Identification and characterization of the first small molecule inhibitor of MDMX

Cellular, Structural Basis, and Recent Progress for Targeting Murine Double Minute X (MDMX) in Tumors

Murine double minute X (MDMX) is an oncoprotein that mainly has a negative regulatory effect on the tumor suppressor p53 to induce tumorigenesis. As MDMX is highly expressed in various types of tumor cells, targeting and inhibiting MDMX are becoming a promising strategy for treating cancers. However, the high degree of structural homology between MDMX and its homologous protein murine double minute 2 (MDM2) is a great challenge for the development of MDMX-targeted therapies. This review introduces the structure, distribution, and regulation of the MDMX, summarizes the structural features and structure-activity relationships (SARs) of MDMX ligands, and focuses on the differences between MDMX and MDM2 in these aspects. Our purpose of this work is to propose potential strategies to achieve the specific targeting of MDMX.

Hdm2 disrupts HdmX-mediated nuclear export of p53 by sequestering it in nucleus

Dysfunction of the tumor suppressor p53 occurs in most human cancers, Hdm2 and HdmX play critical roles in p53 inactivation and degradation. Under unstressed conditions, HdmX binds to p53 like Hdm2, but HdmX cannot directly induce p53 degradation. Moreover, HdmX has been reported to stimulate Hdm2-mediated ubiquitination and degradation of p53. Here we reported that HdmX promoted the nuclear export of p53 independent of Hdm2 in living cells using FRET technology. Whereas, Hdm2 impeded HdmX-mediated nuclear export of p53 by sequestering it in nucleus. Interestingly, the C-terminal RING domain mutant Hdm2C464A formed heterooligomers with p53 in nucleus, which was inhibited by HdmX. The heterooligomers were located near PML-NBs. This study indicate that the nuclear Hdm2-HdmX interaction aborts the HdmX-mediated nuclear export of p53.

Targeting Hdm2 and Hdm4 in Anticancer Drug Discovery: Implications for Checkpoint Inhibitor Immunotherapy

Hdm2 and Hdm4 are structural homologs that regulate the tumor suppressor protein, p53. Since some tumors express wild-type p53, Hdm2 and Hdm4 are plausible targets for anticancer drugs, especially in tumors that express wild-type p53. Hdm4 can enhance and antagonize the activity of Tp53, thereby playing a critical role in the regulation of p53's activity and stability. Moreover, Hdm2 and Hdm4 are overexpressed in many cancers, some expressing wild-type Tp53. Due to experimental evidence suggesting that the activation of wild-type Tp53 can augment the antitumor activity by some checkpoint inhibitors, drugs targeting Hdm2 and Hdm4 may be strong candidates for combining with checkpoint inhibitor immunotherapy. However, other evidence suggests that the overexpression of Hdm2 and Hdm4 may indicate poor response to immune checkpoint inhibitors. These findings require careful examination and scrutiny. In this article, a comprehensive analysis of the Hdm2/Hdm4 partnership will be conducted. Furthermore, this article will address the current progress of drug development regarding molecules that target the Hdm2/Hdm4/Tp53 partnership.

Discovery and optimization of (2-naphthylthio)acetic acid derivative as selective Bfl-1 inhibitor

Bcl-2 anti-apoptotic protein family suppresses cell death by deploying a surface groove to capture the critical BH3 α-helix of pro-apoptotic members. Bfl-1 is a relatively understudied member of this family, though it has been implicated in the pathogenesis and chemoresistance of a variety of human cancers. Reported small molecular Bfl-1 inhibitors encountered the issue of either lack in potency or poor selectivity against its most homologous member Mcl-1. In order to tackle this issue, compound library was screened and a hit compound UMI-77 was identified. We modified its chemical structure to remove the characteristic of PAINS (pan-assay interference compounds), demonstrated the real binding affinity and achieved selectivity against Mcl-1 under the guidance of computational modeling. After optimization 15 was obtained as leading compound to block Bfl-1/BIM interaction in vitro with more than 10-fold selectivity over Mcl-1. We believe 15 is of great value for the exploration of Bfl-1 biological function and its potential as therapeutic target.

Pharmacological inhibition of MDM4 alleviates pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive and fatal lung disease of unknown etiology with no cure. A better understanding of the disease processes and identification of druggable targets will benefit the development of effective therapies for IPF. We previously reported that MDM4 promoted lung fibrosis through the MDM4-p53-dependent pathway. However, it remained unclear whether targeting this pathway would have any therapeutic potential. In this study, we evaluated the efficacy of XI-011, a small molecular inhibitor of MDM4, for treating lung fibrosis. We found that XI-011 significantly reduced MDM4 expression and increased the expression of total and acetylated p53 in primary human myofibroblasts and a murine fibrotic model. XI-011 treatment resulted in the resolution of lung fibrosis in mice with no notable impact on normal fibroblast death or the morphology of healthy lungs. Based on these findings, we propose that XI-011 might be a promising therapeutic drug candidate for treating pulmonary fibrosis.

Computational approaches for the design of modulators targeting protein-protein interactions

BACKGROUND

Protein-protein interactions (PPIs) are intriguing targets for designing novel small-molecule inhibitors. The role of PPIs in various infectious and neurodegenerative disorders makes them potential therapeutic targets . Despite being portrayed as undruggable targets, due to their flat surfaces, disorderedness, and lack of grooves. Recent progresses in computational biology have led researchers to reconsider PPIs in drug discovery.

AREAS COVERED

In this review, we introduce in-silico methods used to identify PPI interfaces and present an in-depth overview of various computational methodologies that are successfully applied to annotate the PPIs. We also discuss several successful case studies that use computational tools to understand PPIs modulation and their key roles in various physiological processes.

EXPERT OPINION

Computational methods face challenges due to the inherent flexibility of proteins, which makes them expensive, and result in the use of rigid models. This problem becomes more significant in PPIs due to their flexible and flat interfaces. Computational methods like molecular dynamics (MD) simulation and machine learning can integrate the chemical structure data into biochemical and can be used for target identification and modulation. These computational methodologies have been crucial in understanding the structure of PPIs, designing PPI modulators, discovering new drug targets, and predicting treatment outcomes.

