Exploring the Structural Compliancy versus Specificity of the Estrogen Receptor Using Isomeric Three-Dimensional Ligands

Discovery of Novel ERα and Aromatase Dual-Targeting PROTAC Degraders to Overcome Endocrine-Resistant Breast Cancer

Estrogen receptor α (ERα) plays a pivotal role in the proliferation, differentiation, and migration of breast cancer (BC) cells, and aromatase (ARO) is a crucial enzyme in estrogen synthesis. Hence, it is necessary to inhibit estrogen production or the activity of ERα for the treatment of estrogen receptor-positive (ER+) BC. Herein, we present a new category of dual-targeting PROTAC degraders designed to specifically target ERα and ARO. Among them, compound 18c bifunctionally degrades and inhibits ERα/ARO, thus effectively suppressing the proliferation of MCF-7 cells while showing negligible cytotoxicity to normal cells. In vivo, 18c promotes the degradation of ERα and ARO and inhibits the growth of MCF-7 xenograft tumors. Finally, compound 18c demonstrates promising antiproliferative and ERα degradation activity against the ERαMUT cells. These findings suggest that 18c, being the inaugural dual-targeting degrader for ERα and ARO, warrants further advancement for the management of BC and the surmounting of endocrine resistance.

Cobalt-Catalyzed Domino Transformations via Enantioselective C-H Activation/Nucleophilic [3 + 2] Annulation toward Chiral Bridged Bicycles

Selective synthesis of chiral bridged (hetero)bicyclic scaffolds via asymmetric C-H activation constitutes substantial challenges due to the multiple reactivities of strained bicyclic structures. Herein, we develop the domino transformations through an unprecedented cobalt-catalyzed enantioselective C-H activation/nucleophilic [3 + 2] annulation with symmetrical bicyclic alkenes. The methods offer straightforward access to a wide range of chiral molecules bearing [2.2.1]-bridged bicyclic cores with four and five consecutive stereocenters in a single step. Two elaborate salicyloxazoline (Salox) ligands were synthesized based on the rational design and mechanistic understanding. The well-defined chiral pockets generated from asymmetric coordination around the trivalent cobalt catalyst direct the orientation of bicyclic alkenes, leading to excellent enantioselectivity.

Identification of Novel Dual-Target Estrogen Receptor α Degraders with Tubulin Inhibitory Activity for the Treatment of Endocrine-Resistant Breast Cancer

Endocrine resistance remains a significant problem in the clinical treatment of estrogen receptor α-positive (ERα+) breast cancer (BC). In this study, we developed a series of novel dual-functional ERα degraders based on a bridged bicyclic scaffold with selenocyano (SeCN) side chains. These compounds displayed potent ERα degradation and tubulin depolymerization activity. Among them, compounds 35s and 35t exhibited the most promising antiproliferative and ERα degradation activity in multiple ERα+ BC cell lines bearing either wild-type or mutant ERα. Meanwhile, compounds 35s and 35t disrupted the microtubule network by restraining tubulin polymerization, evidenced by 35t inducing cell cycle arrest in the G2/M phase. In MCF-7 and LCC2 xenograft models, compounds 35s and 35t remarkably suppressed tumor growth without noticeable poisonousness. Finally, this study provided guidance for developing new dual-target antitumor drug candidates for the ERα+ BC therapy, especially for the resistant variant.

Dual-mechanism estrogen receptor inhibitors

Efforts to improve estrogen receptor-α (ER)-targeted therapies in breast cancer have relied upon a single mechanism, with ligands having a single side chain on the ligand core that extends outward to determine antagonism of breast cancer growth. Here, we describe inhibitors with two ER-targeting moieties, one of which uses an alternate structural mechanism to generate full antagonism, freeing the side chain to independently determine other critical properties of the ligands. By combining two molecular targeting approaches into a single ER ligand, we have generated antiestrogens that function through new mechanisms and structural paradigms to achieve antagonism. These dual-mechanism ER inhibitors (DMERIs) cause alternate, noncanonical structural perturbations of the receptor ligand-binding domain (LBD) to antagonize proliferation in ER-positive breast cancer cells and in allele-specific resistance models. Our structural analyses with DMERIs highlight marked differences from current standard-of-care, single-mechanism antiestrogens. These findings uncover an enhanced flexibility of the ER LBD through which it can access nonconsensus conformational modes in response to DMERI binding, broadly and effectively suppressing ER activity.

