Selective Estrogen Receptor Degraders (SERDs): A Promising Strategy for Estrogen Receptor Positive Endocrine-Resistant Breast Cancer

Estrogen receptor (ER) plays important roles in gene transcription and the proliferation of ER positive breast cancers. Selective modulation of ER has been a therapeutic target for this specific type of breast cancer for more than 30 years. Selective estrogen receptor modulators (SERMs) and aromatase inhibitors (AIs) have been demonstrated to be effective therapeutic approaches for ER positive breast cancers. Unfortunately, 30-50% of ER positive tumors become resistant to SERM/AI treatment after 3-5 years. Fulvestrant, the only approved selective estrogen receptor degrader (SERD), is currently an important therapeutic approach for the treatment of endocrine-resistant breast cancers. The poor pharmacokinetic properties of fulvestrant have inspired the development of a new generation of oral SERDs to overcome drug resistance. In this review, we describe recent advances in ERα structure, functions, and mechanisms of endocrine resistance and summarize the development of oral SERDs in both academic and industrial areas.

Recent progress in selective estrogen receptor downregulators (SERDs) for the treatment of breast cancer

Selective estrogen receptor downregulators (SERDs) are a novel class of compounds capable of reducing the ERα protein level and blocking ER activity. Therefore, SERDs are considered as a significant therapeutic approach to treat ER+ breast cancer in both early stage and more advanced drug-resistant cases. After the FDA approval of a steroidal drug, fulvestrant, as a SERD for the treatment of breast cancer in patients who have progressed on antihormonal agents, several molecules with diverse chemical structures have been rapidly developed, studied and evaluated for selective estrogen receptor downregulation activity. Here we compile the promising SERDs reported in recent years and discuss the chemical structure and pharmacological profile of the most potent compound of the considered series. Because of the availability of only a limited number of effective drugs for the treatment of breast cancer, the quest for a potent SERD with respectable activity and bioavailability is still ongoing. The goal of this article is to make available to the reader an overview of the current progress in SERDs and provide clues for the future discovery and development of novel pharmacological potent SERDs for the treatment of breast cancer.

Carcinogenic potential of fluorinated estrogens in mammary tumorigenesis

Fluorination preventing metabolic hydroxylation of 17β-estradiol (E₂) was applied to investigate the mechanisms underlying estrogen-induced carcinogenesis. Either 2-fluoro-17β-estradiol (2-FE₂) or 4-fluoro-17β-estradiol (4-FE₂) was administered subcutaneously for 52 weeks to August Copenhagen Irish (ACI) rats, the preferred animal model for human breast cancer. 4-FE₂ induced frequent mammary tumors whereas 2-FE₂ did not. The cumulative incidence of mammary tumors in rats treated with 4-FE₂ was comparable to that observed with E₂. The carcinogenic results were supported by histological examination of mammary glands of fluorinated estrogen-treated ACI rats. To evaluate the estrogenic potential of the fluorinated estrogens, 2-FE₂ or 4-FE₂ was administrated subcutaneously to ovariectomized rats. Both 4-FE₂ and 2-FE₂ showed high uterotrophic potency. Our results indicate that estrogenic potential may not be the sole factor driving mammary tumorigenesis. Since fluorination inhibits metabolic hydroxylation of E₂ at the substituted position, the carcinogenic effect may occur through the metabolic activation of 2-hydroxylated E₂, in combination with the compound's estrogenic potency.

Development of novel and safer anti-breast cancer agents, SS1020 and SS5020, based on a fundamental carcinogenic research

Tamoxifen (TAM) has been prescribed worldwide to patients with and women at high-risk of breast cancer. However, long-term use of TAM increases the incidence of endometrial cancer. The carcinogenic mechanisms of TAM have been extensively investigated. TAM is hydroxylated and sulfonated at α-carbon to form α-hydroxytamoxifen-O-sulfonate. This metabolite readily reacts with genomic DNA, particularly with 2′-deoxyguanosine, leading to DNA replication error. TAM also exerts estrogenic activity at endometrial tissue to induce endometrial hyperplasia. Therefore, our efforts focused on the development of novel and safer anti-estrogens to diminish carcinogenic potential of TAM based on chemical modifications. In this review, we describe a crucial idea of our drug design and introduce our compounds SS1020 and SS5020, possessing high effectiveness, and no genotoxic and estrogenic activities.

