Development and optimization of anti-HIV nucleoside analogs and prodrugs: A review of their cellular pharmacology, structure-activity relationships and pharmacokinetics

Prodrugs of γ-Alkyl-Modified Nucleoside Triphosphates: Improved Inhibition of HIV Reverse Transcriptase

The development of nucleoside triphosphate prodrugs is one option to apply nucleoside reverse transcriptase inhibitors. Herein, we report the synthesis and evaluation of d4TTP analogues, in which the γ-phosphate was modified covalently by lipophilic alkyl residues, and acyloxybenzyl prodrugs of these γ-alkyl-modified d4TTPs, with the aim of delivering of γ-alkyl-d4TTP into cells. Selective formation of γ-alkyl-d4TTP was proven with esterase and in CD4+ -cell extracts. In contrast to d4TTP, γ-alkyl-d4TTPs proved highly stable against dephosphorylation. Primer extension assays with HIV reverse transcriptase (RT) and DNA-polymerases α, β or γ showed that γ-alkyl-d4TTPs were substrates for HIV-RT only. In antiviral assays, compounds were highly potent inhibitors of HIV-1 and HIV-2 also in thymidine-kinase-deficient T-cell cultures (CEM/TK- ). Thus, the intracellular delivery of such γ-alkyl-nucleoside triphosphates may potentially lead to nucleoside triphosphates with a higher selectivity towards the viral polymerase that can act in virus-infected cells.

γ-Ketobenzyl-Modified Nucleoside Triphosphate Prodrugs as Potential Antivirals

The antiviral activity of nucleoside reverse transcriptase inhibitors is often hampered by insufficient phosphorylation. Nucleoside triphosphate analogues are presented, in which the γ-phosphate was covalently modified by a non-bioreversible, lipophilic 4-alkylketobenzyl moiety. Interestingly, primer extension assays using human immunodeficiency virus reverse transcriptase (HIV-RT) and three DNA-polymerases showed a high selectivity of these γ-modified nucleoside triphosphates to act as substrates for HIV-RT, while they proved to be nonsubstrates for DNA-polymerases α, β, and γ. In contrast to d4TTP, the γ-modified d4TTPs showed a high resistance toward dephosphorylation in cell extracts. A series of acyloxybenzyl-prodrugs of these γ-ketobenzyl nucleoside triphosphates was prepared. The aim was the intracellular delivery of a stable γ-modified nucleoside triphosphate to increase the selectivity of such compounds to act in infected versus noninfected cells. Delivery of γ-ketobenzyl-d4TTPs was proven in T-lymphocyte cell extracts. The prodrugs were potent inhibitors of HIV-1/2 in cultures of infected CEM/0 cells and more importantly in thymidine kinase-deficient CD4⁺ T-cells.

γ-Non-Symmetrically Dimasked TriPPPro Prodrugs as Potential Antiviral Agents against HIV

Nucleoside analogue reverse transcriptase inhibitors (NRTI) and nucleoside analogue monophosphate prodrugs are used in combination antiretroviral therapy (cART). The design of antivirally active nucleoside triphosphate prodrugs is a recent and an important advancement in the field of nucleoside analogue drug development. Here, we report on TriPPPro‐derivatives of nucleoside analogue triphosphates (NTPs) that comprised two different acyloxybenzyl‐masks at the γ‐phosphate of the NTP aiming to achieve the metabolic bypass. Thus, γ‐non‐symmetrically dimasked TriPPPro‐compounds (γ‐(AB,ab)‐d4TTPs) were synthesized and they proved to be active against HIV‐1 and HIV‐2 in cultures of infected wild‐type human CD4⁺ T‐lymphocyte (CEM/0) cells and more importantly also in thymidine kinase‐deficient CD4⁺ T‐cells (CEM/TK‐). From hydrolysis studies both in phosphate buffer (PB, pH 7.3) and CEM cell extracts, there was surprisingly no differentiation in the cleavage of the two acyloxybenzyl prodrug‐masks. However, if within one of the two acyloxybenzyl groups a short PEG‐type methoxytriglycol group was introduced, the “standard” acyloxybenzyl‐mask was cleaved with high preference.

