Molecular basis for the high-affinity binding and stabilization of firefly luciferase by PTC124

Suppressor tRNA in gene therapy

Suppressor tRNAs are engineered or naturally occurring transfer RNA molecules that have shown promise in gene therapy for diseases caused by nonsense mutations, which result in premature termination codons (PTCs) in coding sequence, leading to truncated, often nonfunctional proteins. Suppressor tRNAs can recognize and pair with these PTCs, allowing the ribosome to continue translation and produce a full-length protein. This review introduces the mechanism and development of suppressor tRNAs, compares suppressor tRNAs with other readthrough therapies, discusses their potential for clinical therapy, limitations, and obstacles. We also summarize the applications of suppressor tRNAs in both in vitro and in vivo, offering new insights into the research and treatment of nonsense mutation diseases.

Tackling assay interference associated with small molecules

Biochemical and cell-based assays are essential to discovering and optimizing efficacious and safe drugs, agrochemicals and cosmetics. However, false assay readouts stemming from colloidal aggregation, chemical reactivity, chelation, light signal attenuation and emission, membrane disruption, and other interference mechanisms remain a considerable challenge in screening synthetic compounds and natural products. To address assay interference, a range of powerful experimental approaches are available and in silico methods are now gaining traction. This Review begins with an overview of the scope and limitations of experimental approaches for tackling assay interference. It then focuses on theoretical methods, discusses strategies for their integration with experimental approaches, and provides recommendations for best practices. The Review closes with a summary of the critical facts and an outlook on potential future developments.

Stop codon readthrough as a treatment option for epidermolysis bullosa-Where we are and where we are going

In the context of rare genetic diseases caused by nonsense mutations, the concept of induced stop codon readthrough (SCR) represents an attractive avenue in the ongoing search for improved treatment options. Epidermolysis bullosa (EB)-exemplary for this group of diseases-describes a diverse group of rare, blistering genodermatoses. Characterized by extreme skin fragility upon minor mechanical trauma, the most severe forms often result from nonsense mutations that lead to premature translation termination and loss of function of essential proteins at the dermo-epidermal junction. Since no curative interventions are currently available, medical care is mainly limited to alleviating symptoms and preventing complications. Complementary to attempts of gene, cell and protein therapy in EB, SCR represents a promising medical alternative. While gentamicin has already been examined in several clinical trials involving EB, other potent SCR inducers, such as ataluren, may also show promise in treating the hitherto non-curative disease. In addition to the extensively studied aminoglycosides and their derivatives, several other substance classes-non-aminoglycoside antibiotics and non-aminoglycoside compounds-are currently under investigation. The extensive data gathered in numerous in vitro experiments and the perspectives they reveal in the clinical setting will be discussed in this review.

P53 together with ferroptosis: a promising strategy leaving cancer cells without escape

TP53, functioning as the keeper of the genome, assumes a pivotal function in the inhibition of tumorigenesis. Recent studies have revealed that p53 regulates ferroptosis pathways within tumor cells and is closely related to tumorigenesis. Therefore, we summarize the pathways and mechanisms by which p53 regulates ferroptosis and identify a series of upstream and downstream molecules involved in this process. Furthermore, we construct a p53-ferroptosis network centered on p53. Finally, we present the progress of drugs to prevent wild-type p53 (wtp53) degeneration and restore wtp53, highlighting the deficiencies of drug development and the prospects for p53 in cancer treatment. These findings provide novel strategies and directions for future cancer therapy.

Therapeutic approaches for Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a monogenic muscle-wasting disorder and a priority candidate for molecular and cellular therapeutics. Although rare, it is the most common inherited myopathy affecting children and so has been the focus of intense research activity. It is caused by mutations that disrupt production of the dystrophin protein, and a plethora of drug development approaches are under way that aim to restore dystrophin function, including exon skipping, stop codon readthrough, gene replacement, cell therapy and gene editing. These efforts have led to the clinical approval of four exon skipping antisense oligonucleotides, one stop codon readthrough drug and one gene therapy product, with other approvals likely soon. Here, we discuss the latest therapeutic strategies that are under development and being deployed to treat DMD. Lessons from these drug development programmes are likely to have a major impact on the DMD field, but also on molecular and cellular medicine more generally. Thus, DMD is a pioneer disease at the forefront of future drug discovery efforts, with these experimental treatments paving the way for therapies using similar mechanisms of action being developed for other genetic diseases.

Structural and functional insights into δ-poly-L-ornithine polymer biosynthesis from Acinetobacter baumannii

Cationic homo-polyamino acid (CHPA) peptides containing isopeptide bonds of diamino acids have been identified from Actinomycetes strains. However, none has been reported from other bacteria. Here, we report a δ-poly-L-ornithine synthetase from Acinetobacter baumannii, which we name PosA. Surprisingly, structural analysis of the adenylation domain and biochemical assay shows L-ornithine as the substrate for PosA. The product from the enzymatic reaction was purified and identified as poly-L-ornithine composed of 7-12 amino acid units. Chemical labeling of the polymer confirmed the isopeptide linkage of δ-poly-L-ornithine. We examine the biological activity of chemically synthesized 12-mer δ-poly-L-ornithine, illustrating that the polymer may act as an anti-fungal agent. Structures of the isolated adenylation domain from PosA are presented with several diamino acids and biochemical assays identify important substrate binding residues. Structurally-guided genome-mining led to the identification of homologs with different substrate binding residues that could activate additional substrates. A homolog from Bdellovibrionales sp. shows modest activity with L-arginine but not with any diamino acids observed to be substrates for previously examined CHPA synthetases. Our study indicates the possibility that additional CHPAs may be produced by various microbes, supporting the further exploration of uncharacterized natural products.

Optical Control of Protein Functions via Genetically Encoded Photocaged Aspartic Acids

Site-specific protein decaging by light has become an effective approach for in situ manipulation of protein activities in a gain-of-function fashion. Although successful decaging of amino acid side chains of Lys, Tyr, Cys, and Glu has been demonstrated, this strategy has not been extended to aspartic acid (Asp), an essential amino acid residue with a range of protein functions and protein-protein interactions. We herein reported a genetically encoded photocaged Asp and applied it to the photocontrolled manipulation of a panel of proteins including firefly luciferase, kinases (e.g., BRAF), and GTPase (e.g., KRAS) as well as mimicking the in situ phosphorylation event on kinases. As a new member of the increasingly expanded amino acid-decaging toolbox, photocaged Asp may find broad applications for gain-of-function study of diverse proteins as well as biological processes in living cells.