Structure-based discovery of novel α-aminoketone derivatives as dual p53-MDM2/MDMX inhibitors for the treatment of cancer

The function of the p53 protein is impaired by the overexpression of its negative regulator murine double minute 2 protein (MDM2) and homologous protein MDMX. Disruption of the p53-MDM2/MDMX interaction to restore the transcriptional function of p53 is considered a promising strategy for cancer therapy. To design dual MDM2/MDMX inhibitors, the binding modes of MDM2 or MDMX with their inhibitors are elucidated. Several hot-spot residues of MDM2 or MDMX are identified by molecular dynamics simulations, alanine scanning and MM-GBSA calculations. Then, focusing on the interaction with hot-spot residues, two series of derivatives bearing 1,3-diketone and α-aminoketone scaffolds are designed and synthesized. Among these compounds, C16 is identified as the most potent compound with low micromolar binding affinities with MDM2 and MDMX. C16 also displays moderate antiproliferative activities against MDM2-overexpressing and MDMX-overexpressing cells, with IC₅₀ values of 0.68 μM in HCT116 cells and 0.54 μM in SH-SY5Y cells. Furthermore, C16 inhibits cell migration and invasion, reactivates the function of p53, arrests the cell cycle and induces cellular apoptosis in HCT116 and SH-SY5Y cells. Collectively, C16 can be developed as a dual MDM2 and MDMX inhibitor for cancer therapy.

Role of p53 in breast cancer progression: An insight into p53 targeted therapy

The transcription factor p53 is an important regulator of a multitude of cellular processes. In the presence of genotoxic stress, p53 is activated to facilitate DNA repair, cell cycle arrest, and apoptosis. In breast cancer, the tumor suppressive activities of p53 are frequently inactivated by either the overexpression of its negative regulator MDM2, or mutation which is present in 30-35% of all breast cancer cases. Notably, the frequency of p53 mutation is highly subtype dependent in breast cancers, with majority of hormone receptor-positive or luminal subtypes retaining the wild-type p53 status while hormone receptor-negative patients predominantly carry p53 mutations with gain-of-function oncogenic activities that contribute to poorer prognosis. Thus, a two-pronged strategy of targeting wild-type and mutant p53 in different subtypes of breast cancer can have clinical relevance. The development of p53-based therapies has rapidly progressed in recent years, and include unique small molecule chemical inhibitors, stapled peptides, PROTACs, as well as several genetic-based approaches using vectors and engineered antibodies. In this review, we highlight the therapeutic strategies that are in pre-clinical and clinical development to overcome p53 inactivation in both wild-type and mutant p53-bearing breast tumors, and discuss their efficacies and limitations in pre-clinical and clinical settings.

MDMX elevation by a novel Mdmx-p53 interaction inhibitor mitigates neuronal damage after ischemic stroke

Mdmx and Mdm2 are two major suppressor factors for the tumor suppressor gene p53. In central nervous system, Mdmx suppresses the transcriptional activity of p53 and enhances the binding of Mdm2 to p53 for degradation. But Mdmx dynamics in cerebral infarction remained obscure. Here we investigated the role of Mdmx under ischemic conditions and evaluated the effects of our developed small-molecule Protein-Protein Interaction (PPI) inhibitors, K-181, on Mdmx-p53 interactions in vivo and in vitro. We found ischemic stroke decreased Mdmx expression with increased phosphorylation of Mdmx Serine 367, while Mdmx overexpression by AAV-Mdmx showed a neuroprotective effect on neurons. The PPI inhibitor, K-181 attenuated the neurological deficits by increasing Mdmx expression in post-stroke mice brain. Additionally, K-181 selectively inhibited HDAC6 activity and enhanced tubulin acetylation. Our findings clarified the dynamics of Mdmx in cerebral ischemia and provide a clue for the future pharmaceutic development of ischemic stroke.

Molecular Targeting of the Most Functionally Complex Gene in Precision Oncology: p53

While chemotherapy is a key treatment strategy for many solid tumors, it is rarely curative, and most tumor cells eventually become resistant. Because of this, there is an unmet need to develop systemic treatments that capitalize on the unique mutational landscape of each patient's tumor. The most frequently mutated protein in cancer, p53, has a role in nearly all cancer subtypes and tumorigenesis stages and therefore is one of the most promising molecular targets for cancer treatment. Unfortunately, drugs targeting p53 have seen little clinical success despite promising preclinical data. Most of these drug compounds target specific aspects of p53 inactivation, such as through inhibiting negative regulation by the mouse double minute (MDM) family of proteins. These treatment strategies fail to address cancer cells' adaptation mechanisms and ignore the impact that p53 loss has on the entire p53 network. However, recent gene therapy successes show that targeting the p53 network and cellular dysfunction caused by p53 inactivation is now possible and may soon translate into successful clinical responses. In this review, we discuss p53 signaling complexities in cancer that have hindered the development and use of p53-targeted drugs. We also describe several current therapeutics reporting promising preclinical and clinical results.

Synthesis and Biological Evaluation of Novel Synthetic Indolone Derivatives as Anti-Tumor Agents Targeting p53-MDM2 and p53-MDMX

A series of novel indolone derivatives were synthesized and evaluated for their binding affinities toward MDM2 and MDMX. Some compounds showed potent MDM2 and moderate MDMX activities. Among them, compound A13 exhibited the most potent affinity toward MDM2 and MDMX, with a K_i of 0.031 and 7.24 μM, respectively. A13 was also the most potent agent against HCT116, MCF7, and A549, with IC₅₀ values of 6.17, 11.21, and 12.49 μM, respectively. Western blot analysis confirmed that A13 upregulated the expression of MDM2, MDMX, and p53 by Western blot analysis. These results indicate that A13 is a potent dual p53-MDM2 and p53-MDMX inhibitor and deserves further investigation.