Membrane Permeable, Bioreversibly Modified Prodrugs of Nucleoside Diphosphate-γ-Phosphonates

Nucleoside reverse transcriptase inhibitors (NRTIs) are widely used as antiviral and anticancer agents, although they require intracellular phosphorylation into their antivirally active form, the triphosphorylated nucleoside analogue metabolites. We report on the synthesis and characterization of a new class of nucleoside triphosphate analogues comprising a C-alkyl-phosphonate moiety replacing the γ-phosphate. These compounds were converted into bioreversibly modified lipophilic prodrugs at the γ-phosphonate by the attachment of an acyloxybenzyl (ester) or an alkoxycarbonyloxybenzyl (carbonate) group. Such compounds formed γ-C-(alkyl)-nucleoside triphosphate analogues with high selectivity because of an enzyme-triggered delivery mechanism. The latter compounds were very stable in CD4⁺ T-lymphocyte (CEM cell) extracts, and they were substrates for HIV-reverse transcriptase without being substrates for DNA-polymerases α, β, and γ. In antiviral assays, the excellent antiviral activity of the prodrugs that was found in CEM/0 cells was completely kept in CEM/TK^- cells. The activity was improved by 3 logs as compared to the parent nucleoside d4T.

Lipophilic Triphosphate Prodrugs of Various Nucleoside Analogues

The antiviral efficacy of many nucleoside analogues is strongly dependent on their intracellular activation by host cellular kinases to yield ultimately the bioactive nucleoside analogue triphosphates (NTP). The metabolic conversion of nucleoside analogues into their triphosphates often proceeds insufficiently. We developed a nucleoside triphosphate (NTP) delivery system (the TriPPPro approach), in which the γ-phosphate is covalently modified by two different biodegradable masking units, one is the acyloxybenzyl (AB) moiety and the other is the alkoxycarbonyloxybenzyl (ACB) group. Such compounds formed NTPs with high selectivity by an enzyme-triggered mechanism in human T-lymphocyte CEM cell extracts loosing first the AB moiety, followed by the ACB group. This enables the bypass of all steps of the intracellular phosphorylation. This approach was applied here to convert some modestly active or even inactive nucleoside analogues into powerful biologically active metabolites. Potent antiviral activity profiles were obtained depending on the lipophilicity of the TriPPPro-NTP prodrugs against HIV-1 and HIV-2 replication in cultures of infected wild-type CD4⁺ CEM T-cells and more importantly in thymidine kinase-deficient CD4⁺ T-cells (CEM/TK^-). This TriPPPro strategy offers high potential for future antiviral and antitumoral chemotherapies.

Anti-HIV-Active Nucleoside Triphosphate Prodrugs

We disclose a study on nucleoside triphosphate (NTP) analogues in which the γ-phosphate is covalently modified by two different biodegradable masking units and d4T as nucleoside analogue that enable the delivery of d4TTP with high selectivity in phosphate buffer (pH 7.3) and by enzyme-triggered reactions in human CD4⁺ T-lymphocyte CEM cell extracts. This allows the bypass of all steps normally needed in the intracellular phosphorylation. These TriPPPro-nucleotides comprising an acyloxybenzyl (AB; ester) or an alkoxycarbonyloxybenzyl (ACB; carbonate) in combination with an ACB moiety are described as NTP delivery systems. The introduction of these two different groups led to the selective formation of γ-(ACB)-d4TTPs by chemical hydrolysis and in particular by cell extract enzymes. γ-(AB)-d4TTPs are faster cleaved than γ-(ACB)-d4TTPs. In antiviral assays, the compounds are highly active against HIV-1 and HIV-2 in wild-type CEM/O cells and more importantly in thymidine kinase-deficient CD4⁺ T-cells (CEM/TK^-).

Nucleotide Reverse Transcriptase Inhibitors: A Thorough Review, Present Status and Future Perspective as HIV Therapeutics

BACKGROUND

Human immunodeficiency virus type-1 (HIV-1) infection leads to acquired immunodeficiency syndrome (AIDS), a severe viral infection that has claimed approximately 658,507 lives in the US between the years 2010-2014. Antiretroviral (ARV) therapy has proven to inhibit HIV-1, but unlike other viral illness, not cure the infection.

OBJECTIVE

Among various Food and Drug Administration (FDA)-approved ARVs, nucleoside/ nucleotide reverse transcriptase inhibitors (NRTIs) are most effective in limiting HIV-1 infection. This review focuses on NRTIs mechanism of action and metabolism.

METHODS

A search of PubMed (1982-2016) was performed to capture relevant articles regarding NRTI pharmacology.

RESULTS

The current classical NRTIs pharmacology for HIV-1 prevention and treatment are presented. Finally, various novel strategies are proposed to improve the efficacy of NRTIs, which will increase therapeutic efficiency of present-day HIV-1 prevention/treatment regimen.

