Remdesivir triphosphate can efficiently inhibit the RNA-dependent RNA polymerase from various flaviviruses.

Remdesivir was shown to inhibit RNA-dependent RNA-polymerases (RdRp) from distinct viral families such as from Filoviridae (Ebola) and Coronaviridae (SARS-CoV, SARS-CoV-2, MERS). In this study, we tested the ability of remdesivir to inhibit RdRps from the Flaviviridae family. Instead of remdesivir, we used the active species that is produced in cells from remdesivir, the appropriate triphosphate, which could be directly tested in vitro using recombinant flaviviral polymerases. Our results show that remdesivir can efficiently inhibit RdRps from viruses causing severe illnesses such as Yellow fever, West Nile fever, Japanese and Tick-borne encephalitis, Zika and Dengue. Taken together, this study demonstrates that remdesivir or its derivatives have the potential to become a broad-spectrum antiviral agent effective against many RNA viruses.

Novel Human Neutral Sphingomyelinase 2 Inhibitors as Potential Therapeutics for Alzheimer's Disease.

Neutral sphingomyelinase 2 (nSMase2) catalyzes the cleavage of sphingomyelin to phosphorylcholine and ceramide, an essential step in the formation and release of exosomes from cells that is critical for intracellular communication. Chronic increase of brain nSMase2 activity and related exosome release have been implicated in various pathological processes, including the progression of Alzheimer's disease (AD), making nSMase2 a viable therapeutic target. Recently, we identified phenyl (R)-(1-(3-(3,4-dimethoxyphenyl)-2,6-dimethylimidazo[1,2-b]pyridazin-8-yl)pyrrolidin-3-yl)carbamate 1 (PDDC), the first nSMase2 inhibitor that possesses both favorable pharmacodynamics and pharmacokinetic (PK) parameters, including substantial oral bioavailability, brain penetration, and significant inhibition of exosome release from the brain in vivo. Herein we demonstrate the efficacy of 1 (PDDC) in a mouse model of AD and detail extensive structure-activity relationship (SAR) studies with 70 analogues, unveiling several that exert similar or higher activity against nSMase2 with favorable pharmacokinetic properties.

Optimization of norbornyl-based carbocyclic nucleoside analogs as cyclin-dependent kinase 2 inhibitors.

We report on the discovery of norbornyl moiety as a novel structural motif for cyclin-dependent kinase 2 (CDK2) inhibitors which was identified by screening a carbocyclic nucleoside analogue library. Three micromolar hits were expanded by the use of medicinal chemistry methods into a series of 16 novel compounds. They had prevailingly micromolar activities against CDK2 and the best compound of the series attained IC50 of 190 nM. The binding modes were explored in molecular details by modeling and docking. Quantum mechanics-based scoring was used to rationalize the affinities. In conclusion, the discovered 9-hydroxymethylnorbornyl moiety was shown by joint experimental-theoretical efforts to be able to serve as a novel substituent for CDK2 inhibitors. This finding opens door to the exploration of chemical space towards more effective derivatives targeting this important class of protein kinases.

Could 5'-N and S ProTide analogues work as prodrugs of antiviral agents?

The nucleoside/nucleotide derived antiviral agents have been the most important components of antiviral therapy used in clinics. Recently, the focus of the medicinal chemists within this exciting research field has been affected mainly by the lack of effective therapies for the Hepatitis C virus (HCV) infection and several other "neglected" diseases caused by viruses such as Zika or Dengue. 2'-Methyl modified nucleosides and their monophosphate prodrugs (ProTides) have revolutionized the therapies for HCV in the last few years and, according to the latest research efforts, have also brought a promise for treatment of diseases caused by other members of Flaviviridae family. Here, we report on the design and synthesis of 5'-N and S modified ProTides derived from 2'-methyladenosine. We studied potential applicability of these derivatives as prodrugs of this archetypal antiviral compound.

Synthesis of tRNA analogues containing a terminal ribose locked in the South conformation to study tRNA-dependent enzymes.

