Structure-guided design, synthesis, and evaluation of guanine-derived inhibitors of the eIF4E mRNA-cap interaction

Proximity-induced SuFEx increases the potency of cytosolic nucleotidase inhibitors and reveals a rare example of covalently targeted histidine

Structure-guided design is one of the most validated solutions for targeting proteins with specific ligands for therapeutic purposes. Nevertheless, it remains challenging to target enzymes with low affinity for their natural ligands and specificities that overlap with those of other proteins. Cytosolic 5'-nucleotidases - involved in the metabolism of nucleic acid derivatives - are an example of such a family. Here we illustrate how precisely designed covalent inhibitors represent a potential solution for selective nucleotidase targeting. We employed the sulfur-fluoride exchange (SuFEx) to develop a covalent inhibitor of cytosolic nucleotidase IIIB (cNIIIB). Using the known inhibitor (7-benzylguanosine monophosphate, Bn7GMP) and computational methods, we designed and synthesized a series of SuFExable inhibitors. One compound indeed covalently bound cNIIIB, which increased the inhibition potency by over 100-fold. The formation of a covalent S-N bond with a non-catalytic His110 residue was confirmed through MS and 15N NMR. The selectivity of the compound in the context of other protein that recognises similar ligands was also confirmed. The study expands the principle of covalent inhibition of nucleotide processing enzymes. It also represents a rare example of histidine tagging by SuFEx. This may facilitate the broader application of SuFEx chemistry in biochemistry and medicinal chemistry.

C-P Bond Formation by Nickel or Cobalt Catalyzed Coupling Reactions

This review discloses nickel- and cobalt-catalyzed coupling reactions that allow C-P bond formation. Activation of C-halide bonds to form phosphonium, pentavalent phosphorus, or trivalent phosphorous compounds has been reported with both metals. However, the conversion of C-O bonds ((activated) ethers, carbonates, acetates) into C-P ones has been only described with Ni. Similarly, there are more examples of C-Y (Y=C, S, N, B) bond activations catalyzed by Ni than by Co. Nevertheless, the cross-dehydrogenative coupling reaction between a P-H reagent and a C-H bond has been reported more often with cobalt than with nickel. In addition, for both metals, electrolytic and photocatalytic processes have been shown to produce a variety of C-P containing molecules. This review aims to provide an overview of the potential of both metals for C-P bond formation and to highlight the remaining challenges.

Chemical Probes for Studying the Eukaryotic Translation Initiation Factor 4E (eIF4E)-Regulated Translatome in Cancer

The dysregulation of translation is a hallmark of cancer that enables rapid changes in the cell proteome to shape oncogenic phenotypes that promote tumor survival. The predominant signaling pathways leading to dysregulation of translational control in cancer are the PI3K-AKT-mTORC1, RAS-RAF-MAPK, and MYC pathways, which all converge on eukaryotic translation initiation factor 4E (eIF4E), an RNA-binding protein that binds to the m7GpppX cap structure at the 5' end of mRNAs to initiate cap-dependent translation. eIF4E is the rate-limiting factor of translation initiation, and its overexpression is known to drive oncogenic transformation, progression, and chemoresistance across many cancers, establishing it as an attractive therapeutic target. Over the last several decades, significant efforts have been made to inhibit eIF4E through the development of mechanistically distinct small-molecule inhibitors that both directly and indirectly act on eIF4E to prevent cap-dependent translation initiation. These inhibitors can serve as powerful chemical tools to improve our understanding of the mechanisms of cap-dependent translation in cancer and to ultimately predict specific cancers that may benefit from eIF4E-targeted therapeutics. This review discusses the progress made in the development of different classes of small-molecule eIF4E inhibitors, the challenges that remain, and their potential as chemical probes to elucidate the complexities of cap-dependent translation in cancer.

Second-Generation Cap Analogue Prodrugs for Targeting Aberrant Eukaryotic Translation Initiation Factor 4E Activity in Cancer

Dysregulation of translation is a hallmark of cancer that enables rapid changes in cellular protein production to shape oncogenic phenotypes. Translation initiation is governed by the m7GpppX cap-binding protein eukaryotic translation initiation factor 4E (eIF4E), the rate-limiting factor of cap-dependent translation initiation. eIF4E is overexpressed in many cancers and drives the production of oncoproteins that promote tumor growth and survival. Accordingly, eIF4E has been established as an attractive albeit challenging therapeutic target. Building upon our previous work of developing cell-permeable cap analogue prodrugs that inhibit eIF4E binding to the m7GpppX cap, herein we disclose the design of second-generation cap analogues with alternative N-9-substituted linkers which exhibit anticancer activity in BRAFV600E mutant melanoma cell lines.

The Proteins Diversity of the eIF4E Family in the eIF4F Complex

In eukaryotes, translation initiation occurs by the cap-dependent mechanism. Each translated mRNA must be pre-bound by the translation initiation factor eIF4E. All isoforms of this factor are combined into one family. The review considers natural diversity of the eIF4E isoforms in different organisms, provides structural information about them, and describes their functional role in the processes, such as oncogenesis, participation in the translation of certain mRNAs under stress, and selective use of the individual isoforms by viruses. In addition, this review briefly describes the mechanism of cap-dependent translation initiation and possible ways to regulate the eIF4E function.

