L-nucleosides: antiviral activity and molecular mechanism

Drug discovery for antiviral chemotherapy has provided the effective treatment of numerous viral diseases. Among antiviral agents used in therapy, nucleoside analogues have been particularly useful. In fact, almost twenty nucleosides are currently used in antiviral therapy, seven of which are for the treatment of HIV infection. In the search for new and effective agents within this class, the focus has recently expanded on L-analogues, characterized by opposite configuration compared to the natural D-nucleosides. The interest in L-nucleosides has risen since the discovery of 3TC, one of the most important drugs used in the treatment of AIDS and hepatitis B infection. This review will discuss the latest advances in L-nucleosides as antiviral agents with a particular focus on the synthesis and molecular mechanism as well as metabolic differences between L- and D-nucleosides.

Determination of acyclovir in maternal plasma, amniotic fluid, fetal and placental tissues by high-performance liquid chromatography

Acyclovir [9-[(2-hydroxyethoxy)-methyl]-guanosine, Zovirax, ACV] is a synthetic purine nucleoside analog active against herpes simplex virus types 1 (HSV-1), 2 (HSV-2), and varicella zoster virus. Acyclovir has frequently been used in HSV-2 seropositive mothers to prevent prenatal transmission of herpes virus to their unborn children. A fast and reproducible HPLC method for the determination of the highly polar acyclvoir in maternal rat plasma, amniotic fluid, placental tissue, and fetal tissue has been developed and validated. Plasma and amniotic fluid samples were prepared by protein precipitation using 2 M perchloric acid and syringe filtering. Tissue samples were homogenized in distilled water, centrifuged, and extracted using a C(18) solid-phase extraction method prior to analysis. Baseline resolution was achieved for acyclovir and the internal standard gancyclovir, an anti-viral of similar structure to acyclovir, using an Agilent Eclipse XDB C(8) column (150 x 2.1 mm, 5 microm). The mobile phase used for the plasma and amniotic fluid was 10 mM acetate/citrate buffer-3.7 mM aqueous octanesulfonic acid (87.5:12.5, v/v) at a flow-rate of 0.2 ml/min. The mobile phase used for the tissue samples was 30 mM acetate/citrate buffer with 5 mM octanesulfonic acid-acetonitrile (99:1, v/v). Both aqueous mobile phase portions were pH adjusted to 3.08. All separations were done using an Agilent 1100 Series HPLC system with UV detection of 254 nm. The assay was validated for each matrix over a range of 0.25-100 microg/ml over 3 days using five replicates of three spiked concentrations. The relative standard deviation and percent error for each validation data set was <15% for middle and high quality control (QC) points and <20% for all low QC points. All calibration curves showed good linearity with an R(2)>0.99. The extraction efficiency for recovery of acyclovir from all matrices was >80%.

Insights into the molecular mechanism of inhibition and drug resistance for HIV-1 RT with carbovir triphosphate

Abacavir (1592U89, or Ziagen) is a powerful and selective inhibitor of HIV-1 viral replication that has been approved by the FDA for treatment of acquired immunodeficiency syndrome. Abacavir is metabolized to the active compound carbovir triphosphate (CBVTP). This compound is a guanosine analogue containing a 2',3'-unsaturation in its planar carbocyclic deoxyribose ring that acts on HIV-1 reverse transcriptase (RT(WT)) as a molecular target, resulting in chain termination of DNA synthesis. A single amino acid change from methionine 184 to valine in HIV-1 RT (RT(M184V)) has been observed clinically in response to abacavir treatment. The ability of the natural substrate, dGTP, or CBVTP to be utilized during DNA- and RNA-directed polymerization by RT(WT) and RT(M184V) was defined by pre-steady-state kinetic parameters. In the case of RT(WT), CBVTP was found to be a surprisingly poor substrate relative to dGTP. In both DNA- and RNA-directed polymerization, a decrease in the efficiency of CBVTP utilization with respect to dGTP was found with RT(M184V), suggesting that this mutation confers resistance at the level of CBVMP incorporation. The relatively low incorporation efficiency for RT(WT) was unanticipated considering earlier studies showing that the triphosphate form of a thymidine nucleoside analogue containing a planar 2',3'-unsaturated ribose ring, D4TTP, was incorporated with high efficiency relative to the natural substrate, dTTP. The difference may be related to the isosteric replacement of oxygen in the deoxyribose ring with carbon. This hypothesis was tested by synthesizing and evaluating D4GTP (the planar 2',3'-unsaturated deoxyribose guanosine analogue that is complementary to D4TTP). In contrast to CBVTP, D4GTP was found to be an excellent substrate for RT(WT) and no resistance was conferred by the M184V mutation, thus providing novel insight into structure-activity relationships for nucleoside-based inhibitors. In this work, we illustrate how an understanding of the molecular mechanism of inhibition and drug resistance led to the discovery of a novel prodrug of D4G. This compound shows promise as a potent antiviral especially with the drug resistant M184V HIV-1 RT that is so often encountered in a clinical setting.

