Caught before Released: Structural Mapping of the Reaction Trajectory for the Sofosbuvir Activating Enzyme, Human Histidine Triad Nucleotide Binding Protein 1 (hHint1)

The activation cascade of the broad-spectrum antiviral bemnifosbuvir characterized at atomic resolution

Bemnifosbuvir (AT-527) and AT-752 are guanosine analogues currently in clinical trials against several RNA viruses. Here, we show that these drugs require a minimal set of 5 cellular enzymes for activation to their common 5'-triphosphate AT-9010, with an obligate order of reactions. AT-9010 selectively inhibits essential viral enzymes, accounting for antiviral potency. Functional and structural data at atomic resolution decipher N6-purine deamination compatible with its metabolic activation. Crystal structures of human histidine triad nucleotide binding protein 1, adenosine deaminase-like protein 1, guanylate kinase 1, and nucleoside diphosphate kinase at 2.09, 2.44, 1.76, and 1.9 Å resolution, respectively, with cognate precursors of AT-9010 illuminate the activation pathway from the orally available bemnifosbuvir to AT-9010, pointing to key drug-protein contacts along the activation pathway. Our work provides a framework to integrate the design of antiviral nucleotide analogues, confronting requirements and constraints associated with activation enzymes along the 5'-triphosphate assembly line.

The Many Faces of Histidine Triad Nucleotide Binding Protein 1 (HINT1)

The histidine triad nucleotide binding protein 1 (HINT1) is a nucleoside phosphoramidase that has garnered interest due to its widespread expression and participation in a broad range of biological processes. Herein, we discuss the role of HINT1 as a regulator of several CNS functions, tumor suppressor, and mast cell activator via its interactions with multiple G-protein-coupled receptors and transcription factors. Importantly, altered HINT1 expression and mutation are connected to the progression of multiple disease states, including several neuropsychiatric disorders, peripheral neuropathy, and tumorigenesis. Additionally, due to its involvement in the activation of several clinically used phosphoramidate prodrugs, tremendous efforts have been made to better understand the interactions behind nucleoside binding and phosphoramidate hydrolysis by HINT1. We detail the substrate specificity and catalytic mechanism of HINT1 hydrolysis, while highlighting the structural biology behind these efforts. The aim of this review is to summarize the multitude of biological and pharmacological functions in which HINT1 participates while addressing the areas of need for future research.

Prodrugs of Nucleoside 5'-Monophosphate Analogues: Overview of the Recent Literature Concerning their Synthesis and Applications

Nucleoside analogues are widely used as anti-infectious and antitumoral agents. However, their clinical use may face limitations associated with their physicochemical properties, pharmacokinetic parameters, and/or their peculiar mechanisms of action. Indeed, once inside the cells, nucleoside analogues require to be metabolized into their corresponding (poly-)phosphorylated derivatives, mediated by cellular and/or viral kinases, in order to interfere with nucleic acid biosynthesis. Within this activation process, the first-phosphorylation step is often the limiting one and to overcome this limitation, numerous prodrug approaches have been proposed. Herein, we will focus on recent literature data (from 2015 and onwards) related to new prodrug strategies, the development of original synthetic approaches and novel applications of nucleotide prodrugs (namely pronucleotides) leading to the intracellular delivery of 5'-monophosphate nucleoside analogues.

Dynamic Long-Range Interactions Influence Substrate Binding and Catalysis by Human Histidine Triad Nucleotide-Binding Proteins (HINTs), Key Regulators of Multiple Cellular Processes and Activators of Antiviral ProTides