Integrating fragment-based screening with targeted protein degradation and genetic rescue to explore eIF4E function

Eukaryotic initiation factor 4E (eIF4E) serves as a regulatory hub for oncogene-driven protein synthesis and is considered a promising anticancer target. Here we screen a fragment library against eIF4E and identify a ligand-binding site with previously unknown function. Follow-up structure-based design yields a low nM tool compound (4, Kd = 0.09 µM; LE 0.38), which disrupts the eIF4E:eIF4G interaction, inhibits translation in cell lysates, and demonstrates target engagement with eIF4E in intact cells (EC50 = 2 µM). By coupling targeted protein degradation with genetic rescue using eIF4E mutants, we show that disruption of both the canonical eIF4G and non-canonical binding sites is likely required to drive a strong cellular effect. This work highlights the power of fragment-based drug discovery to identify pockets in difficult-to-drug proteins and how this approach can be combined with genetic characterization and degrader technology to probe protein function in complex biological systems.

Palladium-Catalyzed Chemoselective Phosphorylation of Poly(pseudo)halides: A Route for Organophosphorus Synthesis

We present an advancement in synthesizing organophosphorus compounds via chemoselective phosphorylation achieved by a palladium and SelectPhos ligand system (Pd/L1). This catalysis system exhibits remarkable chemoselectivity, even in poly(pseudo)halide substrates and overcoming toxicity and substrate scope limitations. The catalytic system is robust, which is demonstrated across diverse substrates such as chloroaryl and bromoaryl triflates. Furthermore, we present a one-pot sequential strategy combining phosphorylation with Suzuki-Miyaura coupling, providing a versatile platform for the efficient synthesis of complex organophosphorus compounds, challenging conventional reactivity paradigms.

Second-Generation Cap Analogue Prodrugs for Targeting Aberrant Eukaryotic Translation Initiation Factor 4E (eIF4E) Activity in Drug-Resistant Melanoma

Melanoma is the deadliest form of skin cancer with a 5-year survival rate of less than 20%. While significant strides have been made in the field of kinase-targeted and immune-based therapies for melanoma, the development of resistance to these therapeutic agents has hindered the success of treatment. Drug-resistant melanoma is particularly reliant on enhanced cap-dependent translation to drive the production of oncoproteins that promote growth and survival. The m7GpppX cap-binding protein eukaryotic translation initiation factor 4E (eIF4E) is the rate-limiting factor of cap-dependent translation initiation, and its overexpression in melanoma tumors has been shown to drive resistance to BRAFV600E kinase-targeted inhibitors. These findings point to eIF4E-targeted therapies as a promising strategy to overcome drug resistance in melanoma. Herein, we build upon our previous work of developing cell-permeable cap analogue inhibitors to design second-generation cap analogues that inhibit eIF4E-mediated cap-dependent translation in drug-resistant melanoma cells.

Chemical and topological design of multicapped mRNA and capped circular RNA to augment translation

Protein and vaccine therapies based on mRNA would benefit from an increase in translation capacity. Here, we report a method to augment translation named ligation-enabled mRNA-oligonucleotide assembly (LEGO). We systematically screen different chemotopological motifs and find that a branched mRNA cap effectively initiates translation on linear or circular mRNAs without internal ribosome entry sites. Two types of chemical modification, locked nucleic acid (LNA) N7-methylguanosine modifications on the cap and LNA + 5 × 2' O-methyl on the 5' untranslated region, enhance RNA-eukaryotic translation initiation factor (eIF4E-eIF4G) binding and RNA stability against decapping in vitro. Through multidimensional chemotopological engineering of dual-capped mRNA and capped circular RNA, we enhanced mRNA protein production by up to tenfold in vivo, resulting in 17-fold and 3.7-fold higher antibody production after prime and boost doses in a severe acute respiratory syndrome coronavirus 2 vaccine setting, respectively. The LEGO platform opens possibilities to design unnatural RNA structures and topologies beyond canonical linear and circular RNAs for both basic research and therapeutic applications.

The Synergistic Effect of N2 and N7 Modifications on the Inhibitory Efficacy of mRNA Cap Analogues

In the fight against cancer, researchers have turned their attention to the eukaryotic initiation factor eIF4E, a protein whose increased level is strongly correlated with the development and progression of various types of cancer. Among the numerous strategies devised to tackle eIF4E overexpression, the use of 5' end mRNA cap analogues has emerged as a promising approach. Here, we present new candidates as potent m7GMP analogues for inhibiting translation and interfacing with eIF4E. By employing an appropriate strategy, we synthesized doubly modified mono- and dinucleotide cap analogues, introducing simultaneous substituents at both the N7 and N2 positions of the guanine ring. This approach was identified as an effective and promising combination. Our findings reveal that these dual modifications increase the potency of the dinucleotide analogue, marking a significant advancement in the development of cancer therapeutics targeting the eIF4E pathway.

