outside the amino-terminal domain is speculated to affect the receptor activation, due to the conformational changes in the ligand that may occur upon binding to the receptor [110] . The subsequent section of the chapter highlights the approaches explored for receptor targeting as well as an overview of mechanisms involved therein. Table 4 .2 provides an overview of drug molecules developed to target receptors predominantly expressed in the cancers of the reproductive system. Fig.4 .3 provides a schematic layout of the classification of the agents (ligands, agonist, antagonists, modulators, etc.), either endogenously present and/or synthesized for targeting to the aforementioned receptors.

Several mechanisms employed for targeting receptors predominantly expressed in the cancers of reproductive system have been summarized and supported by relevant case studies in this section. [112] [113] [114] [115] [116] [117] [118] 3 Natural agents targeted to AR Drugs isolated from natural products have also been found to have a substantial impact on reducing the growth of prostate cancer. Natural drug products are usually preferred as they are regarded as safe and effective in cancer treatments and exhibit lower side effects than the synthetic counterparts.

Selenium, Soy isoflavones, Epi-brassinolide (EBR), Curcumin, etc.

[119] (continued) 6 Aromatase inhibitors (AIs) They are useful either as a mono or as a combination therapy and act by blocking the synthesis of estrogen by inhibiting the aromatase activity.

Letrozole (Femara®), Anastrozole (Arimidex®) [120, 121] 7 Selective progesterone receptor modulators (SPRMs) SPRMs act as partial agonists/antagonists and are majorly employed for emergency contraception, termination of pregnancy, dysmenorrhea, premenstrual syndrome, etc. However, due to the associated safety concerns, their usage has been discontinued.

Asoprisnil, Ulipistal acetate (UPA), Vilaprisan [122] 8 Progesterone receptor antagonist (PRA) PRAs or antiprogestins compete with P4 for its receptor and eventually prevent the latter from binding to and activating the receptor.

Mifepristone, Onapristone [89, 123] 9 GnRHR agonists They act by interfering with the activities of the epidermal growth factor (EGF) and insulin-like growth factor 1 (IGF-1) and inducing cell apoptosis. These agents are 60 times more potent than the native molecule. They are characterized by prolonged half-life and enhanced receptor binding, in comparison to the natural ligand. These agonists have been utilized to treat advanced prostate cancer along with management of ovarian and endometrial cancers. Fig. 4.3 Overview of the classification of several classes of agents, either endogenous or synthesized and used for targeting nuclear receptors and GPCRs, predominantly expressed in the cancers of the reproductive system M. Gore et al.

The limitations of chemotherapy, such as toxicity and lack of selectivity, can be addressed with different approaches that selectively target the existing drugs to the malignant cells and through the development of nontoxic forms of anticancer agents, which may be specifically activated in the tumor tissues. Selective activation of prodrugs into active anticancer agents, in the vicinity of tumor tissues, can be mediated either by metabolic activity or by spontaneous chemical breakdown.

Investigations have been carried out for targeting advanced mCRPC by prodrug approach by means of targeting prostate cancer-specific antigens such as prostatespecific antigens (PSA), prostate-specific membrane antigens (PSMA), CD147, heat shock proteins (HSPs), leutinizing hormone-releasing hormone (LHRH) receptor, epithelial cell adhesion molecules, etc., and prostate-specific enzymes such as cathepsin or matrix metalloproteinase. Numerous PSMA-targeting molecules, which include monoclonal antibodies (mAbs), antibody-drug conjugates (ADCs), antibody fragments (Fabs), peptides, and aptamers, have been developed in the form of prodrugs or nanoparticles. In recent times, a prodrug, namely vinblastine-Noxide (CPD100), was developed for the treatment of ovarian cancer by Cascade Prodrug, a US-based pharmaceutical company. This compound, formulated as sphingomyelin-cholesterol liposomes, is converted into its parent compound, vinblastine, under hypoxic conditions. This formulation was found to be highly successful in the preliminary studies and if successful in clinical trials, it will be the first-of-its-kind of chemotherapeutic agent to demonstrate anticancer activity due to the hypoxic microenvironment present in the solid tumors [126] [127] [128] .

