Advances in the development of nucleoside and nucleotide analogues for cancer and viral diseases

The search for nucleoside/nucleotide analog inhibitors of dengue virus

Nucleoside analogs represent the largest class of antiviral agents and have been actively pursued for potential therapy of dengue virus (DENV) infection. Early success in the treatment of human immunodeficiency virus (HIV) infection and the recent approval of sofosbuvir for chronic hepatitis C have provided proof of concept for this class of compounds in clinics. Here we review (i) nucleoside analogs with known anti-DENV activity; (ii) challenges of the nucleoside antiviral approach for dengue; and (iii) potential strategies to overcome these challenges. This article forms part of a symposium in Antiviral Research on flavivirus drug discovery.

Phospholipid derivatives of cladribine and fludarabine: synthesis and biological properties

Phospholipid derivatives of anticancer nucleosides cladribine and fludarabine (F-ara-A) bearing 1,2- and 1,3-diacylglycerol moieties have been prepared by the H-phosphonate approach using 1,1,3,3-tetraisopropyldisiloxane-1,3-diyl protecting group for cladribine and a combination of tert-butyldimethylsilyl and levulinyl protecting groups for 2-fluoroadenine nucleosides. The synthesized conjugates exhibited lower in vitro antiproliferative activity against human tumor cell lines in comparison with the same concentrations of the parent cladribine and fludarabine phosphate. In the course of biokinetic study, it was found that intragastric administration of phospholipid F-ara-A derivatives to Wistar rats and ICR outbred male mice led to a slow release of F-ara-A into the bloodstream, a smooth increase in nucleoside concentration, and prolonged serum circulation of liberated nucleoside. The oral bioavailability of F-ara-A from 1,2-dimyristoylglycerophosphate derivative 29 was similar to its oral bioavailability from fludarabine phosphate.

A Multidrug-resistant Engineered CAR T Cell for Allogeneic Combination Immunotherapy

The adoptive transfer of chimeric antigen receptor (CAR) T cell represents a highly promising strategy to fight against multiple cancers. The clinical outcome of such therapies is intimately linked to the ability of effector cells to engraft, proliferate, and specifically kill tumor cells within patients. When allogeneic CAR T-cell infusion is considered, host versus graft and graft versus host reactions must be avoided to prevent rejection of adoptively transferred cells, host tissue damages and to elicit significant antitumoral outcome. This work proposes to address these three requirements through the development of multidrug-resistant T cell receptor αβ-deficient CAR T cells. We demonstrate that these engineered T cells displayed efficient antitumor activity and proliferated in the presence of purine and pyrimidine nucleoside analogues, currently used in clinic as preconditioning lymphodepleting regimens. The absence of TCRαβ at their cell surface along with their purine nucleotide analogues-resistance properties could prevent their alloreactivity and enable them to resist to lymphodepleting regimens that may be required to avoid their ablation via HvG reaction. By providing a basic framework to develop a universal T cell compatible with allogeneic adoptive transfer, this work is laying the foundation stone of the large-scale utilization of CAR T-cell immunotherapies.

Use of designed experiments for the improvement of pre-analytical workflow for the quantification of intracellular nucleotides in cultured cell lines

The present study is focused on the development of a pre-analytical strategy for the quantification of intracellular nucleotides from cultured cell lines. Different protocols, including cell recovery, nucleotide extraction and purification, were compared on a panel of nucleoside mono-, di- and triphosphates from four cell lines (adherent and suspension cells). The quantification of nucleotides was performed using a validated technique with on-line solid-phase extraction coupled with liquid chromatography-triple quadrupole tandem mass spectrometry (LC-MS/MS). Designed experiments were implemented to investigate, in a rigorous and limited-testing experimental approach, the influence of several operating parameters. Results showed that the technique used to harvest adherent cells drastically affected the amounts of intracellular nucleotides. Scraping cells was deleterious because of a major leakage (more than 70%) of intracellular nucleotides during scraping. Moreover, some other tested conditions should be avoided, such as using pure methanol as extraction solvent (decrease over 50% of intracellular nucleotides extracted from NCI-H292 cells) or adding a purification step with chloroform. Designed experiments allowed identifying an interaction between the percentage of methanol and the presence of chloroform. The mixture methanol/water (70/30, v/v) was considered as the best compromise according to the nucleoside mono-, di-, or triphosphates and the four cell lines studied. This work highlights the importance of pre-analytical step combined with the cell lines studied associated to sensitive and validated assay for the quantification of nucleotides in biological matrices.