Photocatalyst-free visible-light-promoted C(sp2)-P coupling: efficient synthesis of aryl phosphonates

A novel and efficient method for the synthesis of aryl phosphonates from aryl halides and trialkylphosphites via EDA complex-based photochemistry has been developed. It is demonstrated that aryl radicals, generated from the photoexcitation of the EDA complex formed by aryl halide and potassium thioacetate, could be intercepted with trialkylphosphite to produce the corresponding aryl phosphonates in moderate to good yields. It should be noted that the reaction is performed at room temperature in the absence of any transition metal catalyst, oxidant and photocatalyst, exhibiting high efficiency, high selectivity, and operational simplicity.

Arylazo sulfones: multifaceted photochemical reagents and beyond

The photochemical action of arylazo sulfones under visible light irradiation has recently gained considerable attention for the construction of carbon-carbon and carbon-heteroatom bonds in organic synthesis. The inherent dyedauxiliary group (-N2SO2R) embedded in the reagent is responsible for the absorption of visible light even in the absence of a photocatalyst, additive or oxidant, leading to the generation of three different radicals, viz. aryl (carbon-centred), sulfonyl (sulphur-centred) and diazenyl (nitrogen-centred) radicals, under different reaction conditions. Encountering a reagent with such a versatile behaviour is quite rare, which makes arylazo sulfones a highly interesting class of compounds. The mild reaction conditions under which these reagents can operate are an added advantage. Recently, they are also being used as non-ionic photoacid generators (PAGs), electron acceptors, and hydrogen atom transfer (HAT) and imination reagents in a number of synthetic transformations. They have displayed substantial damaging effect on the structure of DNA in the presence of light which can lead to their use as phototoxic pharmaceuticals for cancer treatment. Moreover, their photochemistry is also being exploited in polymerization reactions (as photoinitiators) and in materials chemistry (surface modification).

Newly Synthesized Anticancer Purine Derivatives Inhibiting p-EIF4E Using Surface-Modified Lipid Nanovesicles

Translation of mRNA is one of the processes adopted by cancer cells to maintain survival via phosphorylated (p)-eIF4E overexpression. Once p-eIF4E binds to the cap structure of mRNA, it advocates a nonstop translation process. In this regard, 15 new-based GMP analogs were synthesized to target eIF4E and restrain its binding to cap mRNA. The compounds were tested against three types of cancer cell lines: Caco-2, HepG-2, MCF-7, and normal kidney cells (Vero cells). Most of the compounds showed high potency against breast cancer cells (MCF-7), characterized by the highest cancer type for overexpression of p-eIF4E. Compound 4b was found to be the most active against three cell lines, colon (Caco-2), hepatic (HepG-2), and breast (MCF-7), with positive IC50 values of 31.40, 27.15, and 21.71 μM, respectively. Then, chitosan-coated niosomes loaded with compound 4b (Cs/4b-NSs) were developed (as kinetically enhanced molecules) to improve the anticancer effects further. The prepared Cs/4b-NSs showed pronounced cytotoxicity compared to the free 4b against Caco2, Hepg2, and MCF-7 with IC50 values of 16.15, 26.66, and 6.90 μM, respectively. Then, the expression of both the phosphorylated and nonphosphorylated western blot techniques was conducted on MCF-7 cells treated with the most active compounds (based on the obtained IC50 values) to determine the total protein expression of both eIF4E and p-eIF4e. Interestingly, the selected most active compounds displayed 35.8-40.7% inhibition of p-eIF4E expression when evaluated on MCF-7 compared to Ribavirin (positive control). CS/4b-NSs showed the best inhibition (40.7%). The findings of the present joint in silico molecular docking, simulation dynamic studies, and experimental investigation suggest the potential use of niosomal nanovesicles as a promising nanocarrier for the targeted delivery of the newly synthesized compound 4b to eukaryotic initiation factor 4E. These outcomes support the possible use of Cs/4b-NSs in targeted cancer therapy.

Copper(II)-catalyzed cascade Csp2-P/C-C bond formation to construct benzo[d]thiazol-2-ylphosphonates

A novel, copper(II)-catalyzed cascade Csp2-P/C-C bond formation in o-haloaryl isothiocyanates with organophosphorus esters has been developed under mild conditions. A series of benzo[d]thiazol-2-ylphosphonates were synthesized in moderate to good yields. Different from the traditional method of obtaining these scaffolds with radical reactions, the method proposed allows accessing them via ionic reactions and has the advantages of easy access to raw materials and simple operation. Finally, we carried out a gram-scale experiment to further demonstrate the scalability of this strategy in the efficient synthesis of benzo[d]thiazol-2-ylphosphonates.