Advanced Strategies for Therapeutic Targeting of Wild-Type and Mutant p53 in Cancer

TP53 is a tumor suppressor gene that encodes a sequence-specific DNA-binding transcription factor activated by stressful stimuli; it upregulates target genes involved in growth suppression, cell death, DNA repair, metabolism, among others. TP53 is the most frequently mutated gene in tumors, with mutations not only leading to loss-of-function (LOF), but also gain-of-function (GOF) that promotes tumor progression, and metastasis. The tumor-specific status of mutant p53 protein has suggested it is a promising target for cancer therapy. We summarize the current progress of targeting wild-type and mutant p53 for cancer therapy through biotherapeutic and biopharmaceutical methods for (1) boosting p53 activity in cancer, (2) p53-dependent and p53-independent strategies for targeting p53 pathway functional restoration in p53-mutated cancer, (3) targeting p53 in immunotherapy, and (4) combination therapies targeting p53, p53 checkpoints, or mutant p53 for cancer therapy.

Leveraging the multivalent p53 peptide-MdmX interaction to guide the improvement of small molecule inhibitors

Overexpressed Mdm2 and its 7homolog MdmX impair p53 activity in many cancers. Small molecules mimicking a p53 peptide can effectively inhibit Mdm2 but not MdmX. Here, we show a strategy for improving lead compounds for Mdm2 and MdmX inhibition based on the multivalency of the p53 peptide. Crystal structures of MdmX complexed with nutlin-3a, a strong Mdm2 inhibitor but a weak one for MdmX, reveal that nutlin-3a fits into the ligand binding pocket of MdmX mimicking the p53 peptide. However, due to distinct flexibility around the MdmX ligand binding pocket, the structures are missing many important intermolecular interactions that exist in the MdmX/p53 peptide and Mdm2/nultin-3a complexes. By targeting these flexible regions, we identify allosteric and additive fragments that enhance the binding affinity of nutlin-3a for MdmX, leading to potent Mdm2/MdmX inhibitors with anticancer activity. Our work provides a practical approach to drug design for signal transduction therapy.

Peptide fragments derived from the interfaces of protein-protein interactions (PPIs) provide useful templates for designing small molecule PPI inhibitors. Here, the authors utilize the multivalency of an MdmX-binding p53 peptide to develop a weak inhibitor of MdmX into potent Mdm2/MdmX inhibitors.

It’s Getting Complicated—A Fresh Look at p53-MDM2-ARF Triangle in Tumorigenesis and Cancer Therapy

Anti-tumorigenic mechanisms mediated by the tumor suppressor p53, upon oncogenic stresses, are our bodies’ greatest weapons to battle against cancer onset and development. Consequently, factors that possess significant p53-regulating activities have been subjects of serious interest from the cancer research community. Among them, MDM2 and ARF are considered the most influential p53 regulators due to their abilities to inhibit and activate p53 functions, respectively. MDM2 inhibits p53 by promoting ubiquitination and proteasome-mediated degradation of p53, while ARF activates p53 by physically interacting with MDM2 to block its access to p53. This conventional understanding of p53-MDM2-ARF functional triangle have guided the direction of p53 research, as well as the development of p53-based therapeutic strategies for the last 30 years. Our increasing knowledge of this triangle during this time, especially through identification of p53-independent functions of MDM2 and ARF, have uncovered many under-appreciated molecular mechanisms connecting these three proteins. Through recognizing both antagonizing and synergizing relationships among them, our consideration for harnessing these relationships to develop effective cancer therapies needs an update accordingly. In this review, we will re-visit the conventional wisdom regarding p53-MDM2-ARF tumor-regulating mechanisms, highlight impactful studies contributing to the modern look of their relationships, and summarize ongoing efforts to target this pathway for effective cancer treatments. A refreshed appreciation of p53-MDM2-ARF network can bring innovative approaches to develop new generations of genetically-informed and clinically-effective cancer therapies.

Organocatalytic (5+1) benzannulation of Morita–Baylis–Hillman carbonates: synthesis of multisubstituted 4-benzylidene pyrazolones

DABCO-catalyzed (5+1) cycloaddition of MBH carbonate undergoes an α-double deprotonation pathway to de novo assemble the benzene ring.

Tumor suppressor p53: Biology, signaling pathways, and therapeutic targeting

TP53 is the most commonly mutated gene in human cancer with over 100,000 literature citations in PubMed. This is a heavily studied pathway in cancer biology and oncology with a history that dates back to 1979 when p53 was discovered. The p53 pathway is a complex cellular stress response network with multiple diverse inputs and downstream outputs relevant to its role as a tumor suppressor pathway. While inroads have been made in understanding the biology and signaling in the p53 pathway, the p53 family, transcriptional readouts, and effects of an array of mutants, the pathway remains challenging in the realm of clinical translation. While the role of mutant p53 as a prognostic factor is recognized, the therapeutic modulation of its wild-type or mutant activities remain a work-in-progress. This review covers current knowledge about the biology, signaling mechanisms in the p53 pathway and summarizes advances in therapeutic development.

Recent Progress and Clinical Development of Inhibitors that Block MDM4/p53 Protein-Protein Interactions

MDM4 is a homologue of MDM2, serving cooperatively as the negative regulator of tumor suppressor p53. Under the shadow of MDM2 inhibitors, limited efforts had been put into the discovery of MDM4 modulators. Recent studies of the experimental drug ALRN-6924, a dual MDM4 and MDM2 inhibitor, suggest that concurrent inhibition of MDM4 and MDM2 might be beneficial over only MDM2 inhibition. In view of the present research progress, we summarized published inhibitors of MDM4/p53 interactions including both peptide-based compounds and small molecules. Cocrystal structures of ligand/MDM4 complexes have been examined, and their structural features were compiled and compared in order to show the molecular basis required for high MDM4 binding affinities. Representative examples of small-molecule MDM4 inhibitors were discussed, followed by clinical results of ALRN-6924, together, providing a consolidated reference for further development of MDM4 inhibitors, either dual or selective.

