**6.1 Androgen deprivation therapy (ADT)**

Ubiquitous and extensive expression of androgen receptor in the bone marrow of both males and females of all ages provides a direct evidence of action of androgen on the bone marrow and offers clues to clinicopathological correlates [39]. Most prostate cancers and their stages depend upon androgen and androgen receptor (AR) for their growth and survival. Androgen receptor is a transcription factor that regulates the expression of several genes in response to binding of androgen (such as testosterone and dihydrotestosterone) and thus regulates the process of proliferation and survival. As mentioned earlier, bone metastasis of prostate cancers majorly results in osteoblastic bone tumors [40]. Therefore, systemic treatment for advanced prostate cancer involves androgen deprivation therapy (ADT) that includes the following approaches:

a.To reduce the levels of circulating androgen by surgical or chemical castration. Surgical castration results in reduction of circulating androgen levels by >90% within 24 hours, whereas chemical castration is achieved by application of analogs of luteinizing hormone-releasing hormone (LH-RH) and results in reduction of circulating levels of testosterone [40].

LH-RH is a neurohormone, secreted by the hypothalamus, and regulates the secretion of gonadotropin, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) from the pituitary. LH-RH acts via binding to its receptor, LH-RHR. These receptors have also been found in the cytoplasm of many tumor cells that involve both reproductive and nonreproductive tissues. LH-RH agonists and antagonists have been found to downregulate these receptors and thus inhibit tumor growth [41]. Leuprolide acetate (Lupron, Eligard), goserelin acetate (Zoladex), triptorelin (Trelstar), and histrelin (Vantas) are some of the LH-RH agonists, whereas degarelix is an antagonist [4, 40]. LH-RHR can also be targeted specifically by peptides conjugated to anticancer drugs, thus developing cytotoxic analogs [41]. AN-152, commercially designated as AEZS-108, has been developed by conjugating 14-OH group of doxorubicin (DOX) to epsilon-amino group of D-Lys side chain of carrier peptide, through a glutaric acid spacer. The drug is endocytosed by cells through receptor-mediated endocytosis and thus selectively acts on cells that express its receptor. After internalization the drug is cleaved from the LH-RH moiety and accumulates in the nucleus. Because of receptor-mediated entry, the drug shows lesser side effects and also overcomes the resistance [41].

Administration of LH-RH agonist or antagonist for ADT not only results in suppression of testosterone to castration levels but also depletes estradiol, because it is derived by aromatization of testosterone [42]. Estradiol deficiency negatively impacts the bone health resulting in decline of bone mineral density (BMD) and increased risk of fractures [42]. This decrease in bone density also results in development of renal stones leading to risk of urinary calculi [43]. Recently parenteral (e.g., intravenous, intramuscular, or transdermal) administration of estradiol has been investigated to suppress androgen production through negative feedback loop involving hypothalamic–pituitary axis and avoids fall in endogenous estradiol levels. This also eliminates the risk of embolic cardiovascular toxicity that was caused due to oral administration of estradiol [42].

**25**

*Bone Tumors: Types and Treatments*

*DOI: http://dx.doi.org/10.5772/intechopen.86550*

b.To prevent binding of androgen to AR, by competitive inhibition using

the side effects associated with administration of abiraterone [44, 45].

toxicity is one of the common adverse effects [40].

truncated constitutively active splice variant of AR [40].

testosterone flare [48, 49].

Flutamide was the first nonsteroidal antiandrogen drug approved by the US Food and Drug Administration (FDA) for prostate cancer and forms the basis for all other nonsteroidal antiandrogens. The recommended dose of flutamide is 250 mg three times per day, so as to achieve a Cmax and Cmin of approximately 1.7 and 0.8 μg/ml, respectively. It acts via blocking the binding of androgen to the ligandbinding pocket of AR, resulting in inhibition of nuclear translocation of androgenbound AR. But improvement in disease upon cessation of flutamide treatment has been observed in patients, due to gain-of-function mutation in the ligand-binding domain of AR, T877A. Flutamide gets eliminated through the kidney, and liver

