Antiviral Activity Spectrum and Target of Action of Different Classes of Nucleoside Analogues

New workflow predicts drug targets against SARS-CoV-2 via metabolic changes in infected cells

COVID-19 is one of the deadliest respiratory diseases, and its emergence caught the pharmaceutical industry off guard. While vaccines have been rapidly developed, treatment options for infected people remain scarce, and COVID-19 poses a substantial global threat. This study presents a novel workflow to predict robust druggable targets against emerging RNA viruses using metabolic networks and information of the viral structure and its genome sequence. For this purpose, we implemented pymCADRE and PREDICATE to create tissue-specific metabolic models, construct viral biomass functions and predict host-based antiviral targets from more than one genome. We observed that pymCADRE reduces the computational time of flux variability analysis for internal optimizations. We applied these tools to create a new metabolic network of primary bronchial epithelial cells infected with SARS-CoV-2 and identified enzymatic reactions with inhibitory effects. The most promising reported targets were from the purine metabolism, while targeting the pyrimidine and carbohydrate metabolisms seemed to be promising approaches to enhance viral inhibition. Finally, we computationally tested the robustness of our targets in all known variants of concern, verifying our targets' inhibitory effects. Since laboratory tests are time-consuming and involve complex readouts to track processes, our workflow focuses on metabolic fluxes within infected cells and is applicable for rapid hypothesis-driven identification of potentially exploitable antivirals concerning various viruses and host cell types.

Advances in Therapeutic L-Nucleosides and L-Nucleic Acids with Unusual Handedness

Nucleic-acid-based small molecule and oligonucleotide therapies are attractive topics due to their potential for effective target of disease-related modules and specific control of disease gene expression. As the non-naturally occurring biomolecules, modified DNA/RNA nucleoside and oligonucleotide analogues composed of L-(deoxy)riboses, have been designed and applied as innovative therapeutics with superior plasma stability, weakened cytotoxicity, and inexistent immunogenicity. Although all the chiral centers in the backbone are mirror converted from the natural D-nucleic acids, L-nucleic acids are equipped with the same nucleobases (A, G, C and U or T), which are critical to maintain the programmability and form adaptable tertiary structures for target binding. The types of L-nucleic acid drugs are increasingly varied, from chemically modified nucleoside analogues that interact with pathogenic polymerases to nanoparticles containing hundreds of repeating L-nucleotides that circulate durably in vivo. This article mainly reviews three different aspects of L-nucleic acid therapies, including pharmacological L-nucleosides, Spiegelmers as specific target-binding aptamers, and L-nanostructures as effective drug-delivery devices.

Profiling Ribonucleotide and Deoxyribonucleotide Pools Perturbed by Remdesivir in Human Bronchial Epithelial Cells

Remdesivir (RDV) has generated much anticipation for its moderate effect in treating severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. However, the unsatisfactory survival rates of hospitalized patients limit its application to the treatment of coronavirus disease 2019 (COVID-19). Therefore, improvement of antiviral efficacy of RDV is urgently needed. As a typical nucleotide analog, the activation of RDV to bioactive triphosphate will affect the biosynthesis of endogenous ribonucleotides (RNs) and deoxyribonucleotides (dRNs), which are essential to RNA and DNA replication in host cells. The imbalance of RN pools will inhibit virus replication as well. In order to investigate the effects of RDV on cellular nucleotide pools and on RNA transcription and DNA replication, cellular RNs and dRNs concentrations were measured by the liquid chromatography-mass spectrometry method, and the synthesis of RNA and DNA was monitored using click chemistry. The results showed that the IC50 values for BEAS-2B cells at exposure durations of 48 and 72 h were 25.3 ± 2.6 and 9.6 ± 0.7 μM, respectively. Ten (10) μM RDV caused BEAS-2B arrest at S-phase and significant suppression of RNA and DNA synthesis after treatment for 24 h. In addition, a general increase in the abundance of nucleotides and an increase of specific nucleotides more than 2 folds were observed. However, the variation of pyrimidine ribonucleotides was relatively slight or even absent, resulting in an obvious imbalance between purine and pyrimidine ribonucleotides. Interestingly, the very marked disequilibrium between cytidine triphosphate (CTP) and cytidine monophosphate might result from the inhibition of CTP synthase. Due to nucleotides which are also precursors for the synthesis of viral nucleic acids, the perturbation of nucleotide pools would block viral RNA replication. Considering the metabolic vulnerability of endogenous nucleotides, exacerbating the imbalance of nucleotide pools imparts great promise to enhance the efficacy of RDV, which possibly has special implications for treatment of COVID-19.