Regulation of Eukaryotic Translation Initiation Factor 4E as a Potential Anticancer Strategy

Eukaryotic translation initiation factors (eIFs) are highly expressed in cancer cells, especially eIF4E, the central regulatory node driving cancer cell growth and a potential target for anticancer drugs. eIF4E-targeting strategies primarily focus on inhibiting eIF4E synthesis, interfering with eIF4E/eIF4G interactions, and targeting eIF4E phosphorylation and peptide inhibitors. Although some small-molecule inhibitors are in clinical trials, no eIF4E inhibitors are available for clinical use. We provide an overview of the regulatory mechanisms of eIF4E and summarize the progress in developing and discovering eIF4E inhibitor strategies. We propose that interference with eIF4E/eIF4G interactions will provide a new perspective for the design of eIF4E inhibitors and may be a preferred strategy.

Design of Cell-Permeable Inhibitors of Eukaryotic Translation Initiation Factor 4E (eIF4E) for Inhibiting Aberrant Cap-Dependent Translation in Cancer

Eukaryotic translation initiation factor 4E (eIF4E) is an RNA-binding protein that binds to the m7GpppX-cap at the 5' terminus of coding mRNAs to initiate cap-dependent translation. While all cells require cap-dependent translation, cancer cells become addicted to enhanced translational capacity, driving the production of oncogenic proteins involved in proliferation, evasion of apoptosis, metastasis, and angiogenesis, among other cancerous phenotypes. eIF4E is the rate-limiting translation factor, and its activation has been shown to drive cancer initiation, progression, metastasis, and drug resistance. These findings have established eIF4E as a translational oncogene and promising, albeit challenging, anti-cancer therapeutic target. Although significant effort has been put forth toward inhibiting eIF4E, the design of cell-permeable, cap-competitive inhibitors remains a challenge. Herein, we describe our work toward solving this long-standing challenge. By employing an acyclic nucleoside phosphonate prodrug strategy, we report the synthesis of cell-permeable inhibitors of eIF4E binding to capped mRNA to inhibit cap-dependent translation.

Four-layer folding framework: design, GAP synthesis, and aggregation-induced emission

The design and synthesis of a type of [1 + 4 + 2] four-layer framework have been conducted by taking advantage of Suzuki-Miyaura cross-coupling and group-assisted purification (GAP) chemistry. The optimized coupling of double-layer diboronic esters with 1-bromo-naphth-2-yl phosphine oxides resulted in a series of multilayer folding targets, showing a broad scope of substrates and moderate to excellent yields. The final products were purified using group-assisted purification chemistry/technology, achieved simply by washing crude products with 95% EtOH without the use of chromatography and recrystallization. The structures were fully characterized and assigned by performing X-ray crystallographic analysis. UV-vis absorption, photoluminescence (PL), and aggregation-induced emission (AIE) were studied for the resulting multilayer folding products.

Site-selective and metal-free C-H phosphonation of arenes via photoactivation of thianthrenium salts

Aryl phosphonates are prevalent moieties in medicinal chemistry and agrochemicals. Their chemical synthesis normally relies on the use of precious metals, harsh conditions or aryl halides as substrates. Herein, we describe a sustainable light-promoted and site-selective C-H phosphonation of arenes via thianthrenation and the formation of an electron donor-acceptor complex (EDA) as key steps. The method tolerates a wide range of functional groups including biomolecules. The use of sunlight also promotes this transformation and our mechanistic investigations support a radical chain mechanism.

Design of Cell-Permeable Inhibitors of Eukaryotic Translation Initiation Factor 4E (eIF4E) for Inhibiting Aberrant Cap-Dependent Translation in Cancer

Eukaryotic translation initiation factor 4E (eIF4E) is an RNA-binding protein that binds to the m 7 GpppX-cap at the 5' terminus of coding mRNAs to initiate cap-dependent translation. While all cells require cap-dependent translation, cancer cells become addicted to enhanced translational capacity, driving the production of oncogenic proteins involved in proliferation, evasion of apoptosis, metastasis, and angiogenesis among other cancerous phenotypes. eIF4E is the rate-limiting translation factor and its activation has been shown to drive cancer initiation, progression, metastasis, and drug resistance. These findings have established eIF4E as a translational oncogene and promising, albeit challenging, anti-cancer therapeutic target. Although significant effort has been put forth towards inhibiting eIF4E, the design of cell-permeable, cap-competitive inhibitors remains a challenge. Herein, we describe our work towards solving this long-standing challenge. By employing an acyclic nucleoside phosphonate prodrug strategy, we report the synthesis of cell-permeable inhibitors of eIF4E binding to capped mRNA to inhibit cap-dependent translation.

Tandem addition of nucleophilic and electrophilic reagents to vinyl phosphinates: the stereoselective formation of organophosphorus compounds with congested tertiary carbons

Carbon anions formed via the addition of Grignard reagents to SP-vinyl phosphinates were modified with electrophilic reagents to afford organophosphorus compounds with diverse carbon skeletons. The electrophiles included acids, aldehydes, epoxy groups, chalcogens and alkyl halides. When alkyl halides were used, bis-alkylated products were afforded. Substitution reactions or polymerization occurred when the reaction was applied to vinyl phosphine oxides.