CONCLUSION

Use of NRTIs will continue to be critical for successful treatment and prevention of HIV-1.

Synthesis and Antiviral Evaluation of Carbocyclic Nucleoside Analogs of Nucleoside Reverse Transcriptase Translocation Inhibitor MK-8591 (4'-Ethynyl-2-fluoro-2'-deoxyadenosine)

MK-8591 (4'-ethynyl-2-fluoro-2'-deoxyadenosine) is a novel nucleoside analog that displays a differentiated mechanism of action as a nucleoside reverse transcriptase translocation inhibitor (NRTTI) compared to approved NRTIs. Herein, we describe our recent efforts to explore the impact of structural changes to the properties of MK-8591 through the synthesis and antiviral evaluation of carbocyclic derivatives. Synthesized analogs were evaluated for their antiviral activity, and the corresponding triphosphates were synthesized and evaluated in a biochemical assay. 4'-Ethynyl-G derivative (±)-29 displayed a promising IC₅₀ of 33 nM in a hPBMC cell-based antiviral assay, and its triphosphate (TP), (±)-29-TP, displayed an IC₅₀ of 324 nM in a biochemical RT-polymerase assay. Improved TP anabolite delivery resulting in improved in vitro potency was achieved by preparing the corresponding phosphoramidate prodrug of single enantiomer 29b, with 6-ethoxy G derivative 34b displaying a significantly improved IC₅₀ of 3.0 nM, paving the way for new directions for this novel class of nucleoside analogs.

Meeting report: 31st International Conference on Antiviral Research

The 31^st International Conference on Antiviral Research (ICAR) was held in Porto, Portugal from June 11–15, 2018. In this report, volunteer rapporteurs provide their summaries of scientific presentations, hoping to effectively convey the speakers' goals and the results and conclusions of their talks. This report provides an overview of the invited keynote and award lectures and highlights of short oral presentations, from the perspective of experts in antiviral research. Of note, a session on human cytomegalovirus included an update on the introduction to the clinic of letermovir for the prevention of CMV infection and disease. The 31^st ICAR successfully promoted new discoveries in antiviral research and drug development. The 32^nd ICAR will be held in Baltimore, Maryland, USA, May 6–10, 2019.

•

The 31st ICAR was held in Porto, Portugal, June 11–15, 2018.

•

This article provides an overview of the invited keynote and award lectures and highlights of short oral presentations.

•

ICAR provided an interdisciplinary forum to review recent developments in all areas of antiviral research.

•

The 32nd ICAR will be held in Baltimore, Maryland, USA, May 6–10, 2019.

Chalcogenozidovudine Derivatives With Antitumor Activity: Comparative Toxicities in Cultured Human Mononuclear Cells

Considering a novel series of zidovudine (AZT) derivatives encompassing selenoaryl moieties promising candidates as therapeutics, we examined the toxicities elicited by AZT and derivatives 5'-(4-Chlorophenylseleno)zidovudine (SZ1); 5'-(Phenylseleno)zidovudine (SZ2); and 5'-(4-Methylphenylseleno)zidovudine (SZ3) in healthy cells and in mice. Resting and stimulated cultured human peripheral blood mononuclear cells (PBMCs) were treated with the compounds at concentrations ranging from 10 to 200 µM for 24 and/or 72 h. Adult mice received a single injection of compounds (100 µmol/kg, s.c.) and 72 h after administration, hepatic/renal biomarkers were analyzed. Resting and stimulated PBMCs exposed to SZ1 displayed loss of viability, increased reactive species production, disruption in cell cycle, apoptosis and increased transcript levels and production of pro-inflammatory cytokines. In a mild way, most of these effects were also induced by SZ2. AZT and SZ3 did not cause significant toxicity towards resting PBMCs. Differently, both compounds elicited apoptosis and S phase arrest in stimulated cells. AZT and derivatives administration did not change the body weight and plasma biochemical markers in mice. However, the absolute weight and organ-to-body weight ratio of liver, kidneys and spleen were altered in AZT, SZ1-, and SZ2-treated mice. Our results highlighted the involvement of derivatives SZ1 and SZ2 in redox and immunological dyshomeostasis leading to activation of apoptotic signaling pathways in healthy cells under different division phases. On the other hand, the derivative SZ3 emerged as a promising candidate for further viral infection/antitumor studies as a new effective therapy with low toxicity for immune cells and after acute in vivo treatment.