2020 update on human coronaviruses: One health, one world

Since human coronavirus (HCoVs) was first described in the 1960s, seven strains of respiratory human coronaviruses have emerged and caused human infections. After the emergence of severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV), a pneumonia outbreak of coronavirus disease 2019 (COVID-19) caused by a novel coronavirus (SARS-CoV-2) has represented a pandemic threat to global public health in the 21st century. Without effectively prophylactic and therapeutic strategies including vaccines and antiviral drugs, these three coronaviruses have caused severe respiratory syndrome and high case-fatality rates around the world. In this review, we detail the emergence event, origin and reservoirs of all HCoVs, compare the differences with regard to structure and receptor usage, and summarize therapeutic strategies for COVID-19 that cause severe pneumonia and global pandemic.

Clinical profile, genotype and management updates of hepatitis B virus

Hepatitis B virus (HBV) is a well known agent of acute and chronic hepatitis, liver cirrhosis and hepatocellular carcinoma. Around 400 million people worldwide carrier of HBV of which more than 250 million reside in Asia, and 1-2 million people have died from it. It has a partially double-stranded DNA, having 3.2-kb genome size and replicate via reverse transcription of RNA intermediate. In the natural history or during the antiviral therapy of chronic HBV infection, seroconversion from HBeAg to anti-HBeAg is usually accompanied by a decrease in viral replication and remission of liver disease. Based on genomic sequence data HBV is classified into eight genotypes A-H and four major serotypes ayw, ayr, adw and adr on the basis of complete genome and S gene sequence analysis. Genotypes and serotypes are useful tools in understanding the epidemiology of HBV infection. HBV genotypes have distinct geographical distributions. The HBV variants appear during HBeAg seroconversion and they bring mutations in the precore region (PC) that prevent HBeAg synthesis. Another common HBeAg variant is the basal core promoter mutant (BCP) characterized by point mutation in the promoter of both HBeAg mRNA and core protein mRNA. The most frequent core promoter mutation is the double A1762T and G1764A nucleotide exchange, which results in a substantial decrease in HBeAg expression but enhanced viral genome replication. The approved antiviral drugs such as Interferon, lamivudine, adefovir dipivoxil, entecavir and telbivudine for purpose of treating chronic HBV infection is to prevent or stop the progression of liver injury by suppressing viral replication or eliminating infection. Sustained losses of viral markers of active viral replication (HBeAg and HBV DNA) are the standard end point of the therapies.

Synthesis of ester prodrugs of 9-(S)-[3-hydroxy-2-(phosphonomethoxy)propyl]-2,6-diaminopurine (HPMPDAP) as anti-poxvirus agents

