Antagonists for Constitutively Active Mutant Estrogen Receptors: Insights into the Roles of Antiestrogen-Core and Side-Chain

Abstract

A major risk for patients having estrogen receptor α (ERα)-positive breast cancer is the recurrence of drug-resistant metastases after initial successful treatment with endocrine therapies. Recent studies have implicated a number of activating mutations in the ligand-binding domain of ERα that stabilize the agonist conformation as a prominent mechanism for this acquired resistance. There are several critical gaps in our knowledge regarding the specific pharmacophore requirements of an antagonist that could effectively inhibit all or most of the different mutant ERs. To address this, we screened various chemotypes for blocking mutant ER-mediated transcriptional signaling and identified RU58668 as a model compound that contains structural elements that support potent ligand-induced inhibition of mutant ERs. We designed and synthesized a focused library of novel antagonists and probed how small and large perturbations in different ligand structural regions influenced inhibitory activity on individual mutant ERs in breast cancer cells. Effective inhibition derives from both non-polar and moderately polar motifs in a multifunctional side chain of the antagonists, with the nature of the ligand core making important contributions by increasing the potency of ligands possessing similar types of side chains. Some of our new antagonists potently blocked the transcriptional activity of the three most common mutant ERs (L536R, Y537S, D538G) and inhibited mutant ER-mediated cell proliferation. Supported by our molecular modeling, these studies provide new insights into the role of specific components, involving both the ligand core and multifunctional side chain, in suppressing wild-type and mutant ER-mediated transcription and breast cancer cell proliferation.

Computer-Aided Ligand Discovery for Estrogen Receptor Alpha

“…Celecoxib side chains in antagonistic activity of their proposed compounds on mutant forms of ERα [204]; Singla and colleagues used docking to study the poses of experimentally active novel indole-based antagonists [205]; and Lou et al used AutoDock to model their best designed hits [206,207]. SBDD using docking programs and pharmacophore models have accelerated the drug discovery process.…”

“…All the conjugates were predicted to bind better than tamoxifen, but this was not the case experimentally as docking scores are not always representative of reality [ 201 ]. Similarly, Katzenellenbogen and collaborators used docking to study the poses and contribution of side chains in antagonistic activity of their proposed compounds on mutant forms of ERα [ 204 ]; Singla and colleagues used docking to study the poses of experimentally active novel indole-based antagonists [ 205 ]; and Lou et al used AutoDock to model their best designed hits [ 206 , 207 ].…”

The Quest for Orally Available Selective Estrogen Receptor Degraders (SERDs)

“…These antiestrogens (AEs) have been hypothesized to overcome tamoxifen/AI breast cancer. [9,13,14] Fulvestrant 3 ( Figure 1A) is a steroidal ERα antagonist originally developed for its lack of agonism across all types of tissues tested, being retrospectively found to also be a SERD which induces ubiquitylation and proteasome-mediated degradation of ERα. [15] Indeed, it is the only FDA-approved therapy effective in postmenopausal women with advanced ERα-positive breast cancer who have progressed on endocrine therapy.…”

“…Selective estrogen receptor degraders (SERDs) offer the ability to not only antagonize ERα but also downregulate ERα protein levels. These antiestrogens (AEs) have been hypothesized to overcome tamoxifen/AI breast cancer [9,13,14] …”

Structural, Thermodynamic, and Kinetic Traits of Antiestrogen-Compounds Selectively Targeting the Y537S Mutant Estrogen Receptor α Transcriptional Activity in Breast Cancer Cell Lines

“…Targeted therapy counteracting mERα is a current object of intense preclinical and early clinical interest, as also evidenced by the significant number of studies aiming at identifying orally bioavailable SERDs, eventually able to overcome resistance (De Savi et al, 2015; Fanning et al, 2018; Hamilton et al, 2018; Sharma et al, 2018; Kahraman et al, 2019; Scott et al, 2019). Among these GDC-810, AZD9496 ( 5 ; Figure 1) (hereafter AZD, a drug in preclinical use as oral SERD, for which a first clinical study have been recently accomplished) (Weir et al, 2016; Hamilton et al, 2018), and LSZ102 have been identified (Scott et al, 2019).…”