Nucleoside diphosphate and triphosphate prodrugs - An unsolvable task?

In this review, our recent advances in the development of nucleoside di- and nucleoside triphosphate prodrugs is summarized. Previously, we had developed a successful membrane-permeable pronucleotide system for the intracellular delivery of nucleoside monophosphates as well, the so-called cycloSal-approach. In contrast to that work in which the delivery is initiated by a chemically driven hydrolysis reaction, for the di- and triphosphate delivery, an enzymatic trigger mechanism involving (carboxy)esterases had to be used. The other features of the new pronucleotide approaches are: (i) lipophilic modification was restricted to the terminal phosphate group leaving charges at the internal phosphate moieties and (ii) appropriate lipophilicity is introduced by long aliphatic residues within the bipartite prodrug moiety. The conceptional design of the di- and triphosphate prodrug systems will be described and the chemical synthesis, the hydrolysis properties, a structure-activity relationship and antiviral activity data will be discussed as well. The advantage of these new approaches is that all phosphorylation steps from the nucleoside analogue into the bioactive nucleoside triphosphate form can be bypassed in the case of the triphosphate prodrugs. Moreover, enzymatic processes like the deamination of nucleosides or nucleoside monophosphates which lead to catabolic clearance of the potential antivirally active compound can be avoided by the delivery of the higher phosphorylated nucleotides.

Comparison of the diastereoisomeric excess of uridine, inosine and adenosine cyanohydrins determined by HPLC-DAD and 1H NMR

The separation of the diastereoisomers of the nucleoside derivatives of uridine, inosine and adenosine was performed by HPLC using chiral and no chiral columns, it was observed with the no chiral columns the resolution was good enough to determine diastereoisomeric excess. These methods were compared with ¹H NMR, and no significant differences were observed between the three techniques. Diastereoisomeric uridine (3a), inosine (3b) and adenosine (4c) cyanohydrins were resolved by ¹H nuclear magnetic resonance (¹H NMR), chiral normal phase-high-performance liquid chromatography-diode array detector (NP-HPLC-DAD) and reversed phase (RP-HPLC-DAD); these methods allowed the assesment of the percent diastereoisomeric excess (% de) of the nucleosidic cyanohydrins of 3a (4, 6 and 4), 3b (10, 8 and 6) and 4c (4, 4 and 4). To the best of our knowledge, there are no reports using analytical techniques for the separation of the epimers of 3a, 3b and 4c.

Validation of a High-Throughput Screening Assay for Identification of Adjunctive and Directly Acting Antimicrobials Targeting Carbapenem-Resistant Enterobacteriaceae

We describe development and validation of a high-throughput screen (HTS) for identifying small molecules that restore the efficacy of carbapenems (adjunctives) and/or directly inhibit growth of carbapenem-resistant Enterobacteriaceae (CRE). Our HTS assay is based on a screen-counterscreen approach using a representative multidrug-resistant CRE strain, Klebsiella pneumoniae BIDMC12A. Specifically, we tested the ability of small molecules to inhibit bacterial growth in the presence (screen) or absence (counterscreen) of meropenem, a representative carbapenem antibiotic. Primary screening of 11,698 known bioactive compounds identified 14 with adjunctive activity and 79 with direct antimicrobial effect. Secondary screening identified triclosan as a strongly synergistic meropenem adjunctive (fractional inhibitory concentration = 0.48) and confirmed azidothymidine (AZT) (minimal inhibitory concentration [MIC] = 4 μg mL(-1)), NH125 (MIC = 4 μg mL(-1)), diphenyleneiodonium chloride (MIC = 8 μg mL(-1)), and spectinomycin (MIC = 32 μg mL(-1)) as potent direct antimicrobials. Spectrum of activity of AZT and spectinomycin was tested against a collection of 103 representative Enterobacteriaceae strains (≈50% CRE). AZT, a nucleoside analog used to treat human immunodeficiency virus, demonstrated an MIC50 of 2 μg mL(-1). Spectinomycin, an antibiotic used to treat gonorrhea, had an MIC50 of 32 μg mL(-1). Therefore, a significant percentage of CRE strains appeared relatively susceptible to these antimicrobials. These data identified AZT and spectinomycin as available agents warranting further study for potential treatment of multidrug-resistant CRE infection. Our results provide proof of principle and impetus for performing a large-scale HTS for discovery of novel, small-molecule adjunctives and antibacterial agents directly targeting CRE.