9-(S)-[3-Hydroxy-2-(phosphonomethoxy)propyl]-2,6-diaminopurine (HPMPDAP) and its cyclic form were selected for further evaluation as potential drug candidates against poxvirus infections. To increase bioavailability of these compounds, synthesis of their structurally diverse ester prodrugs was carried out: alkoxyalkyl (hexadecyloxypropyl, octadecyloxyethyl, hexadecyloxyethyl), pivaloyloxymethyl (POM), 2,2,2-trifluoroethyl, butylsalicylyl, and prodrugs based on peptidomimetics. Most HPMPDAP prodrugs were synthesized in the form of monoesters as well as the corresponding cyclic phosphonate esters. The activity was evaluated not only against vaccinia virus but also against different herpes viruses. The most potent and active prodrugs against vaccinia virus were the alkoxyalkyl ester derivatives of HPMPDAP, with 50% effective concentrations 400-600-fold lower than those of the parent compound. Prodrugs based on peptidomimetics, the 2,2,2-trifluoroethyl, the POM, and the butylsalicylyl derivatives, were able to inhibit vaccinia virus replication at 50% effective concentrations that were equivalent or ∼10-fold lower than those observed for the parent compounds.

Synthesis of some novel hydrazono acyclic nucleoside analogues

The syntheses of novel hydrazono acyclic nucleosides similar to miconazole scaffolds are described. In this series of acyclic nucleosides, pyrimidine as well as purine and other azole derivatives replaced the imidazole function in miconazole and the ether group was replaced with a hydrazone moiety using phenylhydrazine. To interpret the dominant formation of (E)-hydrazone derivatives rather than (Z)-isomers, PM3 semiempirical quantum mechanic calculations were carried out which indicated that the (E)-isomers had the lower heats of formation.

Carbocyclic thymidine analogues for use as potential therapeutic agents

The discovery of azidothymidine's (AZT) activity against human immunodeficiency virus (HIV) provided strong rationale for the design of additional thymidine analogues. One drawback of many nucleoside analogues is the toxicity that often arises due to phosphorylation by cellular kinases. In order to overcome this problem, a number of truncated nucleosides lacking the 4'-hydroxymethyl group have been synthesized. In that regard, the synthesis and preliminary biological results for two truncated carbocyclic thymidine analogues are presented herein.

Carbocyclic pyrimidine nucleosides as inhibitors of S-adenosylhomocysteine hydrolase

The design, synthesis, and unexpected inhibitory activity against S-adenosyl-homocysteine (SAH) hydrolase (SAHase, EC 3.3.1.1) for a series of truncated carbocyclic pyrimidine nucleoside analogues is presented. Of the four nucleosides obtained, 10 was found to be active with a Ki value of 5.0 microM against SAHase.

The current management of HBV drug resistance

Lamivudine and adefovir have potent inhibitory effects on hepatitis B virus (HBV) replication. Although short-term therapy is feasible in a selected subgroup of patients, prolonged therapy is required for sustained suppression in the majority of patients. However, HBV species with mutations in the tyrosine-methionine-aspartate-aspartate (YMDD) locus of the HBV RNA-dependent DNA polymerase (rtM204 I/V) conferring resistance to lamivudine may emerge after 9-10 months therapy. The incidence increases with duration of therapy up to 70% after 5 years lamivudine therapy. Serum alanine aminotransferase (ALT) elevation and HBV DNA rebound ( I log) occur in >90% of the patients with rtM204 I/V during continued lamivudine therapy. Marked flare of serum ALT may occur, sometimes severe, and may be complicated with decompensation or even fatality. The initial clinical and histologic improvement may also reverse after emergence of rtM204 IN Studies do suggest that stopping seems better than continuing lamivudine therapy. Limited data show that interferon therapy seems ineffective. Add-on adefovir therapy is effective in suppressing rtM204 I/V in both compensated and decompensated patients. Switching to adefovir monotherapy is effective and safe even in cirrhotic patients with decompensation. The overall incidence of adefovirresistant rtN236T and A181V is low, being 0 after one year and 5.9% after 3 years' therapy. The impact of adefovir resistance is less clear but is responsive to lamivudine therapy. In conclusion, monitoring of viral breakthrough to start effective intervention is mandatory during direct antiviral therapy.