Human histidine triad nucleotide-binding (hHINT) proteins catalyze nucleotide phosphoramidase and acyl-phosphatase reactions that are essential for the activation of antiviral proTides, such as Sofosbuvir and Remdesivir. hHINT1 and hHINT2 are highly homologous but exhibit disparate roles as regulators of opioid tolerance (hHINT1) and mitochondrial activity (hHINT2). NMR studies of hHINT1 reveal a pair of dynamic surface residues (Q62, E100), which gate a conserved water channel leading to the active site 13 Å away. hHINT2 crystal structures identify analogous residues (R99, D137) and water channel. hHINT1 Q62 variants significantly alter the steady-state k_cat and K_m for turnover of the fluorescent substrate (TpAd), while stopped-flow kinetics indicate that K_D also changes. hHINT2, like hHINT1, exhibits a burst phase of adenylation, monitored by fluorescent tryptamine release, prior to rate-limiting hydrolysis and nucleotide release. hHINT2 exhibits a much smaller burst-phase amplitude than hHINT1, which is further diminished in hHINT2 R99Q. Kinetic simulations suggest that amplitude variations can be accounted for by a variable fluorescent yield of the E·S complex from changes in the environment of bound TpAd. Isothermal titration calorimetry measurements of inhibitor binding show that these hHINT variants also alter the thermodynamic binding profile. We propose that these altered surface residues engender long-range dynamic changes that affect the orientation of bound ligands, altering the thermodynamic and kinetic characteristics of hHINT active site function. Thus, studies of the cellular roles and proTide activation potential by hHINTs should consider the importance of long-range interactions and possible protein binding surfaces far from the active site.

Remdesivir: Mechanism of Metabolic Conversion from Prodrug to Drug

BACKGROUND

Remdesivir (GS-5734) has emerged as a promising drug during the challenging times of COVID-19 pandemic. Being a prodrug, it undergoes several metabolic reactions before converting to its active triphosphate metabolite. It is important to establish the atomic level details and explore the energy profile of the prodrug to drug conversion process.

METHODS

In this work, Density Functional Theory (DFT) calculations were performed to explore the entire metabolic path. Further, the potential energy surface (PES) diagram for the conversion of prodrug remdesivir to its active metabolite was established. The role of catalytic triad of Hint1 phosphoramidase enzyme in P-N bond hydrolysis was also studied on a model system using combined molecular docking and quantum mechanics approach.

RESULTS

The overall energy of reaction is 11.47 kcal/mol exergonic and the reaction proceeds through many steps requiring high activation energies. In the absence of a catalyst, the P-N bond breaking step requires 41.78 kcal/mol, which is reduced to 14.26 kcal/mol in a catalytic environment.

CONCLUSION

The metabolic pathways of model system of remdesivir (MSR) were completely explored completely and potential energy surface diagrams at two levels of theory, B3LYP/6-311++G(d, p) and B3LYP/6-31+G(d), were established and compared. The results highlight the importance of an additional water molecule in the metabolic reaction. The P-N bond cleavage step of the metabolic process requires the presence of an enzymatic environment.

Biochemical, crystallographic and biophysical characterization of histidine triad nucleotide-binding protein 2 with different ligands including a non-hydrolyzable analog of Ap4A

BACKGROUND

Human HINT2 is an important mitochondrial enzyme involved in many processes such as apoptosis and bioenergetics, but its endogenous substrates and the three-dimensional structure of the full-length protein have not been identified yet.

METHODS

An HPLC assay was used to test the hydrolytic activity of HINT2 against various adenosine, guanosine, and 2'-deoxyguanosine derivatives containing phosphate bonds of different types and different leaving groups. Data on binding affinity were obtained by microscale thermophoresis (MST). Crystal structures of HINT2, in its apo form and with a dGMP ligand, were resolved to atomic resolution.

RESULTS

HINT2 substrate specificity was similar to that of HINT1, but with the major exception of remarkable discrimination against substrates lacking the 2'-hydroxyl group. The biochemical results were consistent with binding affinity measurements. They showed a similar binding strength of AMP and GMP to HINT2, and much weaker binding of dGMP, in contrast to HINT1. A non-hydrolyzable analog of Ap4A (JB419) interacted with both proteins with similar K_d and Ap4A is the signaling molecule that can interact with hHINT1 and regulate the activity of some transcription factors.

CONCLUSIONS

Several forms of homo- and heterodimers of different lengths of N-terminally truncated polypeptides resulting from degradation of the full-length protein were described. Ser144 in HINT2 appeared to be functionally equivalent to Ser107 in HINT1 by supporting the protonation of the leaving group in the hydrolytic mechanism of HINT2.