Pharmaceuticals Promoting Premature Termination Codon Readthrough: Progress in Development

Around 11% of all known gene lesions causing human genetic diseases are nonsense mutations that introduce a premature stop codon (PTC) into the protein-coding gene sequence. Drug-induced PTC readthrough is a promising therapeutic strategy for treating hereditary diseases caused by nonsense mutations. To date, it has been found that more than 50 small-molecular compounds can promote PTC readthrough, known as translational readthrough-inducing drugs (TRIDs), and can be divided into two major categories: aminoglycosides and non-aminoglycosides. This review summarizes the pharmacodynamics and clinical application potential of the main TRIDs discovered so far, especially some newly discovered TRIDs in the past decade. The discovery of these TRIDs brings hope for treating nonsense mutations in various genetic diseases. Further research is still needed to deeply understand the mechanism of eukaryotic cell termination and drug-induced PTC readthrough so that patients can achieve the greatest benefit from the various TRID treatments.

Semi-Synthesis of Different Pyranoflavonoid Backbones and the Neurogenic Potential

Flavonoids and chalcones are known for their manifold biological activities, of which many affect the central nervous system. Pyranochalcones were recently shown to have a great neurogenic potential, which is partly due to a specific structural motif-the pyran ring. Accordingly, we questioned if other flavonoid backbones with a pyran ring as structural moiety would also show neurogenic potential. Different semi-synthetic approaches starting with the prenylated chalcone xanthohumol, isolated from hops, led to pyranoflavanoids with different backbones. We identified the chalcone backbone as the most active backbone with pyran ring using a reporter gene assay based on the promoter activity of doublecortin, an early neuronal marker. Pyranochalcones therefore appear to be promising compounds for further development as a treatment strategy for neurodegenerative diseases.

Readthrough compounds for nonsense mutations: bridging the translational gap

Approximately 10% of all pathological mutations are nonsense mutations that are responsible for several severe genetic diseases for which no treatment regimens are currently available. The most widespread strategy for treating nonsense mutations is by enhancing ribosomal readthrough of premature termination codons (PTCs) to restore the production of the full-length protein. In the past decade several compounds with readthrough potential have been identified. However, although preclinical results on these compounds are promising, clinical studies have not yielded positive outcomes. We review preclinical and clinical research related to readthrough compounds and characterize factors that contribute to the observed translational gap.

Effects of synonymous mutations on kinetic properties and structure of firefly luciferase: Molecular dynamics simulation, molecular docking, RNA folding, and experimental study

Although synonymous mutations have long been thought to lack striking results, a growing body of research shows these mutations have highly variable effects. In this study, the impact of synonymous mutations in the development of thermostable luciferase was investigated using a combination of experimental and theoretical approaches. Using bioinformatics analysis, the codon usage features in the Lampyridae family's luciferases were studied and four synonymous mutations of Arg in luciferase were created. An exciting result was that the analysis of kinetic parameters showed a slight increase in the thermal stability of the mutant luciferase. AutoDock Vina, %MinMax algorithm, and UNAFold Server were used to perform molecular docking, folding rate, and RNA folding, respectively. Here, it was assumed that in the region (Arg337) with a moderate propensity for coil, synonymous mutation altered the rate of translation, which in turn may lead to a slight change in the structure of the enzyme. According to the molecular dynamics simulation data, local minor global flexibility is observed in the context of the protein conformation. A plausible explanation is that this flexibility may strengthen hydrophobic interactions due to its sensitivity to a molecular collision. Accordingly, thermostability originated mainly from hydrophobic interaction.

PTC124 Rescues Nonsense Mutation of Two Tumor Suppressor Genes NOTCH1 and FAT1 to Repress HNSCC Cell Proliferation

(1) Background: PTC124 (Ataluren) is an investigational drug for the treatment of nonsense mutation-mediated genetic diseases. With the exception of the TP53 tumor suppressor gene, there has been little research on cancers with nonsense mutation. By conducting a database search, we found that another two tumor suppressor genes, NOTCH1 and FAT1, have a high nonsense mutation rate in head and neck squamous cell carcinoma (HNSCC). PTC124 may re-express the functional NOTCH1 or FAT1 in nonsense mutation NOTCH1 or FAT1 in HSNCC (2) Methods: DOK (with NOTCH1 Y550X) or HO-1-u-1 (with FAT1 E378X) HNSCC cells were treated with PTC124, and the NOTCH1 or FAT1 expression, cell viability, and NOTCH1- or FAT1-related downstream gene profiles were assayed. (3) Results: PTC124 was able to induce NOTCH1 or FAT1 expression in DOK and HO-1-u-1 cells. PTC124 was able to upregulate NOTCH downstream genes HES5, AJUBA, and ADAM10 in DOK cells. PTC124 enhanced DDIT4, which is under the control of the FAT1-YAP1 pathway, in HO-1-u-1 cells. FLI-06 (a NOTCH signaling inhibitor) reversed PTC124-mediated cell growth inhibition in DOK cells. PTC124 could reverse TT-10 (a YAP signaling activator)-mediated HO-1-u-1 cell proliferation. (4) Conclusions: PTC124 can rescue nonsense mutation of NOTCH1 and FAT1 to repress HNSCC cell proliferation.

Spectrochemistry of Firefly Bioluminescence

The chemical reactions underlying the emission of light in fireflies and other bioluminescent beetles are some of the most thoroughly studied processes by scientists worldwide. Despite these remarkable efforts, fierce academic arguments continue around even some of the most fundamental aspects of the reaction mechanism behind the beetle bioluminescence. In an attempt to reach a consensus, we made an exhaustive search of the available literature and compiled the key discoveries on the fluorescence and chemiluminescence spectrochemistry of the emitting molecule, the firefly oxyluciferin, and its chemical analogues reported over the past 50+ years. The factors that affect the light emission, including intermolecular interactions, solvent polarity, and electronic effects, were analyzed in the context of both the reaction mechanism and the different colors of light emitted by different luciferases. The collective data points toward a combined emission of multiple coexistent forms of oxyluciferin as the most probable explanation for the variation in color of the emitted light. We also highlight realistic research directions to eventually address some of the remaining questions related to firefly bioluminescence. It is our hope that this extensive compilation of data and detailed analysis will not only consolidate the existing body of knowledge on this important phenomenon but will also aid in reaching a wider consensus on some of the mechanistic details of firefly bioluminescence.