We report here the synthetic route of two constrained dinucleotides and the determination of the sugar puckering by NMR analyses of the starting nucleosides. Enzymatic ligation to microhelix-RNAs provide access to tRNA analogues containing a 3' terminal A76 locked in South conformation. Biological evaluation of our tRNA analogues has been performed using amino-acyl tRNA-dependent transferase FemXWv, which mediates non-ribosomal incorporation of amino acids into the bacterial cell wall. We have shown that our tRNA analogues inhibited the aminoacyl transfer reaction catalyzed by FemXWv with IC50s of 10 and 8 µM. These results indicate that FemXWv displays a moderate preference for tRNAs containing a terminal A76 locked in the South conformation and that a South to North switch in the conformation of the terminal ribose might contribute to the release of the uncharged tRNAAla product of the aminoacyl transfer reaction catalyzed by FemXwv.

Control of α/ß Anomer Formation by a 2',5' Bridge: Toward Nucleoside Derivatives Locked in the South Conformation.

We describe a novel stereoselective synthesis of nucleoside derivatives with the ribose ring locked in the South conformation by a bridge between C2' and C5'. Despite the intrinsic constraints of the bicyclic structure, we demonstrate that their synthesis can be achieved by ring closing metathesis of readily accessible precursors. The obtained ribose derivatives are, however, very poor substrates for further installation of the nucleobases, and even simple nucleophiles, such as azido or cyano anions, react with unexpected stereo- or regioselectivity under standard glycosylation conditions. Here we explain this behavior by employing density functional theory (DFT) computations and devise an alternative approach resulting in isomers with the desired orientation of the nucleobase.

Adenosine triphosphate analogs can efficiently inhibit the Zika virus RNA-dependent RNA polymerase.

We describe the expression and purification of an active recombinant Zika virus RNA-dependent RNA polymerase (RdRp). Next, we present the development and optimization of an in vitro assay to measure its activity. We then applied the assay to selected triphosphate analogs and discovered that 2'-C-methylated nucleosides exhibit strong inhibitory activity. Surprisingly, also carbocyclic derivatives with the carbohydrate locked in a North-like conformation as well as a ribonucleotide with a South conformation exhibited strong activity. Our results suggest that the traditional 2'-C-methylated nucleosides pursued in the race for anti-HCV treatment can be superseded by brand new scaffolds in the case of the Zika virus.

Rational Design of Novel Highly Potent and Selective Phosphatidylinositol 4-Kinase IIIß (PI4KB) Inhibitors as Broad-Spectrum Antiviral Agents and Tools for Chemical Biology.

Phosphatidylinositol 4-kinase IIIß (PI4KB) is indispensable for the replication of various positive-sense single stranded RNA viruses, which hijack this cellular enzyme to remodel intracellular membranes of infected cells to set up the functional replication machinery. Therefore, the inhibition of this PI4K isoform leads to the arrest of viral replication. Here, we report on the synthesis of novel PI4KB inhibitors, which were rationally designed based on two distinct structural types of inhibitors that bind in the ATP binding side of PI4KB. These "hybrids" not only excel in outstanding inhibitory activity but also show high selectivity to PI4KB compared to other kinases. Thus, these compounds exert selective nanomolar or even subnanomolar activity against PI4KB as well as profound antiviral effect against hepatitis C virus, human rhinovirus, and coxsackievirus B3. Our crystallographic analysis unveiled the exact position of the side chains and explains their extensive contribution to the inhibitory activity.

9-Norbornyl-6-chloropurine (NCP) induces cell death through GSH depletion-associated ER stress and mitochondrial dysfunction.