SIGNIFICANCE

Our results should be considered in future studies on the natural function of HINT2 and its role in nucleotide prodrug processing.

Tissue-Specific Proteomics Analysis of Anti-COVID-19 Nucleoside and Nucleotide Prodrug-Activating Enzymes Provides Insights into the Optimization of Prodrug Design and Pharmacotherapy Strategy

Nucleoside and nucleotide analogs are an essential class of antivirals for COVID-19 treatment. Several nucleoside/nucleotide analogs have shown promising effects against SARS-CoV-2 in vitro; however, their in vivo efficacy is limited. Nucleoside/nucleotide analogs are often formed as ester prodrugs to improve pharmacokinetics (PK) performance. After entering cells, the prodrugs undergo several enzymatic metabolism steps to form the active metabolite triphosphate nucleoside (TP-Nuc); prodrug activation is therefore associated with the abundance and catalytic activity of the corresponding activating enzymes. Having the activation of nucleoside/nucleotide prodrugs occur at the target site of action, such as the lung, is critical for anti-SARS-CoV-2 efficacy. Herein, we conducted an absolute quantitative proteomics study to determine the expression of relevant activating enzymes in human organs related to the PK and antiviral efficacy of nucleoside/nucleotide prodrugs, including the lung, liver, intestine, and kidney. The protein levels of prodrug-activating enzymes differed significantly among the tissues. Using catalytic activity values reported previously for individual enzymes, we calculated prodrug activation profiles in these tissues. The prodrugs evaluated in this study include nine McGuigan phosphoramidate prodrugs, two cyclic monophosphate prodrugs, two l-valyl ester prodrugs, and one octanoate prodrug. Our analysis showed that most orally administered nucleoside/nucleotide prodrugs were primarily activated in the liver, suggesting that parenteral delivery routes such as inhalation and intravenous infusion could be better options when these antiviral prodrugs are used to treat COVID-19. The results also indicated that the l-valyl ester prodrug design can plausibly improve drug bioavailability and enhance effects against SARS-CoV-2 intestinal infections. This study further revealed that an octanoate prodrug could provide a long-acting antiviral effect targeting SARS-CoV-2 infections in the lung. Finally, our molecular docking analysis suggested several prodrug forms of favipiravir and GS-441524 that are likely to exhibit favorable PK features over existing prodrug forms. In sum, this study revealed the activation mechanisms of various nucleoside/nucleotide prodrugs relevant to COVID-19 treatment in different organs and shed light on the development of more effective anti-COVID-19 prodrugs.

"Switching On" Enzyme Substrate Specificity Analysis with a Fluorescent Competitive Inhibitor

Enzymatically driven change to the spectroscopic properties of a chemical substrate or product has been a linchpin in the development of continuous enzyme kinetics assays. These assays inherently necessitate substrates or products that naturally comply with the constraints of the spectroscopic technique being used, or they require structural changes to the molecules involved to make them observable. Here we demonstrate a new analytical kinetics approach with enzyme histidine triad nucleotide binding protein 1 (HINT1) that allows us to extract both useful k_cat values and a rank-ordered list of substrate specificities without the need to track substrates or products directly. Instead, this is accomplished indirectly using a "switch on" competitive inhibitor that fluoresces maximally only when bound to the HINT1 enzyme active site. Kinetic information is extracted from the duration of the diminished fluorescence when the monitorable inhibitor-bound enzyme is challenged with saturating concentrations of a nonfluorescent substrate. We refer to the loss of fluorescence, while the substrate competes for the fluorescent probe in the active site, as the substrate's residence transit time (RTT). The ability to assess k_cat values and substrate specificity by monitoring the RTTs for a set of substrates with a competitive "switch on" inhibitor should be broadly applicable to other enzymatic reactions in which the "switch on" inhibitor has sufficient binding affinity over the enzymatic product.

Potential molecular targets of nonstructural proteins for the development of antiviral drugs against SARS-CoV-2 infection

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The pandemic of SARS-CoV-2 has posed significant threats to public health worldwide.