A Modular Synthesis of Modified Phosphoanhydrides

Phosphoanhydrides (P-anhydrides) are ubiquitously occurring modifications in nature. Nucleotides and their conjugates, for example, are among the most important building blocks and signaling molecules in cell biology. To study and manipulate their biological functions, a diverse range of analogues have been developed. Phosphate-modified analogues have been successfully applied to study proteins that depend on these abundant cellular building blocks, but very often both the preparation and purification of these molecules are challenging. This study discloses a general access to P-anhydrides, including different nucleotide probes, that greatly facilitates their preparation and isolation. The convenient and scalable synthesis of, for example, (18) O labeled nucleoside triphosphates holds promise for future applications in phosphoproteomics.

Palladium-catalyzed modification of unprotected nucleosides, nucleotides, and oligonucleotides

Synthetic modification of nucleoside structures provides access to molecules of interest as pharmaceuticals, biochemical probes, and models to study diseases. Covalent modification of the purine and pyrimidine bases is an important strategy for the synthesis of these adducts. Palladium-catalyzed cross-coupling is a powerful method to attach groups to the base heterocycles through the formation of new carbon-carbon and carbon-heteroatom bonds. In this review, approaches to palladium-catalyzed modification of unprotected nucleosides, nucleotides, and oligonucleotides are reviewed. Polar reaction media, such as water or polar aprotic solvents, allow reactions to be performed directly on the hydrophilic nucleosides and nucleotides without the need to use protecting groups. Homogeneous aqueous-phase coupling reactions catalyzed by palladium complexes of water-soluble ligands provide a general approach to the synthesis of modified nucleosides, nucleotides, and oligonucleotides.

Branched-chain sugar nucleosides: stereocontrolled synthesis and bioevaluation of novel 3'-C-trifluoromethyl and 3'-C-methyl pyranonucleosides

A new series of 3'-C-trifluoromethyl- and 3'-C-methyl-β-d-allopyranonucleosides of 5-fluorouracil and their deoxy derivatives has been designed and synthesized. Treatment of ketosugar 1 with trifluoromethyltrimethylsilane under catalytic fluoride activation and methyl magnesium bromide, gave 1,2:5,6-di-O-isopropylidene-3-C-trifluoromethyl (2a) and 3-C-methyl (2b)-α-D-allofuranose, respectively, in a virtually quantitative yield and with complete stereoselectivity. Hydrolysis followed by acetylation led to the 1,2,4,6-tetra-O-acetyl-3-C-trifluoromethyl (3a) and 3-C-methyl (3b)-β-D-allopyranose. Compounds 3a,b were then condensed with silylated 5-fluorouracil and deacetylated to afford the target nucleosides 5a,b. Deoxygenation of the peracylated allopyranoses 3a,b followed by condensation with silylated 5-fluorouracil and subsequent deacetylation yielded the target 3'-deoxy-3'-C-trifluoromethyl and 3'-deoxy-3'-C-methyl-β-d-glucopyranonucleosides 14a,b. The newly synthesized compounds were evaluated for their potential antiviral and cytostatic activities. The 3'-deoxy-3'-C-methyl- ribonucleoside 11b showed significant cytotoxic activity (∼7 μM) almost equally active against a variety of tumor cell lines.

DNA repair mechanisms in cancer development and therapy

DNA damage has been long recognized as causal factor for cancer development. When erroneous DNA repair leads to mutations or chromosomal aberrations affecting oncogenes and tumor suppressor genes, cells undergo malignant transformation resulting in cancerous growth. Genetic defects can predispose to cancer: mutations in distinct DNA repair systems elevate the susceptibility to various cancer types. However, DNA damage not only comprises a root cause for cancer development but also continues to provide an important avenue for chemo- and radiotherapy. Since the beginning of cancer therapy, genotoxic agents that trigger DNA damage checkpoints have been applied to halt the growth and trigger the apoptotic demise of cancer cells. We provide an overview about the involvement of DNA repair systems in cancer prevention and the classes of genotoxins that are commonly used for the treatment of cancer. A better understanding of the roles and interactions of the highly complex DNA repair machineries will lead to important improvements in cancer therapy.

