Synthesis and evaluation of 11β-(4-substituted phenyl) estradiol analogs: transition from estrogen receptor agonists to antagonists

Novel 11β-substituted estradiol conjugates: Transition from ERα agonizts to effective PROTAC degraders

Endocrine therapy is widely used in clinic for breast cancer treatment, but long-term treatment inevitably causes drug resistance. Most of endocrine therapy-resistant breast cancers continue to depend on ERα signaling for growth and survival. In this regard, small molecule-induced ERα degradation, i.e. proteolysis targeting chimeras (PROTACs), represents an effective strategy to overcome endocrine resistance. Herein, we describe the design, synthesis, and biological evaluation of novel ERα-targeting PROTACs, wherein a E3 ligase ligand was attached to the 11β-position of estradiol via various linkers. Our efforts have identified a potent ERα PROTAC 15b that achieved excellent ERα degradation activity (DC₅₀ = 67 nM) and induced comparable inhibition of cell growth to that of fulvestrant in MCF-7 cells. Besides, 15b displayed antagonistic effects in uterine cells and favorable physicochemical properties, making it as a good lead compound for further development as anti-breast agents.

C-Ring Oxidized Estrone Acetate Derivatives: Assessment of Antiproliferative Activities and Docking Studies

C-Ring oxidized estrone acetate derivatives as antiproliferative agents were prepared and tested against five cancer cell lines by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Flow cytometry assays to evaluate cell viability and modifications in cell cycle phases and molecular docking research against estrogen receptor α, steroid sulfatase, and 17β-hydroxysteroid dehydrogenase type 1 were performed. 9α-Hydroxy,11β-nitrooxyestrone acetate was the most cytotoxic molecule against hormone-dependent cancer cells. Furthermore, flow cytometry experiments revealed that this 9α-hydroxy,11β-nitrooxy derivative markedly reduced HepaRG cells viability (~92%) after 24 h of treatment. However, 9α-hydroxyestrone acetate led to selective inhibition of HepaRG cells growth, inducing a G0/G1 cycle arrest, and did not originate a proliferation effect on T47-D cancer cells. Docking studies estimated a generally lower affinity of these compounds to estrogen receptor α than predicted for estrone and 17β-estradiol. Therefore, this structural modification can be of interest to develop new anticancer estrane derivatives devoid of estrogenic action.

Antiestrogens in combination with immune checkpoint inhibitors in breast cancer immunotherapy

Breast cancers (BCs) with expression of estrogen receptor-alpha (ERα) occur in more than 70% of newly-diagnosed patients in the U.S. Endocrine therapy with antiestrogens or aromatase inhibitors is an important intervention for BCs that express ERα, and it remains one of the most effective targeted treatment strategies. However, a substantial proportion of patients with localized disease, and essentially all patients with metastatic BC, become resistant to current endocrine therapies. ERα is present in most resistant BCs, and in many of these its activity continues to regulate BC growth. Fulvestrant represents one class of ERα antagonists termed selective ER downregulators (SERDs). Treatment with fulvestrant causes ERα down-regulation, an event that helps overcome several resistance mechanisms. Unfortunately, full antitumor efficacy of fulvestrant is limited by its poor bioavailability in clinic. We have designed and tested a new generation of steroid-like SERDs. Using ERα-positive BC cells in vitro, we find that these compounds suppress ERα protein levels with efficacy similar to fulvestrant. Moreover, these new SERDs markedly inhibit ERα-positive BC cell transcription and proliferation in vitro even in the presence of estradiol-17β. In vivo, the SERD termed JD128 significantly inhibited tumor growth in MCF-7 xenograft models in a dose-dependent manner (P < 0.001). Further, our findings indicate that these SERDs also interact with ER-positive immune cells in the tumor microenvironment such as myeloid-derived suppressor cells (MDSC), tumor infiltrating lymphocytes and other selected immune cell subpopulations. SERD-induced inhibition of MDSCs and concurrent actions on CD8+ and CD4 + T-cells promotes interaction of immune checkpoint inhibitors with BC cells in preclinical models, thereby leading to enhanced tumor killing even among highly aggressive BCs such as triple-negative BC that lack ERα expression. Since monotherapy with immune checkpoint inhibitors has not been effective for most BCs, combination therapies with SERDs that enhance immune recognition may increase immunotherapy responses in BC and improve patient survival. Hence, ERα antagonists that also promote ER downregulation may potentially benefit patients who are unresponsive to current endocrine therapies.

