Asymmetric allostery in estrogen receptor-α homodimers drives responses to the ensemble of estrogens in the hormonal milieu.

The estrogen receptor-α (ER) is thought to function only as a homodimer but responds to a variety of environmental, metazoan, and therapeutic estrogens at subsaturating doses, supporting binding mixtures of ligands as well as dimers that are only partially occupied. Here, we present a series of flexible ER ligands that bind to receptor dimers with individual ligand poses favoring distinct receptor conformations-receptor conformational heterodimers-mimicking the binding of two different ligands. Molecular dynamics simulations showed that the pairs of different ligand poses changed the correlated motion across the dimer interface to generate asymmetric communication between the dimer interface, the ligands, and the surface binding sites for epigenetic regulatory proteins. By examining the binding of the same ligand in crystal structures of ER in the agonist vs. antagonist conformers, we also showed that these allosteric signals are bidirectional. The receptor conformer can drive different ligand binding modes to support agonist vs. antagonist activity profiles, a revision of ligand binding theory that has focused on unidirectional signaling from the ligand to the coregulator binding site. We also observed differences in the allosteric signals between ligand and coregulator binding sites in the monomeric vs. dimeric receptor, and when bound by two different ligands, states that are physiologically relevant. Thus, ER conformational heterodimers integrate two different ligand-regulated activity profiles, representing different modes for ligand-dependent regulation of ER activity.

Asymmetric Allostery in Estrogen Receptor-α Homodimers Drives Responses to the Ensemble of Estrogens in the Hormonal Milieu.

The estrogen receptor-α (ER) is thought to function only as a homodimer, but responds to a variety of environmental, metazoan, and therapeutic estrogens at sub-saturating doses, supporting binding mixtures of ligands as well as dimers that are only partially occupied. Here, we present a series of flexible ER ligands that bind to receptor dimers with individual ligand poses favoring distinct receptor conformations -receptor conformational heterodimers-mimicking the binding of two different ligands. Molecular dynamics simulations showed that the pairs of different ligand poses changed the correlated motion across the dimer interface to generate asymmetric communication between the dimer interface, the ligands, and the surface binding sites for epigenetic regulatory proteins. By examining binding of the same ligand in crystal structures of ER in the agonist versus antagonist conformers, we also showed that these allosteric signals are bidirectional. The receptor conformer can drive different ligand binding modes to support agonist versus antagonist activity profiles, a revision of ligand binding theory that has focused on unidirectional signaling from ligand to the coregulator binding site. We also observed differences in the allosteric signals between ligand and coregulator binding sites in the monomeric versus dimeric receptor, and when bound by two different ligands, states that are physiologically relevant. Thus, ER conformational heterodimers integrate two different ligand-regulated activity profiles, representing new modes for ligand-dependent regulation of ER activity. Significance: The estrogen receptor-α (ER) regulates transcription in response to a hormonal milieu that includes low levels of estradiol, a variety of environmental estrogens, as well as ER antagonists such as breast cancer anti-hormonal therapies. While ER has been studied as a homodimer, the variety of ligand and receptor concentrations in different tissues means that the receptor can be occupied with two different ligands, with only one ligand in the dimer, or as a monomer. Here, we use X-ray crystallography and molecular dynamics simulations to reveal a new mode for ligand regulation of ER activity whereby sequence-identical homodimers can act as functional or conformational heterodimers having unique signaling characteristics, with ligand-selective allostery operating across the dimer interface integrating two different signaling outcomes.

ESR1 F404 Mutations and Acquired Resistance to Fulvestrant in ESR1-Mutant Breast Cancer.

Fulvestrant is used to treat patients with hormone receptor-positive advanced breast cancer, but acquired resistance is poorly understood. PlasmaMATCH Cohort A (NCT03182634) investigated the activity of fulvestrant in patients with activating ESR1 mutations in circulating tumor DNA (ctDNA). Baseline ESR1 mutations Y537S are associated with poor outcomes and Y537C with good outcomes. Sequencing of baseline and EOT ctDNA samples (n = 69) revealed 3/69 (4%) patients acquired novel ESR1 F404 mutations (F404L, F404I, and F404V), in cis with activating mutations. In silico modeling revealed that ESR1 F404 contributes to fulvestrant binding to estrogen receptor-alpha (ERα) through a pi-stacking bond, with mutations disrupting this bond. In vitro analysis demonstrated that single F404L, E380Q, and D538G models were less sensitive to fulvestrant, whereas compound mutations D538G + F404L and E380Q + F404L were resistant. Several oral ERα degraders were active against compound mutant models. We have identified a resistance mechanism specific to fulvestrant that can be targeted by treatments in clinical development. SIGNIFICANCE: Novel F404 ESR1 mutations may be acquired to cause overt resistance to fulvestrant when combined with preexisting activating ESR1 mutations. Novel combinations of mutations in the ER ligand binding domain may cause drug-specific resistance, emphasizing the potential of similar drug-specific mutations to impact the efficacy of oral ER degraders in development. This article is featured in Selected Articles from This Issue, p. 201.

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.

Development of novel phenoxy-diketopiperazine-type plinabulin derivatives as potent antimicrotubule agents based on the co-crystal structure.

