Selective estrogen receptor modulators (SERMs) and selective estrogen receptor degraders (SERDs) in cancer treatment

The Emerging Role of ESR1 Mutations in Luminal Breast Cancer as a Prognostic and Predictive Biomarker of Response to Endocrine Therapy

Mutations in the hotspot ligand-binding domain of the estrogen receptor (ER) gene ESR1 have recently been recognized as mechanisms of endocrine resistance in endocrine receptor-positive metastatic breast cancer (MBC). Accumulating data suggest these mutations develop under the selective pressure of endocrine treatments, and are infrequent in untreated ER-positive breast cancers. In vitro studies show that these mutations confer ligand-independent activity, resistance to estrogen deprivation, and relative resistance to tamoxifen and fulvestrant. Post-hoc retrospective and prospective analyses of ESR1 mutations in patients with MBC have consistently found that these mutations are markers of poor prognosis and predict resistance to aromatase inhibitors (AIs). These results warrant further investigation and prospective validation in dedicated studies. Moreover, studies are ongoing to clarify the activity of novel drugs in the context of metastatic endocrine resistant luminal breast cancer harboring ESR1 mutations. In this review, we summarize the pre-clinical and clinical findings defining the characteristics of ESR1 mutant breast cancer, and highlight the potential clinical developments in this field.

Targeting transcription factors in cancer - from undruggable to reality

Mutated or dysregulated transcription factors represent a unique class of drug targets that mediate aberrant gene expression, including blockade of differentiation and cell death gene expression programmes, hallmark properties of cancers. Transcription factor activity is altered in numerous cancer types via various direct mechanisms including chromosomal translocations, gene amplification or deletion, point mutations and alteration of expression, as well as indirectly through non-coding DNA mutations that affect transcription factor binding. Multiple approaches to target transcription factor activity have been demonstrated, preclinically and, in some cases, clinically, including inhibition of transcription factor-cofactor protein-protein interactions, inhibition of transcription factor-DNA binding and modulation of levels of transcription factor activity by altering levels of ubiquitylation and subsequent proteasome degradation or by inhibition of regulators of transcription factor expression. In addition, several new approaches to targeting transcription factors have recently emerged including modulation of auto-inhibition, proteolysis targeting chimaeras (PROTACs), use of cysteine reactive inhibitors, targeting intrinsically disordered regions of transcription factors and combinations of transcription factor inhibitors with kinase inhibitors to block the development of resistance. These innovations in drug development hold great promise to yield agents with unique properties that are likely to impact future cancer treatment.

Design and Synthesis of Novel Breast Cancer Therapeutic Drug Candidates Based upon the Hydrophobic Feedback Approach of Antiestrogens

Based upon hydrophobic feedback approaches, we designed and synthesized novel sulfur-containing ERα modulators (4 and 5) as breast cancer therapeutic drug candidates. The tetrahydrothiepine derivative 5a showed the highest binding affinity toward ERα because of its high hydrophobicity, and it acted as an agonist toward MCF-7 cell proliferation. The corresponding alkylamino derivative 5d maintained high binding affinity to ERα and potently inhibited MCF-7 cell proliferation (IC₅₀: 0.09 μM). Docking simulation studies of compound 5d with the ERα BD revealed that the large hydrophobic moiety of compound 5d fit well into the hydrophobic pocket of the ERα LBD and that the sulfur atom of compound 5d formed a sulfur–π interaction with the amino acid residue His524 of the ERα LBD. These interactions play important roles for the binding affinity of compound 5d to the ERα LBD.

Estrogen Receptor α (ERα)-targeting Compounds and Derivatives: Recent Advances in Structural Modification and Bioactivity

Breast cancer is the most common cancer suffered by female, and the second highest cause of cancer-related death among women worldwide. At present, hormone therapy is still the main treatment route and can be divided into three main categories: selective estrogen receptor modulators (SERMs), selective estrogen receptor downregulators (SERDs), and aromatase inhibitors (AIs). However, breast cancer is difficult to cure even after several rounds of anti-estrogen therapy and most drugs have serious side-effects. Here, we review the literature published over the past five years regarding the isolation and synthesis of analogs and their derivatives.

Building Bridges in a Series of Estrogen Receptor Degraders: An Application of Metathesis in Medicinal Chemistry

Herein we report the use of metathesis to construct a novel tetracyclic core in a series of estrogen receptor degraders. This improved the chemical stability, as assessed using an NMR-MS based assay, and gave a molecule with excellent physicochemical properties and pharmacokinetics in rat. X-ray crystallography established minimal perturbation of the bridged compounds relative to the unbridged analogues in the receptor binding pocket. Unfortunately, despite retaining excellent binding to ERα, this adversely affected the ability of the compounds to degrade the receptor.