Synthesis of L-oxetanocin

[reaction: see text] Hitherto unknown L-oxetanocin has been synthesized from L-xylose in 16 steps via a ribonolactone derivative.

Advances in antiviral agents for hepatitis B virus

Hepatitis B virus (HBV) is the third most common disease after venereal diseases and chickenpox. HBV currently infects 2 billion people in the world, of which 350 million are chronic carriers. At least 1 million chronically infected individuals die each year due to HBV-related diseases, especially cirrhosis and liver cancer. The greatest concern about the diffusion of this virus is in endemic regions in central and southern Africa, South-East Asia and South America, where neonatal exposure results in high mortality rates. Anti-HBV therapy has made important progresses in the last decade, with two approved drugs and a number of other potent agents in the pharmaceutical industry pipeline. Nevertheless, resistance and viral rebound are still major issues in devising a winning strategy, and there is a continuous need of developing new active compounds, as well as therapeutic protocols based on combination therapy and a prophylactic approach. This review will summarize the latest advances in anti-HBV therapy, with particular regard to the latest clinical data on the most significant anti-HBV agents. Issues such as viral resistance and combination therapy will be highlighted.

Structure-activity relationships of 2'-fluoro-2',3'-unsaturated D-nucleosides as anti-HIV-1 agents

We studied the structure-activity relationships of a series of 2'-fluoro-2',3'-unsaturated D-nucleosides against HIV-1 in human peripheral blood mononuclear (PBM) cells. The target compounds 10-21 and 28-33 were prepared by N-glycosylation of the acetate 4, which was readily prepared from 2,3-O-isopropylidene-D-glyceraldehyde in five steps. Among the newly synthesized nucleosides, 2-amino-6-chloropurine (11), adenine (14), inosine (16), guanine (18), 2,6-diaminopurine (20), and 5-fluorocytosine (30) derivatives were found to exhibit interesting anti-HIV activities with EC(50) values of 4.3, 0.44, 1.0, 2.6, 3.0, and 0.82 microM, respectively. The implications for drug resistance of the titled nucleosides with respect to lamivudine-resistant variants (M184V) were also examined, and no significant cross-resistance with the variants was observed with the D-series.

Efficient synthesis of 2-deoxy-L-erythro-pentose (2-deoxy-L-ribose) from L-arabinose

An efficient and practical route for the large-scale synthesis of 2-deoxy-L-erythro-pentose (2-deoxy-L-ribose) starting from L-arabinose was developed using Barton-type free-radical deoxygenation reaction as a key step. The radical precursor, a phenoxythiocarbonyl ester, was prepared in situ, and the most efficient deoxygenation was achieved by slow addition of tributyltin hydride to the reaction mixture.

Novel use of a guanosine prodrug approach to convert 2',3'-didehydro-2',3'-dideoxyguanosine into a viable antiviral agent

Transient kinetic studies with human immunodeficiency virus (HIV) type 1 reverse transcriptase suggest that nucleotide analogs containing the 2',3'-didehydro-2',3'-dideoxy ribose ring structure present in D4T (stavudine) triphosphate are among the most effective alternative substrates. For unclear reasons, however, the corresponding purine nucleoside, 2',3'-didehydro-2',3'-dideoxyguanosine (D4G), was found to be inactive in cell culture. We have found that the previously reported lack of activity of D4G is primarily due to solution instability, and in this report we describe a novel use of a guanosine prodrug approach to stabilize the nucleoside. D4G was modified at the 6 position of the purine ring to contain a cyclopropylamino group yielding the prodrug, cyclo-D4G. An evaluation of cyclo-D4G revealed that the prodrug possessed anti-HIV activity. In addition, cyclo-D4G had increased stability, lipophilicity, and solubility, as well as decreased toxicity relative to D4G, suggesting that further study is warranted.