DNA replication fidelity in Mycobacterium tuberculosis is mediated by an ancestral prokaryotic proofreader

The DNA replication machinery is an important target for antibiotic development for increasingly drug resistant bacteria including Mycobacterium tuberculosis¹. While blocking DNA replication leads to cell death, disrupting the processes used to ensure replication fidelity can accelerate mutation and the evolution of drug resistance. In E. coli, the proofreading subunit of the replisome, the ε-exonuclease, is essential for high fidelity DNA replication²; however, we find that it is completely dispensable in M. tuberculosis. Rather, the mycobacterial replicative polymerase, DnaE1, encodes a novel editing function that proofreads DNA replication, mediated by an intrinsic 3′-5′ exonuclease activity within its PHP domain. Inactivation of the DnaE1 PHP domain increases the mutation rate by greater than 3,000 fold. Moreover, phylogenetic analysis of DNA replication proofreading in the bacterial kingdom suggests that E. coli is a phylogenetic outlier and that PHP-domain mediated proofreading is widely conserved and indeed may be the ancestral prokaryotic proofreader.

Antiviral therapies and prospects for a cure of chronic hepatitis B

Current therapies of chronic hepatitis B (CHB) remain limited to either pegylated interferon-α (Peg-IFN-α), or one of the five approved nucleoside analog (NA) treatments. Although viral suppression can be achieved in the majority of patients with high-barrier-to-resistance new-generation NAs (i.e., entecavir and tenofovir), HBsAg loss is achieved in only 10% of patients with both classes of drugs after a follow-up of 5 years. Attempts to improve the response by administering two different NAs or a combination of NA and Peg-IFN-α have been unsuccessful. Therefore, there is a renewed interest to investigate a number of steps in the hepatitis B virus (HBV) replication cycle and specific virus-host cell interactions as potential targets for new antivirals. Novel targets and compounds could readily be evaluated using both relevant in vitro and newly developed in vivo models of HBV infection. The addition of one or several new drugs to current regimens should offer the prospect of markedly improving the response to therapy, thus reducing the burden of drug resistance, as well as the incidence of cirrhosis and hepatocellular carcinoma (HCC).

Cytosolic 5'-nucleotidase II interacts with the leucin rich repeat of NLR family member Ipaf

IMP/GMP preferring cytosolic 5'-nucleotidase II (cN-II) is a bifunctional enzyme whose activities and expression play crucial roles in nucleotide pool maintenance, nucleotide-dependent pathways and programmed cell death. Alignment of primary amino acid sequences of cN-II from human and other organisms show a strong conservation throughout the entire vertebrata taxon suggesting a fundamental role in eukaryotic cells. With the aim to investigate the potential role of this homology in protein-protein interactions, a two hybrid system screening of cN-II interactors was performed in S. cerevisiae. Among the X positive hits, the Leucin Rich Repeat (LRR) domain of Ipaf was found to interact with cN-II. Recombinant Ipaf isoform B (lacking the Nucleotide Binding Domain) was used in an in vitro affinity chromatography assay confirming the interaction obtained in the screening. Moreover, co-immunoprecipitation with proteins from wild type Human Embryonic Kidney 293 T cells demonstrated that endogenous cN-II co-immunoprecipitated both with wild type Ipaf and its LRR domain after transfection with corresponding expression vectors, but not with Ipaf lacking the LRR domain. These results suggest that the interaction takes place through the LRR domain of Ipaf. In addition, a proximity ligation assay was performed in A549 lung carcinoma cells and in MDA-MB-231 breast cancer cells and showed a positive cytosolic signal, confirming that this interaction occurs in human cells. This is the first report of a protein-protein interaction involving cN-II, suggesting either novel functions or an additional level of regulation of this complex enzyme.

Targeting Glutamine Metabolism in Breast Cancer with Aminooxyacetate

PURPOSE

Glutamine addiction in c-MYC-overexpressing breast cancer is targeted by the aminotransferase inhibitor, aminooxyacetate (AOA). However, the mechanism of ensuing cell death remains unresolved.