Identification of Secondary Binding Sites on Protein Surfaces for Rational Elaboration of Synthetic Protein Mimics

Minimal mimics of protein conformations provide rationally designed ligands to modulate protein function. The advantage of minimal mimics is that they can be chemically synthesized and coaxed to be proteolytically resistant; a key disadvantage is that minimization of the protein binding epitope may be associated with loss of affinity and specificity. Several approaches to overcome this challenge may be envisioned, including deployment of covalent warheads and use of nonnatural residues to improve contacts with the binding surface. Herein, we describe our computational and experimental efforts to enhance the minimal protein mimics with fragments that can contact undiscovered binding pockets on Mdm2 and MdmX-two well-studied protein partners of p53.

A Diarylpentanoid with Potential Activation of the p53 Pathway: Combination of in silico Screening Studies, Synthesis, and Biological Activity Evaluation

In silico studies of a library of diarylpentanoids led us to the identification of potential new MDM2/X ligands. The diarylpentanoids with the best docking scores obeying the druglikeness and ADMET prediction properties were subsequently synthesized and evaluated for their antiproliferative activity on colon cancer HCT116 and fibroblasts HFF-1 cells. The effect on p53-MDM2/X interactions was evaluated through yeast-based assays for compounds showing potent antiproliferative activity in HCT116 cells and low toxicity in normal cells, resulting in the identification of a potential dual inhibitor. Moreover, its antiproliferative effect was significantly reduced in the absence of p53 and in MDA-MB-231 cells expressing a mutant p53 form. The antiproliferative effect of this compound was associated with induction of cell cycle arrest, apoptosis, PARP cleavage and increased p53 and its transcriptional targets, p21 and PUMA, in HCT116 cells. Docking poses and residues involved in the inhibition of p53-MDM2/X interactions were predicted by docking studies.

Computational Modeling as a Tool to Investigate PPI: From Drug Design to Tissue Engineering

Protein-protein interactions (PPIs) mediate a large number of important regulatory pathways. Their modulation represents an important strategy for discovering novel therapeutic agents. However, the features of PPI binding surfaces make the use of structure-based drug discovery methods very challenging. Among the diverse approaches used in the literature to tackle the problem, linear peptides have demonstrated to be a suitable methodology to discover PPI disruptors. Unfortunately, the poor pharmacokinetic properties of linear peptides prevent their direct use as drugs. However, they can be used as models to design enzyme resistant analogs including, cyclic peptides, peptide surrogates or peptidomimetics. Small molecules have a narrower set of targets they can bind to, but the screening technology based on virtual docking is robust and well tested, adding to the computational tools used to disrupt PPI. We review computational approaches used to understand and modulate PPI and highlight applications in a few case studies involved in physiological processes such as cell growth, apoptosis and intercellular communication.

p73: From the p53 shadow to a major pharmacological target in anticancer therapy

p73, along with p53 and p63, belongs to the p53 family of transcription factors. Besides the p53-like tumor suppressive activities, p73 has unique roles, namely in neuronal development and differentiation. In addition, the TP73 gene is rarely mutated in tumors. This makes p73 a highly appealing therapeutic target, particularly towards cancers with a null or disrupted p53 pathway. Distinct isoforms are transcribed from the TP73 locus either with (TAp73) and without (ΔNp73) the N-terminal transactivation domain. Conversely to TA tumor suppressors, ΔN proteins exhibit oncogenic properties by inhibiting p53 and TA protein functions. As such, p73 isoforms compose a puzzled and challenging regulatory pathway. This state-of-the-art review affords an update overview on p73 structure, biological functions and pharmacological regulation. Importantly, it addresses the relevance of p73 isoforms in carcinogenesis, highlighting their potential as drug targets in anticancer therapy. A critical discussion of major pharmacological approaches to promote p73 tumor suppressive activities, with relevant survival outcomes for cancer patients, is also provided.

Retinoblastoma: Etiology, Modeling, and Treatment

Retinoblastoma is a retinal cancer that is initiated in response to biallelic loss of RB1 in almost all cases, together with other genetic/epigenetic changes culminating in the development of cancer. RB1 deficiency makes the retinoblastoma cell-of-origin extremely susceptible to cancerous transformation, and the tumor cell-of-origin appears to depend on the developmental stage and species. These are important to establish reliable preclinical models to study the disease and develop therapies. Although retinoblastoma is the most curable pediatric cancer with a high survival rate, advanced tumors limit globe salvage and are often associated with high-risk histopathological features predictive of dissemination. The advent of chemotherapy has improved treatment outcomes, which is effective for globe preservation with new routes of targeted drug delivery. However, molecularly targeted therapeutics with more effectiveness and less toxicity are needed. Here, we review the current knowledge concerning retinoblastoma genesis with particular attention to the genomic and transcriptomic landscapes with correlations to clinicopathological characteristics, as well as the retinoblastoma cell-of-origin and current disease models. We further discuss current treatments, clinicopathological correlations, which assist in guiding treatment and may facilitate globe preservation, and finally we discuss targeted therapeutics for future treatments.

Targeting MDMX for Cancer Therapy: Rationale, Strategies, and Challenges

The oncogene MDMX, also known as MDM4 is a critical negative regulator of the tumor suppressor p53 and has been implicated in the initiation and progression of human cancers. Increasing evidence indicates that MDMX is often amplified and highly expressed in human cancers, promotes cancer cell growth, and inhibits apoptosis by dampening p53-mediated transcription of its target genes. Inhibiting MDMX-p53 interaction has been found to be effective for restoring the tumor suppressor activity of p53. Therefore, MDMX is becoming one of the most promising molecular targets for developing anticancer therapeutics. In the present review, we mainly focus on the current MDMX-targeting strategies and known MDMX inhibitors, as well as their mechanisms of action and in vitro and in vivo anticancer activities. We also propose other potential targeting strategies for developing more specific and effective MDMX inhibitors for cancer therapy.