Ataluren suppresses a premature termination codon in an MPS I-H mouse

Abstarct

Suppressing translation termination at premature termination codons (PTCs), termed readthrough, is a potential therapy for genetic diseases caused by nonsense mutations. Ataluren is a compound that has shown promise for clinical use as a readthrough agent. However, some reports suggest that ataluren is ineffective at suppressing PTCs. To further evaluate the effectiveness of ataluren as a readthrough agent, we examined its ability to suppress PTCs in a variety of previously untested models. Using NanoLuc readthrough reporters expressed in two different cell types, we found that ataluren stimulated a significant level of readthrough. We also explored the ability of ataluren to suppress a nonsense mutation associated with Mucopolysaccharidosis I-Hurler (MPS I-H), a genetic disease that is caused by a deficiency of α-L-iduronidase that leads to lysosomal accumulation of glycosaminoglycans (GAGs). Using mouse embryonic fibroblasts (MEFs) derived from Idua-W402X mice, we found that ataluren partially rescued α-L-iduronidase function and significantly reduced GAG accumulation relative to controls. Two-week oral administration of ataluren to Idua-W402X mice led to significant GAG reductions in most tissues compared to controls. Together, these data reveal important details concerning the efficiency of ataluren as a readthrough agent and the mechanisms that govern its ability to suppress PTCs.

Key messages

Ataluren promotes readthrough of PTCs in a wide variety of contexts.

Ataluren reduces glycosaminoglyan storage in MPS I-H cell and mouse models.

Ataluren has a bell-shaped dose–response curve and a narrow effective range.

Development of a High-Throughput Screening Assay for Small-Molecule Inhibitors of Androgen Receptor Splice Variants

The role of the androgen receptor (AR) in the progression of prostate cancer (PCa) is well established and competitive inhibition of AR ligand binding domain (LBD) has been the mainstay of antiandrogen therapies for advanced and metastatic disease. However, the efficacy of such drugs is often limited by the emergence of resistance, mediated through point mutations and receptor splice variants lacking the AR-LBD. As a result, the prognosis for patients with malignant, castrate-resistant disease remains poor. The amino terminal domain (NTD) of the AR has been shown to be critical for AR function. Its modular activation function (AF-1) is important for both gene regulation and participation in protein–protein interactions. However, due to the intrinsically disordered structure of the domain, its potential as a candidate for therapeutic intervention has been generally overlooked. In this article, we describe the design and development of a functional cell-based assay aimed at identifying small-molecule inhibitors of the AR-NTD. We demonstrate the suitability of the assay for high-throughput screening platforms and validate two initial hits emerging from a small, targeted, library screen in PCa cells.

Identification of 2-Benzylidene-tetralone Derivatives as Highly Potent and Reversible Firefly Luciferase Inhibitors

The extensive applications of Firefly luciferase (Fluc) in numerous biological, biomedical, and clinical investigations rendered an urgent need for efficient and biocompatible Fluc inhibitors for the construction of novel assay platforms. Herein we describe the identification of 2-benzylidene-tetralone derivatives as highly potent and reversible Firefly luciferase inhibitors by competing with d-luciferin. The most active compound 48 was found to have >7000 fold higher potency (IC₅₀ = 0.25 nM) than that of the well-known luciferase inhibitor resveratrol (IC₅₀ = 1.9 μM) biochemically with sub- to low nanomolar IC₅₀ values, and it can efficiently block the Fluc generated bioluminescence in vivo.

Synthesis and biological evaluation of thiazolidinedione derivatives with high ligand efficiency to P. aeruginosa PhzS

The thiazolidinone ring is found in compounds that have widespan biology activity and there is mechanism-based evidence that compounds bearing this moiety inhibit P. aeruginosa PhzS (PaPzhS), a key enzyme in the biosynthesis of the virulence factor named pyocyanin. Ten novel thiazolidinone derivatives were synthesised and screened against PaPhzS, using two orthogonal assays. The biological results provided by these and 28 other compounds, whose synthesis had been described, suggest that the dihydroquinazoline ring, found in the previous hit (A- Kd = 18 µM and LE = 0.20), is not required for PaPzhS inhibition, but unsubstituted nitrogen at the thiazolidinone ring is. The molecular simplification approach, pursued in this work, afforded an optimised lead compound (13- 5-(2,4-dimethoxyphenyl)thiazolidine-2,4-dione) with 10-fold improvement in affinity (Kd= 1.68 µM) and more than 100% increase in LE (0.45), which follows the same inhibition mode as the original hit compound (competitive to NADH).Executive summaryPhzS is a key enzyme in the pyocyanin biosynthesis pathway in P. aeruginosa.Orthogonal assays (TSA and FITC) show that fragment-like thiazolidinedione derivatives bind to PaPhzS with one-digit micromolar affinity.Fragment-like thiazolidinedione derivatives bind to the cofactor (NADH) binding site in PaPhzS.The molecular simplification optimised the ligand efficiency and affinity of the lead compound.

Ataluren-Promising Therapeutic Premature Termination Codon Readthrough Frontrunner

Around 12% of hereditary disease-causing mutations are in-frame nonsense mutations. The expression of genes containing nonsense mutations potentially leads to the production of truncated proteins with residual or virtually no function. However, the translation of transcripts containing premature stop codons resulting in full-length protein expression can be achieved using readthrough agents. Among them, only ataluren was approved in several countries to treat nonsense mutation Duchenne muscular dystrophy (DMD) patients. This review summarizes ataluren’s journey from its identification, via first in vitro activity experiments, to clinical trials in DMD, cystic fibrosis, and aniridia. Additionally, data on its pharmacokinetics and mechanism of action are presented. The range of diseases with underlying nonsense mutations is described for which ataluren therapy seems to be promising. What is more, experiments in which ataluren did not show its readthrough activity are also included, and reasons for their failures are discussed.

A small molecule that induces translational readthrough of CFTR nonsense mutations by eRF1 depletion

Premature termination codons (PTCs) prevent translation of a full-length protein and trigger nonsense-mediated mRNA decay (NMD). Nonsense suppression (also termed readthrough) therapy restores protein function by selectively suppressing translation termination at PTCs. Poor efficacy of current readthrough agents prompted us to search for better compounds. An NMD-sensitive NanoLuc readthrough reporter was used to screen 771,345 compounds. Among the 180 compounds identified with readthrough activity, SRI-37240 and its more potent derivative SRI-41315, induce a prolonged pause at stop codons and suppress PTCs associated with cystic fibrosis in immortalized and primary human bronchial epithelial cells, restoring CFTR expression and function. SRI-41315 suppresses PTCs by reducing the abundance of the termination factor eRF1. SRI-41315 also potentiates aminoglycoside-mediated readthrough, leading to synergistic increases in CFTR activity. Combining readthrough agents that target distinct components of the translation machinery is a promising treatment strategy for diseases caused by PTCs.