Correction of a genetic deficiency in pantothenate kinase 1 using phosphopantothenate replacement therapy

Coenzyme A (CoA) is a ubiquitous cofactor involved in numerous essential biochemical transformations, and along with its thioesters is a key regulator of intermediary metabolism. Pantothenate (vitamin B5) phosphorylation by pantothenate kinase (PanK) is thought to control the rate of CoA production. Pantothenate kinase associated neurodegeneration is a hereditary disease that arises from mutations that inactivate the human PANK2 gene. Aryl phosphoramidate phosphopantothenate derivatives were prepared to test the feasibility of using phosphopantothenate replacement therapy to bypass the genetic deficiency in the Pank1(-/-) mouse model. The efficacies of candidate compounds were first compared by measuring the ability to increase CoA levels in Pank1(-/-) mouse embryo fibroblasts. Administration of selected candidate compounds to Pank1(-/-) mice corrected their deficiency in hepatic CoA. The PanK bypass was confirmed by the incorporation of intact phosphopantothenate into CoA using triple-isotopically labeled compound. These results provide strong support for PanK as a master regulator of intracellular CoA and illustrate the feasibility of employing PanK bypass therapy to restore CoA levels in genetically deficient mice.

Lipophilic prodrugs of nucleoside triphosphates as biochemical probes and potential antivirals

The antiviral activity of nucleoside reverse transcriptase inhibitors is often limited by ineffective phosphorylation. We report on a nucleoside triphosphate (NTP) prodrug approach in which the γ-phosphate of NTPs is bioreversibly modified. A series of TriPPPro-compounds bearing two lipophilic masking units at the γ-phosphate and d4T as a nucleoside analogue are synthesized. Successful delivery of d4TTP is demonstrated in human CD4⁺ T-lymphocyte cell extracts by an enzyme-triggered mechanism with high selectivity. In antiviral assays, the compounds are potent inhibitors of HIV-1 and HIV-2 in CD4⁺ T-cell (CEM) cultures. Highly lipophilic acyl residues lead to higher membrane permeability that results in intracellular delivery of phosphorylated metabolites in thymidine kinase-deficient CEM/TK⁻ cells with higher antiviral activity than the parent nucleoside.

Charged phosphorylated metabolite such as nucleoside tri-phosphates exhibit poor membrane permeability due to their high polarity, limiting their utility as drugs or cellular probes. Here the authors develop a method to render nucleoside triphosphates cell permeable and allows their release by an enzyme-triggered mechanism.

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.

A facile method for synthesizing water-soluble and superior sustained release anti-HIV prodrug SCs-d4T

To efficiently deliver stavudine (d4T) for AIDS therapy, chitosan-stavudine conjugate (Cs-d4T) was synthesized. However, its poor water-solubility limited its clinical application. In this study, a sulphonated chitosan-stavudine conjugate (SCs-d4T) was synthesized with a mild SO3·Py complex sulphonation strategy. Chemical characteristics and morphology of Cs-d4T and SCs-d4T were performed by NMR, XRD, FTIR, ICP-AES and SEM. SCs-d4T demonstrated satisfactory solubility (106-bold of Cs-d4T solubility), good anti-HIV activity (6-fold of d4T anti-HIV activity), and well sustained release ability. The major release product O-isopropyl-5'-H-phosphonate of d4T (d4T-P-H) showed higher anti-HIV activity than d4T. For further evaluating the influence of linker and sulphonation strategy on anti-HIV activity, chitosan grafted with d4T by succinyl linker (Cs-sd4T) and SCs-d4T sulphonated by oleum were also prepared. The result showed that the O-isopropyl monophosphate linker of Cs-d4T and SO3·Py complex sulphonation strategy revealed higher anti-HIV activity than succinyl linker of Cs-sd4T and oleum sulphonation strategy, respectively.

Novel acyclic phosphonylated 1,2,3-triazolonucleosides with an acetamidomethyl linker: synthesis and biological activity

A new series of 4-substituted [(1,2,3-triazol-1-yl)acetamido]methylphosphonates as acyclic nucleotide analogs were synthesized from diethyl (2-chloroacetamido)methylphosphonate via azidation followed by 1,3-dipolar cycloaddition with selected alkynes derived from natural nucleobases or their mimetics. All compounds were tested for their antiviral activities against DNA and RNA viruses as well as for cytostatic activity or cytotoxicity. Among all tested compounds, [(1,2,3-triazol-1-yl)acetamido]methylphosphonate 6e substituted with the N(3)-Bz-benzuracil moiety showed activity against the vesicular stomatitis virus (EC50 = 45 µM) in HeLa cell cultures.