"Magic Chloro": Profound Effects of the Chlorine Atom in Drug Discovery

Chlorine is one of the most common atoms present in small-molecule drugs beyond carbon, hydrogen, nitrogen, and oxygen. There are currently more than 250 FDA-approved chlorine-containing drugs, yet the beneficial effect of the chloro substituent has not yet been reviewed. The seemingly simple substitution of a hydrogen atom (R = H) with a chlorine atom (R = Cl) can result in remarkable improvements in potency of up to 100,000-fold and can lead to profound effects on pharmacokinetic parameters including clearance, half-life, and drug exposure in vivo. Following the literature terminology of the "magic methyl effect" in drugs, the term "magic chloro effect" has been coined herein. Although reports of 500-fold or 1000-fold potency improvements are often serendipitous discoveries that can be considered "magical" rather than planned, hypotheses made to explain the magic chloro effect can lead to lessons that accelerate the cycle of drug discovery.

Therapeutic targeting of eukaryotic initiation factor (eIF) 4E

Fundamental studies unraveled the role of eukaryotic initiation factor (eIF) 4E in mRNA translation and its control. Under physiological conditions, regulation of translation by eIF4E is essential to cellular homeostasis. Under stress, gene flow information is parsed by eIF4E to support adaptive mechanisms that favor cell survival. Dysregulated eIF4E activity fuels tumor formation and progression and modulates response to therapy. Thus, there has been heightened interest in understanding eIF4E function in controlling gene expression as well as developing strategies to block its activity to treat disease.

Synthesis of ortho-Phosphated (Hetero)Arylamines through Cascade Atherton-Todd Reaction/[3,3]-Rearrangement from Arylhydroxylamines and Dialkyl Phosphites

A practical and facile strategy for the synthesis of ortho-phosphated (hetero)arylamines from readily available arylhydroxylamines and dialkyl phosphites via cascade Atherton-Todd reaction/[3,3]-rearrangement was developed. This method is amenable to various arylhydroxylamines such as phenylhydroxylamines, naphthylhydroxylamines, and pyridylhydroxylamines, has mild reaction conditions, is oxidant-free, and has good functional-group compatibility and excellent regioselectivity.

HuR as a molecular target for cancer therapeutics and immune-related disorders

The control of eukaryotic gene expression occurs at multiple levels, from transcription to messenger RNA processing, transport, localization, turnover, and translation. RNA-binding proteins control gene expression and are involved in different stages of mRNA processing, including splicing, maturation, turnover, and translation. A ubiquitously expressed RBP Human antigen R is engaged in the RNA processes mentioned above but, most importantly, controls mRNA stability and turnover. Dysregulation of HuR is linked to many diseases, including cancer and other immune-related disorders. HuR targets mRNAs containing AU-rich elements at their 3'untranslated region, which encodes proteins involved in cell growth, proliferation, tumor formation, angiogenesis, immune evasion, inflammation, invasion, and metastasis. HuR overexpression has been reported in many tumor types, which led to a poor prognosis for patients. Hence, HuR is considered an appealing drug target for cancer treatment. Therefore, multiple attempts have been made to identify small molecule inhibitors for blocking HuR functions. This article reviews the current prospects of drugs that target HuR in numerous cancer types, their mode of action, and off-target effects. Furthermore, we will summarize drugs that interfered with HuR-RNA interactions and established themselves as novel therapeutics. We will also highlight the significance of HuR overexpression in multiple cancers and discuss its role in immune functions. This review provides evidence of a new era of HuR-targeted small molecules that can be used for cancer therapeutics either as a monotherapy or in combination with other cancer treatment modalities.

Phosphonic acid-containing inhibitors of tyrosyl-DNA phosphodiesterase 1

Tyrosyl-DNA phosphodiesterase 1 (TDP1) repairs stalled type I topoisomerase (TOP1)-DNA complexes by hydrolyzing the phosphodiester bond between the TOP1 Y723 residue and the 3′-phosphate of its DNA substrate. Although TDP1 antagonists could potentially reduce the dose of TOP1 inhibitors needed to achieve effective anticancer effects, the development of validated TDP1 inhibitors has proven to be challenging. This may, in part, be due to the open and extended nature of the TOP1 substrate binding region. We have previously reported imidazopyrazines and imidazopyridines that can inhibit TDP1 catalytic function in vitro. We solved the TDP1 crystal structures with bound inhibitors of this class and found that the dicarboxylic acid functionality within the N-(3,4-dicarboxyphenyl)-2-diphenylimidazo [1,2-a]pyridin-3-amine platform overlaps with aspects of phosphoryl substrate recognition. Yet phosphonic acids could potentially better-replicate cognate TOP1-DNA substrate binding interactions than carboxylic acids. As reported herein, we designed phosphonic acid-containing variants of our previously reported carboxylic acid-containing imidazopyrazine and imidazopyridine inhibitors and effected their synthesis using one-pot Groebke–Blackburn–Bienayme multicomponent reactions. We obtained crystal structures of TDP1 complexed with a subset of inhibitors. We discuss binding interactions of these inhibitors within the context of phosphate-containing substrate and carboxylic acid-based inhibitors. These compounds represent a new structural class of small molecule ligands that mimic aspects of the 3′-processed substrate that results from TDP1 catalysis.