Premature termination codons can cause early translation termination and lead to disease. Here the authors perform a screen to identify compounds with readthrough activity and show that these reduce eRF1 levels to suppress premature termination associated with cystic fibrosis.

The Impact of Assay Design on Medicinal Chemistry: Case Studies

GRAPHICAL ABSTRACT

Evaluation of Firefly and Renilla Luciferase Inhibition in Reporter-Gene Assays: A Case of Isoflavonoids

Firefly luciferase is susceptible to inhibition and stabilization by compounds under investigation for biological activity and toxicity. This can lead to false-positive results in in vitro cell-based assays. However, firefly luciferase remains one of the most commonly used reporter genes. Here, we evaluated isoflavonoids for inhibition of firefly luciferase. These natural compounds are often studied using luciferase reporter-gene assays. We used a quantitative structure–activity relationship (QSAR) model to compare the results of in silico predictions with a newly developed in vitro assay that enables concomitant detection of inhibition of firefly and Renilla luciferases. The QSAR model predicted a moderate to high likelihood of firefly luciferase inhibition for all of the 11 isoflavonoids investigated, and the in vitro assays confirmed this for seven of them: daidzein, genistein, glycitein, prunetin, biochanin A, calycosin, and formononetin. In contrast, none of the 11 isoflavonoids inhibited Renilla luciferase. Molecular docking calculations indicated that isoflavonoids interact favorably with the D-luciferin binding pocket of firefly luciferase. These data demonstrate the importance of reporter-enzyme inhibition when studying the effects of such compounds and suggest that this in vitro assay can be used to exclude false-positives due to firefly or Renilla luciferase inhibition, and to thus define the most appropriate reporter gene.

Challenges with Methods for Detecting and Studying the Transcription Factor Nuclear Factor Kappa B (NF-κB) in the Central Nervous System

The transcription factor nuclear factor kappa B (NF-κB) is highly expressed in almost all types of cells. NF-κB is involved in many complex biological processes, in particular in immunity. The activation of the NF-κB signaling pathways is also associated with cancer, diabetes, neurological disorders and even memory. Hence, NF-κB is a central factor for understanding not only fundamental biological presence but also pathogenesis, and has been the subject of intense study in these contexts. Under healthy physiological conditions, the NF-κB pathway promotes synapse growth and synaptic plasticity in neurons, while in glia, NF-κB signaling can promote pro-inflammatory responses to injury. In addition, NF-κB promotes the maintenance and maturation of B cells regulating gene expression in a majority of diverse signaling pathways. Given this, the protein plays a predominant role in activating the mammalian immune system, where NF-κB-regulated gene expression targets processes of inflammation and host defense. Thus, an understanding of the methodological issues around its detection for localization, quantification, and mechanistic insights should have a broad interest across the molecular neuroscience community. In this review, we summarize the available methods for the proper detection and analysis of NF-κB among various brain tissues, cell types, and subcellular compartments, using both qualitative and quantitative methods. We also summarize the flexibility and performance of these experimental methods for the detection of the protein, accurate quantification in different samples, and the experimental challenges in this regard, as well as suggestions to overcome common challenges.

Properties of Non-Aminoglycoside Compounds Used to Stimulate Translational Readthrough of PTC Mutations in Primary Ciliary Dyskinesia

Primary ciliary dyskinesia (PCD) is a rare disease with autosomal recessive inheritance, caused mostly by bi-allelic gene mutations that impair motile cilia structure and function. Currently, there are no causal treatments for PCD. In many disease models, translational readthrough of premature termination codons (PTC-readthrough) induced by aminoglycosides has been proposed as an effective way of restoring functional protein expression and reducing disease symptoms. However, variable outcomes of pre-clinical trials and toxicity associated with long-term use of aminoglycosides prompt the search for other compounds that might overcome these problems. Because a high proportion of PCD-causing variants are nonsense mutations, readthrough therapies are an attractive option. We tested a group of chemical compounds with known PTC-readthrough potential (ataluren, azithromycin, tylosin, amlexanox, and the experimental compound TC007), collectively referred to as non-aminoglycosides (NAGs). We investigated their PTC-readthrough efficiency in six PTC mutations found in Polish PCD patients, in the context of cell and cilia health, and in comparison to the previously tested aminoglycosides. The NAGs did not compromise the viability of the primary nasal respiratory epithelial cells, and the ciliary beat frequency was retained, similar to what was observed for gentamicin. In HEK293 cells transfected with six PTC-containing inserts, the tested compounds stimulated PTC-readthrough but with lower efficiency than aminoglycosides. The study allowed us to select compounds with minimal negative impact on cell viability and function but still the potential to induce PTC-readthrough.

Nonsense suppression therapies in human genetic diseases

About 11% of all human disease-associated gene lesions are nonsense mutations, resulting in the introduction of an in-frame premature translation-termination codon (PTC) into the protein-coding gene sequence. When translated, PTC-containing mRNAs originate truncated and often dysfunctional proteins that might be non-functional or have gain-of-function or dominant-negative effects. Therapeutic strategies aimed at suppressing PTCs to restore deficient protein function-the so-called nonsense suppression (or PTC readthrough) therapies-have the potential to provide a therapeutic benefit for many patients and in a broad range of genetic disorders, including cancer. These therapeutic approaches comprise the use of translational readthrough-inducing compounds that make the translational machinery recode an in-frame PTC into a sense codon. However, most of the mRNAs carrying a PTC can be rapidly degraded by the surveillance mechanism of nonsense-mediated decay (NMD), thus decreasing the levels of PTC-containing mRNAs in the cell and their availability for PTC readthrough. Accordingly, the use of NMD inhibitors, or readthrough-compound potentiators, may enhance the efficiency of PTC suppression. Here, we review the mechanisms of PTC readthrough and their regulation, as well as the recent advances in the development of novel approaches for PTC suppression, and their role in personalized medicine.