Malignancies specific to prostate gland can also be controlled by indirect targeting of AR-targeted drugs such as agonists, antagonists, partial agonists, and antagonists. Indirect targeting approach can also be considered such as targeting small molecules like 17α-hydroxylase involved in the de-novo synthesis of androgens by drugs such as Abiraterone. Abiraterone, in combination with prednisone, was approved in 2012 as the first-line therapy against mCRPCs, before commencing the administration of the conventional chemotherapy. Tamoxifen is the oldest and the most extensively studied SERM. It is a prodrug with low ER affinity, and gets converted into active metabolites such as endoxifen or afimoxifene through metabolism in the liver. These active metabolites demonstrate 30-100 times greater affinity toward the target receptors, as compared to tamoxifen. Substances involved in the synthesis of estrogen, such as aromatase, can also be considered as alternative targets for the treatment of ovarian cancers. The AIs blocks the protein involved in estrogen synthesis, but their usage is limited for the postmenopausal women, as they do not inhibit estrogen synthesis in the ovaries of the premenopausal women. Fulvestrant (Faslodex®), Anastrozole (Arimidex®), Letrozole (Femara®), and Exemestane (Aromasin®) are examples of the AIs, which are useful either as a monotherapy or combinatorial therapy, during the treatment of ovarian cancer [121, [129] [130] [131] .

Gene therapy has been explored as one of the advanced ER-targeting strategies.

Recently, an innovative effort provided a strong confirmation that enhanced estrogen signaling is responsible for the growth of the cervical tumor. Enhanced expression of Cyclin D1, ERs, and aromatase is significantly associated with the tumor growth. Hence, blocking the estrogen pathway, particularly to decrease the ER activity, can be a rational approach to activate p53 and retinoblastoma protein (pRb), along with lowering the expression of HPV E6 and E7. This approach has been utilized to block ER-mediated tumor growth in cervical cancer cells, by transfection using adenovirus (AD) as a gene carrier [132] .

Different approaches of drug delivery are being employed to target receptors expressed in reproductive neoplasia. Promising results of raloxifene in clinical trials has suggested for an improvement in the efficacy of this drug by exploring novel delivery systems like nanoformulations or through the development of raloxifene analogs. Raloxifene has been effectively encapsulated in nanoparticles, such as styrene-maleic acid (SMA) micelles, which demonstrated superior pharmacokinetic profile than the free drug [95, 133] . As discussed in Table 4 .2, Progesterone and GnRH receptors can be directly targeted by means of agonists, antagonists, or partial agonists and antagonists. Extensive research is in progress for finding numerous other potential approaches for targeting these receptor for the treatment of malignancies related to organs of reproductive system. 

Carcinogenesis in the different organs of the reproductive system, particularly, prostate, ovarian, and cervical tissues, involves aberrant expression of various physiological receptors belonging to different superfamilies. Structural and pharmacological role of four predominant receptors, namely, AR, ER, PR (sex steroid nuclear receptors), and GnRHR (GPCRs) has been highlighted in this chapter. Moreover, strategies and molecules developed for targeting these receptors, for formulating clinically relevant anticancer therapeutics, have been put forth and supported by the ongoing preclinical and clinical studies. We speculate that a combinatorial therapy comprising receptor-targeted ligands/agents, with clinically acceptable cytotoxic drugs, as well as targeting moieties such as antibodies (antibody-drug conjugates) and use of nano-and novel carriers for drug delivery, will enhance the overall antineoplastic effect. Considerable research has been conducted in deciphering the role of the AR and GnRHR in the cancers of the reproductive system and efforts to target these receptors have been commenced. We anticipate similar investigations to be conducted for ER and PR receptors in the near future.