EXPERIMENTAL DESIGN

A correlation between glutamine dependence for growth and c-MYC expression was studied in breast cancer cell lines. The cytotoxic effects of AOA, its correlation with high c-MYC expression, and effects on enzymes in the glutaminolytic pathway were investigated. AOA-induced cell death was assessed by measuring changes in metabolite levels by magnetic resonance spectroscopy (MRS), the effects of amino acid depletion on nucleotide synthesis by cell-cycle and bromodeoxyuridine (BrdUrd) uptake analysis, and activation of the endoplasmic reticulum (ER) stress-mediated pathway. Antitumor effects of AOA with or without common chemotherapies were determined in breast cancer xenografts in immunodeficient mice and in a transgenic MMTV-rTtA-TetO-myc mouse mammary tumor model.

RESULTS

We established a direct correlation between c-MYC overexpression, suppression of glutaminolysis, and AOA sensitivity in most breast cancer cells. MRS, cell-cycle analysis, and BrdUrd uptake measurements indicated depletion of aspartic acid and alanine leading to cell-cycle arrest at S-phase by AOA. Activation of components of the ER stress-mediated pathway, initiated through GRP78, led to apoptotic cell death. AOA inhibited growth of SUM159, SUM149, and MCF-7 xenografts and c-myc-overexpressing transgenic mouse mammary tumors. In MDA-MB-231, AOA was effective only in combination with chemotherapy.

CONCLUSIONS

AOA mediates its cytotoxic effects largely through the stress response pathway. The preclinical data of AOA's effectiveness provide a strong rationale for further clinical development, particularly for c-MYC-overexpressing breast cancers.

Amide-controlled, one-pot synthesis of tri-substituted purines generates structural diversity and analogues with trypanocidal activity

A novel one-pot synthesis of tri-substituted purines and the discovery of purine analogues with trypanocidal activity are reported. The reaction is initiated by a metal-free oxidative coupling of primary alkoxides and diaminopyrimidines with Schiff base formation and subsequent annulation in the presence of large N,N-dimethylamides (e.g. N,N-dimethylpropanamide or larger). This synthetic route is in competition with a reaction previously-reported by our group, allowing the generation of a combinatorial library of tri-substituted purines by the simple modification of the amide and the alkoxide employed. Among the variety of structures generated, two purine analogues displayed trypanocidal activity against the protozoan parasite Trypanosoma brucei with IC50 < 5 μM, being each of those compounds obtained through each of the synthetic pathways.

An astute synthesis of locked nucleic acid monomers

Novel attributes of Locked Nucleic Acid (LNA) makes it preferable over most of the other classes of modified nucleic acid analogues and therefore, it has been extensively explored in different synthetic oligonucleotide based therapeutics. In addition to five oligonucleotides of this class undergoing clinical trials, a healthy pipeline in pre-clinical studies validates the tenacity of LNA. Due to the increasing demand, an efficient biocatalytic methodology has recently been devised for the convergent synthesis of LNA monomers via selective enzymatic monoacetylation of diastereotopic hydroxymethyl functions of 3-O-benzyl-4-C-hydroxymethyl-1,2-O-isopropylidene-α-D-ribofuranose. This commentary article provides an insight into the different synthetic strategies followed for the synthesis of LNA monomers and their triumphs in clinical biotechnology.

Regulation of mammalian nucleotide metabolism and biosynthesis

Nucleotides are required for a wide variety of biological processes and are constantly synthesized de novo in all cells. When cells proliferate, increased nucleotide synthesis is necessary for DNA replication and for RNA production to support protein synthesis at different stages of the cell cycle, during which these events are regulated at multiple levels. Therefore the synthesis of the precursor nucleotides is also strongly regulated at multiple levels. Nucleotide synthesis is an energy intensive process that uses multiple metabolic pathways across different cell compartments and several sources of carbon and nitrogen. The processes are regulated at the transcription level by a set of master transcription factors but also at the enzyme level by allosteric regulation and feedback inhibition. Here we review the cellular demands of nucleotide biosynthesis, their metabolic pathways and mechanisms of regulation during the cell cycle. The use of stable isotope tracers for delineating the biosynthetic routes of the multiple intersecting pathways and how these are quantitatively controlled under different conditions is also highlighted. Moreover, the importance of nucleotide synthesis for cell viability is discussed and how this may lead to potential new approaches to drug development in diseases such as cancer.