EMERALD: Phase III trial of elacestrant (RAD1901) vs endocrine therapy for previously treated ER+ advanced breast cancer

Elacestrant is a novel, nonsteroidal, orally bioavailable selective estrogen receptor degrader (SERD) that has demonstrated activity in patients with estrogen receptor (ER)-positive/HER2-negative breast cancer previously treated with endocrine therapies including fulvestrant and/or CDK 4/6 inhibitor therapy, and in those with ESR1 mutations (ESR1-mut) known to confer endocrine resistance. Herein, we describe the design and methodology of EMERALD, an international, multicenter, randomized, open-label, active-controlled, Phase III clinical study comparing the efficacy and safety of elacestrant to standard-of-care endocrine monotherapy treatment (fulvestrant or an aromatase inhibitor, per investigator's choice) in patients with ER-positive/HER2-negative advanced breast cancer. Primary end points are progression-free survival in ESR1-mut patients and in all patients (NCT03778931; EudraCT 2018-002990-24).

Distinct Roles for BET Family Members in Estrogen Receptor α Enhancer Function and Gene Regulation in Breast Cancer Cells

The bromodomain family member proteins (BRD; BET proteins) are key coregulators for estrogen receptor alpha (ERα)-mediated transcriptional enhancers. The use of BRD-selective inhibitors has gained much attention as a potential treatment for various solid tumors, including ER-positive breast cancers. However, the roles of individual BET family members have largely remained unexplored. Here, we describe the role of BRDs in estrogen (E2)-dependent gene expression in ERα-positive breast cancer cells. We observed that chemical inhibition of BET family proteins with JQ1 impairs E2-regulated gene expression and growth in breast cancer cells. In addition, RNAi-mediated depletion of each BET family member (BRDs 2, 3, and 4) revealed partially redundant roles at ERα enhancers and for target gene transcription. Furthermore, we found a unique role of BRD3 as a molecular sensor of total BET family protein levels and activity through compensatory control of its own protein levels. Finally, we observed that BRD3 is recruited to a subset of ERα-binding sites (ERBS) that are enriched for active enhancer features, located in clusters of ERBSs likely functioning as "super enhancers," and associated with highly E2-responsive genes. Collectively, our results illustrate a critical and specific role for BET family members in ERα-dependent gene transcription. IMPLICATIONS: BRD3 is recruited to and controls the activity of a subset ERα transcriptional enhancers, providing a therapeutic opportunity to target BRD3 with BET inhibitors in ERα-positive breast cancers.

ER subtype selectivity of m-carborane-containing phenols: C-alkyl groups on the m-carborane cage enhance ERα selectivity

Estrogen receptor (ER) exhibits two subtypes, ERα and ERβ, whose biological functions are quite different despite expression in the same tissues. We developed diiodo-m-carborane derivative 3a, which showed 14-fold selectivity for ERβ with high binding affinity toward ERβ. Interestingly, introduction of an alkyl group into the carbon atom of the m-carborane cage of 3a markedly enhanced the binding affinity toward ERα and decreased affinity toward ERβ. C-n-propyl derivative 3d showed 28-fold selectivity for ERα in an ER binding assay and promoted proliferation of MCF-7 breast cancer cells. Docking simulation studies suggest that the directions of the n-propyl group and the diiodo substituent introduced on the m-carborane cage play important roles for the control of ER subtype selectivity. As 3a and 3d showed ERβ and ERα selectivity with high binding affinity, respectively, these ligands may be useful as biological tools to aid in understanding the different roles of ER subtypes.

A high AR:ERα or PDEF:ERα ratio predicts a sub-optimal response to tamoxifen therapy in ERα-positive breast cancer

PURPOSE

Approximately 30% oestrogen receptor alpha (ERα)-positive breast cancer (BC) patients exhibit intrinsic or recurrent resistance to adjuvant endocrine therapy with tamoxifen. The androgen receptor (AR) is expressed in about 90% of ERα-positive patients, with particularly high expression in tamoxifen-resistant tumours. Prostate-derived Ets factor (PDEF), which is a co-regulator of AR, plays a role in tamoxifen resistance in ERα-positive BC. The purpose of this research was to analyse the potential roles of AR, PDEF and ERα levels in the response to tamoxifen resistance in ERα-positive BC.

METHODS

The nuclear AR:ERα and PDEF:ERα ratios were examined immunohistochemically in a cohort of 225 ERα-positive pre-menopausal BC patients who had received adjuvant tamoxifen therapy.