UNLABELLED: 9-Norbornyl-6-chloropurine (NCP) is a representative of a series of antienteroviral bicycle derivatives with selective cytotoxicity towards leukemia cell lines. In this work we explored the mechanism of the antileukemic activity of NCP in T-cell lymphoblast cells (CCRF-CEM). Specifically, we searched for a potential link between its ability to induce cell death on the one hand and to modulate intracellular glutathione (GSH) that is necessary to its metabolic transformation via glutathione-S-transferase on the other hand. We have observed that GSH levels decreased rapidly in NCP-treated cells. Despite a complete regeneration following 24h of incubation with NCP, this profound drop in cellular GSH content triggered ER stress, ROS production and lipid peroxidation leading to the loss of mitochondrial membrane potential (MMP). These events induced concentration-dependent cell cycle arrest in G2/M phase and apoptosis. Both MMP loss and apoptosis were reversed by sulfhydryl-containing compounds (GSH, N-acetyl-l-cysteine). Furthermore, we have also shown that NCP-induced GSH decrease activated the Nrf2 pathway and its downstream targets NAD(P)H: quinone oxidoreductase (NQO-1) and glutamate cysteine ligase modifier subunit (GCLm), thus explaining the fast restoration of GSH pool and ROS decrease. Importantly, we confirmed that the cell death-inducing properties of the compounds were co-dependent on their ability to diminish cellular GSH level by analyzing the relationships between the GSH-depleting potency and cytotoxicity in a series of other norbornylpurine analogs. Altogether, the results demonstrated that in CCRF-CEM cells NCP triggered apoptosis through GSH depletion-associated oxidative and ER stress and mitochondrial depolarization.

Purine analogs as phosphatidylinositol 4-kinase IIIß inhibitors.

We report on an extensive structure-activity relationship study of novel PI4K IIIß inhibitors. The purine derivative of the potent screening hit T-00127-HEV1 has served as a suitable starting point for a thorough investigation of positions 8 and 2. While position 8 of the purine scaffold can only bear a small substituent to maintain the inhibitory activity, position 2 is opened for extensive modification and can accommodate even substituted phenyl rings without the loss of PI4K IIIß inhibitory activity. These empirical observations nicely correlate with the results of our docking study, which suggests that position 2 directs towards solution and can provide the necessary space for the interaction with remote residues of the enzyme, whereas the cavity around position 8 is strictly limited. The obtained compounds have also been subjected to antiviral screening against a panel of (+)ssRNA viruses.

Synthesis and biological evaluation of conformationally restricted adenine bicycloribonucleosides.

We prepared a novel series of conformationally restricted bicyclonucleosides and nucleotides. The synthetic approach employed a ring closing metathesis to provide access to both 6 and 7 membered saturated and unsaturated rings linking the 3' to 5' methylene groups of the sugar. The bicyclonucleosides were also transformed to the corresponding phosphoramidate prodrugs by an innovative one-pot protocol of boronate ester protection, coupling of the phosphoryl chloridate and deprotection of the boronate. A similar strategy was also employed for the synthesis of the corresponding monophosphates as crucial intermediates for the synthesis of selected triphosphates. The biological properties of the nucleosides and monophosphate prodrugs were assessed for antiviral and cytostatic activities in cell based assays whilst the triphosphates were evaluated in enzymatic assays. The lack of significant effects suggests that the linkage of the 3' to 5'via a ring system and the subsequent conformational restriction of the ribose ring to the South conformation are incompatible with the kinases and polymerases that recognize nucleosides and their metabolites.

The high-resolution crystal structure of phosphatidylinositol 4-kinase IIß and the crystal structure of phosphatidylinositol 4-kinase IIα containing a nucleoside analogue provide a structural basis for isoform-specific inhibitor design.

Phosphatidylinositol 4-phosphate (PI4P) is the most abundant monophosphoinositide in eukaryotic cells. Humans have four phosphatidylinositol 4-kinases (PI4Ks) that synthesize PI4P, among which are PI4K IIß and PI4K IIα. In this study, two crystal structures are presented: the structure of human PI4K IIß and the structure of PI4K IIα containing a nucleoside analogue. The former, a complex with ATP, is the first high-resolution (1.9 Å) structure of a PI4K. These structures reveal new details such as high conformational heterogeneity of the lateral hydrophobic pocket of the C-lobe and together provide a structural basis for isoform-specific inhibitor design.