Enzalutamide (previously called MDV3100) also acts via inhibiting the binding of androgen to AR, thus blocking its nuclear translocation and interaction with co-activators [4, 29, 40, 46]. Its recommended dose is 160 mg/day [40, 47]. However, clinical resistance due to gain-of-function mutation in AR ligand-binding domain (F876 L) and constitutive expression of active spice variants of AR that lack ligand-binding domain results in poor survival rates. Apalutamide and darolutamide also belong to the second generation of nonsteroidal antiandrogen that blocks the androgen binding to AR [40]. A novel first class of drug, ralaniten, is currently under clinical trials for patients

who have previously received abiraterone, enzalutamide, or both. This class of drug binds to the unique region in the N-terminal domain of both full-length and

c.ADT that effectively reduces the serum testosterone levels has been a core tool for treating metastatic and advanced prostate cancer [48]. However, neoadjuvant ADT has been suggested to have several advantages in prostate cancer patients undergoing transperineal prostate brachytherapy. The agents that are mainly used as adjuvant ADT include estrogens, antiandrogen monotherapy, and combined androgen blockade (CAB) using antiandrogen plus a gonadotropin-releasing hormone receptor (GnRH) agonist. It has been reported that in comparison to GnRH agonist, degarelix, a GnRH receptor antagonist, is more efficient in achieving castration levels of testosterone and PSA, without risk of

antiandrogens. These molecules compete with androgen for the ligand-binding domain of AR [40]. The antiandrogens can be of two categories, steroidal and nonsteroidal. Cyproterone acetate, a derivative of hydroxyprogesterone, is a steroidal antiandrogen and an antigonadotropin, which has a binding affinity for AR. But it has been found that it is not a pure antagonist but rather a partial agonist that adversely affects the survival of prostate cancer patients when combined with castration [40]. Among the nonsteroidal antiandrogens are the first-generation flutamide, nilutamide, and bicalutamide and second-generation enzalutamide and the cytochrome P450 c17 (CYP17, a critical enzyme in testosterone synthesis) inhibitor, abiraterone acetate, which prevents synthesis of androgens. Abiraterone inhibits 17-α-hydroxylase/17,20 lyase, a testosterone synthesis enzyme found in the adrenals, testis, and tumor [40, 44]. All these nonsteroidal antiandrogens are similar in terms of the chemical structure of their moiety that binds to the ligand-binding pocket [40]. It has been found that treatment with abiraterone acetate plus prednisone prolongs survival among patients with metastatic castration-resistant prostate cancer [44], though back pain, nausea, constipation, bone pain, arthralgia, urinary tract infection, edema, cardiac events, and elevation in levels of aminotransferase are some of

### *Bone Tumors: Types and Treatments DOI: http://dx.doi.org/10.5772/intechopen.86550*

*Hormone Therapy and Replacement in Cancer and Aging-Related Diseases*

**6.1 Androgen deprivation therapy (ADT)**

includes the following approaches:

already been known that the sex hormones play a major role in tumor cell survival and metastases. Thus, hormone therapy for curing tumors would basically involve hormone deprivation approaches. These strategies might inhibit the action of hormones responsible to bone metastases, resulting in osteolytic or osteoblastic tumors.

Ubiquitous and extensive expression of androgen receptor in the bone marrow of both males and females of all ages provides a direct evidence of action of androgen on the bone marrow and offers clues to clinicopathological correlates [39]. Most prostate cancers and their stages depend upon androgen and androgen receptor (AR) for their growth and survival. Androgen receptor is a transcription factor that regulates the expression of several genes in response to binding of androgen (such as testosterone and dihydrotestosterone) and thus regulates the process of proliferation and survival. As mentioned earlier, bone metastasis of prostate cancers majorly results in osteoblastic bone tumors [40]. Therefore, systemic treatment for advanced prostate cancer involves androgen deprivation therapy (ADT) that

a.To reduce the levels of circulating androgen by surgical or chemical castration. Surgical castration results in reduction of circulating androgen levels by >90% within 24 hours, whereas chemical castration is achieved by application of analogs of luteinizing hormone-releasing hormone (LH-RH) and results in