Direct phosphorylation of benzylic C-H bonds under transition metal-free conditions forming sp3C-P bonds

Direct phosphorylation of benzylic C-H bonds was achieved in a biphasic system under transition metal-free conditions. A selective radical/radical sp3C-H/P(O)-H cross coupling was proposed, and various substituted toluenes were applicable. The transformation provided a promising method for constructing sp3C-P bonds.

I2-mediated Csp2–P bond formation via tandem cyclization of o-alkynylphenyl isothiocyanates with organophosphorus esters

A highly efficient molecular-iodine-catalyzed cascade cyclization reaction has been developed, creating a series of 4H-benzo[d][1,3]thiazin-2-yl phosphonates in moderate to excellent yields. This approach benefits from metal-free catalysts and available raw materials.

Visible-light-initiated regio- and stereoselective C(sp2)–H phosphorylation of enamides under transition-metal-free conditions

A visible-light-induced stereo- and regioselective phosphorylation of enamides with phosphine oxides under transition-metal-free conditions has been disclosed.

Development of a Novel Peptide Aptamer that Interacts with the eIF4E Capped-mRNA Binding Site using Peptide Epitope Linker Evolution (PELE)

Identifying new binding sites and poses that modify biological function are an important step towards drug discovery. We have identified a novel disulphide constrained peptide that interacts with the cap-binding site of eIF4E, an attractive therapeutic target that is commonly overexpressed in many cancers and plays a significant role in initiating a cancer specific protein synthesis program though binding the 5′cap (7′methyl-guanoisine) moiety found on mammalian mRNAs. The use of disulphide constrained peptides to explore intracellular biological targets is limited by their lack of cell permeability and the instability of the disulphide bond in the reducing environment of the cell, loss of which results in abrogation of binding. To overcome these challenges, the cap-binding site interaction motif was placed in a hypervariable loop on an VH domain, and then selections performed to select a molecule that could recapitulate the interaction of the peptide with the target of interest in a process termed Peptide Epitope Linker Evolution (PELE). A novel VH domain was identified that interacted with the eIF4E cap binding site with a nanomolar affinity and that could be intracellularly expressed in mammalian cells. Additionally, it was demonstrated to specifically modulate eIF4E function by decreasing cap-dependent translation and cyclin D1 expression, common effects of eIF4F complex disruption.

Identifying new binding sites and poses that modify biological function are an important step towards drug discovery.

Highly Efficient Production of Heteroarene Phosphonates by Dichromatic Photoredox Catalysis

A new strategy to achieve efficient aerobic phosphorylation of five-membered heteraroenes with excellent yields using dichromatic photoredox catalysis in a gel-based nanoreactor is described here. The procedure involves visible aerobic irradiation (cold white LEDs) of a mixture containing the heteroarene halide, trisubstituted phospite, N,N-diisopropylethylamine (DIPEA) as sacrificial agent, and catalytic amounts of 9,10-dicyanoanthracene (DCA) in the presence of an adequate gelator, which permits a faster process than at the homogeneous phase. The methodology, which operates by a consecutive photoinduced electron transfer (ConPET) mechanism, has been successfully applied to the straightforward and clean synthesis of a number of different heteroarene (furan, thiophene, selenophene, pyrrole, oxazole, or thioxazole) phosphonates, extending to the late-stage phosphonylation of the anticoagulant rivaroxaban. Strategically, employment of cold white light is critical since it provides both selective wavelengths for exciting first DCA (blue region) and subsequently its corresponding radical anion DCA^•- (green region). The resultant strongly reducing excited agent DCA^•-* is capable of even activate five-membered heteroarene halides (Br, Cl) with high reduction potentials (∼-2.7 V) to effect the C(sp²)-P bond formation. Spectroscopic and thermodynamic studies have supported the proposed reaction mechanism. Interestingly, the rate of product formation has been clearly enhanced in gel media because reactants can be presumably localized not only in the solvent pools but also through to the fibers of the viscoelastic gel network. This has been confirmed by field-emission scanning electron microscopy images where a marked densification of the network has been observed, modifying its fibrillary morphology. Finally, rheological measurements have shown the resistance of the gel network to the incorporation of the reactants and the formation of the desired products.

Control of the eIF4E activity: structural insights and pharmacological implications

The central role of eukaryotic translation initiation factor 4E (eIF4E) in controlling mRNA translation has been clearly assessed in the last decades. eIF4E function is essential for numerous physiological processes, such as protein synthesis, cellular growth and differentiation; dysregulation of its activity has been linked to ageing, cancer onset and progression and neurodevelopmental disorders, such as autism spectrum disorder (ASD) and Fragile X Syndrome (FXS). The interaction between eIF4E and the eukaryotic initiation factor 4G (eIF4G) is crucial for the assembly of the translational machinery, the initial step of mRNA translation. A well-characterized group of proteins, named 4E-binding proteins (4E-BPs), inhibits the eIF4E–eIF4G interaction by competing for the same binding site on the eIF4E surface. 4E-BPs and eIF4G share a single canonical motif for the interaction with a conserved hydrophobic patch of eIF4E. However, a second non-canonical and not conserved binding motif was recently detected for eIF4G and several 4E-BPs. Here, we review the structural features of the interaction between eIF4E and its molecular partners eIF4G and 4E-BPs, focusing on the implications of the recent structural and biochemical evidence for the development of new therapeutic strategies. The design of novel eIF4E-targeting molecules that inhibit translation might provide new avenues for the treatment of several conditions.