Nucleoside transporter proteins as biomarkers of drug responsiveness and drug targets

Nucleoside and nucleobase analogs are currently used in the treatment of solid tumors, lymphoproliferative diseases, viral infections such as hepatitis and AIDS, and some inflammatory diseases such as Crohn. Two gene families are implicated in the uptake of nucleosides and nucleoside analogs into cells, SCL28 and SLC29. The former encodes hCNT1, hCNT2, and hCNT3 proteins. They translocate nucleosides in a Na⁺ coupled manner with high affinity and some substrate selectivity, being hCNT1 and hCNT2 pyrimidine- and purine-preferring, respectively, and hCNT3 a broad selectivity transporter. SLC29 genes encode four members, being hENT1 and hENT2 the only two which are unequivocally implicated in the translocation of nucleosides and nucleobases (the latter mostly via hENT2) at the cell plasma membrane. Some nucleoside-derived drugs can also interact with and be translocated by members of the SLC22 gene family, particularly hOCT and hOAT proteins. Inter-individual differences in transporter function and perhaps, more importantly, altered expression associated with the disease itself might modulate the transporter profile of target cells, thereby determining drug bioavailability and action. Drug transporter pharmacology has been periodically reviewed. Thus, with this contribution we aim at providing a state-of-the-art overview of the clinical evidence generated so far supporting the concept that these membrane proteins can indeed be biomarkers suitable for diagnosis and/or prognosis. Last but not least, some of these transporter proteins can also be envisaged as drug targets, as long as they can show “transceptor” functions, in some cases related to their role as modulators of extracellular adenosine levels, thereby providing a functional link between P1 receptors and transporters.

High-Throughput Hit Screening Cascade to Identify Respiratory Syncytial Virus (RSV) Inhibitors

Respiratory syncytial virus (RSV) infects 99% of children by age 2 years and is a leading cause of serious lower respiratory tract infection (LRTI) and infant hospitalization in the United Kingdom. Identification of efficacious RSV therapeutics has been hindered by the lack of a robust and appropriate primary assay for high-throughput screening (HTS). Here we report an HTS cascade that identified inhibitors of RSV replication using a robust RSV replicon luminescence-reporter assay for the primary campaign. The performance of the assay was consistent and reliable at scale, with Z' of 0.55 ± 0.08 across 150 assay plates and signal-to-background ratios >40. The HTS assay was used to screen the AstraZeneca compound library of 1 million compounds at a single concentration of 10 µM. Hits specifically targeting the RSV replicon were determined using a series of hit generation assays. Compounds nonspecifically causing cell toxicity were removed, and hits were confirmed in live viral inhibition assays exhibiting greater physiological relevance than the primary assay. In summary, we developed a robust screening cascade that identified hit molecules that specifically targeted RSV replication.

Catalytic C–H bond functionalisation of purine and pyrimidine nucleosides: a synthetic and mechanistic perspective

C–H bond functionalisation of heteroarenes, especially nucleosides, has received a lot of attention in the past few years. This review describes the state-of the art in this area with a global aspiration for possibly functionalising purine and pyrimidine moieties in complex biomolecular systems.

Nuclear Receptors in Drug Metabolism, Drug Response and Drug Interactions

Orally delivered small-molecule therapeutics are metabolized in the liver and intestine by phase I and phase II drug-metabolizing enzymes (DMEs), and transport proteins coordinate drug influx (phase 0) and drug/drug-metabolite efflux (phase III). Genes involved in drug metabolism and disposition are induced by xenobiotic-activated nuclear receptors (NRs), i.e. PXR (pregnane X receptor) and CAR (constitutive androstane receptor), and by the 1α, 25-dihydroxy vitamin D3-activated vitamin D receptor (VDR), due to transactivation of xenobiotic-response elements (XREs) present in phase 0-III genes. Additional NRs, like HNF4-α, FXR, LXR-α play important roles in drug metabolism in certain settings, such as in relation to cholesterol and bile acid metabolism. The phase I enzymes CYP3A4/A5, CYP2D6, CYP2B6, CYP2C9, CYP2C19, CYP1A2, CYP2C8, CYP2A6, CYP2J2, and CYP2E1 metabolize >90% of all prescription drugs, and phase II conjugation of hydrophilic functional groups (with/without phase I modification) facilitates drug clearance. The conjugation step is mediated by broad-specificity transferases like UGTs, SULTs, GSTs. This review delves into our current understanding of PXR/CAR/VDR-mediated regulation of DME and transporter expression, as well as effects of single nucleotide polymorphism (SNP) and epigenome (specified by promoter methylation, histone modification, microRNAs, long non coding RNAs) on the expression of PXR/CAR/VDR and phase 0-III mediators, and their impacts on variable drug response. Therapeutic agents that target epigenetic regulation and the molecular basis and consequences (overdosing, underdosing, or beneficial outcome) of drug-drug/drug-food/drug-herb interactions are also discussed. Precision medicine requires understanding of a drug's impact on DME and transporter activity and their NR-regulated expression in order to achieve optimal drug efficacy without adverse drug reactions. In future drug screening, new tools such as humanized mouse models and microfluidic organs-on-chips, which mimic the physiology of a multicellular environment, will likely replace the current cell-based workflow.