LH-RH is a neurohormone, secreted by the hypothalamus, and regulates the secretion of gonadotropin, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) from the pituitary. LH-RH acts via binding to its receptor, LH-RHR. These receptors have also been found in the cytoplasm of many tumor cells that involve both reproductive and nonreproductive tissues. LH-RH agonists and antagonists have been found to downregulate these receptors and thus inhibit tumor growth [41]. Leuprolide acetate (Lupron, Eligard), goserelin acetate (Zoladex), triptorelin (Trelstar), and histrelin (Vantas) are some of the LH-RH agonists, whereas degarelix is an antagonist [4, 40]. LH-RHR can also be targeted specifically by peptides conjugated to anticancer drugs, thus developing cytotoxic analogs [41]. AN-152, commercially designated as AEZS-108, has been developed by conjugating 14-OH group of doxorubicin (DOX) to epsilon-amino group of D-Lys side chain of carrier peptide, through a glutaric acid spacer. The drug is endocytosed by cells through receptor-mediated endocytosis and thus selectively acts on cells that express its receptor. After internalization the drug is cleaved from the LH-RH moiety and accumulates in the nucleus. Because of receptor-mediated entry, the drug shows lesser side effects and also overcomes the resistance [41]. Administration of LH-RH agonist or antagonist for ADT not only results in suppression of testosterone to castration levels but also depletes estradiol, because it is derived by aromatization of testosterone [42]. Estradiol deficiency negatively impacts the bone health resulting in decline of bone mineral density (BMD) and increased risk of fractures [42]. This decrease in bone density also results in development of renal stones leading to risk of urinary calculi [43]. Recently parenteral (e.g., intravenous, intramuscular, or transdermal) administration of estradiol has been investigated to suppress androgen production through negative feedback loop involving hypothalamic–pituitary axis and avoids fall in endogenous estradiol levels. This also eliminates the risk of embolic cardiovascular toxicity that was caused

reduction of circulating levels of testosterone [40].

**24**

due to oral administration of estradiol [42].

b.To prevent binding of androgen to AR, by competitive inhibition using antiandrogens. These molecules compete with androgen for the ligand-binding domain of AR [40]. The antiandrogens can be of two categories, steroidal and nonsteroidal. Cyproterone acetate, a derivative of hydroxyprogesterone, is a steroidal antiandrogen and an antigonadotropin, which has a binding affinity for AR. But it has been found that it is not a pure antagonist but rather a partial agonist that adversely affects the survival of prostate cancer patients when combined with castration [40]. Among the nonsteroidal antiandrogens are the first-generation flutamide, nilutamide, and bicalutamide and second-generation enzalutamide and the cytochrome P450 c17 (CYP17, a critical enzyme in testosterone synthesis) inhibitor, abiraterone acetate, which prevents synthesis of androgens. Abiraterone inhibits 17-α-hydroxylase/17,20 lyase, a testosterone synthesis enzyme found in the adrenals, testis, and tumor [40, 44]. All these nonsteroidal antiandrogens are similar in terms of the chemical structure of their moiety that binds to the ligand-binding pocket [40]. It has been found that treatment with abiraterone acetate plus prednisone prolongs survival among patients with metastatic castration-resistant prostate cancer [44], though back pain, nausea, constipation, bone pain, arthralgia, urinary tract infection, edema, cardiac events, and elevation in levels of aminotransferase are some of the side effects associated with administration of abiraterone [44, 45].

Flutamide was the first nonsteroidal antiandrogen drug approved by the US Food and Drug Administration (FDA) for prostate cancer and forms the basis for all other nonsteroidal antiandrogens. The recommended dose of flutamide is 250 mg three times per day, so as to achieve a Cmax and Cmin of approximately 1.7 and 0.8 μg/ml, respectively. It acts via blocking the binding of androgen to the ligandbinding pocket of AR, resulting in inhibition of nuclear translocation of androgenbound AR. But improvement in disease upon cessation of flutamide treatment has been observed in patients, due to gain-of-function mutation in the ligand-binding domain of AR, T877A. Flutamide gets eliminated through the kidney, and liver toxicity is one of the common adverse effects [40].

Enzalutamide (previously called MDV3100) also acts via inhibiting the binding of androgen to AR, thus blocking its nuclear translocation and interaction with co-activators [4, 29, 40, 46]. Its recommended dose is 160 mg/day [40, 47]. However, clinical resistance due to gain-of-function mutation in AR ligand-binding domain (F876 L) and constitutive expression of active spice variants of AR that lack ligand-binding domain results in poor survival rates. Apalutamide and darolutamide also belong to the second generation of nonsteroidal antiandrogen that blocks the androgen binding to AR [40].