Nucleoside inhibitors of tick-borne encephalitis virus.

Tick-borne encephalitis virus (TBEV) is a leading cause of human neuroinfections in Europe and Northeast Asia. There are no antiviral therapies for treating TBEV infection. A series of nucleoside analogues was tested for the ability to inhibit the replication of TBEV in porcine kidney cells and human neuroblastoma cells. The interactions of three nucleoside analogues with viral polymerase were simulated using advanced computational methods. The nucleoside analogues 7-deaza-2'-C-methyladenosine (7-deaza-2'-CMA), 2'-C-methyladenosine (2'-CMA), and 2'-C-methylcytidine (2'-CMC) inhibited TBEV replication. These compounds showed dose-dependent inhibition of TBEV-induced cytopathic effects, TBEV replication (50% effective concentrations [EC50]of 5.1 ± 0.4 µM for 7-deaza-2'-CMA, 7.1 ± 1.2 µM for 2'-CMA, and 14.2 ± 1.9 µM for 2'-CMC) and viral antigen production. Notably, 2'-CMC was relatively cytotoxic to porcine kidney cells (50% cytotoxic concentration [CC50] of ∼50 µM). The anti-TBEV effect of 2'-CMA in cell culture diminished gradually after day 3 posttreatment. 7-Deaza-2'-CMA showed no detectable cellular toxicity (CC50 > 50 µM), and the antiviral effect in culture was stable for >6 days posttreatment. Computational molecular analyses revealed that compared to the other two compounds, 7-deaza-2'-CMA formed a large cluster near the active site of the TBEV polymerase. High antiviral activity and low cytotoxicity suggest that 7-deaza-2'-CMA is a promising candidate for further investigation as a potential therapeutic agent in treating TBEV infection.

Highly Selective Phosphatidylinositol 4-Kinase IIIß Inhibitors and Structural Insight into Their Mode of Action.

Phosphatidylinositol 4-kinase IIIß is a cellular lipid kinase pivotal to pathogenesis of various RNA viruses. These viruses hijack the enzyme in order to modify the structure of intracellular membranes and use them for the construction of functional replication machinery. Selective inhibitors of this enzyme are potential broad-spectrum antiviral agents, as inhibition of this enzyme results in the arrest of replication of PI4K IIIß-dependent viruses. Herein, we report a detailed study of novel selective inhibitors of PI4K IIIß, which exert antiviral activity against a panel of single-stranded positive-sense RNA viruses. Our crystallographic data show that the inhibitors occupy the binding site for the adenine ring of the ATP molecule and therefore prevent the phosphorylation reaction.

Synthesis of locked cyclohexene and cyclohexane nucleic acids (LCeNA and LCNA) with modified adenosine units.

We describe here the preparation of conformationally locked cyclohexane nucleic acids designed as hybrids between locked nucleic acids (LNAs) and cyclohexene nucleic acids (CeNAs), both of which excel in hybridization with complementary RNAs. We have accomplished the synthesis of these adenine derivatives starting from a simple ketoester and installed all four chiral centres by means of total synthesis. The acquired monomers were incorporated into nonamer oligonucleotides.

Norbornane-based nucleoside and nucleotide analogues locked in North conformation.

We report on the synthesis of novel conformationally locked nucleoside and nucleotide derivatives, which are structurally closely related to clinically used antivirals such as didanosine and abacavir. As a suitable conformationally rigid substitute of the sugar/pseudosugar ring allowing a permanent stabilization of the nucleoside in North conformation we employed bicyclo[2.2.1]heptane (norbornane) substituted in the bridgehead position with a hydroxymethyl group and in the C-3 position with a nucleobase. Prepared nucleoside derivatives were also converted into appropriate phosphoramidate prodrugs (ProTides) in order to increase delivery of the compounds in the cells. All target compounds were evaluated in a broad antiviral and cytostatic assay panel.

Transport mechanisms of a novel antileukemic and antiviral compound 9-norbornyl-6-chloropurine.