Targeting female hormone receptors as cervical cancer therapy

The role of endocrine G proteincoupled receptors in ovarian cancer progression

Data on specific cancers: World Cancer Research Fund International

HER2 gene amplification in patients with prostate cancer: evaluating a CISH-based method

The global burden of women's cancers: a grand challenge in global health

Cervical cancer

The G protein-coupled estrogen receptor (GPER/GPR30) may serve as a prognostic marker in early-stage cervical cancer

The role of reproductive hormones in epithelial ovarian carcinogenesis

Endogenous hormones and ovarian cancer: epidemiology and current hypotheses

The genesis and evolution of high-grade serous ovarian cancer

Epithelial ovarian cancer. Risk factors, screening and the role of prophylactic oophorectomy

Human prostate cancer risk factors

P2 receptors and cancer

Understanding the roles of the P2X7 receptor in solid tumour progression and therapeutic perspectives

The human papilloma virus-ion channel link in cancer: an alternative opportunity for diagnosis and therapy. Human papillomavirus and related diseases-from bench to bedside-research aspects

N-methyl-D-aspartate receptor in human prostate cancer

Expression of gamma-aminobutyric acid receptor (subtype A) in prostate cancer

Expression of endothelin 1 and endothelin A receptor in HPV-associated cervical carcinoma: new potential targets for anticancer therapy

Endothelin B receptor, a new target in cancer immune therapy

Targeting endothelin receptor type A in human cervical carcinoma cells

Protease-activated receptor-2 (PAR-2) in cervical cancer proliferation

Gastrin-releasing peptide receptor expression in cervical cancer

The gastrin-releasing peptide receptor as a marker of dysplastic alterations in cervical epithelial cells

Folate receptor alpha is associated with cervical carcinogenesis and regulates cervical cancer cells growth by activating ERK1/2/c-Fos/c-Jun

Expression of endothelin 1 and endothelin A receptor in ovarian carcinoma: evidence for an autocrine role in tumor growth

Emerging role of the endothelin axis in ovarian tumor progression

Potential importance of protease activated receptor (PAR)-1 expression in the tumor stroma of nonsmall-cell lung cancer

Role of protease activated receptor-2 in tumor advancement of ovarian cancers

Adrenergic system, a backstage manipulator regulating tumour progression and drug target in cancer therapy. Seminars in cancer biology

Overexpression of gastrin-releasing peptide receptors in tumor-associated blood vessels of human ovarian neoplasms

Gastrin-releasing peptide receptor as a molecular target in experimental anticancer therapy

Minireview: G protein-coupled estrogen receptor-1, GPER-1: its mechanism of action and role in female reproductive cancer, renal and vascular physiology

Function of G-protein-coupled estrogen receptor-1 in reproductive system tumors

Paradoxical impact of two folate receptors, FRα and RFC, in ovarian cancer: effect on cell proliferation, invasion and clinical outcome

Expression of serotonin receptors 5-HT1A, 5-HT1B, 5-HT2B and 5-HT4 in ovary and in ovarian tumours

Expression of endothelin receptor a associated with prostate cancer progression

Endothelin-1 inhibits apoptosis in prostate cancer

Overexpression of protease-activated receptors-1,-2, and-4 (PAR-1,-2, and-4) in prostate cancer

Increased expression of protease-activated receptor 4 and trefoil factor 2 in human colorectal cancer

Protease-activated receptor 1: a role in prostate cancer metastasis

β-adrenergic receptor signaling in prostate cancer

Immunohistochemical analysis of gastrin-releasing peptide receptor (GRPR) and possible regulation by estrogen receptor βcx in human prostate carcinoma

Prostate specific G protein coupled receptor is associated with prostate cancer prognosis and affects cancer cell proliferation and invasion

PSGR2, a novel G-protein coupled receptor, is overexpressed in human prostate cancer

Expression and functional role of orphan receptor GPR158 in prostate cancer growth and progression

Expression and function of lysophosphatidic acid LPA1 receptor in prostate cancer cells

Gonadotropin-releasing hormone receptors as molecular therapeutic targets in prostate cancer: current options and emerging strategies

Expression of serotonin receptors 2B and 4 in human prostate cancer tissue and effects of their antagonists on prostate cancer cell lines