Tricyclic Indazoles-A Novel Class of Selective Estrogen Receptor Degrader Antagonists

Herein, we report the identification and synthesis of a series of tricyclic indazoles as a novel class of selective estrogen receptor degrader antagonists. Replacement of a phenol, present in our previously reported tetrahydroisoquinoline scaffold, with an indazole group led to the removal of a reactive metabolite signal in an in vitro glutathione trapping assay. Further optimization, guided by X-ray crystal structures and NMR conformational work, varied the alkyl side chain and pendant aryl group and resulted in compounds with low turnover in human hepatocytes and enhanced chemical stability. Compound 9 was profiled as a representative of the series in terms of pharmacology and demonstrated the desired estrogen receptor α degrader-antagonist profile and demonstrated activity in a xenograft model of breast cancer.

Metabolomic analysis uncovered an association of serum phospholipid levels with estrogen-induced mammary tumors in female ACI/Seg rats

Estrogen is reported to be involved in mammary tumorigenesis. To unveil metabolic signatures for estrogen-induced mammary tumorigenesis, we carried out serum metabolomic analysis in an estrogen-induced mammary tumor model, female August Copenhagen-Irish/Segaloff (ACI/Seg) rats, using liquid chromatography-mass spectrometry. In contrast to the control group, all rats with an implanted 17β-estradiol (E2) pellet developed mammary tumors during this experiment. E2 treatment significantly suppressed body weight gain. But no significant differences in food consumption were observed between the two groups, suggesting that metabolic alteration depended on E2 treatment. Serum metabolomic analysis detected 116 features that were statistically different (p < 0.01) between the groups. Quantitation analysis revealed that several phospholipids such as phosphatidylcholines and lysophosphatidylcholines (LPCs) were identified as significantly different metabolites. E2-treated rat serum stimulated the proliferation of human breast cancer MDA-MB-231 cells. In addition, the proliferation effect was diminished by pretreating cells with either autotaxin inhibitor or antagonist for lysophosphatidic acid receptor whose ligands are metabolites of LPCs via autotaxin-mediated hydrolysis. In summary, our results suggest that not only are phospholipids potential biomarkers for mammary tumors but importantly, LPCs themselves could be associated with E2-induced mammary tumorigenesis in female ACI/Seg rats.

Full antagonism of the estrogen receptor without a prototypical ligand side chain

Resistance to endocrine therapies remains a major clinical problem for the treatment of estrogen receptor-α (ERα)-positive breast cancer. On-target side effects limit therapeutic compliance and use for chemoprevention, highlighting an unmet need for new therapies. Here we present a full-antagonist ligand series lacking the prototypical ligand side chain that has been universally used to engender antagonism of ERα through poorly understood structural mechanisms. A series of crystal structures and phenotypic assays reveal a structure-based design strategy with separate design elements for antagonism and degradation of the receptor, and access to a structurally distinct space for further improvements in ligand design. Understanding structural rules that guide ligands to produce diverse ERα-mediated phenotypes has broad implications for the treatment of breast cancer and other estrogen-sensitive aspects of human health including bone homeostasis, energy metabolism, and autoimmunity.

Recent development of targeted approaches for the treatment of breast cancer

Breast cancer is the most prominent cause of cancer death in women worldwide. The highlights of this review are to provide an overview of the targeted therapeutic agents, challenges with metastatic breast cancer (MBCa), mechanisms of action through Hedgehog/Gli 1 signaling pathway and future prospective. Over a decade of success, several drugs have been approved and are in the advanced stages of clinical trials that target the receptors such as estrogen receptor, growth factor receptor, receptor activator of nuclear factor kappa-B, etc. Currently, several monoclonal antibodies are also used for the treatment of breast cancer. Advances in understanding tumor biology, particularly signaling pathways such as Notch signaling pathway, Hedgehog/Gli 1 signaling pathway, and inhibitors are considered to be important for bone metastasis. These studies may provide vital information for the design and development of new strategies with respect to efficacy, reduction of the side effects, and treatment strategies.