A method for quantitating the intracellular metabolism of AZT amino acid phosphoramidate pronucleotides by capillary high-performance liquid chromatography-electrospray ionization mass spectrometry

A methodology has been developed for the analysis of the intracellular metabolism of 3'-azido-3'-deoxythymidine (AZT) amino acid phosphoramidates utilizing reverse-phase high-performance liquid chromatography interfaced with negative ion electrospray ionization mass spectrometry (LC/ESI(-) -MS). The presented work demonstrates the potential of capillary LC/MS and LC/MS/MS to identify and quantitate the cellular uptake and metabolism of nucleoside phosphoramidate. Significant intracellular amounts of D- and L-phenylalanine methyl ester or D- and L-tryptophan methyl ester AZT phosphoramidates were observed for human T-lymphoblastoid leukemia (CEM) cells incubated for 2 and 4 h with the prodrugs. AZT-MP was the primary metabolite observed for human T-lymphoblastoid leukemia (CEM) cells. In this paper, the details of using LC/MS to analyze AZT amino acid phosphoramidates in biological samples are discussed. LC/MS is an efficient method for analyzing multiple samples containing several analytes in a short period of time. The method also provides high selectivity and sensitivity, and requires minimal sample preparation. This approach should be broadly applicable for the analysis of the intracellular metabolism of nucleoside prodrugs and pronucleotides.

Synthesis of azole nucleoside 5'-monophosphate mimics (P1Ms) and their inhibitory properties of IMP dehydrogenases

IMPDH inhibitors have potential antimicrobial, anticancer and immunomodulatory effects. Nucleoside inhibitors of IMPDH exert their inhibitory effects via nucleoside 5'-MPs. Conversion of nucleoside analogs to NMPs by cellular nucleoside kinases is not assured, and usually is inefficient. In order to bypass cellular phosphorylation, a series of azole nucleoside 5'-MP mimics (P1Ms) based on ribavirin, EICAR and bredinin were synthesized and screened against human and C. albicans IMP dehydrogenises. P1Ms 8, 16, 25, 28 and 29 demonstrated substantial IMPDH inhibition with Ki values in low micromolar range.

Monitoring the intracellular metabolism of nucleoside phosphoramidate pronucleotides by 31P NMR

The intracellular metabolism of 3'-azido-3'-deoxythymidine (AZT)-(L)-tryptophan methyl ester phosphoramidate (L-ATO) and AZT-(L)-phenylalanine methyl ester phosphoramidate (L-APO) by the human T-lymphoblastoid cell line CCRF-CEM (CEM-1.3) and peripheral blood mononuclear cell line (PBMC) was investigated with high field 31P NMR spectroscopy. The AZT amino acid phosphoramidates were shown to accumulate intracellularly and to be readily converted into AZT-MP by both tissues types. Thus, the efficient delivery of nucleoside monophosphates to cells can be facilitated by nucleoside phosphoramidate pronucleotides.

Chemoenzymatic Synthesis and Synthetic Application of Enantiopure Aminocyclopentenols: Total Synthesis of Carbocyclic (+)-Uracil Polyoxin C and Its α-Epimer

Carbocyclic uracil polyoxin C (+)-2 and its alpha-epimer (-)-3 were synthesized in an efficient fashion from cis-4-(N-tert-butylcarbamoyl)cyclopent-2-en-1-ol (+/-)-7. The synthesis incorporates a concise, inexpensive chemoenzymatic synthesis of enantiopure aminocyclopentenols, a Pd(0)-catalyzed substitution reaction, and a mild reduction of an alpha-nitro ester by TiCl(3)/sodium borohydride. Significantly, this process demonstrates the synthetic utility of the versatile enantiopure aminocyclopentenol building block (-)-4.

Unexpected inhibition of S-adenosyl-L-homocysteine hydrolase by a guanosine nucleoside

A series of shape-modified flexible nucleosides ('fleximers', 1, 2, and 3) was modeled, synthesized and subsequently assayed against S-adenosyl-L-homocysteine hydrolase (SAHase). No inhibitory activity was observed for the adenosine fleximer, which served as a substrate, but moderate inhibitory activity was exhibited by the guanosine fleximers. This is the first known report of a guanosine nucleoside analogue possessing activity against SAHase.