Nuisance compounds in cellular assays

Compounds that exhibit assay interference or undesirable mechanisms of bioactivity ("nuisance compounds") are routinely encountered in cellular assays, including phenotypic and high-content screening assays. Much is known regarding compound-dependent assay interferences in cell-free assays. However, despite the essential role of cellular assays in chemical biology and drug discovery, there is considerably less known about nuisance compounds in more complex cell-based assays. In our view, a major obstacle to realizing the full potential of chemical biology will not just be difficult-to-drug targets or even the sheer number of targets, but rather nuisance compounds, due to their ability to waste significant resources and erode scientific trust. In this review, we summarize our collective academic, government, and industry experiences regarding cellular nuisance compounds. We describe assay design strategies to mitigate the impact of nuisance compounds and suggest best practices to efficiently address these compounds in complex biological settings.

Inhibitors of Eukaryotic Translational Machinery as Therapeutic Agents

Inhibiting eukaryotic protein translation with small molecules is emerging as a powerful therapeutic strategy. The advantage of targeting cellular translational machinery is that it is required for the highly proliferative state of many neoplastic cells, replication of certain viruses, and ultimately the expression of a wide variety of protein targets. Although, this approach has been exploited to develop clinical agents, such as homoharringtonine (HHT, 1), used to treat chronic myeloid leukemia (CML), inhibiting components of the translational machinery is often associated with cytotoxic phenotypes. However, recent studies have demonstrated that certain small molecules can inhibit the translation of specific subsets of proteins, leading to lower cytotoxicity, and opening-up therapeutic opportunities for translation inhibitors to be deployed in indications beyond oncology and infectious disease. This review summarizes efforts to develop inhibitors of the eukaryotic translational machinery as therapeutic agents and highlights emerging opportunities for translation inhibitors in the future.

The elusive relationship between structure and colour emission in beetle luciferases

In beetles, luciferase enzymes catalyse the conversion of chemical energy into light through bioluminescence. The principles of this process have become a fundamental biotechnological tool that revolutionized biological research. Different beetle species can emit different colours of light, despite using the same substrate and highly homologous luciferases. The chemical reasons for these different colours are hotly debated yet remain unresolved. This Review summarizes the structural, biochemical and spectrochemical data on beetle bioluminescence reported over the past three decades. We identify the factors that govern what colour is emitted by wild-type and mutant luciferases. This topic is controversial, but, in general, we note that green emission requires cationic residues in a specific position near the benzothiazole fragment of the emitting molecule, oxyluciferin. The commonly emitted green-yellow light can be readily changed to red by introducing a variety of individual and multiple mutations. However, complete switching of the emitted light from red to green has not been accomplished and the synergistic effects of combined mutations remain unexplored. The minor colour shifts produced by most known mutations could be important in establishing a 'mutational catalogue' to fine-tune emission of beetle luciferases, thereby expanding the scope of their applications.

Examining the evidence of non-monotonic dose-response in Androgen Receptor agonism high-throughput screening assay

Modern High-Throughput Screening (HTS) techniques allow to determine in vitro bioactivity of tens of thousands of chemicals within a relatively short period of time and tested compounds are usually interpreted as either active or inactive. The interpretation is mostly based on the assumption of monotonic dose-response. This approach ignores potential abnormal dose-response relationships, such as non-monotonic dose-response (NMDR). NMDR presents a serious challenge to toxicologists and pharmacologists, since they undermine the usefulness of such concepts as lowest-observed-adverse-effect level (LOAEL) and no-observed-adverse-effect level (NOAEL). The possible presence of the NMDR in Androgen receptor (AR) agonism was examined for a structurally diverse set of chemicals (~8 300 unique compounds) from Tox21 project library. The source of activity data is Tox21 AR agonism luciferase-based HTS on the MDA-MB-453 cell line. The examination of curve fitting for 35,328 dose-response data entries was based on modified version of existing criteria for determination of NMDR. The bias that arises from compounds' cytotoxicity and interference with firefly luciferase protein was also studied. The examination has shown evidence of NMDR for several compounds, including known AR antagonists (e. g. Cyproterone acetate) and other known endocrine disruptors (e. g. Tranilast). Compounds were divided into 3 groups based on chemical class, known biological activity profile and the shape of dose-response curve. The challenges of using HTS data to determine NMDR and benefits of this analysis are discussed.

How We Think about Targeting RNA with Small Molecules

RNA offers nearly unlimited potential as a target for small molecule chemical probes and lead medicines. Many RNAs fold into structures that can be selectively targeted with small molecules. This Perspective discusses molecular recognition of RNA by small molecules and highlights key enabling technologies and properties of bioactive interactions. Sequence-based design of ligands targeting RNA has established rules for affecting RNA targets and provided a potentially general platform for the discovery of bioactive small molecules. The RNA targets that contain preferred small molecule binding sites can be identified from sequence, allowing identification of off-targets and prediction of bioactive interactions by nature of ligand recognition of functional sites. Small molecule targeted degradation of RNA targets (ribonuclease-targeted chimeras, RIBOTACs) and direct cleavage by small molecules have also been developed. These growing technologies suggest that the time is right to provide small molecule chemical probes to target functionally relevant RNAs throughout the human transcriptome.

Targeting Nonsense: Optimization of 1,2,4-Oxadiazole TRIDs to Rescue CFTR Expression and Functionality in Cystic Fibrosis Cell Model Systems

Cystic fibrosis (CF) patients develop a severe form of the disease when the cystic fibrosis transmembrane conductance regulator (CFTR) gene is affected by nonsense mutations. Nonsense mutations are responsible for the presence of a premature termination codon (PTC) in the mRNA, creating a lack of functional protein. In this context, translational readthrough-inducing drugs (TRIDs) represent a promising approach to correct the basic defect caused by PTCs. By using computational optimization and biological screening, we identified three new small molecules showing high readthrough activity. The activity of these compounds has been verified by evaluating CFTR expression and functionality after treatment with the selected molecules in cells expressing nonsense–CFTR–mRNA. Additionally, the channel functionality was measured by the halide sensitive yellow fluorescent protein (YFP) quenching assay. All three of the new TRIDs displayed high readthrough activity and low toxicity and can be considered for further evaluation as a therapeutic approach toward the second major cause of CF.