Expression of serotonin receptors and role of serotonin in human prostate cancer tissue and cell lines

HER2 as a novel therapeutic target for cervical cancer

Epidermal growth factor receptor as a biomarker for cervical cancer

Immunohistochemical evaluation of insulin-like growth factor I receptor status in cervical cancer specimens

Vascular endothelial growth factor (VEGF) pathway as a therapeutic target in gynecologic malignancies

High expression of prolactin receptor is associated with cell survival in cervical cancer cells

Diagnosis and prognostic significance of c-Met in cervical cancer: a meta-analysis

Expression of fibroblast growth factor receptor family members is associated with prognosis in early stage cervical cancer patients

HER2 as a therapeutic target in ovarian cancer. Ovarian cancer-clinical and therapeutic perspectives

EGFR as a prognostic biomarker and therapeutic target in ovarian cancer: evaluation of patient cohort and literature review

Insulin-like growth factor receptor I targeting in epithelial ovarian cancer

Hepatocyte growth factor/MET in cancer progression and biomarker discovery

Basic fibroblast growth factor and receptor expression in human ovarian cancer

HER2: an emerging biomarker in non-breast and non-gastric cancers

HER2 and EGFR overexpression support metastatic progression of prostate cancer to bone

Insulin-like growth factor receptor-1 (IGF-IR) as a target for prostate cancer therapy

Increased expressions of vascular endothelial growth factor (VEGF), VEGF-C and VEGF receptor-3 in prostate cancer tissue are associated with tumor progression

The role of fibroblast growth factors and their receptors in prostate cancer

Vitamin D and VDR in gynecological cancers-a systematic review

Methylation and silencing of the retinoic acid receptor-β2 gene in cervical cancer

Hormone receptors in serous ovarian carcinoma: prognosis, pathogenesis, and treatment considerations

Retinoid receptors in ovarian cancer: expression and prognosis

Androgen receptor: structure, role in prostate cancer and drug discovery

The role of estrogen receptor β in prostate cancer

Progesterone receptor in the prostate: a potential suppressor for benign prostatic hyperplasia and prostate cancer

The role of peroxisome proliferator-activated receptor gamma in prostate cancer

A practical guide to rational drug design

The nuclear receptor superfamily

Nuclear receptors in skeletal homeostasis

The roles of sex steroid receptor coregulators in cancer

The role of sex hormones and steroid receptors on female reproductive cancers

Estrogen biosynthesis and action in ovarian cancer

Apoptotic death of prostate cancer cells by a gonadotropin-releasing hormone-II antagonist

GnRH receptors in cancer: from cell biology to novel targeted therapeutic strategies

Targeting gonadotropin-releasing hormone receptor inhibits the early step of ovarian cancer metastasis by modulating tumormesothelial adhesion

Chemistry and structural biology of androgen receptor

Androgen receptor signaling in prostate cancer development and progression

Clinical relevance of androgen receptor alterations in prostate cancer

The role of the androgen receptor in ovarian cancer carcinogenesis and its clinical implications

Androgen receptors beyond prostate cancer: an old marker as a new target

Foye's principles of medicinal chemistry

New opportunities for targeting the androgen receptor in prostate cancer

Androgen receptor structure, function and biology: from bench to bedside

Estrogen receptors and human disease

Functions and physiological roles of two types of estrogen receptors, ERα and ERβ, identified by estrogen receptor knockout mouse

Estrogen and its receptors in cancer

Estrogens and their receptors in prostate cancer: therapeutic implications

Estradiol-17β. Handbook of hormones

Structural and functional diversity of estrogen receptor ligands

Resveratrol, a polyphenolic compound found in grapes and wine, is an agonist for the estrogen receptor

Structure-activity relationships of estrogens

Progesterone action in breast, uterine, and ovarian cancers

Membrane progesterone receptors in reproduction and cancer

Handbook of hormones

Progesterone receptor action: defining a role in breast cancer

Expression and function of the progesterone receptor in human prostate stroma provide novel insights to cell proliferation control