The co-crystal structure of Compound 6b with tubulin was prepared and solved for indicating the binding mode and for further optimization. Based on the co-crystal structures of tubulin with plinabulin and Compound 6b, a total of 27 novel A/B/C-rings plinabulin derivatives were designed and synthesized. Their biological activities were evaluated against human lung cancer NCI-H460 cell line. The optimum phenoxy-diketopiperazine-type Compound 6o exhibited high potent cytotoxicity (IC50 = 4.0 nM) through SAR study of three series of derivatives, which was more potent than plinabulin (IC50 = 26.2 nM) and similar to Compound 6b (IC50 = 3.8 nM) against human lung cancer NCI-H460 cell line. Subsequently, the Compound 6o was evaluated against other four human cancer cell lines. Both tubulin polymerization assay and immunofluorescence assay showed that Compound 6o could inhibit microtubule polymerization efficiently. Furthermore, theoretical calculation of the physical properties and molecular docking were elucidated for these plinabulin derivatives. The binding mode of Compound 6o was similar to Compound 6b based on the result of molecular docking. The theoretical calculated LogPo/w and PCaco of Compound 6o were better than Compound 6b, which could enhance its cytostatic activity. Therefore, Compound 6o might be developed as a novel potent anti-microtubule agent.

In vitro and in vivo pharmacokinetic and pharmacodynamic study of MBRI-001, a deuterium-substituted plinabulin derivative as a potent anti-cancer agent.

MBRI-001 was demonstrated preliminary better pharmacokinetics and antitumor effects than that of plinabulin in vivo. In this approach, we further carried out systematic pharmacokinetic and pharmacodynamic study of MBRI-001 in vitro and in vivo. MBRI-001 was tested stable in rat plasma and more stable in liver microsomes than plinabulin in vitro. In vivo, MBRI-001 could be distributed rapidly and widely in various tissues, especially the concentration of MBRI-001 in lung was remarkably higher than other tissues. Excretion study indicated that MBRI-001 might been decomposed and excreted as metabolites. Additionally, the combination treatment of MBRI-001 and gefitinib revealed better antitumor inhibition rate than monotherapy in vivo. Therefore, we suggest that MBRI-001 could be developed as a promising anti-cancer agent in near future.

Design, synthesis and biological evaluation of anti-pancreatic cancer activity of plinabulin derivatives based on the co-crystal structure.

Based on the co-crystal structures of tubulin with plinabulin and Compound 1 (a derivative of plinabulin), a total of 18 novel plinabulin derivatives were designed and synthesized. Their biological activities were evaluated against human pancreatic cancer BxPC-3 cell lines. Two novel Compounds 13d and 13e exhibited potent activities with IC50 at 1.56 and 1.72 nM, respectively. The tubulin polymerization assay indicated that these derivatives could inhibit microtubule polymerization. Furthermore, the interaction between tubulin and these compounds were elucidated by molecular docking. The binding modes of Compounds 13d and 13e were similar to the co-crystal structure of Compound 1. H-π interaction was observed between the aromatic hydrogen of thiophene moiety with Phe20, which could enhance their binding affinities.

Synthesis of deuterium-enriched and fluorine-substituted plinabulin derivatives and evaluation of their antitumor activities.

Deuterium-enriched and fluorine-substituted compounds have been widely applied in drug discovery due to their advantages in the studies of clinical pharmacokinetics and metabolic profiles. Herein we synthesized a library of deuterated and fluorine-substituted plinabulin derivatives, and all 15 D- or F-compounds were characterized by MS, [Formula: see text] NMR and [Formula: see text]. Their antitumor activities were evaluated against human Jurkat T lymphocyte cells.

Development of MBRI-001, a deuterium-substituted plinabulin derivative as a potent anti-cancer agent.

Plinabulin, a drug targeting microtubule of cancer cells, has been currently tried in its phase III clinical study. However, low efficacy caused by poor pharmacokinetic (PK) properties has been considered to be the main obstacle to approved by the Food and Drug Administration. Herein, we introduced a deuterium atom as an isostere in its structure to become a new compound named (MBRI-001, No. 9 in a series of deuterium-substituted compounds). The structure of MBRI-001 was characterized by HRMS, NMR, IR and a single crystal analysis. MBRI-001 exhibited better pharmacokinetic characteristics than that of plinabulin. Additionally, its antitumor activity is in a low nanomolar level for a variety of cancer cell lines and high activity against human NCI-H460 xenograted in mice intravenous administration. Importantly, continuous administration of MBRI-001 exhibited lower toxicity compared to docetaxel. We thus suggest that MBRI-001 could be developed as a promising anti-cancer agent in near future.

First synthesis of 12-oxosoladulcidine and its derivatives as potential antitumor steroidal alkaloids.

The first synthesis of 12-oxosoladulcidine (= (3beta,5alpha,22alpha,25R)-3-hydroxyspirosolan-12-one; 4) is reported, and its structure was confirmed by single-crystal X-ray diffraction analysis. Compound 4 was readily obtained in five steps in an overall yield of 31%, starting from hecogenin (5). By slightly modifying the synthetic protocol, eight analogues of 4 were also prepared. The title compound and its derivatives are expected to be potent antitumor alkaloids, since structurally closely related to the known antitumor agents soladulcidine (2) and hecogenin (5).