RNA-based therapies in animal models of Leber congenital amaurosis causing blindness

Leber congenital amaurosis (LCA) is a severe, genetically heterogeneous recessive eye disease in which ~ 35% of gene mutations are in-frame nonsense mutations coding for loss-of-function premature termination codons (PTCs) in mRNA. Nonsense suppression therapy allows read-through of PTCs leading to production of full-length protein. A limitation of nonsense suppression is that nonsense-mediated decay (NMD) degrades PTC-containing RNA transcripts. The purpose of this study was to determine whether inhibition of NMD could improve nonsense suppression efficacy in vivo. Using a high-throughput approach in the recessive cep290 zebrafish model of LCA (cep290;Q1223X), we first tested the NMD inhibitor Amlexanox in combination with the nonsense suppression drug Ataluren. We observed reduced retinal cell death and improved visual function. With these positive data, we next investigated whether this strategy was also applicable across species in two mammalian models: Rd12 (rpe65;R44X) and Rd3 (rd3;R107X) mouse models of LCA. In the Rd12 model, cell death was reduced, RPE65 protein was produced, and in vivo visual function testing was improved. We establish for the first time that the mechanism of action of Amlexanox in Rd12 retina was through reduced UPF1 phosphorylation. In the Rd3 model, however, no beneficial effect was observed with Ataluren alone or in combination with Amlexanox. This variation in response establishes that some forms of nonsense mutation LCA can be targeted by RNA therapies, but that this needs to be verified for each genotype. The implementation of precision medicine by identifying better responders to specific drugs is essential for development of validated retinal therapies.

Pharmacophore-Based Design of New Chemical Scaffolds as Translational Readthrough-Inducing Drugs (TRIDs)

Translational readthrough-inducing drugs (TRIDs) rescue the functional full-length protein expression in genetic diseases, such as cystic fibrosis, caused by premature termination codons (PTCs). Small molecules have been developed as TRIDs to trick the ribosomal machinery during recognition of the PTC. Herein we report a computational study to identify new TRID scaffolds. A pharmacophore approach was carried out on compounds that showed readthrough activity. The pharmacophore model applied to screen different libraries containing more than 87000 compounds identified four hit-compounds presenting scaffolds with diversity from the oxadiazole lead. These compounds have been synthesized and tested using the Fluc reporter harboring the UGA PTC. Moreover, the cytotoxic effect and the expression of the CFTR protein were evaluated. These compounds, a benzimidazole derivative (NV2899), a benzoxazole derivative (NV2913), a thiazole derivative (NV2909), and a benzene-1,3-disulfonate derivative (NV2907), were shown to be potential new lead compounds as TRIDs, boosting further efforts to address the optimization of the chemical scaffolds.

Effect of Ataluren on dystrophin mutations

Duchenne muscular dystrophy is a severe muscle wasting disease caused by mutations in the dystrophin gene (dmd). Ataluren has been approved by the European Medicines Agency for treatment of Duchenne muscular dystrophy. Ataluren has been reported to promote ribosomal read‐through of premature stop codons, leading to restoration of full‐length dystrophin protein. However, the mechanism of Ataluren action has not been fully described. To evaluate the efficacy of Ataluren on all three premature stop codons featuring different termination strengths (UAA > UAG > UGA), novel dystrophin‐deficient zebrafish were generated. Pathological assessment of the muscle by birefringence quantification, a tool to directly measure muscle integrity, did not reveal a significant effect of Ataluren on any of the analysed dystrophin‐deficient mutants at 3 days after fertilization. Functional analysis of the musculature at 6 days after fertilization by direct measurement of the generated force revealed a significant improvement by Ataluren only for the UAA‐carrying mutant dmd^ta222a. Interestingly however, all other analysed dystrophin‐deficient mutants were not affected by Ataluren, including the dmd^pc3 and dmd^pc2 mutants that harbour weaker premature stop codons UAG and UGA, respectively. These in vivo results contradict reported in vitro data on Ataluren efficacy, suggesting that Ataluren might not promote read‐through of premature stop codons. In addition, Ataluren had no effect on dystrophin transcript levels, but mild adverse effects on wild‐type larvae were identified. Further assessment of N‐terminally truncated dystrophin opened the possibility of Ataluren promoting alternative translation codons within dystrophin, thereby potentially shifting the patient cohort applicable for Ataluren.

Structural Analysis and Identification of False Positive Hits in Luciferase-Based Assays

Luciferase-based bioluminescence detection techniques are highly favored in high-throughput screening (HTS), in which the firefly luciferase (FLuc) is the most commonly used variant. However, FLuc inhibitors can interfere with the activity of luciferase, which may result in false positive signals in HTS assays. In order to reduce the unnecessary cost of time and money, an in silico prediction model for FLuc inhibitors is highly desirable. In this study, we built an extensive data set consisting of 20 888 FLuc inhibitors and 198 608 noninhibitors, and then developed a group of classification models based on the combination of three machine learning (ML) algorithms and four types of molecular representations. The best prediction model based on XGBoost and ECFP4 and MOE2d descriptors yielded a balanced accuracy (BA) of 0.878 and an area under the receiver operating characteristic curve (AUC) value of 0.958 for the validation set, and a BA of 0.886 and an AUC of 0.947 for the test set. Three external validation sets, including set 1 (3231 FLuc inhibitors and 69 783 noninhibitors), set 2 (695 FLuc inhibitors and 75 913 noninhibitors), and set 3 (1138 FLuc inhibitors and 8155 noninhibitors), were used to verify the predictive ability of our models. The BA values for the three external validation sets given by the best model are 0.864, 0.845, and 0.791, respectively. In addition, the important features or structural fragments related to FLuc inhibitors were recognized by the Shapley additive explanations (SHAP) method along with their influences on predictions, which may provide valuable clues to detecting undesirable luciferase inhibitors. Based on the important and explanatory features, 16 rules were proposed for detecting FLuc inhibitors, which can achieve a correction rate of 70% for FLuc inhibitors. Furthermore, a comparison with existing prediction rules and models for FLuc inhibitors used in virtual screening verified the high reliability of the models and rules proposed in this study. We also used the model to screen three curated chemical databases, and almost 10% of the molecules in the evaluated databases were predicted as inhibitors, highlighting the potential risk of false positives in luciferase-based assays. Finally, a public web server called ChemFLuc was developed (http://admet.scbdd.com/chemfluc/index/), and it offers a free available service to predict potential FLuc inhibitors.

Synthesis of SMT022357 enantiomers and in vivo evaluation in a Duchenne muscular dystrophy mouse model

Following on from ezutromid, the first-in-class benzoxazole utrophin modulator that progressed to Phase 2 clinical trials for the treatment of Duchenne muscular dystrophy, a new chemotype was designed to optimise its physicochemical and ADME profile. Herein we report the synthesis of SMT022357, a second generation utrophin modulator preclinical candidate, and an asymmetric synthesis of its constituent enantiomers. The pharmacological properties of both enantiomers were evaluated in vitro and in vivo. No significant difference in the activity or efficacy was observed between the two enantiomers; activity was found to be comparable to the racemic mixture.