Synthesis of benzoylbenzamide derivatives of 17α-E-vinyl estradiol and evaluation as ligands for the estrogen receptor-α ligand binding domain

A series consisting of substituted benzoylbenzamide derivatives of 17α-E-vinyl estradiol 6a-i and 7a-d was prepared in good overall yields from the corresponding novel iodinated benzoylbenzamide precursors using Pd(0)-catalyzed Stille coupling. Biological evaluation using competitive binding assays indicated that all compounds were effective ligands for the ERα- and ERβ-LBD (RBA = 0.5-10.0% of estradiol). Most of the compounds expressed lower stimulatory (agonist) potency (RSA <0.2-0.5%) compared to their binding affinity, however, the meta-substituted isomer 6h demonstrated a level of efficacy (RSA = 5.7%) comparable to its affinity (RBA = 9.5%). Docking studies of 6b, 6h, and 6i with the 2YAT crystal structure suggested that higher affinity and efficacy of 6h are due to an effective set of interactions with exposed receptor sidechains not observed with the ortho- and para- isomers. In this binding model, the terminal ring of the ligand is exposed to the solvent space, which would explain both the small variation in RBA values and the narrow SAR for the diverse structural features.

Review of fluorescent steroidal ligands for the estrogen receptor 1995-2018

The development of fluorescent ligands for the estrogen receptor (ER) continues to be of interest. Over the past 20 years, most efforts have focused on appending an expanding variety of fluorophores to the B-, C- and D-rings of the steroidal scaffold. This review highlights the synthesis and evaluation of derivatives substituted primarily at the 6-, 7α- and 17α-positions, culminating with our recent work on 11β-substituted estradiols, and proposes an approach to new fluorescent imaging agents that retain high ER affinity.

Design, synthesis and biological evaluation of novel 2-methoxyestradiol analogs as dual selective estrogen receptor modulators (SERMs) and antiangiogenic agents

2-methoxyestradiol is a novel agent showing both anti-angiogenic and vascular disrupting properties. In this study, a series of 11α-substituted 2-methoxyestradiol analogs have been designed and synthesized targeting dual ERα and microtubulin. Biological evaluation was performed on their anti-proliferative activities against 5 different cell lines. The results indicated that most compounds exhibited good activities, in which compound 24c and 30c showed the best activity with low micromolar IC₅₀ (2.73 μM -7.75 μM) in all cell lines. The investigation of ER affinity showed that the majority of the compounds displayed good activity at the concentration of 50 μM. In further mechanism study, it was observed that 24c and 30c could induce G2/M cell cycle arrest as well as significant anti-estrogenic activity. In CAM assay, compound 24c and 30c presented significantly anti-angiogenesis activity comparable with 2-methoxyestradiol. Overall, based on biological activities data, 24c and 30c can be identified as a potential lead molecule which might be of therapeutic importance for cancer treatment.

Small Molecules for Active Targeting in Cancer

For the purpose of this review, active targeting in cancer research encompasses strategies wherein a ligand for a cell surface receptor expressed on tumor cells is used to deliver a cytotoxic or imaging cargo. This area of research is more than two decades old, but in those 20 and more years, how many receptors have been studied extensively? What kinds of the ligands are used for active targeting? Are they mostly naturally occurring molecules such as folic acid, or synthetic substances developed in campaigns for medicinal chemistry efforts? This review outlines the most important receptor or ligand combinations that have been used in active targeting to answer these questions, and therefore to address the most important one of all: is research in active targeting affording diminishing returns, or is this an area for which the potential far exceeds progress made so far?