Molecular mechanism of DApd/DXG against zidovudine- and lamivudine- drug resistant mutants: a molecular modelling approach

In order to understand molecular mechanism of antiviral drug resistance of HIV-1 reverse transcriptase (RT) as well as potent antiviral activity of 2,6-diaminopurine dioxolane (DAPD) [prodrug of (-)-beta-D-dioxolane guanine (DXG)] against drug-resistant RTs, molecular modelling studies of three structurally distinct nucleoside RT inhibitor (NRTI)-triphosphates (TP) [zidovudine (AZT)-TP, lamivudine (3TC)-TP and DXG-TP] complexed with the wild-type (WT) and mutated RT were conducted. The computational analyses indicated that the antiviral activity and the calculated relative binding energy of the RT inhibitor triphosphates can be correlated, and the minimized structures gave information on the molecular mechanism of drug resistance conferred by mutations. The interactions between the NRTI-TP and adjacent amino acid residues (Lys65, Lys70, Arg72, Tyr115 and/or Gln151) played important roles in stabilizing the enzyme-inhibitor complex. Particularly, Arg72 was found to stabilize the dioxolane and oxathiolane sugar moiety through hydrogen bonding, which was responsible for favourable binding affinity of DXG-TP to AZT- as well as 3TC-resistant mutants. The conformational changes in these amino acid residues caused by mutation always affected the changes in the tertiary structures of enzyme-inhibitor complexes through either closing or opening the gap between the fingers and palm domains. The enzyme-inhibitor complexes with good binding affinity showed tight binding modes by closing the gap between the two domains, whereas weak inhibitors gave open and loose complexes.

Synthesis and potent anti-HIV activity of L-2',3'-didehydro-2',3'-dideoxy-2'-fluoro-4'-thiocytidine

[reaction: see text] L-2'-Fluoro-4'-thio-2',3'-unsaturated cytidine 11 was synthesized from (R)-2-fluorobutenolide 2, which was prepared from 2,3-O-isopropylidene-L-glyceraldehyde 1. The synthesized compound 11 shows potent antiviral activity against HIV-1.

Synthesis and potent anti-HIV activity of L-3'-fluoro-2',3'-unsaturated cytidine

L-2',3'-Didehydro-2',3'-dideoxy-3'-fluorocytidine (L-3'-Fd4C), a novel potent anti-HIV agent (EC(50) 0.03 microM in PBM cells), has been synthesized from L-xylose in 14 steps. [reaction: see text]

Synthesis of carbocyclic orotidine analogs as potential orotidine decarboxylase inhibitors

An asymmetric synthesis of carbocyclic orotidine 15 and its monophosphate 16 were accomplished via the key intermediate cyclopentanone 4, which was prepared from D-gamma-ribonolactone in steps. None of synthesized the compounds inhibited orotidine 5'-monophosphate decarboxylase (EC 4.1.1.23) or orotate phosphoribosyltransferase (EC 2.4.2.10).

Enantiomeric synthesis of D- and L-cyclopentenyl nucleosides and their antiviral activity against HIV and West Nile virus

Enantiomeric synthesis of D- and L-cyclopentenyl nucleosides and their antiviral activity against HIV and West Nile virus are described. The key intermediate (-)- and (+)-cyclopentenyl alcohols (7 and 15) were prepared from D-gamma-ribonolactone and D-ribose, respectively. Coupling of 7 with appropriately blocked purine and pyrimidine bases via the Mitsunobu reaction followed by deprotection afforded the target L-(+)-cyclopentenyl nucleosides (24-28, 31, 33, and 36). D-(-)-Cyclopentenyl nucleosides (1, 40, 43, and 52-56) were also prepared by a similar procedure for L-isomers from 15. The synthesized compounds were evaluated for their antiviral activity against two RNA viruses: HIV and West Nile virus. Among the synthesized D-(-)-nucleosides, adenine (1, neplanocin A), cytosine (55, CPE-C), and 5-fluorocytosine (56) analogues exhibited moderate to potent anti-HIV activity (EC(50) 0.1, 0.06, and 5.34 microM, respectively) with significant cytotoxicity in PBM, Vero, and CEM cells. Also, cytosine (55) and 5-fluorocytosine (56) analogues exhibited the most potent anti-West Nile virus activity (EC(50) 0.2-3.0 and 15-20 microM, respectively). Among L-(+)-nucleosides, only the cytosine (27) analogue exhibited weak anti-HIV activity (EC(50) 58.9 microM).

L-Nucleosides as chemotherapeutic agents

Nucleoside analogues have been the cornerstone of antiviral therapy over the past thirty years and, currently, 16 commonly used antiviral drugs belong to this category. Although for long time it was believed that only D-nucleosides, possessing a 'natural' stereochemistry, could elicit biological activity, in the last decade this has been proven not to be true. 3TC, a L-nucleoside analogue, is one of the most effective anti-HIV and anti-hepatitis B virus drugs, and nine other L-nucleosides are currently undergoing clinical trials and/or preclinical studies as antiviral or antitumoral agents. This minireview summarizes some biological features and the current status of these promising L-nucleoside analogues.