Difficulties in the deprotection of 1,2-ketals in nucleosides containing alkylidencarbazoyl groups

The synthesis of unprotected alkylidencarbazoyl nucleoside derivatives 8a-8d is shown. A direct deprotection route from readily available 2',3'-isopropylidene protected nucleosides 5a-5d. prepared from a chemoenzymatic procedure, did not give the selective cleavage of the ketal function without affecting the C=N bond. The next option tried was to look at the previous compound in the retrosynthetic route: 2',3'-protected carbazoyl nucleoside 4. However, in all cases we obtained unsatisfactory results. Stepping further back, the hydrolysis of compound 3a led us to unprotected carbonate nucleoside 9 in quantitative yield. With this compound available, the synthesis towards derivatives 8 was accomplished through a known procedure.

Stereoselective synthesis of a novel apio analogue of neplanocin A as potential S-adenosylhomocysteine hydrolase inhibitor

A total synthesis of apio-neplanocin A, which combines properties of apio nucleoside and neplanocin A and is a potential inhibitor of S-adenosylhomocysteine hydrolase, was accomplished starting from D-ribose via stereoselective hydroxymethylation and RCM reaction. [reaction: see text]

Comparative analysis of DNA breakage, chromosomal aberrations and apoptosis induced by the anti-herpes purine nucleoside analogues aciclovir, ganciclovir and penciclovir

Nucleoside analogues have been used in antiviral therapy and suicide cancer gene therapy. Therefore, it is of importance to compare their potential cytotoxic and genotoxic action. Using metabolically competent CHO cells expressing the thymidine kinase gene of herpes simplex virus type 1 (CHO-HSVtk cells) as a model system, the induction of DNA breaks was compared with the induction of structural chromosomal aberrations and apoptosis/necrosis after exposure to the anti-herpes nucleoside analogues aciclovir (ACV), ganciclovir (GCV) and penciclovir (PCV). After continuous treatment of CHO-HSVtk cells with the drugs, LD(10) in a colony-forming assay was 50, 0.5 and 1 microM for ACV, GCV and PCV, respectively, with GCV to be the most potent agent as determined at a given dose level. There was a remarkable difference in the activity of the agents to kill HSVtk expressing and non-expressing cells: the difference in cellular sensitivity of HSVtk(+) versus HSVtk(-) cells at LD(10) level was 7-fold for ACV, 60-fold for GCV and 400-fold for PCV. The drugs were shown to be strong inducers of apoptosis that was analysed as to concentration- and time-dependence; they induced to only very low extent necrosis. The agents were also highly potent in the induction of DNA single-strand breaks (SSBs) and double-strand breaks (DSBs) (as measured by single cell gel electrophoresis (SCGE)) and chromosomal aberrations. Although PCV induced DNA DSBs with a kinetics and frequency similar to that of GCV, it caused mostly condensation defects instead of "typical" structural chromosomal aberrations. For the drugs used, the frequency of apoptotic cells and the induction of abnormal mitoses appear to be related indicating genotoxic effects induced by the agents to be involved in cell killing due to apoptosis.

Preparation of 4'-substituted thymidines by substitution of the thymidine 5'-esters