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Target-based drug development is a promising approach against SARS-CoV-2 infection.

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Nonstructural proteins may play critical roles from drug design perspectives.

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Insights into NSPs of different viruses could streamline novel drug development.

Outbreaks of severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV), and SARS-CoV-2 have produced high pathogenicity and mortality rates in human populations. However, to meet the increasing demand for treatment of these pathogenic coronaviruses, accelerating novel antiviral drug development as much as possible has become a public concern. Target-based drug development may be a promising approach to achieve this goal. In this review, the relevant features of potential molecular targets in human coronaviruses (HCoVs) are highlighted, including the viral protease, RNA-dependent RNA polymerase, and methyltransferases. Additionally, recent advances in the development of antivirals based on these targets are summarized. This review is expected to provide new insights and potential strategies for the development of novel antiviral drugs to treat SARS-CoV-2 infection.

ATI-2173, a Novel Liver-Targeted Non-Chain-Terminating Nucleotide for Hepatitis B Virus Cure Regimens

ATI-2173 is a novel liver-targeted molecule designed to deliver the 5′-monophosphate of clevudine for the treatment of chronic hepatitis B infection. Unlike other nucleos(t)ides, the active clevudine-5′-triphosphate is a noncompetitive, non-chain-terminating inhibitor of hepatitis B virus (HBV) polymerase that delivers prolonged reduction of viremia in both a woodchuck HBV model and in humans for up to 6 months after cessation of treatment. However, long-term clevudine treatment was found to exhibit reversible skeletal myopathy in a small subset of patients and was subsequently discontinued from development.

ATI-2173 is a novel liver-targeted molecule designed to deliver the 5′-monophosphate of clevudine for the treatment of chronic hepatitis B infection. Unlike other nucleos(t)ides, the active clevudine-5′-triphosphate is a noncompetitive, non-chain-terminating inhibitor of hepatitis B virus (HBV) polymerase that delivers prolonged reduction of viremia in both a woodchuck HBV model and in humans for up to 6 months after cessation of treatment. However, long-term clevudine treatment was found to exhibit reversible skeletal myopathy in a small subset of patients and was subsequently discontinued from development. ATI-2173 was designed by modifying clevudine with a 5′-phosphoramidate to deliver the 5′-monophosphate to the liver. Bypassing the first phosphorylation step of clevudine, the 5′-monophosphate is converted to the active 5′-triphosphate in the liver. ATI-2173 is a selective inhibitor of HBV with an anti-HBV 50% effective concentration (EC₅₀) of 1.31 nM in primary human hepatocytes, with minimal to no toxicity in hepatocytes, skeletal muscle, liver, kidney, bone marrow, and cardiomyocytes. ATI-2173 activity was decreased by viral polymerase mutations associated with entecavir, lamivudine, and adefovir resistance, but not capsid inhibitor resistance mutations. A single oral dose of ATI-2173 demonstrated 82% hepatic extraction, no food effect, and greatly reduced peripheral exposure of clevudine compared with equimolar oral dosing of clevudine. Despite reduced plasma clevudine exposure, liver concentrations of the 5′-triphosphate were equivalent following ATI-2173 versus clevudine administration. By selectively delivering the 5′-monophosphate to the liver, while retaining the unique anti-HBV activity of the 5′-triphosphate, ATI-2173 may provide an improved pharmacokinetic profile for clinical use, reducing systemic exposure of clevudine and potentially eliminating skeletal myopathy.