Expression of progesterone receptor is a favorable prognostic marker in ovarian cancer

Role of nuclear progesterone receptor isoforms in uterine pathophysiology

Gonadotropin-releasing hormone receptor system: modulatory role in aging and neurodegeneration

Gonadotropin-releasing hormone (GnRH) receptor structure and GnRH binding

Functional domains of the gonadotropin-releasing hormone receptor

Molecular mechanisms of ligand interaction with the gonadotropin-releasing hormone receptor

Selective androgen receptor modulators (SARMs) as function promoting therapies

Antiandrogens in prostate cancer

Androgen receptor antagonists (antiandrogens) structureactivity relationships

Recent developments in antiandrogens and selective androgen receptor modulators

SAR based design of nicotinamides as a novel class of androgen receptor antagonists for prostate cancer

Effect of small molecules modulating androgen receptor (SARMs) in human prostate cancer models

Arylisothiocyanato selective androgen receptor modulators (SARMs) for prostate cancer

Sintokamide A is a novel antagonist of androgen receptor that uniquely binds activation function-1 in its aminoterminal domain

Overview on the complexity of androgen receptor-targeted therapy for prostate cancer

Aromatase inhibitors in ovarian cancer: is there a role?

progesterone and epithelial ovarian cancer

Selective progesterone receptor modulators for fertility preservation in women with symptomatic uterine fibroids

Selective progesterone receptor modulators and progesterone antagonists: mechanisms of action and clinical applications

Hormone therapy for ovarian cancer

GnRH agonists and antagonists in prostate cancer

Drug-targeting strategies for prostate cancer

Liposomal formulation of hypoxia activated prodrug for the treatment of ovarian cancer

Prostate cancer relevant antigens and enzymes for targeted drug delivery

Androgen receptor targeted therapies in castration-resistant prostate cancer: bench to clinic

Conversion of abiraterone to D4A drives anti-tumour activity in prostate cancer

Antiestrogen therapy in recurrent ovarian cancer resulting in 28 months of stable disease: a case report and review of the literature

Inhibition of growth of cervical cancer cells using a dominant negative estrogen receptor gene

Raloxifene: promises and challenges as a drug treatment for castrate resistant prostate cancer

Preclinical development of ONC1-13B, novel antiandrogen for prostate cancer treatment

Cisplatin cytotoxicity is increased by mifepristone in cervical carcinoma: an in vitro and in vivo study

Antihormonal agents as a strategy to improve the effect of chemo-radiation in cervical cancer: in vitro and in vivo study

Active estrogen receptor-alpha signaling in ovarian cancer models and clinical specimens

Preclinical evaluation of targeted cytotoxic luteinizing hormone-releasing hormone analogue AN-152 in androgen-sensitive and insensitive prostate cancers

Ribociclib (LEE011): mechanism of action and clinical impact of this selective cyclin-dependent kinase 4/6 inhibitor in various solid tumors

Ribociclib and letrozole in treating patients with relapsed ER positive ovarian, fallopian tube, primary peritoneal, or endometrial cancer

Phase 1-2 study of onapristone in patients with progesterone receptor expressing cancers

Enzalutamide in patients with androgen receptor positive (AR+) ovarian, primary peritoneal or fallopian tube cancer and one, two or three prior therapies

Memorial Sloan Kettering Cancer Center

A trial of tamoxifen and letrozole in recurrent and persistent squamous cell carcinoma of the cervix (TGOG1005)

Buddhist Tzu Chi General Hospital

Metformin hydrochloride and doxycycline in treating patients with localized breast or uterine cancer

Cancer of Reproductive System: Receptors and Targeting Strategies 140

Dynamics of androgen receptor genomics and transcriptomics after neoadjuvant androgen ablation (DARANA)

The Netherlands Cancer Institute

A safety and pharmacokinetics study of niraparib plus an androgen receptor-targeted therapy in men with metastatic castration-resistant prostate cancer (BEDIVERE)