Synthesis and Biological Evaluation of 5'-O-Dicarboxylic Fatty Acyl Monoester Derivatives of Anti-HIV Nucleoside Reverse Transcriptase Inhibitors

A number of 5'-O-dicarboxylic fatty acyl monoester derivatives of 3'-azido-3'-deoxythymidine (zidovudine, AZT), 2',3'-didehydro-2',3'-dideoxythymidine (stavudine, d4T), and 3'-fluoro-3'-deoxythymidine (alovudine, FLT) were synthesized to improve the lipophilicity and potentially the cellular delivery of parent polar 2', 3'-dideoxynucleoside (ddN) analogues. The compounds were evaluated for their anti-HIV activity. Three different fatty acids with varying chain length of suberic acid (octanedioic acid), sebacic acid (decanedioic acid), and dodecanedioic acid were used for the conjugation with the nucleosides. The compounds were evaluated for anti-HIV activity and cytotoxicity. All dicarboxylic ester conjugates of nucleosides exhibited significantly higher anti-HIV activity than that of the corresponding parent nucleoside analogs. Among all the tested conjugates, 5'-O-suberate derivative of AZT (EC50 = 0.10 nM) was found to be the most potent compound and showed 80-fold higher anti-HIV activity than AZT without any significant toxicity (TC50 > 500 nM).

Biological evaluation and molecular modelling of didanosine derivatives

These prodrugs of DDI with increased lipophilicity and good antiviral performance should be of interest in HIV therapy.

Design, synthesis, and biological evaluation of new 2'-deoxy-2'-fluoro-4'-triazole cytidine nucleosides as potent antiviral agents

A series of 4'-[1,2,3]triazole-2'-deoxy-2'-fluoro-β-d-arabinofuranosylcytosines (9-17) were prepared by Cu(I)-mediated [3 + 2] cycloaddition reactions (CuAAC) of 1-(4'-azido-2'-deoxy-2'-fluoro-β-d-arabinofuranosyl)cytosine (1) with appropriate alkynes in good yields. Their structures were fully established by (1)H NMR, (13)C NMR, HRMS, and elemental analysis. Most of these nucleoside analogs exhibited potent anti-HIV-1 activity with no cytotoxicity observed at the highest tested concentration up to 25 μM. Among them, compounds 9, 10 and 13 exhibited extremely potent antiviral activity, thus had a great potential for further development as novel nucleoside reverse transcriptase inhibitors (NRTIs) for the treatment of HIV-1 infection. Besides, the anti-HBV activity of compounds 10, 11 and 17 had been investigated.

Parallel solution-phase synthesis of an adenosine antibiotic analog library

A library of eighty one adenosine antibiotic analogs was prepared under the Pilot Scale Library Program of the NIH Roadmap initiative from 5'-amino-5'-deoxy-2',3'-O-isopropylidene-adenosine 3. Diverse aldehyde, sulfonyl chloride and carboxylic acid reactant sets were condensed to 3, in solution-phase fashion, leading after acid-mediated hydrolysis to the targeted compounds in good yields and high purity. No marked antituberculosis or anticancer activity was noted on preliminary cellular testing, but these nucleoside analogs should be useful candidates for other types of biological activity.

Synthesis of Oligonucleotide Conjugates and Phosphorylated Nucleotide Analogues: An Improvement to a Solid Phase Synthetic Approach

An improvement to our solid phase strategy to generate pharmacologically interesting molecule libraries is proposed here. The synthesis of newo-chlorophenol-functionalised solid supports with very high loading (0.18–0.22 meq/g for control pore glass (CPG) and 0.25–0.50 meq/g for TG) is reported. To test the efficiency of these supports, we prepared nucleotide and oligonucleotide models, and their coupling yields and the purity of the crude detached materials were comparable to previously available results. These supports allow the facile and high-yield preparation of highly pure phosphodiester and phosphoramidate monoester nucleosides, conjugated oligonucleotides, and other yet unexplored classes of phosphodiester and phosphoramidate molecules.

Self-assembled nucleolipids: from supramolecular structure to soft nucleic acid and drug delivery devices

This short review aims at presenting some recent illustrative examples of spontaneous nucleolipids self-assembly. High-resolution structural investigations reveal the diversity and complexity of assemblies formed by these bioinspired amphiphiles, resulting from the interplay between aggregation of the lipid chains and base–base interactions. Nucleolipids supramolecular assemblies are promising soft drug delivery systems, particularly for nucleic acids. Regarding prodrugs, squalenoylation is an innovative concept for improving efficacy and delivery of nucleosidic drugs.