In Vitro and In Vivo Characterization of the Anti-Zika Virus Activity of ProTides of 2'-C-β-Methylguanosine

The ProTide approach has emerged as a powerful tool to improve the intracellular delivery of nucleotide analogs with antiviral and anticancer activity. Here, we characterized the anti-ZIKV (ZIKV, Zika virus) activity of two ProTides of 2'-C-β-methylguanosine. ProTide UMN-1001 is a 2'-C-β-methylguanosine tryptamine phosphoramidate monoester, and ProTide UMN-1002 is a 2-(methylthio)-ethyl-2'-C-β-methylguanosine tryptamine phosphoramidate diester. UMN-1002 undergoes stepwise intracellular activation to the corresponding nucleotide monophosphate followed by P-N bond cleavage by intracellular histidine triad nucleotide binding protein 1 (Hint1). UMN-1001 is activated by Hint1 but is less cell-permeable than UMN-1002. UMN-1001 and UMN-1002 were found to be more potent than 2'-C-β-methylguanosine against ZIKV in human-derived microvascular endothelial and neuroblastoma cells and in reducing ZIKV RNA replication. Studies with a newborn mouse model of ZIKV infection demonstrated that, while treatment with 2'-C-β-methylguanosine and UMN-1001 was lethal, treatment with UMN-1002 was nontoxic and significantly reduced ZIKV infection. Our data suggests that anchimeric activated ProTides of 2'-C-β-methyl nucleosides should be further investigated for their potential as anti-ZIKV therapeutics.

Histidine-Triad Hydrolases Provide Resistance to Peptide-Nucleotide Antibiotics

The Escherichia coli microcin C (McC) and related compounds are potent Trojan horse peptide-nucleotide antibiotics. The peptide part facilitates transport into sensitive cells. Inside the cell, the peptide part is degraded by nonspecific peptidases releasing an aspartamide-adenylate containing a phosphoramide bond. This nonhydrolyzable compound inhibits aspartyl-tRNA synthetase. In addition to the efficient export of McC outside the producing cells, special mechanisms have evolved to avoid self-toxicity caused by the degradation of the peptide part inside the producers. Here, we report that histidine-triad (HIT) hydrolases encoded in biosynthetic clusters of some McC homologs or by standalone genes confer resistance to McC-like compounds by hydrolyzing the phosphoramide bond in toxic aspartamide-adenosine, rendering them inactive.IMPORTANCE Uncovering the mechanisms of resistance is a required step for countering the looming antibiotic resistance crisis. In this communication, we show how universally conserved histidine-triad hydrolases provide resistance to microcin C, a potent inhibitor of bacterial protein synthesis.

Current and future use of nucleo(s)tide prodrugs in the treatment of hepatitis C virus infection

This review describes the current state of discovery of past most important nucleoside and nucleotide prodrugs in the treatment of hepatitis C virus infection as well as future potential drugs currently in discovery or clinical evaluation. I highlight first generation landmark prodrug compounds which have been the foundations of incremental improvements toward the discovery and approval milestone of Sofosbuvir. Sofosbuvir is the first nucleotide prodrug marketed for hepatitis C virus treatment and the backbone of current combination therapies. Since this approval, new nucleotide prodrugs using the same design of Sofosbuvir McGuigan prodrug have emerged, some of them progressing through advanced clinical trials and may become available as new incremental alternative hepatitis C virus treatments in the future. Although since Sofosbuvir success, only minimal design efforts have been invested in finding better liver targeted prodrugs, a few novel prodrugs are being studied and their different modes of activation may prove beneficial over the heart/liver targeting ratio to reduce potential drug–drug interaction in combination therapies and yield safer treatment to patients. Prodrugs have long been avoided as much as possible in the past by development teams due to their metabolism and kinetic characterization complexity, but with their current success in hepatitis C virus treatment, and the knowledge gained in this endeavor, should become a first choice in future tissue targeting drug discovery programs beyond the particular case of nucleos(t)ide analogs.

Crystal Structure of Histidine Triad Nucleotide-Binding Protein from the Pathogenic Fungus Candida albicans