Small-molecule MDM2/X inhibitors and PROTAC degraders for cancer therapy: advances and perspectives

Blocking the MDM2/X–P53 protein–protein interaction has been widely recognized as an attractive therapeutic strategy for the treatment of cancers. Numerous small-molecule MDM2 inhibitors have been reported since the release of the structure of the MDM2–P53 interaction in 1996, SAR405838, NVP-CGM097, MK-8242, RG7112, RG7388, DS-3032b, and AMG232 currently undergo clinical evaluation for cancer therapy. This review is intended to provide a comprehensive and updated overview of MDM2 inhibitors and proteolysis targeting chimera (PROTAC) degraders with a particular focus on how these inhibitors or degraders are identified from starting points, strategies employed, structure–activity relationship (SAR) studies, binding modes or co-crystal structures, biochemical data, mechanistic studies, and preclinical/clinical studies. Moreover, we briefly discuss the challenges of designing MDM2/X inhibitors for cancer therapy such as dual MDM2/X inhibition, acquired resistance and toxicity of P53 activation as well as future directions.

Identification of a Structural Determinant for Selective Targeting of HDMX

p53 is a critical tumor-suppressor protein that guards the human genome against mutations by inducing cell-cycle arrest or apoptosis. Cancer cells subvert p53 by deletion, mutation, or overexpression of the negative regulators HDM2 and HDMX. For tumors that retain wild-type p53, its reactivation by pharmacologic targeting of HDM2 and/or HDMX represents a promising strategy, with a series of selective small-molecule HDM2 inhibitors and a dual HDM2/HDMX stapled-peptide inhibitor being evaluated in clinical trials. Because selective HDM2 targeting can cause hematologic toxicity, selective HDMX inhibitors could provide an alternative p53-reactivation strategy, but clinical candidates remain elusive. Here, we applied a mutation-scanning approach to uncover p53-based stapled peptides that are selective for HDMX. Crystal structures of stapled-peptide/HDMX complexes revealed a molecular mechanism for the observed specificity, which was validated by HDMX mutagenesis. Thus, we provide a blueprint for the development of HDMX-selective inhibitors to dissect and target the p53/HDMX interaction.

Recent Synthetic Approaches towards Small Molecule Reactivators of p53

The tumor suppressor protein p53 is often called "the genome guardian" and controls the cell cycle and the integrity of DNA, as well as other important cellular functions. Its main function is to trigger the process of apoptosis in tumor cells, and approximately 50% of all cancers are related to the inactivation of the p53 protein through mutations in the TP53 gene. Due to the association of mutant p53 with cancer therapy resistance, different forms of restoration of p53 have been subject of intense research in recent years. In this sense, this review focus on the main currently adopted approaches for activation and reactivation of p53 tumor suppressor function, focusing on the synthetic approaches that are involved in the development and preparation of such small molecules.

Revealing binding selectivity of ligands toward murine double minute 2 and murine double minute X based on molecular dynamics simulations and binding free energy calculations

It is well known that the interactions of p53 with murine double minute 2 and murine double minute X, namely MDM2 and MDMX, have been significant targets of efficient anti-cancer drug design. In this study, molecular dynamics (MD) simulations, principal component (PC) analysis and binding free energy calculations are combined to recognize binding selectivity of three ligands to MDM2 and MDMX. The binding free energies were estimated by using molecular mechanics generalized Born surface area (MM-GBSA) method and the obtained results display that the increase in the binding enthalpy of three ligands to MDM2 relative to MDMX mainly drives the binding selectivity of them toward MDM2 and MDMX. The information obtained from PC analysis shows that the associations of ligands exert important impacts on internal dynamics of MDM2 and MDMX. Meanwhile, the calculations of residue-based free energy decomposition not only identify the hot interaction spots of ligands with MDM2 and MDMX, but also show the residues (L54, M53), (Y67, Y66), (V93, V92), (H96, P95), (I99, I98) and (Y100, Y99) in (MDM2, MDMX) are responsible for most contributions to the binding selectivity of three ligands toward MDM2 and MDMX. It is believed that this work can provide useful information for design of highly selective and dual inhibitors targeting MDM2 and MDMX.Communicated by Ramaswamy H. Sarma.

Focused Library Generator: case of Mdmx inhibitors

We present a Focused Library Generator that is able to create from scratch new molecules with desired properties. After training the Generator on the ChEMBL database, transfer learning was used to switch the generator to producing new Mdmx inhibitors that are a promising class of anticancer drugs. Lilly medicinal chemistry filters, molecular docking, and a QSAR IC₅₀ model were used to refine the output of the Generator. Pharmacophore screening and molecular dynamics (MD) simulations were then used to further select putative ligands. Finally, we identified five promising hits with equivalent or even better predicted binding free energies and IC₅₀ values than known Mdmx inhibitors. The source code of the project is available on https://github.com/bigchem/online-chem.

Inhibition of p53 inhibitors: progress, challenges and perspectives

p53 is the major tumor suppressor and the most frequently inactivated gene in cancer. p53 could be disabled either by mutations or by upstream negative regulators, including, but not limited to MDM2 and MDMX. p53 activity is required for the prevention as well as for the eradication of cancers. Restoration of p53 activity in mouse models leads to the suppression of established tumors of different origin. These findings provide a strong support to the anti-cancer strategy aimed for p53 reactivation. In this review, we summarize recent progress in the development of small molecules, which restore the tumor suppressor function of wild-type p53 and discuss their clinical advance. We discuss different aspects of p53-mediated response, which contribute to suppression of tumors, including non-canonical p53 activities, such as regulation of immune response. While targeting p53 inhibitors is a very promising approach, there are certain limitations and concerns that the intensive research and clinical evaluation of compounds will hopefully help to overcome.

p53 modifications: exquisite decorations of the powerful guardian

The last 40 years have witnessed how p53 rose from a viral binding protein to a central factor in both stress responses and tumor suppression. The exquisite regulation of p53 functions is of vital importance for cell fate decisions. Among the multiple layers of mechanisms controlling p53 function, posttranslational modifications (PTMs) represent an efficient and precise way. Major p53 PTMs include phosphorylation, ubiquitination, acetylation, and methylation. Meanwhile, other PTMs like sumoylation, neddylation, O-GlcNAcylation, adenosine diphosphate (ADP)-ribosylation, hydroxylation, and β-hydroxybutyrylation are also shown to play various roles in p53 regulation. By independent action or interaction, PTMs affect p53 stability, conformation, localization, and binding partners. Deregulation of the PTM-related pathway is among the major causes of p53-associated developmental disorders or diseases, especially in cancers. This review focuses on the roles of different p53 modification types and shows how these modifications are orchestrated to produce various outcomes by modulating p53 activities or targeted to treat different diseases caused by p53 dysregulation.