tert-Butyl thymidylate 3 was prepared from thymidine 1 in six steps and 67% overall yield. When the lithium trianion of 3 (prepared by treatment of 3 with excess LDA and then excess tert-butyllithum) is reacted with electrophiles, trapping occurs stereoselectively from either the alpha- or beta-face depending on the electrophile (Scheme 1). Deuterioacetic acid in deuteriomethanol affords mainly the alpha-deuterated product (4a/4b = 2.4:1) while all other electrophiles, e.g., phenylselenenyl chloride, allyl bromide, and N-fluorobenzenesulfonimide (NFSI), give predominately (or completely) the products of attack from the beta-face (5bcd/4bcd = 3.7:1 to 100:0). The structures of the products were determined by coupling constant analysis of both the initial compounds and the diols 6bcd prepared by ester reduction and by formation of the acetonides 7bc. The methyl ester of the 3'-epimer of thymidylic acid 9 was also prepared from thymidine 1 in nine steps and 74% overall yield. When the lithium trianion of 9 (prepared by treatment of 9 with excess LDA and then excess tert-butyllithum) is reacted with electrophiles, trapping also occurs stereoselectively with attack on either the alpha- or beta-face depending on the electrophile (Scheme 2). Again, deuterioacetic acid in deuteriomethanol affords mainly the beta-deuterated product (11a/10a = 1.6:1) while all other electrophiles, e.g., phenylselenenyl chloride, methyl iodide, allyl bromide, and NFSI, gave predominately (or completely) the product of attack from the alpha-face (8.7:1 to 100: 0). Again, the structures of the products were determined by coupling constant analysis of both the initial compounds, and the diols 12b-e were prepared by reduction of the ester and by formation of the acetonides 13bcd. A rationale has been developed using molecular mechanics calculations to explain the diastereoselectivity that involves staggered axial attack on the sp(2) carbon opposite to the pseudoaxial alkoxy group in the most stable half-chair conformation of the enolates, as shown in Schemes 3-5.

Vaccinia virus inhibitors as a paradigm for the chemotherapy of poxvirus infections

Poxviruses continue to pose a major threat to human health. Monkeypox is endemic in central Africa, and the discontinuation of the vaccination (with vaccinia virus) has rendered most humans vulnerable to variola virus, the etiologic agent of smallpox, should this virus be used in biological warfare or terrorism. However, a large variety of compounds have been described that are potent inhibitors of vaccinia virus replication and could be expected to be active against other poxviruses as well. These compounds could be grouped in different classes: (i) IMP dehydrogenase inhibitors (e.g., EICAR); (ii) SAH hydrolase inhibitors (e.g., 5'-noraristeromycin, 3-deazaneplanocin A, and various neplanocin A derivatives); (iii) OMP decarboxylase inhibitors (e.g., pyrazofurin) and CTP synthetase inhibitors (e.g., cyclopentenyl cytosine); (iv) thymidylate synthase inhibitors (e.g., 5-substituted 2'-deoxyuridines); (v) nucleoside analogues that are targeted at viral DNA synthesis (e.g., Ara-A); (vi) acyclic nucleoside phosphonates [e.g., (S)-HPMPA and (S)-HPMPC (cidofovir)]; and (vii) polyanionic substances (e.g., polyacrylic acid). All these compounds could be considered potential candidate drugs for the therapy and prophylaxis of poxvirus infections at large. Some of these compounds, in particular polyacrylic acid and cidofovir, were found to generate, on single-dose administration, a long-lasting protective efficacy against vaccinia virus infection in vivo. Cidofovir, which has been approved for the treatment of cytomegalovirus retinitis in immunocompromised patients, was also found to protect mice, again when given as a single dose, against a lethal aerosolized or intranasal cowpox virus challenge. In a biological warfare scenario, it would be advantageous to be able to use a single treatment for an individual exposed to an aerosolized poxvirus. Cidofovir thus holds great promise for treating human smallpox, monkeypox, and other poxvirus infections. Anecdotal experience points to the efficacy of cidofovir in the treatment of the poxvirus infections molluscum contagiosum and orf (ecthyma contagiosum) in immunosuppressed patients.