Histidine triad nucleotide-binding protein (HINT) is a member of the histidine triad (HIT) superfamily, which has hydrolase activity owing to a histidine triad motif. The HIT superfamily can be divided to five classes with functions in galactose metabolism, DNA repair, and tumor suppression. HINTs are highly conserved from archaea to humans and function as tumor suppressors, translation regulators, and neuropathy inhibitors. Although the structures of HINT proteins from various species have been reported, limited structural information is available for fungal species. Here, to elucidate the structural features and functional diversity of HINTs, we determined the crystal structure of HINT from the pathogenic fungus Candida albicans (CaHINT) in complex with zinc ions at a resolution of 2.5 Å. Based on structural comparisons, the monomer of CaHINT overlaid best with HINT protein from the protozoal species Leishmania major. Additionally, structural comparisons with human HINT revealed an additional helix at the C-terminus of CaHINT. Interestingly, the extended C-terminal helix interacted with the N-terminal loop (α1-β1) and with the α3 helix, which appeared to stabilize the dimerization of CaHINT. In the C-terminal region, structural and sequence comparisons showed strong relationships among 19 diverse species from archea to humans, suggesting early separation in the course of evolution. Further studies are required to address the functional significance of variations in the C-terminal region. This structural analysis of CaHINT provided important insights into the molecular aspects of evolution within the HIT superfamily.

Tunable Supramolecular Assemblies from Amphiphilic Nucleoside Phosphoramidate Nanofibers by Enzyme Activation

Enzymes possess unique qualities that make them ideal regulators of supramolecular assembly. They are uniquely sensitive to biomolecules and biological compartments, catalytic in effecting chemical reactions, and present a biocompatible and degradable platform for assembly regulation. We demonstrate the novel utility of Histidine Triad Nucleotide Binding Protein 1 (HINT1) in regulating supramolecular hydrogel formation. We synthesized nucleoside-phosphoramidate-functionalized self-assembling peptides that we observed to form nanofibers. We found HINT1's catalytic hydrolysis of the nucleoside phosphoramidate moieties within the nanofiber structures to induce nanofiber organization and higher ordered assembly. The role of HINT1 in effecting this structural change was confirmed with experiments utilizing a high-affinity HINT1 inhibitor and catalytically dead HINT1 mutant. In addition, the kinetics and morphology of hydrogel formation were found to be dependent on the structure of the released nucleoside monophosphate. This work highlights the self-assembly of phosphoramidate nanofibers and their higher organization triggered by HINT1 enzymatic activity.

Structure and Functional Characterization of Human Histidine Triad Nucleotide-Binding Protein 1 Mutations Associated with Inherited Axonal Neuropathy with Neuromyotonia

Inherited peripheral neuropathies are a group of neurodegenerative disorders that clinically affect 1 in 2500 individuals. Recently, genetic mutations in human histidine nucleotide-binding protein 1 (hHint1) have been strongly and most frequently associated with patients suffering from axonal neuropathy with neuromyotonia. However, the correlation between the impact of these mutations on the hHint1 structure, enzymatic activity and in vivo function has remained ambiguous. Here, we provide detailed biochemical characterization of a set of these hHint1 mutations. Our findings indicate that half of the mutations (R37P, G93D and W123*) resulted in a destabilization of the dimeric state and a significant decrease in catalytic activity and HINT1 inhibitor binding affinity. The H112N mutant was found to be dimeric, but devoid of catalytic activity, due to the loss of the catalytically essential histidine; nevertheless, it exhibited high affinity to AMP and a HINT1 inhibitor. In contrast to the active-site mutants, the catalytic activity and dimeric structure of the surface mutants, C84R and G89V, were found to be similar to the wild-type enzyme. Taken together, our results suggest that the pathophysiology of inherited axonal neuropathy with neuromyotonia can be induced by conversion of HINT1 from a homodimer to monomer, by modification of select surface residues or by a significant reduction of the enzyme's catalytic efficiency.

5'-Phosphorothiolate Dinucleotide Cap Analogues: Reagents for Messenger RNA Modification and Potent Small-Molecular Inhibitors of Decapping Enzymes