Photoinduced transition-metal and external photosensitizer free cross-coupling of aryl triflates with trialkyl phosphites

Photoinduced phosphonation of aryl triflates with trialkyl phosphites via a tandem single-electron-transfer, C-O bond cleavage and Arbuzov rearrangement process in the absence of transition-metal and external photosensitizer is reported herein. The protocol features good functional group compatibility and mild reaction conditions, providing various aryl phosphates in good to high yields. Furthermore, this strategy allows the late-stage phosphonation of complex and biologically active compounds.

Recent advances in the carbon-phosphorus (C-P) bond formation from unsaturated compounds by s- and p-block metals

Researchers around the globe have witnessed several breakthroughs in s- and p-block metal chemistry. Over the past few years, several applications in catalysis associated with these main group metals have been established, and owing to their abundance and low cost and they have proved to be essential alternatives to transition metal catalysts. In this review, we present a detailed discussion on the catalytic addition of P-H bonds from various phosphine reagents to multiple bonds of unsaturated substrates for the synthesis of organophosphorus compounds with C-P bonds promoted by various s- and p-block metal catalysts, as published in the last decade.

Visible-light-facilitated P-center radical addition to C[double bond, length as m-dash]X (X = C, N) bonds results in cyclizations

Visible-light-facilitated phosphorus radical reactions have been developed as a powerful and sustainable tool for the synthesis of various organophosphorus compounds. In general, these reactions require stoichiometric amounts of oxidants, and reductants, bases, and radical initiators, leading to uneconomical and complicated processes. Progress has been made over the past few years toward using reactions that proceed under eco-benign and mild reaction conditions. Furthermore, these reactions have broad functional group tolerance, with some facile and economical pathways. Herein, we summarize the discoveries and achievements pertaining to C-P bond formation through a visible light photocatalysis procedure with high atom economy, made by our group and other research groups. It was established that greener and more environmentally friendly approaches do not require an additional oxidant or base. Moreover, we have designed and synthesized a new type of P-radical precursor, which can take part in reactions without the requirement for any additional bases, oxidants, and additives. This breakthrough, pertaining to novel visible-light-induced transformations, will be discussed and a plausible mechanism is proposed, based on corresponding experiments and the literature.

Highly Regioselective Tandem Reaction of Ene-Yne-Oxazolones Induced by H-Phosphonates: Construction of Phosphinylindane Derivatives

A highly regioselective divergent approach for the phosphine-containing indane/indene derivatives from the ene-yne-oxazolone precursors was reported. An insight into the reaction mechanism involving the phospha-1,4-addition followed by 5-exo-dig ring closure with a concomitant C-P/C-C bond formation was also proposed. This promising protocol utilized H-phosphonate as the phosphonating reagent in a silver-catalyzed or base-mediated cascade cyclization to construct the corresponding phosphorylated spiroindenoxazolones and amidoindenes, respectively, in good yields (up to 88% yield).

Copper-Catalyzed Phosphorylation of 2,3-Allenoic Acids and Phosphine Oxide: Access to Phosphorylated Butenolides

We investigated a novel Cu-catalyzed annulation of 2,3-allenoic acids with diphenylphosphine oxide, leading to the formation of 4-phosphate butenolides in up to 88% yield. The formation of the C-P bond provides new avenues for the functionalization of different furan-2(5H)-ones, with favorable features such as suitable functional group tolerance and mild synthesis conditions.

New approaches to target RNA binding proteins

RNA binding proteins (RBPs) are a large and diverse class of proteins that regulate all aspects of RNA biology. As RBP dysregulation has been implicated in a number of human disorders, including cancers and neurodegenerative disease, small molecule chemical probes that target individual RBPs represent useful tools for deciphering RBP function and guiding the production of new therapeutics. While RBPs are often thought of as tough-to-drug, the discovery of a number of small molecules that target RBPs has spurred considerable recent interest in new strategies for RBP chemical probe discovery. Here we review current and emerging technologies for high throughput RBP-small molecule screening that we expect will help unlock the full therapeutic potential of this exciting protein class.

The Synthesis of Biarylmonophosphonates via Palladium-Catalyzed Phosphonation, Iridium-Catalyzed C-H Borylation, Palladium-Catalyzed Suzuki–Miyaura Cross-Coupling

Abstract

The iridium-catalyzed C-H borylation of diethyl phenylphosphonate results in nonselective mono and bisborylation to afford a near statistical mixture of 3-, 3,5- and 4-boryl substituted aryl phosphonates whereas 3-substituted aryl phosphonates undergo highly regioselective C-H borylation to afford the corresponding meta-phosphonate substituted arylboronic esters as the sole product; the resulting boronic esters were used as nucleophilic reagents in a subsequent palladium-catalyzed Suzuki–Miyaura cross-coupling to generate a range of biarylmonophosphonates. Gratifyingly, the Suzuki–Miyaura cross-coupling can be conducted without purifying the boronic ester which greatly simplifies the synthetic procedure.