Chronic hepatitis B virus infection: treatment strategies for the next millennium

Chronic hepatitis B virus (HBV) infection is a leading cause of cirrhosis and hepatocellular carcinoma worldwide. Its prevalence approaches 10% in hyperendemic areas, such as southeast Asia, China, and Africa. Although chronic HBV infection is seen less frequently in North America and Europe, an estimated 1.25 million persons in the United States are infected. In the past decade, revolutionary strides have been made toward the treatment of chronic HBV infection. Interferon-alpha was once the only available therapy but has recently been joined by the nucleoside analogues, the most extensively studied of which is lamivudine. Interferon therapy continues to have a role in the treatment of a carefully selected group of patients. Lamivudine therapy, which has less stringent selection criteria, suppresses HBV DNA in almost all treated patients: Seventeen percent to 33% experience loss of hepatitis B e antigen, and 53% to 56% have a histologic response. Extended lamivudine treatment leads to the development of a specific lamivudine-resistant virus with base-pair substitutions at the YMDD locus of the DNA polymerase. Newer nucleoside analogues and other immunomodulator therapies are being investigated. In the future, combination therapy with different classes of agents may yield improved response rates and delay the development of resistance.

Synthesis and anti-HIV evaluation of 2',3'-dideoxy imidazo- and nu-triazolo[4,5-d]pyridazine nucleosides

The syntheses of the 2'-deoxy and 2',3'-dideoxynucleosides of 2,8-diaza-3-deazainosine and the 2',3'-dideoxynucleoside of 2-aza-3-deazainosine were achieved and the pathways leading to these novel nucleosides are described. The preparation of the 2',3'-dideoxynucleoside (1) of 2-aza-3-deazainosine involved deoxygenation of the 2'-deoxy-3'-imidazolide intermediate with n-Bu3SnH and AlBN. The latter nucleoside was synthesized from the known 2'-deoxy derivative of 2-aza-3-deazainosine. The three-step synthesis of 1 from the 2'-deoxy analogue was accomplished in 40% overall yield. Rather than synthesize the corresponding 2',3'-dideoxynucleoside (2) of 2,8-diaza-3-deazainosine in the same manner, i.e. deoxygenation of the 2'-deoxynucleoside, a more cost-effective route was chosen. This pathway involved reductive cleavage of the 5'-protected, 2',3'-thiocarbonate derivative to furnish a mixture of the 2'- and 3'-deoxy isomers. This mixture was not separated, but was deoxygenated by the aforementioned imidazolide method. Using this methodology, 2 was prepared in 23% overall yield from 2,8-diaza-3-deazainosine. Nucleosides 1 and 2 were evaluated for antiretroviral activity and were found to be inactive.

Biphasic clearance kinetics of hepatitis B virus from patients during adefovir dipivoxil therapy

In a recent phase II clinical study, 13 chronic hepatitis B-infected patients treated daily with 30 mg adefovir dipivoxil for 12 weeks displayed a median 4.1-log10 decrease in plasma hepatitis B virus (HBV)-DNA levels. The decline of viral load during therapy displayed a biphasic kinetic profile that was modeled to determine the efficacy of inhibition of viral production, as well as kinetic constants for the clearance of free virus and the loss of infected cells. Viral production was suppressed with an efficacy of 0.993 +/- 0.008, indicating that only 0.7% of viral production persisted during therapy. The initial, faster phase of viral load decline reflects the clearance of HBV particles from plasma with a half-life of 1.1 +/- 0.3 days, translating to a 48% daily turnover of the free virus. The second, slower phase of viral load decline closely mirrors the rate-limiting process of infected cell loss, with a half-life of 18 +/- 7 days. The duration of therapy required to completely eliminate the virus from plasma or suppress it to levels sufficient to induce seroconversion is a function of the half-life of the free virus, the half-life of infected cells, and the efficacy of inhibition of virus production from infected cells. These quantitative analyses provide a more detailed picture of the dynamics of HBV infection and therapy, and can be used to compare the efficacy of various doses and inhibitors of HBV replication for the treatment of HBV infections.