Cytotoxic Profiling of Annotated and Diverse Chemical Libraries Using Quantitative High-Throughput Screening

Cell-based phenotypic screening is a commonly used approach to discover biological pathways, novel drug targets, chemical probes, and high-quality hit-to-lead molecules. Many hits identified from high-throughput screening campaigns are ruled out through a series of follow-up potency, selectivity/specificity, and cytotoxicity assays. Prioritization of molecules with little or no cytotoxicity for downstream evaluation can influence the future direction of projects, so cytotoxicity profiling of screening libraries at an early stage is essential for increasing the likelihood of candidate success. In this study, we assessed the cell-based cytotoxicity of nearly 10,000 compounds in the National Institutes of Health, National Center for Advancing Translational Sciences annotated libraries and more than 100,000 compounds in a diversity library against four normal cell lines (HEK 293, NIH 3T3, CRL-7250, and HaCat) and one cancer cell line (KB 3-1, a HeLa subline). This large-scale library profiling was analyzed for overall screening outcomes, hit rates, pan-activity, and selectivity. For the annotated library, we also examined the primary targets and mechanistic pathways regularly associated with cell death. To our knowledge, this is the first study to use high-throughput screening to profile a large screening collection (>100,000 compounds) for cytotoxicity in both normal and cancer cell lines. The results generated here constitute a valuable resource for the scientific community and provide insight into the extent of cytotoxic compounds in screening libraries, allowing for the identification and avoidance of compounds with cytotoxicity during high-throughput screening campaigns.

Kinetic Target-Guided Synthesis: Reaching the Age of Maturity

Kinetic target-guided synthesis (KTGS) is an original discovery strategy allowing a target to catalyze the irreversible synthesis of its own ligands from a pool of reagents. Although pioneered almost two decades ago, it only recently proved its usefulness in medicinal chemistry, as exemplified by the increasing number of protein targets used, the wider range of target and pocket types, and the diversity of therapeutic areas explored. In recent years, two new leads for in vivo studies were released. Amidations and multicomponent reactions expanded the armamentarium of reactions beyond triazole formation and two new examples of in cellulo KTGS were also disclosed. Herein, we analyze the origins and the chemical space of both KTGS ligands and warhead-bearing reagents. We review the KTGS timeline focusing on recent cases in order to give medicinal chemists the full scope of this strategy which has great potential for hit discovery and hit or lead optimization.

Characterisation of utrophin modulator SMT C1100 as a non-competitive inhibitor of firefly luciferase

Firefly luciferase (FLuc) is a powerful tool for molecular and cellular biology, and popular in high-throughput screening and drug discovery. However, FLuc assays have been plagued with positive and negative artefacts due to stabilisation and inhibition by small molecules from a range of chemical classes. Here we disclose Phase II clinical compound SMT C1100 for the treatment of Duchenne muscular dystrophy as an FLuc inhibitor (K_D of 0.40 ± 0.15 µM). Enzyme kinetic studies using SMT C1100 and other non-competitive inhibitors including resveratrol and NFκBAI4 identified previously undescribed modes of inhibition with respect to FLuc's luciferyl adenylate intermediate. Employing a photoaffinity strategy to identify SMT C1100's binding site, a photolabelled SMT C1100 probe instead underwent FLuc-dependent photooxidation. Our findings support novel binding sites on FLuc for non-competitive inhibitors.

Strategies against Nonsense: Oxadiazoles as Translational Readthrough-Inducing Drugs (TRIDs)

This review focuses on the use of oxadiazoles as translational readthrough-inducing drugs (TRIDs) to rescue the functional full-length protein expression in mendelian genetic diseases caused by nonsense mutations. These mutations in specific genes generate premature termination codons (PTCs) responsible for the translation of truncated proteins. After a brief introduction on nonsense mutations and their pathological effects, the features of various classes of TRIDs will be described discussing differences or similarities in their mechanisms of action. Strategies to correct the PTCs will be presented, particularly focusing on a new class of Ataluren-like oxadiazole derivatives in comparison to aminoglycosides. Additionally, recent results on the efficiency of new candidate TRIDs in restoring the production of the cystic fibrosis transmembrane regulator (CFTR) protein will be presented. Finally, a prospectus on complementary strategies to enhance the effect of TRIDs will be illustrated together with a conclusive paragraph about perspectives, opportunities, and caveats in developing small molecules as TRIDs.

Improving the Utility of the Tox21 Dataset by Deep Metadata Annotations and Constructing Reusable Benchmarked Chemical Reference Signatures

The Toxicology in the 21st Century (Tox21) project seeks to develop and test methods for high-throughput examination of the effect certain chemical compounds have on biological systems. Although primary and toxicity assay data were readily available for multiple reporter gene modified cell lines, extensive annotation and curation was required to improve these datasets with respect to how FAIR (Findable, Accessible, Interoperable, and Reusable) they are. In this study, we fully annotated the Tox21 published data with relevant and accepted controlled vocabularies. After removing unreliable data points, we aggregated the results and created three sets of signatures reflecting activity in the reporter gene assays, cytotoxicity, and selective reporter gene activity, respectively. We benchmarked these signatures using the chemical structures of the tested compounds and obtained generally high receiver operating characteristic (ROC) scores, suggesting good quality and utility of these signatures and the underlying data. We analyzed the results to identify promiscuous individual compounds and chemotypes for the three signature categories and interpreted the results to illustrate the utility and re-usability of the datasets. With this study, we aimed to demonstrate the importance of data standards in reporting screening results and high-quality annotations to enable re-use and interpretation of these data. To improve the data with respect to all FAIR criteria, all assay annotations, cleaned and aggregate datasets, and signatures were made available as standardized dataset packages (Aggregated Tox21 bioactivity data, 2019).