A novel first class of drug, ralaniten, is currently under clinical trials for patients who have previously received abiraterone, enzalutamide, or both. This class of drug binds to the unique region in the N-terminal domain of both full-length and truncated constitutively active splice variant of AR [40].

c.ADT that effectively reduces the serum testosterone levels has been a core tool for treating metastatic and advanced prostate cancer [48]. However, neoadjuvant ADT has been suggested to have several advantages in prostate cancer patients undergoing transperineal prostate brachytherapy. The agents that are mainly used as adjuvant ADT include estrogens, antiandrogen monotherapy, and combined androgen blockade (CAB) using antiandrogen plus a gonadotropin-releasing hormone receptor (GnRH) agonist. It has been reported that in comparison to GnRH agonist, degarelix, a GnRH receptor antagonist, is more efficient in achieving castration levels of testosterone and PSA, without risk of testosterone flare [48, 49].

ADT is the mainstay of treatment for advanced prostate cancer, but eventual development of castration-resistant prostate cancer (CRPC) reduces the survival rates. One of the main reasons for development of CRPC is the sustained levels of androgen within the tumor due to suboptimal androgen suppression by primary ADT. Moreover, apart from the hormone-independent subsets, the other subsets of CRPC cells adapt themselves to the low testosterone environment induced by ADT and become hypersensitive to even lower concentrations of testosterone and other androgen precursors. Therefore, secondary hormone therapies are proving to be more efficient to achieve maximum suppression of testosterone. GTx-758 (3-fluoro-N-(4-fluorophenyl)-4-hydroxy-N-(4-hydroxyphenyl) benzamide) is an oral nonsteroidal selective estrogen receptor (ERα) agonist that lowers the free testosterone and PSA levels by increasing sex hormone-binding globulin (SHBG). This also helps to avoid side effects related to estrogen deficiency [50].

Finasteride and dutasteride (5α-reductase enzyme inhibitors) are found to inhibit 5α-reductase-mediated conversion of testosterone to the high affinity androgen receptor ligand, 5α-dihydrotestosterone [51].

#### **6.2 Selective estrogen receptor modulators (SERMs)**

Estrogen is chemically related compounds derived from androgen precursors but contain a defining aromatic and hydroxyl group at the 17th position. Estrogens comprise the natural ligands for estrogen receptors (ERs), with 17β-estradiol being a potent agonist. 17β-estradiol has been reported to inhibit metastasis-associated lung adenocarcinoma transcript 1 (MALAT-1)-mediated osteosarcoma migration, invasion, metastasis, and induction of cell apoptosis, in an estrogen receptor α (ERα)-independent manner [15]. Earlier it was thought that binding of ER agonists induces a conformational change in the receptors, conferring the ability for coactivators to bind, whereas ER antagonists were thought to compete for binding [52]. But later studies with tamoxifen revealed that the same molecule can behave as an agonist (tamoxifen acts as an estrogen agonist in the uterus, promoting hypertrophy) as well as an antagonist (tamoxifen exhibited estrogenic activity in the bone, thus protecting against bone loss), depending on the tissue context. Recently it has been found that oxysterols such as 27-hydroxycholesterol (27 HC) also modulate the activity of estrogen receptors (ERs) and are therefore classified as endogenous SERMs. 27 HC is derived from cholesterol in the presence of enzyme CYP27A1 (cytochrome P450 enzyme). Breast cancer is the most common cancer in women, and its metastasis is majorly hormone (estrogen receptor) dependent. Some of the reports emphasize on the role of 27 HC in cancer progression and drug resistance, but several other reports also highlight its beneficial role in inhibiting proliferation and invasion of prostate cancer cells by blocking sterol-regulatory element-binding protein 2 (SREBP2). However, their different affinities for the different subtypes of ERs (α and β) and different relative expressions of these subtypes in tissues may explain some the of SERMs' pharmacology. Recent evidence also suggests that binding of the receptor even by structurally related compounds could result in unique conformational changes, thus allowing recruitment of distinct sets of co-activators and/or corepressors to the receptor [52].

SERMs such as genistein, daidzein, and 4-hydroxytamoxifen have been reported to downregulate the expression of epidermal growth factor (EGFR) in vitro in osteosarcoma cells in an ER-dependent manner. The reduction in EGFR expression resulted in upregulation of markers for osteoblast differentiation, thus resulting in suppression of tumor cell proliferation [53].

**27**

*Bone Tumors: Types and Treatments*

risk of breast cancer [54].