Exploiting the nucleotide substrate specificity of repair DNA polymerases to develop novel anticancer agents

The genome is constantly exposed to mutations that can originate during replication or as a result of the action of both endogenous and/or exogenous damaging agents [such as reactive oxygen species (ROS), UV light, genotoxic environmental compounds, etc.]. Cells have developed a set of specialized mechanisms to counteract this mutational burden. Many cancer cells have defects in one or more DNA repair pathways, hence they rely on a narrower set of specialized DNA repair mechanisms than normal cells. Inhibiting one of these pathways in the context of an already DNA repair-deficient genetic background, will be more toxic to cancer cells than to normal cells, a concept recently exploited in cancer chemotherapy by the synthetic lethality approach. Essential to all DNA repair pathways are the DNA pols. Thus, these enzymes are being regarded as attractive targets for the development of specific inhibitors of DNA repair in cancer cells. In this review we examine the current state-of-the-art in the development of nucleotide analogs as inhibitors of repair DNA polymerases.

Application of kinase bypass strategies to nucleoside antivirals

Nucleoside and nucleotide analogs have served as the cornerstones of antiviral therapy for many viruses. However, the requirement for intracellular activation and side-effects caused by distribution to off-target sites of toxicity still limit the efficacy of the current generation of drugs. Kinase bypass strategies, where phosphorylated nucleosides are delivered directly into cells, thereby, removing the requirement for enzyme catalyzed phosphorylation steps, have already changed the face of antiviral therapy in the form of the acyclic nucleoside phosphonates, cidofovir, adefovir (given orally as its dipivoxil prodrug) and tenofovir (given orally as its disoproxil prodrug), currently used clinically. These strategies hold further promise to advance the field of antiviral therapy with at least 10 kinase bypass and tissue targeted prodrugs, representing seven distinct prodrug classes, currently in clinical trials. This article reviews the history of kinase bypass strategies applied to nucleoside antivirals and the evolution of different tissue targeted prodrug strategies, highlighting clinically relevant examples.

Novel interactions of fluorinated nucleotide derivatives targeting orotidine 5'-monophosphate decarboxylase

Fluorinated nucleosides and nucleotides are of considerable interest to medicinal chemists because of their antiviral, anticancer, and other biological activities. However, their direct interactions at target binding sites are not well understood. A new class of 2'-deoxy-2'-fluoro-C6-substituted uridine and UMP derivatives were synthesized and evaluated as inhibitors of orotidine 5'-monophosphate decarboxylase (ODCase or OMPDCase). These compounds were synthesized from the key intermediate, fully protected 2'-deoxy-2'-fluorouridine. Among the synthesized compounds, 2'-deoxy-2'-fluoro-6-iodo-UMP covalently inhibited human ODCase with a second-order rate constant of 0.62 ± 0.02 M(-1) s(-1). Interestingly, the 6-cyano-2'-fluoro derivative covalently interacted with ODCase defying the conventional thinking, where its ribosyl derivative undergoes transformation into BMP by ODCase. This confirms that the 2'-fluoro moiety influences the chemistry at the C6 position of the nucleotides and thus interactions in the active site of ODCase. Molecular interactions of the 2'-fluorinated nucleotides are compared to those with the 3'-fluorinated nucleotides bound to the corresponding target enzyme, and the carbohydrate moieties were shown to bind in different conformations.

A Review of Electroanalytical Techniques for Determination of Anti-HIV Drugs

Until now after the human immunodeficiency virus (HIV) was discovered as the then tentative aetiological agent of acquired immune deficiency syndrome (AIDS), exactly 25 anti-HIV compounds have been formally approved for clinical use in the treatment of AIDS. These compounds fall into six categories: nucleoside reverse transcriptase inhibitors (NRTIs: zidovudine, didanosine, zalcitabine, lamivudine, abacavir, stavudine, and emtricitabine), nucleotide reverse transcriptase inhibitors (NtRTIs: tenofovir), nonnucleoside reverse transcriptase inhibitors (NNRTIs: efavirenz, nevirapine, delavirdine, and etravirine), protease inhibitors (PIs: ritonavir, indinavir, saquinavir, nelfinavir, amprenavir, lopinavir, fosamprenavir, atazanavir, tipranavir and darunavir), fusion inhibitors (FIs: enfuvirtide), coreceptor inhibitors (CRIs: maraviroc), and integrase inhibitors (INIs: raltegravir). The present paper submitted the use of various electroanalytical techniques for the determination of anti-HIV drugs. This paper covers the time period from 1990 to 2010 including voltammetric techniques that were reported. Presented application concerns analysis of anti-HIV drugs from pharmaceutical dosage forms and biological samples.