Selective Human Estrogen Receptor Partial Agonists (ShERPAs) for Tamoxifen-Resistant Breast Cancer

Almost 70% of breast cancers are estrogen receptor α (ERα) positive. Tamoxifen, a selective estrogen receptor modulator (SERM), represents the standard of care for many patients; however, 30-50% develop resistance, underlining the need for alternative therapeutics. Paradoxically, agonists at ERα such as estradiol (E2) have demonstrated clinical efficacy in patients with heavily treated breast cancer, although side effects in gynecological tissues are unacceptable. A drug that selectively mimics the actions of E2 in breast cancer therapy but minimizes estrogenic effects in other tissues is a novel, therapeutic alternative. We hypothesized that a selective human estrogen receptor partial agonist (ShERPA) at ERα would provide such an agent. Novel benzothiophene derivatives with nanomolar potency in breast cancer cell cultures were designed. Several showed partial agonist activity, with potency of 0.8-76 nM, mimicking E2 in inhibiting growth of tamoxifen-resistant breast cancer cell lines. Three ShERPAs were tested and validated in xenograft models of endocrine-independent and tamoxifen-resistant breast cancer, and in contrast to E2, ShERPAs did not cause significant uterine growth.

Synthesis and evaluation of 2-halogenated-1,1-bis(4-hydroxyphenyl)-2-(3-hydroxyphenyl)-ethylenes as potential estrogen receptor-targeted radiodiagnostic and radiotherapeutic agents

A series of three 1,1-bis(4-hydroxyphenyl)-2-(3-hydroxyphenyl)-ethylene derivatives was prepared and evaluated as potential estrogen receptor imaging agents. The compounds display high binding affinity compared to estradiol, with the 2-iodo and 2-bromo-derivatives expressing higher affinity than the parent 2-nonhalogenated derivative. Evaluation in immature female rats also indicate that the compounds were all full estrogenic agonists with potencies in the same order of activity (I∼Br>H). Computational analysis of the interactions between the ligands and ERα-LBD demonstrated positive contribution of halide to binding properties. In preparation for studies using the radiohalogenated analogs, the corresponding protected 2-(tributylstannyl) derivative was prepared and converted to the corresponding 2-iodo-product.

Synthesis and preliminary evaluation steroidal antiestrogen-geldanamycin conjugates

Three novel steroidal antiestrogen-geldanamycin conjugates were prepared using a convergent strategy. The antiestrogenic component utilized the 11β-(4-functionalized-oxyphenyl) estradiol scaffold, while the geldanamycin component was derived by replacement of the 17-methoxy group with an appropriately functionalized amine. Ligation was achieved in high yield using azide alkyne cyclization reactions. Evaluation of the products against two breast cancer cell lines indicated that the conjugates retained significant antiproliferative activity.

Anticancer steroids: linking natural and semi-synthetic compounds

Steroids, a widespread class of natural organic compounds occurring in animals, plants and fungi, have shown great therapeutic value for a broad array of pathologies. The present overview is focused on the anticancer activity of steroids, which is very representative of a rich structural molecular diversity and ability to interact with various biological targets and pathways. This review encompasses the most relevant discoveries on steroid anticancer drugs and leads through the last decade and comprises 668 references.

Convergent synthesis of a steroidal antiestrogen-mitomycin C hybrid using "click" chemistry

A convergent synthesis of a novel estrogen receptor-targeted drug hybrid was developed based on structures of the potent anti-proliferative mitomycin C and the steroidal anti-estrogen RU 39411. The steroidal antiestrogen was prepared with an azido-triethylene glycoloxy linker while the mitomycin C derivative (porfirimycin) incorporated a complementary 7-N-terminal alkyne. The two components were ligated using the Huisgen [3 + 2] cycloaddition ("click") reaction. Preliminary biological assays demonstrated that the final hybrid compound retained both potent anti-estrogenic and anti-proliferative activities.