The facile and visualizable identification of broad-spectrum inhibitors of MDM2/p53 using co-expressed protein complexes

MDM2 is a well-known oncoprotein overexpressed in a variety of cancers, and the identification of inhibitors that disrupt the MDM2/p53 interaction is of great interest in anticancer drug development. Here we designed a platform for the facile and visualizable identification of inhibitors of MDM2 using co-expressed protein complexes of MDM2/p53. A hexahistidine-tag on MDM2 allows the binding of the protein complex to the Ni-NTA affinity resin, while the fluorescent protein fused to p53 enables the direct visualization of the interaction of p53 with MDM2. Hence, the inhibition of the MDM2/p53 interaction can be observed with the naked eye. The assay can be set up by directly loading cell lysate to the Ni-NTA affinity resin, and no chemical modification of proteins is needed. In addition to the qualitative analyses, the binding affinity of inhibitors to the MDM2 protein can be quantified by fluorescence titration. The applications of this system have been verified using small molecules and peptide inhibitors. As a proof of concept, we screened a small library using this platform. Interestingly, two types of novel inhibitors of MDM2, including cyclohexyl-triphenylamine derivatives and platinum complexes, were identified and their binding affinities were obtained. Quantitative measurements show that these new types of inhibitors demonstrate a high binding affinity (up to K_d = 51.9 nM) to MDM2.

Identification of selective protein-protein interaction inhibitors using efficient in silico peptide-directed ligand design

Development of selective hDM2/X p53 inhibitors is key to further develop this anticancer target. This method displayed a 50% success rate and identified hDMX selective compounds.

The development of protein–protein interaction (PPI) inhibitors with therapeutic value is of increasing importance as the first clinical agent has now been approved, but PPIs remain difficult targets for the development of small molecule ligands. This article describes a highly efficient approach to the development of inhibitors of the p53/hDMX or hDM2 interaction that involves the design of small molecules in silico based upon a peptide/protein structure. The process for molecule design, starting from a virtual library of just over 1200 fragments, led to the eventual synthesis of twenty compounds, of which ten bound to either hDM2, hDMX or both in in vitro binding assays. This 50% success rate is extremely efficient compared to traditional high throughput screening. The identification of two selective hDMX inhibitors from twenty compounds highlights this efficiency as, to date, only two other hDMX-selective agents exist in the literature. Preliminary biological studies show that 20% of the compounds identified have cellular activity and activate downstream pathways associated with p53 activation.

The long and the short of it: the MDM4 tail so far

The mouse double minute 4 (MDM4) is emerging from the shadow of its more famous relative MDM2 and is starting to steal the limelight, largely due to its therapeutic possibilities. MDM4 is a vital regulator of the tumor suppressor p53. It restricts p53 transcriptional activity and also, at least in development, facilitates MDM2's E3 ligase activity toward p53. These functions of MDM4 are critical for normal cell function and a proper response to stress. Their importance for proper cell maintenance and proliferation identifies them as a risk for deregulation associated with the uncontrolled growth of cancer. MDM4 tails are vital for its function, where its N-terminus transactivation domain engages p53 and its C-terminus RING domain binds to MDM2. In this review, we highlight recently identified cellular functions of MDM4 and survey emerging therapies directed to correcting its dysregulation in disease.

Chemical Instability and Promiscuity of Arylmethylidenepyrazolinone-Based MDMX Inhibitors

Targeting the protein-protein interaction between p53 and MDM2/MDMX (MDM4) represents an attractive anticancer strategy for the treatment of p53-competent tumors. Several selective and potent MDM2 inhibitors have been developed and entered the clinic; however, the repertoire of MDMX antagonists is still limited. The arylmethylidenepyrazolinone SJ-172550 has been reported as a selective MDMX antagonist; yet, uncertainties about its mechanism of action have raised doubts about its use as a chemical probe. Here, we show that, in addition to its unclear mode of action, SJ-172550 is unstable in aqueous buffers, giving rise to side products of unknown biological activity. Using an SJ-172550-derived affinity probe, we observed promiscuous binding to cellular proteins whereas cellular thermal shift assays did not reveal a stabilizing effect on MDMX. Overall, our results raise further questions about the interpretation of data using SJ-172550 and related compounds to investigate cellular phenotypes.

Improving anticancer activity towards colon cancer cells with a new p53-activating agent

BACKGROUND AND PURPOSE

Impairment of the tumour suppressor p53 pathway is a major event in human cancers, making p53 activation one of the most attractive therapeutic strategies to halt cancer. Here, we have identified a new selective p53 activator and investigated its potential as an anticancer agent.

EXPERIMENTAL APPROACH

Anti-proliferative activity of the (R)-tryptophanol-derived bicyclic lactam SYNAP was evaluated in a range of human cancer cells with different p53 status. The anticancer activity and mechanism of action of SYNAP was studied in two- and three-dimensional models of human colon adenocarcinoma HCT116 cells with wild-type p53 and corresponding p53-null isogenic derivative cells, alone and in combination with known chemotherapeutic agents.

KEY RESULTS

SYNAP showed anti-proliferative effect in human cancer cells dependent on p53 status. In HCT116 cells, SYNAP caused p53-dependent growth inhibition, associated with cell cycle arrest and apoptosis, anti-migratory activity and regulation of the expression of p53 transcriptional targets. Data also indicated that SYNAP targeted p53, inhibiting its interaction with its endogenous inhibitors, murine double minute (MDM)2 and MDMX. Moreover, SYNAP sensitized colon cancer cells to the cytotoxic effect of known chemotherapeutic agents. SYNAP did not induce acquired or cross-resistance and re-sensitized doxorubicin-resistant colon cancer cells to chemotherapy. Additionally, SYNAP was non-genotoxic and had low cytotoxicity against normal cells.