Diastereoselective synthesis of P-chirogenic phosphoramidate prodrugs of nucleoside analogues (ProTides) via copper catalysed reaction

The first example of transition metal assisted diastereoselective synthesis of phosphoramidate of nucleoside analogues is presented.

Synthesis and biological evaluation of nitric oxide-releasing hybrids from gemcitabine and phenylsulfonyl furoxans as anti-tumor agents

Novel furoxan/gemcitabine hybrids displayed significant antitumor activities, in particular 10e, which could be independent of the nucleoside transporter, release high levels of NO, and induce cell apoptosis by regulating apoptotic related proteins in tumor cells in vitro.

Synthesis and antiviral evaluation of 2',3'-dideoxy-2',3'-difluoro-D-arabinofuranosyl 2,6-disubstituted purine nucleosides

The synthesis of new 2,6-disubstituted purine 2',3'-dideoxy-2',3'-difluoro-D-arabino nucleosides is reported. Their ability to block HIV and HCV replication along with their cytotoxicity toward Huh-7 cells, human lymphocyte, CEM and Vero cells was also assessed. Among them, β-2,6-diaminopurine nucleoside 25 and guanosine derivative 27 demonstrate potent anti-HIV-1 activity (EC₅₀ = 0.56 and 0.65 μM; EC₉₀ = 4.2 and 3.1 μM) while displaying only moderate cytotoxicity in primary human lymphocytes.

Lead tetraacetate mediated one pot oxidative cleavage and acetylation reaction: an approach to apio and homologated apio pyrimidine nucleosides and their anticancer activity

An efficient and versatile strategy towards apio and homologated apio pyrimidines has been described via one pot oxidative cleavage and acetylation using Pb(OAc)₄.

Redesigning the synthesis of vidarabine via a multienzymatic reaction catalyzed by immobilized nucleoside phosphorylases

The comparison between the biocatalyzed synthesis of araA here described and the chemical synthesis of this nucleoside showed that the enzymatic route is superior (less steps, milder conditions and reagents, easier downstream, lower E-factor).

Prodrugs of phosphonates and phosphates: crossing the membrane barrier

A substantial portion of metabolism involves transformation of phosphate esters, including pathways leading to nucleotides and oligonucleotides, carbohydrates, isoprenoids and steroids, and phosphorylated proteins. Because the natural substrates bear one or more negative charges, drugs that target these enzymes generally must be charged as well, but small charged molecules can have difficulty traversing the cell membrane by means other than endocytosis. The resulting dichotomy has stimulated a great deal of effort to develop effective prodrugs, compounds that carry little or no charge to enable them to transit biological membranes, but able to release the parent drug once inside the target cell. This chapter presents recent studies on advances in prodrug forms, along with representative examples of their application to marketed and developmental drugs.

Microwave promoted C–O coupling for synthesizing O-aryloxytriazole nucleoside analogues

A convenient and effective microwave-promoted synthesis of O-aryloxytriazole nucleosides was established, leading to an interesting candidate with anticancer activity.

Aspartic acid based nucleoside phosphoramidate prodrugs as potent inhibitors of hepatitis C virus replication

A series of novel nucleoside phosphoramidate protides has been synthesized and shown as potent inhibitors of hepatitis C virus replication. The conjugates are having a diverse structural variation in the promoiety part and can be catalytically processed to deliver active nucleotides.