RESULTS

For both AR:ERα and PDEF:ERα ratios, the optimal cutoff value was 2.0. Patients receiving adjuvant tamoxifen treatment who had a high AR:ERα (≥ 2.0) (HR = 3.90) or PDEF:ERα ratio (≥ 2.0) (HR = 2.77) had a beyond twofold increased risk of failure. Both the AR:ERα ratio (P = 0.001) and PDEF:ERα ratio (P = 0.002) were independently associated with the risk of tamoxifen treatment failure. Furthermore, both a high ratio of AR:ERα (≥ 2.0) and PDEF:ERα (≥ 2.0) were associated with shorter disease-free survival (DFS) and shorter disease-specific survival (DSS). In addition, both the AR:ERα ratio and PDEF:ERα ratio were independent predictors of DFS (both P < 0.0001) and DSS (P = 0.001 and P < 0.0001, respectively).

CONCLUSIONS

AR:ERα and PDEF:ERα ratios are independent predictors of the response to conventional ERα-directed tamoxifen endocrine therapy.

The potential influence of breast cancer estrogen receptors' distribution on active DNA demethylation

Alterations in DNA methylation may cause disturbances in regulation of gene expression, including drug metabolism and distribution. Moreover, many cancers, including breast cancer, are characterized by DNA hypomethylation and a decreased 5-hydroxymethylcytosine level. The abnormal cell growth found in breast carcinoma might be the result of impaired up-regulation of breast cancer receptors. Receptors’ expression in breast cancer determines clinical outcome, and it is possible that they lead to different DNA methylation patterns. Excessive steroid exposure can affect DNA methylation by promoting demethylation of CpG islands in promoter regions of genes, and hence may have an impact on promotion and progression of breast cancer cells. Tamoxifen, as a leading drug in breast cancer hormone therapy, has an ability to act like estrogen or antiestrogen depending on the type and localization of the breast cancer receptor. Further studies are needed to determine whether tamoxifen, similarly to steroids, may evoke changes in methylation pattern.

Targeted protein degradation in vivo with Proteolysis Targeting Chimeras: Current status and future considerations

Proteolysis Targeting Chimeras (PROTACs) are a rapidly expanding new therapeutic modality inducing selective protein degradation and offering the potential of a differentiated pharmacological profile across multiple therapeutic areas. As the repertoire of protein targets and E3 ligases available for incorporation into PROTACs continues to grow, understanding the drug- and system-dependent parameters for PROTACs will be critical for achieving tissue/cell specific pharmacology. The review discusses the current knowledge and future direction of in vivo PROTAC study evaluation. The importance of establishing the quantitative relationship between loss of protein target and biological function in vivo, coupled with building mechanistic PK/PD and ultimately PBPK/PD models, is emphasised with the aim to aid translation from preclinical to clinical space.

Dietary walnut altered gene expressions related to tumor growth, survival, and metastasis in breast cancer patients: a pilot clinical trial

Consumption of walnuts has slowed breast cancer growth and/or reduced the risk of mammary cancer in mice. The benefit against cancer was associated with altered expression of genes for cancer growth and survival. We hypothesized that walnut consumption would alter gene expression in pathologically confirmed breast cancers of women in a direction that would be expected to decrease breast cancer growth and survival, as was seen in mice. The study was a nonplacebo, 2-arm, clinical trial. Women with breast lumps large enough for research and pathology biopsies were recruited and randomized to walnut consuming or control groups. Immediately after biopsy collection, women in the walnut group began to consume 2 oz of walnuts per day until follow-up surgery. Pathological studies confirmed that lumps were breast cancer in all women who remained in the trial. At surgery, about 2 weeks after biopsy, additional specimens were taken from the breast cancers. Changes in gene expression in the surgical specimen compared to baseline were determined in each individual woman in walnut-consuming (n = 5) and control (n = 5) groups. RNA sequencing expression profiling revealed that expression of 456 identified genes was significantly changed in the tumor due to walnut consumption. Ingenuity Pathway Analysis showed activation of pathways that promote apoptosis and cell adhesion, and inhibition of pathways that promote cell proliferation and migration. These results support the hypothesis that, in humans, walnut consumption could suppress growth and survival of breast cancers.

History of Estrogen: Its Purification, Structure, Synthesis, Biologic Actions, and Clinical Implications