CONCLUSION AND IMPLICATIONS

SYNAP revealed encouraging anticancer activity, either alone or in combination with known chemotherapeutic agents, in colon cancer cells. Apart from its promising application in cancer therapy, SYNAP may provide a starting point for improved p53 activators.

Novel isatin-derived molecules activate p53 via interference with Mdm2 to promote apoptosis

The p53 protein is a key tumor suppressor in mammals. In response to various forms of genotoxic stress p53 stimulates expression of genes whose products induce cell cycle arrest and/or apoptosis. An E3-ubiquitin ligase, Mdm2 (mouse-double-minute 2) and its human ortholog Hdm2, physically interact with the amino-terminus of p53 to mediate its ubiquitin-mediated degradation via the proteasome. Thus, pharmacological inhibition of the p53-Mdm2 interaction leads to overall stabilization of p53 and stimulation of its anti-tumorigenic activity. In this study we characterize the biological effects of a novel class of non-genotoxic isatin Schiff and Mannich base derivatives (ISMBDs) that stabilize p53 on the protein level. The likely mechanism behind their positive effect on p53 is mediated via the competitive interaction with Mdm2. Importantly, unlike Nutlin, these compounds selectively promoted p53-mediated cell death. These novel pharmacological activators of p53 can serve as valuable molecular tools for probing p53-positive tumors and set up the stage for development of new anti-cancer drugs.

Simultaneous Targeting of RGD-Integrins and Dual Murine Double Minute Proteins in Glioblastoma Multiforme

In the fight against Glioblastoma Multiforme, recent literature data have highlighted that integrin α5β1 and p53 are part of convergent pathways in the control of glioma apoptosis. This observation prompted us to seek a molecule able to simultaneously modulate both target families. Analyzing the results of a previous virtual screening against murine double minute 2 protein (MDM2), we envisaged that Arg-Gly-Asp (RGD)-mimetic molecules could be inhibitors of MDM2/4. Herein, we present the discovery of compound 7, which inhibits both MDM2/4 and α5β1/αvβ3 integrins. A lead optimization campaign was carried out on 7 with the aim to preserve the activities on integrins while improving those on MDM proteins. Compound 9 turned out to be a potent MDM2/4 and α5β1/αvβ3 blocker. In p53-wild type glioma cells, 9 arrested cell cycle and proliferation and strongly reduced cell invasiveness, emerging as the first molecule of a novel class of integrin/MDM inhibitors, which might be especially useful in subpopulations of patients with glioblastoma expressing a functional p53 concomitantly with a high level of α5β1 integrin.

Small Molecule Modulators of RING-Type E3 Ligases: MDM and Cullin Families as Targets

Ubiquitin-proteasome system (UPS) is a primary signaling pathway for regulation of intracellular protein levels. E3 ubiquitin ligases, substrate-specific members of the UPS, represent highly attractive protein targets for drug discovery. The importance of E3 ligases as prospective targets for small molecule modulation is reinforced by ever growing evidence of their role in cancer and other diseases. To date the number of potent compounds targeting E3 ligases remains rather low and their rational design constitutes a challenging task. To successfully address this problem one must take into consideration the multi-subunit nature of many E3 ligases that implies multiple druggable pockets and protein-protein interfaces. In this review, we briefly cover the current state of drug discovery in the field of RING-type E3 ligases with focus on MDM and Cullin families as targets. We also provide an overview of small molecule chimeras that induce RING-type E3-mediated proteasomal degradation of substrate proteins of interest.

Optical Biosensors Based on Photonic Crystals Supporting Bound States in the Continuum

A novel optical label-free bio-sensing platform based on a new class of resonances supported in a photonic crystal metasurface is reported herein. Molecular binding is detected as a shift in the resonant wavelength of the bound states in the continuum of radiation modes. The new configuration is applied to the recognition of the interaction between protein p53 and its protein regulatory partner murine double minute 2 (MDM2). A detection limit of 66 nM for the protein p53 is found. The device provides an excellent interrogation stability and loss-free operation, requires minimal optical interrogation equipment and can be easily optimized to work in a wide wavelength range.

Nutlin-3, A p53-Mdm2 Antagonist for Nasopharyngeal Carcinoma Treatment

Nasopharyngeal carcinoma (NPC) is a form of head and neck cancer of multifactorial etiolo-gies that is highly prevalent among men in the population of Southern China and Southeast Asia. NPC has claimed many thousands of lives worldwide; but the low awareness of NPC remains a hindrance in early diagnosis and prevention of the disease. NPC is highly responsive to radiotherapy and chemothera-py, but radiocurable NPC is still dependent on concurrent treatment of megavoltage radiotherapy with chemotherapy. Despite a significant reduction in loco-regional and distant metastases, radiotherapy alone has failed to provide a significant improvement in the overall survival rate of NPC, compared to chemo-therapy. In addition, chemo-resistance persists as the major challenge in the management of metastatic NPC although the survival rate of advanced metastatic NPC has significantly improved with the admin-istration of chemotherapy adjunctive to radiotherapy. In this regard, targeted molecular therapy could be explored for the discovery of alternative NPC therapies. Nutlin-3, a small molecule inhibitor that specifi-cally targets p53-Mdm2 interaction offers new therapeutic opportunities by enhancing cancer cell growth arrest and apoptosis through the restoration of the p53-mediated tumor suppression pathway while pro-ducing minimal cytotoxicity and side effects. This review discusses the potential use of Nutlin-3 as a p53-activating drug and the future directions of its clinical research for NPC treatment.