Clinical potential of elacytarabine in patients with acute myeloid leukemia

Acute myeloid leukemia (AML) has been treated for over four decades with standard induction chemotherapy including seven days of cytosine arabinoside (cytarabine, ara-C) infusion. Cytarabine, while effective in killing leukemic cells, is subject to development of several resistance mechanisms rendering the drug ineffective in many patients. Elacytarabine, a lipophilic 5'-elaidic acid ester or nucleoside analogue of cytosine arabinoside, was created with the intent of overcoming resistance mechanisms including reduced expression of the human equilibrative nucleoside transporter 1 (hENT1) required for cytarabine entry into cells, as well as increased activity of cytidine deaminase (CDA) which breaks down the active metabolite of cytarabine, ara-CTP. Elacytarabine enters cells independently of transporters, has a longer half life compared with cytarabine and is not subject to deactivation by CDA. Preclinical data were encouraging although subsequent clinical studies have failed to show superiority of elacytarabine compared with standard of care as monotherapy in patients with AML. Clinical trials utilizing elacytarabine in combination with anthracyclines are ongoing. Use of hENT1 expression as a predictive marker for cytarabine or elacytarabine response has been studied with no conclusive validation to date. Despite promising early results, the jury is still out in regards to this novel agent as an effective alternative to standard cytarabine therapy in acute leukemias, especially in combination with additional agents such as anthracyclines.

RW, Parang K. Carbocyclodipeptides as modified nucleosides: synthesis and anti-HIV activities

A new class of nucleoside analogues were synthesized using cyclic dipeptides and modified 2′-deoxyfuranoribose sugars to introduce flexibility by peptides in place of common nucleoside bases and to determine their biological properties. The synthesis was carried out by coupling of a protected ribose sugar with synthesized dipeptides in the presence of hexamethyldisilazane and trimethylsilyltriflate. The final products were characterized by NMR and high-resolution MS-TOF spectroscopy. The compounds were evaluated for anti-HIV activities. 1-(4-Azido-5-(hydroxymethyl)tetrahydrofuran-2-yl)-3,6-diisopropylpiperazine-2,5-dione (compound 14) containing 3- and 6-isopropyl groups in the base and 3′-azide (EC50 = 1.96 μmol/L) was the most potent compound among all of the synthesized analogs.

Design, synthesis, and biological evaluation of unconventional aminopyrimidine, aminopurine, and amino-1,3,5-triazine methyloxynucleosides

Herein we describe a class of unconventional nucleosides (methyloxynucleosides) that combine unconventional nucleobases such as substituted aminopyrimidines, aminopurines, or aminotriazines with unusual sugars in their structures. The allitollyl or altritollyl derivatives were pursued as ribonucleoside mimics, whereas the tetrahydrofuran analogues were pursued as their dideoxynucleoside analogues. The compounds showed poor, if any, activity against a broad range of RNA and DNA viruses, including human immunodeficiency virus (HIV). This inactivity may be due to lack of an efficient metabolic conversion into their corresponding 5'-triphosphates and poor affinity for their target enzymes (DNA/RNA polymerases). Several compounds showed cytostatic activity against proliferating human CD4(+) T-lymphocyte CEM cells and against several other tumor cell lines, including murine leukemia L1210 and human prostate PC3, kidney CAKI-1, and cervical carcinoma HeLa cells. A few compounds were inhibitory to Moloney murine sarcoma virus (MSV) in C3H/3T3 cell cultures, with the 2,6-diaminotri-O-benzyl-D-allitolyl- and -D-altritolyl pyrimidine analogues being the most potent among them. This series of unconventional nucleosides may represent a novel family of potential antiproliferative agents.

Investigation of diastereoselective acyclic α-alkoxydithioacetal substitutions involving thiacarbenium intermediates

Reported herein is an experimental and theoretical study that elucidates why silylated nucleobase additions to acyclic α-alkoxythiacarbenium intermediates proceed with high 1,2-syn stereocontrol (anti-Felkin-Anh), which is opposite to what would be expected with corresponding activated aldehydes. The acyclic thioaminals formed undergo intramolecular cyclizations to provide nucleoside analogues with anticancer and antiviral properties. The factors influencing the selectivity of the substitution reaction have been examined thoroughly. Halothioether species initially form, ionize in the presence (low dielectric media) or absence (higher dielectric media) of the nucleophile, and react through SN2-like transition structures (TS A and D), where the α-alkoxy group is gauche to the thioether moiety. An important, and perhaps counterintuitive, observation in this work was that calculations done in the gas phase or low dielectric media (toluene) are essential to locate the product- and rate-determining transition structures (C-N bond formation) that allow the most reasonable prediction of selectivity and isotope effects for more polar solvents (THF, MeCN). The ΔΔG(⧧) (G(TSA-TSD)) obtained in silico are consistent with the preferential formation of 1,2-syn product and with the trends of stereocontrol displayed by 2,3-anti and 2,3-syn α,β-bis-alkoxydithioacetals.