Synthesis and biological evaluation of a carbocyclic azanoraristeromycin siderophore conjugate

Synthesis and biological evaluation of a carbocyclic azanoraristeromycin siderophore conjugate 22 is reported. Coupling of previously prepared L-alanyl-4'-azanoraristeromycin 19 with protected tripeptide trihydroxamate 20, followed by hydrogenolytic removal of all protecting groups, provided the first carbocylic azanoraristeromycin siderophore conjugate (22, 8 with iron). Compounds 19 and 22 showed inhibitory activity against tumor cells, and conjugate 22, in particular, displayed significant activity against those viruses (i.e. reo, parainfluenza, vaccinia, cytomegalo) that are known to be inhibited by S-adenosylhomocysteine hydrolase inhibitors.

Treatment of chronic hepatitis B: new antiviral therapies

Chronic hepatitis B infection is the most important cause of cirrhosis and hepatocellular carcinoma worldwide. Interferon-alpha has been shown to be effective in approximately one third of patients, and response seems to be sustained in long-term follow-up studies in Western countries. New treatments using lamivudine and other nucleoside analogues such as famciclovir, lobucavir, and adfovir showed promising results although sustained suppression of viral replication is unusual after discontinuation of therapy. The results of recent clinical studies using these nucleoside analogues are discussed in detail in this review. Other important issues such as drug resistance and the role of combination therapy are also addressed.

Mutations in hepatitis B DNA polymerase associated with resistance to lamivudine do not confer resistance to adefovir in vitro

To determine whether adefovir is active against lamivudine-resistant hepatitis B virus (HBV), the inhibition constants of adefovir diphosphate and lamivudine triphosphate for wild-type and mutant human HBV DNA polymerases, which contain amino acid substitutions associated with lamivudine resistance, were compared. Recombinant wild-type and mutant human HBV DNA polymerases were expressed and substantially purified using a baculovirus expression system and immunoaffinity chromatography. HBV DNA polymerase mutants M552I, M552V, and L528M/M552V showed resistance to lamivudine triphosphate with inhibition constants (Ki) increased by 8.0-fold, 19.6-fold, and 25.2-fold compared with that of wild-type HBV DNA polymerase. However, these mutants remained sensitive to adefovir diphosphate with the inhibition constants increasing by 1.3-fold and 2.2-fold or decreasing by 0.79-fold. The L528M single mutation, identified in patients with increasing HBV DNA levels during therapy with famciclovir, also remained sensitive to adefovir diphosphate with the inhibition constant increased by only 2.3-fold.

Respiratory syncytial virus (RSV) disease and prospects for its control

Respiratory syncytial virus (RSV) is a major virus pathogen of infants and young children, an important cause of disease in adults and is responsible for a significant amount of excess morbidity and mortality in the elderly. It also can be devastating in immunosuppressed populations. Vaccines are being developed, but none are currently licensed. Moreover, even if one or more are approved, they may not be suitable for some populations vulnerable to RSV (e.g. very young infants and the immunosuppressed). Ribavirin and immunoglobulin preparations with high titers of RSV-specific neutralizing antibodies are currently approved for use to treat and prevent RSV infection. However, neither of these is cost-effective or simple to administer. New agents are needed to reduce the impact of RSV. This review is concerned with the means currently available for controlling RSV, the search for new agents effective against this virus, and future prospects for preventing and treating RSV infections.

New antiherpetic nucleoside from a Basidiomycete

Antiviral activity was characterized from the culture broth of the Basidiomycete Macrocystidia cucumis (Pers. ex Fr.) Heim. When the stationary phase was reached (21 d), the culture broth was shown through an ELISA assay to contain antiviral activity against herpes simplex virus type 1 (HSV-1), as assessed in baby hamster kidney cells (BHK-21). Once the presence of the anti-HSV-1 activity in the culture broth was demonstrated, we proceeded with the purification and isolation of the active principle using a semi-preparative HPLC technique. The activity was associated with a purine nucleoside designated McA. This compound displayed no cytotoxicity at antivirally effective concentrations and proved to be a novel nucleoside analogue.