Synthesis of beta-hydroxyphosphonate and 1,2-dihydroxy acyclic nucleoside analogs via 1,3-dipolar cycloaddition strategy

A convenient synthetic approach toward nucleoside analogs where beta-hydroxyphosphonate- or 1,2-dihydroxy units are connected to the nucleic acid base through a triazole spacer is discussed.

Synthesis of 3'-amino-3'-deoxyguanosine and 3'-amino-3'-deoxyxyloguanosine monophosphate HepDirect prodrugs from guanosine

The synthesis of 3'-amino-3'-deoxyguanosine and 3'-amino-3'-deoxyxyloguanosine monophosphate HepDirect prodrugs from guanosine is reported. Initial incorporation of N,N-dibenzylformamidino protection of the C2-amino of guanosine masked the reactivity of that group and simplified purification of subsequent analogues. The first key intermediate, 9-(2,5-bis-O-tert-butyldimethylsilyl-beta-D-ribofuranosyl)-2-N-(N,N-dibenzylformamidino)guanine (3a), was prepared in 60% yield after recycling of the undesired 3',5'-bis-O-protected byproduct (4a) by simple equilibration in methanol to a mixture of the two bis-O-protected compounds. Thus, protected, the 3'-position was manipulated to form the 3'-deoxyribo- or 3'-deoxyxylo-3'-azido derivatives (9 or 16, respectively). Further selective manipulations provided the cis-5'-monophosphate (3-chlorophenyl)-1,3-propanyl diester prodrugs (HepDirect prodrugs), 15 and 21. These HepDirect prodrugs were demonstrated to activate to their respective NTPs in rat hepatocytes.

Synthesis, nanosizing and in vitro drug release of a novel anti-HIV polymeric prodrug: chitosan-O-isopropyl-5'-O-d4T monophosphate conjugate

A novel approach to improve the antiviral efficacy of nucleoside reverse transcriptase inhibitors (NRTIs) and reduce their side effects was developed by constructing a nanosized NRTI monophosphate-polymer conjugate using d4T as a model NRTI. Firstly, a novel chitosan-O-isopropyl-5'-O-d4T monophosphate conjugate with a phosphoramidate linkage was efficiently synthesized through Atherton-Todd reaction under mild conditions. The anti-HIV activity and cytotoxicity of the polymeric conjugate were evaluated in MT4 cell line. Then the conjugate nanoparticles were prepared by the process of ionotropic gelation between TPP and chitosan-d4T conjugate to improve their delivery to viral reservoirs, and their physicochemical properties were characterized by transmission electron microscopy (TEM), dynamic light scattering (DLS) techniques and X-ray diffraction (XRD). In vitro drug release studies in pH 1.1 and pH 7.4 suggested that both chitosan-d4T conjugate and its nanoparticles prefer to release d4T 5'-(O-isopropyl) monophosphate than free d4T for prolonged periods, which resulted in the enhancement of anti-HIV selectivity of the polymeric conjugate relative to free d4T due to bypassing the metabolic bottleneck of monophosphorylation. Additionally, the crosslinked conjugate nanoparticles can prevent the coupled drug from leaking out of the nanoparticles before entering the target viral reservoirs and provide a mild sustained release of d4T 5'-(O-isopropyl) monophosphate without the burst release. The results suggested that this kind of chitosan-O-isopropyl-5'-O-d4T monophosphate conjugate nano-prodrugs may be used as a targeting and sustained polymeric prodrugs for improving therapy efficacy and reducing side effects in antiretroviral treatment.

Electrospray ionization tandem mass spectrometric characteristics of d4T H-phosphonate and distamycin conjugates

The study of the dissociation of the protonated molecular species [M+H](+) and selected fragment ions allowed proposals for the main fragmentation pathways of the title compounds. The main fragments are formed by expelling a molecule of thymine, thymidine (d4T) or isopropyl. The most striking feature of the tandem mass (MS/MS) spectra is the cleavage of C-CO bonds between N-methylpyrrole and carbonyl groups in the presence of the amidine. Electrospray ionization is proven to be a good method for the structural characterization and identification of these kinds of compounds.