Current and future targets of antiviral therapy in the hepatitis C virus life cycle

ABSTRACT 

Advances in our understanding of the hepatitis C virus (HCV) life cycle have enabled the development of numerous clinically advanced direct-acting antivirals. Indeed, the recent approval of first-generation direct-acting antivirals that target the viral NS3–4A protease and NS5B RNA-dependent RNA polymerase brings closer the possibility of universally efficacious and well-tolerated antiviral therapies for this insidious infection. However, the complexities of comorbidities, unforeseen side effects or drug–drug interactions, viral diversity, the high mutation rate of HCV RNA replication and the elegant and constantly evolving mechanisms employed by HCV to evade host and therapeutically implemented antiviral strategies remain as significant obstacles to this goal. Here, we review advances in our understanding of the HCV life cycle and associated opportunities for antiviral therapy.

Cyclic-disulfide-based prodrugs for cytosol-specific drug delivery

The cytosolic conversion of therapeutically relevant nucleosides into bioactive triphosphates is often hampered by the inefficiency of the first kinase-mediated step. Nucleoside monophosphate prodrugs can be used to bypass this limitation. Herein we describe a novel cyclic-disulfide class of nucleoside monophosphate prodrugs with a cytosol-specific, reductive release trigger. The key event, a charge-dissipating reduction-triggered cyclodeesterification leads to robust cytosolic production of the cyclic 3',5'-monophosphate for downstream enzymatic processing. The antiviral competence of the platform was demonstrated with an O-benzyl-1,2-dithiane-4,5-diol ester of 2'-C-methyluridine-3',5'-phosphate. Both in vitro and in vivo comparison with the clinically efficacious ProTide prodrug of 2'-deoxy-2'-α-fluoro-β-C-methyluridine is provided. The cytosolic specificity of the release allows for a wide range of potential applications, from tissue-targeted drug delivery to intracellular imaging.

Stereoselective synthesis of 4'-selenonucleosides via seleno-Michael reaction as potent antiviral agents

Based on the hypothesis that the bulky selenium atom, with 4p orbitals, can sterically hinder the approach of a cellular kinase to 5'-OH for phosphorylation, 4'-selenonucleosides with one-carbon homologation were designed and synthesized via a novel seleno-Michael reaction, with the stereoselectivity controlled by steric effects. 5'-Homo-4'-selenonucleosides (n = 2) demonstrated potent antiherpes simplex virus (HSV-1) activity, indicating that the bulky selenium atom might play a key role in preventing phosphorylation by cellular kinases, resulting in no antiviral activity.

Synthesis and biological evaluation of 2'-substituted-4'-selenoribofuranosyl pyrimidines as antitumor agents

The 2'-substituted-4'-selenoribofuranosyl pyrimidines 3a-3j were synthesized from D-ribose and assayed for anticancer activity. The 2'-azido and 2'-fluoro groups with a ribo configuration were introduced by the regioselective opening of the O2,2'-anhydronucleosides with sodium azide and (HF)x-pyridine, respectively. Among the compounds tested, only 2'-fluoro derivative 3j was found to exhibit significant anticancer activity, but was much less potent than the corresponding 2'-arabino analogue 2c. This study will provide medicinal chemists with the insight into the identification of structural requirements for the anticancer activity for the developments of biologically active nucleosides.

Synthesis and antiviral properties of spirocyclic [1,2,3]-triazolooxazine nucleosides

An efficient synthesis of spirocyclic triazolooxazine nucleosides is described. This was achieved by the conversion of β-D-psicofuranose to the corresponding azido-derivative, followed by alkylation of the primary alcohol with a range of propargyl bromides, obtained by Sonogashira chemistry. The products of these reactions underwent 1,3-dipolar addition smoothly to generate the protected spirocyclic adducts. These were easily deprotected to give the corresponding ribose nucleosides. The library of compounds obtained was investigated for its antiviral activity using MHV (mouse hepatitis virus) as a model wherein derivative 3 f showed the most promising activity and tolerability.