Rescuing the CFTR protein function: Introducing 1,3,4-oxadiazoles as translational readthrough inducing drugs

Nonsense mutations in the CFTR gene prematurely terminate translation of the CFTR mRNA leading to the production of a truncated protein that lacks normal function causing a more severe form of the cystic fibrosis (CF) disease. About 10% of patients affected by CF show a nonsense mutation. A potential treatment of this alteration is to promote translational readthrough of premature termination codons (PTCs) by Translational Readthrough Inducing Drugs (TRIDs) such as PTC124. In this context we aimed to compare the activity of PTC124 with analogues differing in the heteroatoms position in the central heterocyclic core. By a validated protocol consisting of computational screening, synthesis and biological tests we identified a new small molecule (NV2445) with 1,3,4-oxadiazole core showing a high readthrough activity. Moreover, we evaluated the CFTR functionality after NV2445 treatment in CF model systems and in cells expressing a nonsense-CFTR-mRNA. Finally, we studied the supramolecular interactions between TRIDs and CFTR-mRNA to assess the biological target/mechanism and compared the predicted ADME properties of NV2445 and PTC124.

Beetle luciferases with naturally red- and blue-shifted emission

New crystal structures of red- and green blue–shifted beetle luciferases reveal that the color emission mechanism is dependent on the active site microenvironment affected by the conformation of loop regions.

The different colors of light emitted by bioluminescent beetles that use an identical substrate and chemiexcitation reaction sequence to generate light remain a challenging and controversial mechanistic conundrum. The crystal structures of two beetle luciferases with red- and blue-shifted light relative to the green yellow light of the common firefly species provide direct insight into the molecular origin of the bioluminescence color. The structure of a blue-shifted green-emitting luciferase from the firefly Amydetes vivianii is monomeric with a structural fold similar to the previously reported firefly luciferases. The only known naturally red-emitting luciferase from the glow-worm Phrixothrix hirtus exists as tetramers and octamers. Structural and computational analyses reveal varying aperture between the two domains enclosing the active site. Mutagenesis analysis identified two conserved loops that contribute to the color of the emitted light. These results are expected to advance comparative computational studies into the conformational landscape of the luciferase reaction sequence.

High-throughput screening with nucleosome substrate identifies small-molecule inhibitors of the human histone lysine methyltransferase NSD2

The histone lysine methyltransferase nuclear receptor-binding SET domain protein 2 (NSD2, also known as WHSC1/MMSET) is an epigenetic modifier and is thought to play a driving role in oncogenesis. Both NSD2 overexpression and point mutations that increase its catalytic activity are associated with several human cancers. Although NSD2 is an attractive therapeutic target, no potent, selective, and bioactive small molecule inhibitors of NSD2 have been reported to date, possibly due to the challenges of developing high-throughput assays for NSD2. Here, to establish a platform for the discovery and development of selective NSD2 inhibitors, we optimized and implemented multiple assays. We performed quantitative high-throughput screening with full-length WT NSD2 and a nucleosome substrate against a diverse collection of bioactive small molecules comprising 16,251 compounds. We further interrogated 174 inhibitory compounds identified in the primary screen with orthogonal and counter assays and with activity assays based on the clinically relevant NSD2 variants E1099K and T1150A. We selected five confirmed inhibitors for follow-up, which included a radiolabeled validation assay, surface plasmon resonance studies, methyltransferase profiling, and histone methylation in cells. We found that all five NSD2 inhibitors bind the catalytic SET domain and one exhibited apparent activity in cells, validating the workflow and providing a template for identifying selective NSD2 inhibitors. In summary, we have established a robust discovery pipeline for identifying potent NSD2 inhibitors from small-molecule libraries.

Characterization and Use of TurboLuc Luciferase as a Reporter for High-Throughput Assays

Luciferase-based reporter assays are powerful tools for monitoring gene expression in cells because of their ultrasensitive detection capacity and wide dynamic range. Here we describe the characterization and use of a luciferase reporter enzyme derived from the marine copepod Metridia luciferase family, referred to as TurboLuc luciferase (TurboLuc). To develop TurboLuc, the wild-type luciferase was modified to decrease its size, increase brightness, slow luminescent signal decay, and provide for efficient intracellular expression. To determine the enzyme susceptibility to compound inhibition and judge the suitability of using of TurboLuc as a reporter in screening assays, purified TurboLuc enzyme was screened for inhibitors using two different compound libraries. No inhibitors of this enzyme were identified in a library representative of typical diverse low molecular weight (LMW) compounds using a purified TurboLuc enzyme assay supporting that such libraries will show very low interference with this enzyme. We were able to identify a few inhibitors from a purified natural product library which can serve as useful tools to validate assays using TurboLuc. In addition to the inhibitor profile for TurboLuc we describe the use of this reporter in cells employing miniaturized assay volumes within 1536-well plates. TurboLuc luciferase is the smallest luciferase reporter enzyme described to date (16 kDa), shows bright luminescence and low interference by LMW compounds, and therefore should provide an ideal reporter in assays applied to high-throughput screening.

Chemi- and Bioluminescence of Cyclic Peroxides

Bioluminescence is a phenomenon that has fascinated mankind for centuries. Today the phenomenon and its sibling, chemiluminescence, have impacted society with a number of useful applications in fields like analytical chemistry and medicine, just to mention two. In this review, a molecular-orbital perspective is adopted to explain the chemistry behind chemiexcitation in both chemi- and bioluminescence. First, the uncatalyzed thermal dissociation of 1,2-dioxetane is presented and analyzed to explain, for example, the preference for triplet excited product states and increased yield with larger nonreactive substituents. The catalyzed fragmentation reaction and related details are then exemplified with substituted 1,2-dioxetanone species. In particular, the preference for singlet excited product states in that case is explained. The review also examines the diversity of specific solutions both in Nature and in artificial systems and the difficulties in identifying the emitting species and unraveling the color modulation process. The related subject of excited-state chemistry without light absorption is finally discussed. The content of this review should be an inspiration to human design of new molecular systems expressing unique light-emitting properties. An appendix describing the state-of-the-art experimental and theoretical methods used to study the phenomena serves as a complement.

Targeting mutant p53 for efficient cancer therapy

The tumour suppressor gene TP53 is the most frequently mutated gene in cancer. Wild-type p53 can suppress tumour development by multiple pathways. However, mutation of TP53 and the resultant inactivation of p53 allow evasion of tumour cell death and rapid tumour progression. The high frequency of TP53 mutation in tumours has prompted efforts to restore normal function of mutant p53 and thereby trigger tumour cell death and tumour elimination. Small molecules that can reactivate missense-mutant p53 protein have been identified by different strategies, and two compounds are being tested in clinical trials. Novel approaches for targeting TP53 nonsense mutations are also underway. This Review discusses recent progress in pharmacological reactivation of mutant p53 and highlights problems and promises with these strategies.