Organophosphate ester tri-o-cresyl phosphate interacts with estrogen receptor α in MCF-7 breast cancer cells promoting cancer growth

Plastic in the ocean degrades to microplastic, thereby enhancing the leaching of incorporated plasticizers due to the increased particle surface. The uptake of microplastic-derived plasticizers by marine animals and the subsequent entry in the food chain raises concerns for adverse health effects in human beings. Frequently used plasticizers as the organophosphate ester tri-o-cresyl phosphate (TOCP) are known to affect the male reproductive system. However, the overall endocrine potential of TOCP and the underlying molecular mechanisms remain elusive as yet. In this study, we investigated the molecular effects of TOCP on estrogen receptor α (ERα)-transfected HEK-ESR1 cells and the human breast cancer cell line MCF-7. Applying virtual screening and molecular docking, we identified TOCP as potent ligand of ERα in silico. Microscale thermophoresis confirmed the binding in vitro with similar intensity as the natural ligand 17-β-estradiol. To identify the molecular mechanisms of TOCP-mediated effects, we used next-generation sequencing to analyze the gene expression pattern of TOCP-treated MCF-7 cells. RNA-sequencing revealed 22 differently expressed genes associated with ESR1 as upstream regulator: CYP1A1, SLC7A11, RUNX2, DDIT4, STC2, KLHL24, CCNG2, CEACAM5, SLC7A2, MAP1B, SLC7A5, IGF1R, CD55, FOSL2, VEGFA, and HSPA13 were upregulated and PRKCD, CCNE1, CEBPA, SFPQ, TNFAIP2, KRT19 were downregulated. The affected genes promote tumor growth by increasing angiogenesis and nutritional supply, favor invasion and metastasis, and interfere with the cell cycle. Based on the gene expression pattern, we conclude TOCP to mediate endocrine effects on MCF-7 cells by interacting with ERα.

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.

The SERM/SERD bazedoxifene disrupts ESR1 helix 12 to overcome acquired hormone resistance in breast cancer cells

Acquired resistance to endocrine therapy remains a significant clinical burden for breast cancer patients. Somatic mutations in the ESR1 (estrogen receptor alpha (ERα)) gene ligand-binding domain (LBD) represent a recognized mechanism of acquired resistance. Antiestrogens with improved efficacy versus tamoxifen might overcome the resistant phenotype in ER +breast cancers. Bazedoxifene (BZA) is a potent antiestrogen that is clinically approved for use in hormone replacement therapies. We found that BZA possesses improved inhibitory potency against the Y537S and D538G ERα mutants compared to tamoxifen and has additional inhibitory activity in combination with the CDK4/6 inhibitor palbociclib. In addition, comprehensive biophysical and structural biology studies show BZA’s selective estrogen receptor degrading (SERD) properties that override the stabilizing effects of the Y537S and D538G ERα mutations.

Novel Hybrid Conjugates with Dual Suppression of Estrogenic and Inflammatory Activities Display Significantly Improved Potency against Breast Cancer

In this work, we developed a small library of novel OBHS-RES hybrid compounds with dual inhibition activities targeting both the estrogen receptor α (ERα) and NF-κB by incorporating resveratrol (RES), a known inhibitor of NF-κB, into a privileged indirect antagonism structural motif (OBHS, oxabicycloheptene sulfonate) of estrogen receptor (ER). The OBHS-RES conjugates could bind well to ER and showed remarkable ERα antagonistic activity, and they also exhibited excellent NO inhibition in macrophage RAW 264.7 cells. Compared with 4-hydroxytamoxifen, some of them showed better antiproliferative efficacy in MCF-7 cell lines with IC₅₀ up to 3.7 μM. In vivo experiments in a MCF-7 breast cancer model in Balb/c nude mice indicated that compound 26a was more potent than tamoxifen. Exploration of the compliancy of the structure against ER specificity utilizing these types of isomeric three-dimensional ligands indicated that one enantiomer had much better biological activity than the other.

Separation of stereoisomers of 7-oxa-bicyclo[2.2.1]heptene sulfonate (OBHS), a Selective Estrogen Receptor Modulator (SERM), via chiral stationary phases using SFC/UV and SFC/MS

The enantiomeric separation of a racemate of 7-oxa-bicyclo[2.2.1]heptene sulfonate (OBHS) derivatives, a Selective Estrogen Receptor Modulator (SERM), was obtained using supercritical fluid chromatography in tandem with UV and mass spectrometry (SFC/UV and SFC/MS, respectively). Supercritical CO₂ modified with methanol or isopropyl alcohol was used with isopropylamine (IPAm), trimethylamine (TEA), or trifluoroacetic acid (TFA) as an additive to obtain the enantiomers separations. Both Chiralpak IC and IA were evaluated for the separation of enantiomers. Results showed enantiomers separation can be achieved in less than 5 min with a resolution greater than 1 and 0.9, respectively, for the different OBHS derivatives (compounds A and B) using supercritical CO₂ modified with 40% isopropyl alcohol containing 0.25% IPAm and IC column applying isocratic conditions. Similar conditions were used with the semi-preparative Chiralpak IC column to isolate more than 50 mg of each enantiomer. SFC/MS and SFC/UV results showed pure enantiomers were isolated. Method development via SFC was much simpler than those reported in the literature using HPLC.