**6.3 Activation of ERα**

**6.4 Estrogen inhibitors**

*DOI: http://dx.doi.org/10.5772/intechopen.86550*

class of phytoestrogens. Phytoestrogens are plant-derived substances that resemble 17β-estradiol and can bind to activate intracellular estrogen receptors. These dietary phytoestrogens have been reported to exhibit bone-protecting effect without the

Genistein has also been demonstrated to elicit different cell responses through different signaling mechanisms. A combination of genistein and 17β-estradiol has been shown to significantly increase apoptosis of breast cancer cells by increasing the BAX/BCL-2 (BCL-2-associated X protein (BAX)) ratio and reducing phosphorylation of extracellular signal-regulated kinase (ERK) ½ and AKT [55].

Osteosarcoma is a malignant tumor in the bone that originates from osteoblasts or osteoblast precursors. The reports clarify that normal osteoblasts express ERα, whereas osteosarcomas do not (due to promoter DNA methylation). Thus a treatment strategy that involves induction of ERα expression in osteosarcoma cells in combination with estrogen administration would reduce proliferation of osteosarcoma and increase cell differentiation. In vitro treatment of osteosarcoma cells with decitabine (DAC, 5-Aza-2′-deoxycytidine) has been found to induce the expression of ERα but reduce the expression of metastasis-associated markers such as vimentin, slug, zeb1, and MMP9, with simultaneous decrease in stem cell markers such as *SOX2*, *OCT4*, and *NANOG*. Subsequent treatment with 17β-estradiol synergized with DAC in reducing cell proliferation and inducing differentiation markers such

The bone is the frequent site for metastasis of breast cancer. Estrogen plays a critical role in development and progression of breast cancer by interacting with ERα and ERβ. In postmenopausal women, estrogens (estrone and estradiol) are synthesized from androgens (androstenedione and testosterone) at extragonadal sites, including the breast. Thus the third generation of therapy involves inhibition of these aromatase enzymes, catalyzing the conversion of androgens to estrogens [56]. The aromatase inhibitors fall into two categories: steroidal and nonsteroidal. Letrozole and anastrozole are the third-generation nonsteroidal aromatase inhibitors that block the extragonadal conversion of androgens to estrogens and give rise to an estrogen-depleted environment [51, 56, 57]. This lowers the estrogen in breast tissues and reduces their metastasis to the bone [56]. But in patients with hormone receptor-positive breast cancer, both the disease and its therapeutic treatment with antiestrogenic agents negatively impact the bone and result in decrease in bone mineral density. Therefore anti-hormonal therapy is considered only in cases where cancer cells express the ERα [58]. However, unlike nonsteroidal aromatase inhibitors, a steroidal aromatase inhibitor, e.g., exemestane (probably due to its steroid structure), has been reported to exert beneficial effects on the bone through its

Fulvestrant, an alkylosulfonian derivative of estradiol, is another category of estrogen inhibitors (estrogen receptor antagonist), which competitively binds to ER with high affinity and downregulates expression of ERβ by functional blockade [59, 60]. Fulvestrant has been reported to induce mitochondrial depolarization at high concentrations that results in release of apoptogenic factors, loss of oxidative

2-Methoxyestradiol (2-ME) belongs to another class of anticancer drugs, which act via induction of neuronal nitric oxide synthase and generation of nitric oxide

as alkaline phosphatase, osterix, and bone sialoproteins [21].

primary metabolite 17-hydroexemestane [51, 57].

phosphorylation, and eventually cell death due to apoptosis [60].

Isoflavones such as genistein and daidzein are abundantly found in soybeans and soy-based food products. Isoflavones, coumestans, and lignans belong to a

*Bone Tumors: Types and Treatments DOI: http://dx.doi.org/10.5772/intechopen.86550*

class of phytoestrogens. Phytoestrogens are plant-derived substances that resemble 17β-estradiol and can bind to activate intracellular estrogen receptors. These dietary phytoestrogens have been reported to exhibit bone-protecting effect without the risk of breast cancer [54].

Genistein has also been demonstrated to elicit different cell responses through different signaling mechanisms. A combination of genistein and 17β-estradiol has been shown to significantly increase apoptosis of breast cancer cells by increasing the BAX/BCL-2 (BCL-2-associated X protein (BAX)) ratio and reducing phosphorylation of extracellular signal-regulated kinase (ERK) ½ and AKT [55].