6-Chloropurines substituted at the position 9 with variously modified bicyclic skeletons represent promising antiviral and anticancer agents. This work aimed to investigate the transport mechanisms of 9-[(1R*,2R*,4S*)-bicyclo[2.2.1]hept-2-yl]-6-chloro-9H-purine (9-norbornyl-6-chloropurine, NCP) and their relationship to the metabolism and biological activity of the compound. Transport experiments were conducted in CCRF-CEM cells using radiolabeled compound ([(3)H]NCP). The pattern of the intracellular uptake of [(3)H]NCP in CCRF-CEM cells pointed to a combination of passive and facilitated diffusion as prevailing transport mechanisms. NCP intracellular metabolism was found to enhance its uptake by modifying NCP concentration gradient. The transport kinetics reached steady state under the conditions of MRP and MDR proteins blockade, indicating that NCP is a substrate for these efflux pumps. Their inhibition also increased the cytotoxicity of NCP. Our findings suggest that the novel nucleoside analog NCP has potential to become a new orally available antileukemic agent due to its rapid membrane permeation.

From norbornane-based nucleotide analogs locked in South conformation to novel inhibitors of feline herpes virus.

A synthetic route toward a series of unique cyclic nucleoside phosphonates locked in South conformation is described. The desired conformation is stabilized by a substitution of the sugar moiety by bicyclo[2.2.1]heptane (norbornane) bearing a purine or pyrimidine nucleobase in the bridgehead position. Although the final phosphonate derivatives are devoid of any significant antiviral activity probably due to the unfavorable conformational properties, several intermediates and their analogs exhibit surprising activity against feline herpes virus. Since these compounds do not possess an appropriate hydroxymethyl function allowing phosphorylation and subsequent incorporation into the polynucleotide chain, it seems to be likely that these compounds act by a novel unknown mechanism of action and may represent a new possible alternative for nucleoside and nucleotide therapeutics of this widely spread feline infection. A number of derivatives exerted also a significant antiviral activity against Coxsackievirus B3 and B4.

Synthesis of novel purine-based coxsackievirus inhibitors bearing polycylic substituents at the N-9 position.

The synthesis of a novel library of purine derivatives bearing various bicyclic and polycylic substituents at the N-9 position is described. The series includes norbornanes, bicyclo[2.2.2]octanes, and bicyclo[3.2.1]octanes attached at the bridgehead position as well as bicyclo[3.1.1]heptanes, tetrahydro-1-naphthalenes, and adamantanes bonded either directly or via a linear chain to the 6-chloropurine nucleobase. A number of prepared derivatives exerted significant activity against the enterovirus. Despite attempts to correlate the activity against picornaviruses with their phosphatidylinositol 4-kinase KIIIß inhibitory activity, it is clear that the inhibition of this host factor cannot explain the observed antiviral potency.

9-Norbornyl-6-chloropurine is a novel antileukemic compound interacting with cellular GSH.

AIM: 6-Chloropurines substituted at position 9 with bicyclic skeletons represent promising chemotherapeutic agents. We explored the metabolism and membrane transport of 9-norbornyl-6-chloropurine (NCP) aiming to understand its mechanism of action. MATERIALS AND METHODS: The metabolism of NCP was studied in vitro in whole cells (CCRF-CEM), cellular extracts, subcellular fractions and purified enzymes. Transport experiments were conducted in Caco-2 cell monolayers. RESULTS: Three metabolites were identified, a glutathione conjugate (NCP-GS), NCP-cysteinylglycine and NCP-cysteine. Both glutathione-S-transferase inhibition and glutathione (GSH) depletion prevented metabolite formation and increased the cytotoxicity of NCP. Transepithelial transport (Caco-2) indicated good permeability, with Papp (12.6±0.3) ×10(-5) cm/s. Importantly, the drug induced glutathione depletion in treated cells and affected the activity of several GSH-dependent enzymes. CONCLUSION: The novel nucleoside analog NCP represents a promising orally available antileukemic agent, acting through lowering of GSH levels in tumor cells.