The 5' cap consists of 7-methylguanosine (m⁷G) linked by a 5'-5'-triphosphate bridge to messenger RNA (mRNA) and acts as the master regulator of mRNA turnover and translation initiation in eukaryotes. Cap analogues that influence mRNA translation and turnover (either as small molecules or as part of an RNA transcript) are valuable tools for studying gene expression, which is often also of therapeutic relevance. Here, we synthesized a series of 15 dinucleotide cap (m⁷GpppG) analogues containing a 5'-phosphorothiolate (5'-PSL) moiety (i.e., an O-to-S substitution within the 5'-phosphoester) and studied their biological properties in the context of three major cap-binding proteins: translation initiation factor 4E (eIF4E) and two decapping enzymes, DcpS and Dcp2. While the 5'-PSL moiety was neutral or slightly stabilizing for cap interactions with eIF4E, it significantly influenced susceptibility to decapping. Replacing the γ-phosphoester with the 5'-PSL moiety (γ-PSL) prevented β-γ-pyrophosphate bond cleavage by DcpS and conferred strong inhibitory properties. Combining the γ-PSL moiety with α-PSL and β-phosphorothioate (PS) moiety afforded first cap-derived hDcpS inhibitor with low nanomolar potency. Susceptibility to Dcp2 and translational properties were studied after incorporation of the new analogues into mRNA transcripts by RNA polymerase. Transcripts containing the γ-PSL moiety were resistant to cleavage by Dcp2. Surprisingly, superior translational properties were observed for mRNAs containing the α-PSL moiety, which were Dcp2-susceptible. The overall protein expression measured in HeLa cells for this mRNA was comparable to mRNA capped with the translation augmenting β-PS analogue reported previously. Overall, our study highlights 5'-PSL as a synthetically accessible cap modification, which, depending on the substitution site, can either reduce susceptibility to decapping or confer superior translational properties on the mRNA. The 5'-PSL-analogues may find application as reagents for the preparation of efficiently expressed mRNA or for investigation of the role of decapping enzymes in mRNA processing or neuromuscular disorders associated with decapping.

Phosphoramidates and phosphonamidates (ProTides) with antiviral activity

Following the first report on the nucleoside phosphoramidate (ProTide) prodrug approach in 1990 by Chris McGuigan, the extensive investigation of ProTide technology has begun in many laboratories. Designed with aim to overcome limitations and the key resistance mechanisms associated with nucleoside analogues used in the clinic (poor cellular uptake, poor conversion to the 5'-monophosphate form), the ProTide approach has been successfully applied to a vast number of nucleoside analogues with antiviral and anticancer activity. ProTides consist of a 5'-nucleoside monophosphate in which the two hydroxyl groups are masked with an amino acid ester and an aryloxy component which once in the cell is enzymatically metabolized to deliver free 5'-monophosphate, which is further transformed to the active 5'-triphosphate form of the nucleoside analogue. In this review, the seminal contribution of Chris McGuigan's research to this field is presented. His technology proved to be extremely successful in drug discovery and has led to two Food and Drug Administration-approved antiviral agents.

A Crystal Structure Based Guide to the Design of Human Histidine Triad Nucleotide Binding Protein 1 (hHint1) Activated ProTides

Nucleotide analogues that incorporate a metabolically labile nucleoside phosphoramidate (a ProTide) have found utility as prodrugs. In humans, ProTides can be cleaved by human histidine triad nucleotide binding protein 1 (hHint1) to expose the nucleotide monophosphate. Activation by this route circumvents highly selective nucleoside kinases that limit the use of nucleosides as prodrugs. To better understand the diversity of potential substrates of hHint1, we created and studied a series of phosphoramidate nucleosides. Using a combination of enzyme kinetics, X-ray crystallography, and isothermal titration calorimetry with both wild-type and inactive mutant enzymes, we have been able to explore the energetics of substrate binding and establish a structural basis for catalytic efficiency. Diverse nucleobases are well tolerated, but portions of the ribose are needed to position substrates for catalysis. Beneficial characteristics of the amine leaving group are also revealed. Structural principles revealed by these results may be exploited to tune the rate of substrate hydrolysis to strategically alter the intracellular release of the product nucleoside monophosphate from the ProTide.

Phosphoramidate hydrolysis catalyzed by human histidine triad nucleotide binding protein 1 (hHint1): a cluster-model DFT computational study

The histidine triad of hHint1 serves as a proton shuttle in the DFT proposed mechanism of the hydrolysis of phosphoramidate.