Graphical Abstract

Evaluation of carboxyfluorescein-labeled 7-methylguanine nucleotides as probes for studying cap-binding proteins by fluorescence anisotropy

Fluorescence anisotropy (FA) is a powerful technique for the discovery of protein inhibitors in a high-throughput manner. In this study, we sought to develop new universal FA-based assays for the evaluation of compounds targeting mRNA 5′ cap-binding proteins of therapeutic interest, including eukaryotic translation initiation factor 4E and scavenger decapping enzyme. For this purpose, a library of 19 carboxyfluorescein probes based on 7-methylguanine nucleotides was evaluated as FA probes for these proteins. Optimal probe:protein systems were further investigated in competitive binding experiments and adapted for high-throughput screening. Using a small in-house library of compounds, we verified and confirmed the accuracy of the developed FA assay to study cap-binding protein binders. The applications of the most promising probes were then extended to include evaluation of allosteric inhibitors as well as RNA ligands. From this analysis, we confirmed the utility of the method to study small molecule ligands and evaluate differently 5′ capped RNAs.

Structural Insights into the Interaction of Clinically Relevant Phosphorothioate mRNA Cap Analogs with Translation Initiation Factor 4E Reveal Stabilization via Electrostatic Thio-Effect

mRNA-based therapies and vaccines constitute a disruptive technology with the potential to revolutionize modern medicine. Chemically modified 5′ cap structures have provided access to mRNAs with superior translational properties that could benefit the currently flourishing mRNA field. Prime examples of compounds that enhance mRNA properties are antireverse cap analog diastereomers that contain an O-to-S substitution within the β-phosphate (β-S-ARCA D1 and D2), where D1 is used in clinically investigated mRNA vaccines. The compounds were previously found to have high affinity for eukaryotic translation initiation factor 4E (eIF4E) and augment translation in vitro and in vivo. However, the molecular basis for the beneficial “thio-effect” remains unclear. Here, we employed multiple biophysical techniques and captured 11 cap analog-eIF4E crystallographic structures to investigate the consequences of the β-O-to-S or -Se substitution on the interaction with eIF4E. We determined the S_P/R_P configurations of β-S-ARCA and related compounds and obtained structural insights into the binding. Unexpectedly, in both stereoisomers, the β-S/Se atom occupies the same binding cavity between Lys162 and Arg157, indicating that the key driving force for complex stabilization is the interaction of negatively charged S/Se with positively charged amino acids. This was observed for all structural variants of the cap and required significantly different conformations of the triphosphate for each diastereomer. This finding explains why both β-S-ARCA diastereomers have higher affinity for eIF4E than unmodified caps. Binding affinities determined for di-, tri-, and oligonucleotide cap analogs suggested that the “thio-effect” was preserved in longer RNAs. Our observations broaden the understanding of thiophosphate biochemistry and enable the rational design of translationally active mRNAs and eIF4E-targeting drugs.

Highly efficient chemical phosphorylation of 6-(4-phenylpiperazine-1-yl)-9-(β-D-ribofuranosyl)-9H-purine

Antimetabolites, which are metabolic antagonists used in the treatment of cancer and viral diseases by replacing metabolites, inhibit the action of metabolic enzymes and disrupt the pathways of synthesis of structural units necessary for the formation of nucleic acids. Purine antagonists, that are subunits of antimetabolites, reduce the production of purine bases, and hence, cause the nucleotide production to stop and bring about the death of cancer cells. Fludarabine (2-fluoro-ara-AMP), which is used in chemotherapy, is an antimetabolite of the purine class containing mono phosphate in its structure. In this study, a protocol was presented to effectively and efficiently synthesis of 6-(4-phenylpiperazine-1-yl)-9-(β-D-ribofuranosyl)-9H-purine-5'- O-phosphate compound in six steps and 25% overall yield starting with commercially available 6-chloropurine.

Selective hydrolysis of phosphorus(V) compounds to form organophosphorus monoacids

An azide and transition metal-free method for the synthesis of elusive phosphonic, phosphinic, and phosphoric monoacids has been developed. Inert pentavalent P(v)-compounds (phosphonate, phosphinate, and phosphate) are activated by triflate anhydride (Tf2O)/pyridine system to form a highly reactive phosphoryl pyridinium intermediate that undergoes nucleophilic substitution with H2O to selectively deprotect one alkoxy group and form organophosphorus monoacids.

Metal-free visible-light-induced photoredox-catalyzed intermolecular pyridylation/phosphinoylation of alkenes

A visible-light-induced and photoredox-catalyzed intermolecular pyridylation/phosphinoylation of alkenes using 4-cyanopyridine and diphenylphosphine oxide under mild metal-free conditions has been reported.