Synthesis and evaluation of raloxifene derivatives as a selective estrogen receptor down-regulator

Estrogen receptors (ERs) play a major role in the growth of human breast cancer cells. A selective estrogen receptor down-regulator (SERD) that acts as not only an inhibitor of ligand binding, but also induces the down-regulation of ER, would be useful for the treatment for ER-positive breast cancer. We previously reported that tamoxifen derivatives, which have a long alkyl chain, had the ability to down-regulate ERα. With the aim of expanding range of the currently available SERDs, we designed and synthesized raloxifene derivatives, which had various lengths of the long alkyl chains, and evaluated their SERD activities. All compounds were able to bind ERα, and RC10, which has a decyl group on the amine moiety of raloxifene, was shown to be the most potent compound. Our findings suggest that the ligand core was replaceable, and that the alkyl length was important for controlling SERD activity. Moreover, RC10 showed antagonistic activity and its potency was superior to that of 4,4'-(heptane-4,4-diyl)bis(2-methylphenol) (18), a competitive antagonist of ER without SERD activity. These results provide information that will be useful for the development of promising SERDs candidates.

Anti-breast cancer potential of daidzein in rodents

AIMS

This study was carried out to explore anti-breast cancer potential of isoflavone daidzein or its related compounds using appropriate animal models and their anti-tumor mechanism.

MAIN METHODS

Daidzein or its major metabolite equol at a dose molar equivalent to tamoxifen [1.0 mg(2.7 μmol)/kg or 10 mg (27 μmol)/kg/day] was treated orally to rats bearing 7,12-dimethylbenz(a)anthracene(DMBA)-induced mammary tumors or ovariectomized athymic nude mice implanted with human MCF-7 breast cancer xenograft and an estrogen pellet. The growth of tumors was monitored for several weeks after the treatment. The cell-cycle and apoptotic stages in mammary tumors collected from rats were analyzed by flow cytometry. Immunohistochemistry analysis was also used to determine the expression of caspase-3.

KEY FINDINGS

Oral treatment with daidzein or equol at a human equivalent dose suppressed the growth of both DMBA-induced mammary tumors and human MCF-7 breast cancer xenografts in rodents, the inhibitory activity being superior to that of genistein or tamoxifen. Strong apoptosis induced by daidzein or equol contributes to the anti-tumor potential.

SIGNIFICANCE

Daidzein and its metabolite equol showed the potential of inhibiting the growth of mammary tumors in rodents. Daidzein or equol could be used as a core structure to design new drugs for breast cancer therapy. Our results indicate that consumption of daidzein may protect against breast cancer.

Anti-breast cancer potential of SS5020, a novel benzopyran antiestrogen

Treatment with tamoxifen (TAM) increases the risk of developing endometrial cancer in women. The carcinogenic effect is thought to involve initiation and/or promotion resulting from DNA damage induced by TAM as well as its estrogenic action. To minimize this serious side-effect while increasing the anti-breast cancer potential, a new benzopyran antiestrogen, 2E-3-{4-[(7-hydroxy-2-oxo-3-phenyl-2H-chromen-4-yl)-methyl]-phenyl}-acrylic acid (SS5020), was synthesized. Unlike TAM, SS5020 exhibits no genotoxic activity to damage DNA. Furthermore, SS5020 does not present significant uterotrophic potential in rats; in contrast, the structurally related compounds, TAM, toremifene, raloxifene (RAL) and SP500263 all have uterotrophic activity. At the human equivalent molar dose of TAM (0.33 or 1.0 mg/kg), SS5020 had much stronger antitumor potential than those same antiestrogens against 7,12-dimethylbenz(a)anthracene-induced mammary carcinoma in rats. The growth of human MCF-7 breast cancer xenograft implanted into athymic nude mice was also effectively suppressed by SS5020. SS5020, lacking genotoxic and estrogenic actions, could be a safer and stronger antiestrogen alternative to TAM and RAL for breast cancer therapy and prevention.