This mini-review summarizes key points from the Clark Sawin Memorial Lecture on the History of Estrogen delivered at Endo 2018 and focuses on the rationales and motivation leading to various discoveries and their clinical applications. During the classical period of antiquity, incisive clinical observations uncovered important findings; however, extensive anatomical dissections to solidify proof were generally lacking. Initiation of the experimental approach followed later, influenced by Claude Bernard's treatise "An Introduction to the Study of Experimental Medicine." With this approach, investigators began to explore the function of the ovaries and their "internal secretions" and, after intensive investigations for several years, purified various estrogens. Clinical therapies for hot flashes, osteoporosis, and dysmenorrhea were quickly developed and, later, methods of hormonal contraception. Sophisticated biochemical methods revealed the mechanisms of estrogen synthesis through the enzyme aromatase and, after discovery of the estrogen receptors, their specific biologic actions. Molecular techniques facilitated understanding of the specific transcriptional and translational events requiring estrogen. This body of knowledge led to methods to prevent and treat hormone-dependent neoplasms as well as a variety of other estrogen-related conditions. More recently, the role of estrogen in men was uncovered by prismatic examples of estrogen deficiency in male patients and by knockout of the estrogen receptor and aromatase in animals. As studies became more extensive, the effects of estrogen on nearly every organ were described. We conclude that the history of estrogen illustrates the role of intellectual reasoning, motivation, and serendipity in advancing knowledge about this important sex steroid.

Immunohistochemical analysis of estrogen receptor in breast cancer with ESR1 mutations detected by hybrid capture-based next-generation sequencing

Estrogen receptor-α (ER-α), encoded by ESR1, is detected by immunohistochemistry in approximately 70% of invasive breast cancers and serves as a strong predictive biomarker. ESR1-activating mutations in the ligand-binding domain have been reported in up to 35-40% of ER-positive metastatic breast cancers and are associated with endocrine therapy resistance and disease progression. At present, it is unclear whether ESR1 mutations alter the immunohistochemical detection of ER performed in routine clinical practice. In this study, ESR1 mutations in breast cancer were identified utilizing Memorial Sloan Kettering-Integrated Mutation Profiling of Actionable Cancer Targets (MSK-IMPACT), a Food and Drug Administration-approved hybridization capture-based next-generation sequencing assay. Five hundred and eighty-six breast cancers from patients with locally advanced or metastatic disease were analyzed using MSK-IMPACT in the study period. ESR1 somatic alterations were identified in 67 breast cancer samples from 66 patients. Immunohistochemical analysis of ER, progesterone receptor, and human epidermal growth factor receptor 2 was performed on the primary and treated breast cancers from these patients at the time of diagnosis. Twenty unique ESR1 mutations were identified involving the ligand-binding domain, all in breast cancer samples from patients previously treated with endocrine therapy. The most frequent mutations were D538G (n = 22), Y537S (n = 7), and E380Q (n = 7). All breast cancer samples with an ESR1 mutation were ER-positive by immunohistochemistry. Review of the ER immunohistochemistry in the paired untreated primary tumor and treated tumor from 34 patients showed no detectable change in the ER-positive immunohistochemical status (median percentage of invasive tumor cells with nuclear staining: untreated primary tumor 90%, treated tumor 95%). We conclude that ESR1 mutations do not appreciably diminish ER-positive staining by immunohistochemistry. In addition to standard biomarker testing by immunohistochemistry, the assessment of ESR1 mutations by molecular testing can help guide the clinical management of patients with ER-positive breast cancer in the setting of endocrine resistance and progression of disease.

C-3- and C-4-Substituted Bicyclic Coumarin Sulfamates as Potent Steroid Sulfatase Inhibitors

Synthetic routes to potent bicyclic nonsteroidal sulfamate-based active-site-directed inhibitors of the enzyme steroid sulfatase (STS), an emerging target in the treatment of postmenopausal hormone-dependent diseases, including breast cancer, are described. Sulfamate analogs 9-27 and 28-46 of the core in vivo active two-ring coumarin template, modified at the 4- and 3-positions, respectively, were synthesized to expand structure-activity relationships. α-Alkylacetoacetates were used to synthesize coumarin sulfamate derivatives with 3-position modifications, and the bicyclic ring of other parent coumarins was primarily constructed via the Pechmann synthesis of hydroxyl coumarins. Compounds were examined for STS inhibition in intact MCF-7 breast cancer cells and in placental microsomes. Low nanomolar potency STS inhibitors were achieved, and some were found to inhibit the enzyme in MCF-7 cells ca. 100-500 more potently than the parent 4-methylcoumarin-7-O-sulfamate 3, with the best compounds close in potency to the tricyclic clinical drug Irosustat. 3-Hexyl-4-methylcoumarin-7-O-sulfamate 29 and 3-benzyl-4-methylcoumarin-7-O-sulfamate 41 were particularly effective inhibitors with IC50 values of 0.68 and 1 nM in intact MCF-7 cells and 8 and 32 nM for placental microsomal STS, respectively. They were docked into the STS active site for comparison with estrone 3-O-sulfamate and Irosustat, showing their sulfamate group close to the catalytic hydrated formylglycine residue and their pendant group lying between the hydrophobic sidechains of L103, F178, and F488. Such highly potent STS inhibitors expand the structure-activity relationship for these coumarin sulfamate-based agents that possess therapeutic potential and may be worthy of further development.