Flexibility is important for inhibition of the MDM2/p53 protein-protein interaction by cyclic β-hairpins

Protein-protein interactions that have large, flat and featureless binding sites are difficult drug targets. In the development of their modulators conventional drug discovery strategies are often unsuccessful. Gaining a detailed understanding of the binding mode of protein-protein interaction inhibitors is therefore of vast importance for their future pharmaceutical use. The MDM2/p53 protein pair is a highly promising target for cancer treatment. Disruption of the protein complex using p53 α-helix mimetics has been shown to be a successful strategy to control p53 activity. To gain further insight into the binding of inhibitors to MDM2, the flexibility of four cyclic β-hairpins that act as α-helical mimetics and potential MDM2/p53 interaction inhibitors was investigated in relation to their inhibitory activity. MDM2-binding of the mimetics was determined using fluorescence polarization and surface plasmon resonance assays, whereas their conformation and dynamics in solution was described by the combined experimental and computational NAMFIS analysis. Molecular flexibility was shown to be important for the activity of the cyclic β-hairpin based MDM2 inhibitors.

Inhibition of WIP1 phosphatase sensitizes breast cancer cells to genotoxic stress and to MDM2 antagonist nutlin-3

PP2C family serine/threonine phosphatase WIP1 acts as a negative regulator of the tumor suppressor p53 and is implicated in silencing of cellular responses to genotoxic stress. Chromosomal locus 17q23 carrying the PPM1D (coding for WIP1) is commonly amplified in breast carcinomas and WIP1 was proposed as potential pharmacological target. Here we employed a cellular model with knocked out PPM1D to validate the specificity and efficiency of GSK2830371, novel small molecule inhibitor of WIP1. We have found that GSK2830371 increased activation of the DNA damage response pathway to a comparable level as the loss of PPM1D. In addition, GSK2830371 did not affect proliferation of cells lacking PPM1D but significantly supressed proliferation of breast cancer cells with amplified PPM1D. Over time cells treated with GSK2830371 accumulated in G1 and G2 phases of the cell cycle in a p21-dependent manner and were prone to induction of senescence by a low dose of MDM2 antagonist nutlin-3. In addition, combined treatment with GSK2830371 and doxorubicin or nutlin-3 potentiated cell death through a strong induction of p53 pathway and activation of caspase 9. We conclude that efficient inhibition of WIP1 by GSK2830371 sensitizes breast cancer cells with amplified PPM1D and wild type p53 to chemotherapy.

Activation of p53 and destabilization of androgen receptor by combinatorial inhibition of MDM2 and MDMX in prostate cancer cells

Castration-resistant prostate cancer (CRPC) frequently develops after initial standard radiation and androgen deprivation therapy, leaving patients with limited further treatment options. Androgen receptor (AR) is a transcription factor that plays a key role in the initiation and progression of prostate cancer. p53, a major tumor suppressor that is rarely mutated in early-stages of prostate cancer, is often deregulated during prostate cancer progression. Here, we report an unusual co-amplification of MDM2 and MDMX, two crucial negative regulators of p53, in CRPC datasets. We demonstrate that combinatorial inhibition of MDM2 and MDMX, with nutlin-3 and NSC207895 respectively, has a profound inhibitory effect on cell proliferation of androgen-responsive, wild-type TP53 gene carrying prostate cancer cells LNCaP and 22Rv1. We further show that the combinatorial inhibition of MDM2 and MDMX not only activates p53, but also decreases cellular levels of AR and represses its function. Additionally, co-expression of MDM2 and MDMX stabilizes AR. Together, our results indicate that combinatorial inhibition of MDM2 and MDMX may offer a novel compelling strategy for prostate cancer therapy.

High-Throughput Screening Identifies 1,4,5-Substituted 1,2,3-Triazole Analogs as Potent and Specific Antagonists of Pregnane X Receptor

Human pregnane X receptor (hPXR) is a nuclear receptor that regulates the expression of phase I and phase II drug-metabolism enzymes, as well as that of drug transporters. hPXR is a "xenobiotics sensor" and can be activated by structurally diverse compounds. The activation of hPXR by its agonists increases the clearance of xenobiotics by increasing the expression of drug-metabolism enzymes and drug transporters, possibly leading to drug toxicity, drug resistance, and other adverse drug reactions. Therefore, hPXR antagonists might attenuate agonist-mediated activation of hPXR and reduce the risk of adverse drug reactions. Several hPXR antagonists have been reported, but none of them is specific for hPXR. In this study, we present the first large-scale, unbiased, cell-based high-throughput screen to identify specific hPXR antagonists. Among the 132,975 compounds screened, we identified the 1,4,5-substituted 1,2,3-triazole analogs as potent and specific hPXR antagonists by sequentially performing primary screening, retesting, and dose-response analysis using cell-based hPXR gene reporter and receptor binding assays, as well as receptor and promoter specificity assays. The compound SJ000076745-1 is the most potent and specific hPXR antagonist in the 1,4,5-substituted 1,2,3-triazole chemical class, having a cell-based hPXR antagonist 50% inhibitory concentration (IC₅₀) value of 377 ± 16 nM and an hPXR binding inhibitory IC₅₀ value of 563 ± 40 nM.

Computer-Aided Identification and Lead Optimization of Dual Murine Double Minute 2 and 4 Binders: Structure-Activity Relationship Studies and Pharmacological Activity

The function of p53 protein, also known as "genome guardian", might be impaired by the overexpression of its primary cellular inhibitor, the murine double minute 2 protein (MDM2). However, the recent finding that MDM2-selective inhibitors induce high levels of its homologue MDM4, prompt us to identify, through a receptor-based virtual screening on an in house database, dual MDM2/MDM4 binders. Compound 1 turned out to possess an IC₅₀ of 93.7 and of 4.6 nM on MDM2 and MDM4, respectively. A series of compounds were synthesized to optimize its activity on MDM2. As a result, compound 12 showed low nanomolar IC₅₀ for both targets. NMR studies confirmed the pocket of binding of 12 as predicted by the Glide docking software. Notably, 12 was able to cause concentration-dependent inhibition of cell proliferation, yielding an IC₅₀ value of 356 ± 21 nM in neuroblastoma SHSY5Y cells and proved even to efficiently block cancer stem cell growth.