Inhibitors of Nucleotide Biosynthesis as Candidates for a Wide Spectrum of Antiviral Chemotherapy

Emerging and re-emerging viruses have been a challenge in public health in recent decades. Host-targeted antivirals (HTA) directed at cellular molecules or pathways involved in virus multiplication represent an interesting strategy to combat viruses presently lacking effective chemotherapy. HTA could provide a wide range of agents with inhibitory activity against current and future viruses that share similar host requirements and reduce the possible selection of antiviral-resistant variants. Nucleotide metabolism is one of the more exploited host metabolic pathways as a potential antiviral target for several human viruses. This review focuses on the antiviral properties of the inhibitors of pyrimidine and purine nucleotide biosynthesis, with an emphasis on the rate-limiting enzymes dihydroorotate dehydrogenase (DHODH) and inosine monophosphate dehydrogenase (IMPDH) for which there are old and new drugs active against a broad spectrum of pathogenic viruses.

Antiviral Treatment: From Concept to Reality

The initial development of antiviral compounds was slow, with the first clinical antiviral agent not available until the 1960s. Early development was hindered by the lack of understanding of virus life-cycles and the absence of an assay system for antiviral activity. The appearance of the first assay system, the fertilized egg yolk sac, led to the identification of methisazone. This was the first antiviral agent to be used clinically. The improvement in antiviral assay systems, and the start of directed research programmes led to the development of the first antiherpes agents idoxuridine, trifluorothymidine (TFT) and vidarabine. However, all of these agents were associated with significant adverse effects. Antiherpes therapy took a major step forward with the development of the acyclic nucleoside analogue, aciclovir. Aciclovir is much more potent than previous antiherpesvirus agents. Its mode of action results in selectivity for herpesvirus-infected cells, thus significantly reducing the side-effects seen with earlier agents. Because of this, it became the standard therapy for herpes simplex virus type 1 (HSV-1), HSV-2 and varicella zoster virus infections. However, the bioavailability of oral aciclovir is poor, requiring high and frequent doses. This led to the search for better absorbed agents and to the development of two prodrugs, famciclovir and valaciclovir. Both famciclovir and valaciclovir offer much improved bioavailability of the nucleosides penciclovir and aciclovir, compared with aciclovir. Once in the body the conversion of valaciclovir to aciclovir means that the two agents have similar pharmacokinetic properties. Similarly, famciclovir is converted to penciclovir in the body. Penciclovir, when phosphorylated in virus-infected cells, persists within the cell for a much longer period than aciclovir triphosphate. Over the last 10 years we have seen an acceleration in the development of antiviral agents and some major advances in antiviral therapy. Many challenges, however, still lie ahead.

Side-Chain Derivatives of Biologically Active Nucleosides. Part 2: Synthesis and anti-HIV Activity of 5′-C-Methyl Derivatives of 3′-Fluoro-3′-Deoxythymidine

1-(3′-Fluoro-2′,3′,6′-trideoxy-β-D-allofuranosyl)thymine [7] and 1-(3′-fluoro-2′,3′,6′-trideoxy-α-L-talofuranosyl) thymine [8] were synthesized starting from the corresponding 2,3′-anhydro nucleoside derivatives. The fluorine was introduced stereoselectively by opening of the anhydro bridge in the presence of HF/AIF3. The 5′-C-methyl derivatives, [7] and [8], of 3′-fluoro-3′-deoxythymidine (FLT) were evaluated for their inhibitory effect against human immunodeficiency virus type 1 (HIV-1) and type 2 (HIV-2). The compounds [7] and [8] had antiviral activity which was three orders of magnitude lower than the reference compound 3′-fluoro-3′-deoxythymidine. None of the compounds showed appreciable activity against other RNA or DNA viruses at subtoxic concentrations.

Low potency toxins reveal dense interaction networks in metabolism

BACKGROUND

The chemicals of metabolism are constructed of a small set of atoms and bonds. This may be because chemical structures outside the chemical space in which life operates are incompatible with biochemistry, or because mechanisms to make or utilize such excluded structures has not evolved. In this paper I address the extent to which biochemistry is restricted to a small fraction of the chemical space of possible chemicals, a restricted subset that I call Biochemical Space. I explore evidence that this restriction is at least in part due to selection again specific structures, and suggest a mechanism by which this occurs.

RESULTS

Chemicals that contain structures that our outside Biochemical Space (UnBiological groups) are more likely to be toxic to a wide range of organisms, even though they have no specifically toxic groups and no obvious mechanism of toxicity. This correlation of UnBiological with toxicity is stronger for low potency (millimolar) toxins. I relate this to the observation that most chemicals interact with many biological structures at low millimolar toxicity. I hypothesise that life has to select its components not only to have a specific set of functions but also to avoid interactions with all the other components of life that might degrade their function.

CONCLUSIONS

The chemistry of life has to form a dense, self-consistent network of chemical structures, and cannot easily be arbitrarily extended. The toxicity of arbitrary chemicals is a reflection of the disruption to that network occasioned by trying to insert a chemical into it without also selecting all the other components to tolerate that chemical. This suggests new ways to test for the toxicity of chemicals, and that engineering organisms to make high concentrations of materials such as chemical precursors or fuels may require more substantial engineering than just of the synthetic pathways involved.

Potential of acyclic nucleoside phosphonates in the treatment of DNA virus and retrovirus infections

The acyclic nucleoside phosphonates [HPMPC: cidofovir, Vistide; PMEA: adefovir dipivoxil, Hepsera; and PMPA: tenofovir, Viread] have proven to be effective in vitro (cell culture systems) and in vivo (animal models and clinical studies) against a wide variety of DNA virus and retrovirus infections, for example, cidofovir against herpesvirus [herpes simplex virus type 1 and 2, varicella-zoster virus, cytomegalovirus, Epstein-Barr virus, human herpesvirus type 6, 7 and 8), polyoma-, papilloma-, adeno- and poxvirus (variola virus, cowpox virus, vaccinia virus, molluscum contagiosum virus and orf) infections; adefovir against herpesvirus, hepadnavirus [human hepatitis B virus] and retrovirus [HIV type-1 and 2, simian immunodeficiency virus and feline immunodeficiency virus] infections; and tenofovir against both hepadna- and retrovirus infections. Cidofovir has been officially approved for the treatment of cytomegalovirus retinitis in AIDS patients, tenofovir disoproxil fumarate (Viread) for the treatment of HIV infections (i.e., AIDS) and adefovir dipivoxil for the treatment of chronic hepatitis B.

Synthesis and Antimicrobial Activity of Bisphosphonates

Dimethyl [(substitutedphenyl)(6-oxo6λ5dibenzo[ d,f][1,3,2]dioxaphophepin-6-yl)methyl]phosphonates (5a-j) were synthesised through a three step process involving preparation of dimethyl hydroxy(substitutedphenyl)methyl-phosphonates (4a-j) and their reaction with 6-bromodibenzo[ d,f][1,3,2]dioxaphosphepine (2) in dry toluene in the presence of triethylamine at 50–60°C. Tetramethylguanidine (TMG) as a catalyst was found to increase the yields and purity of the products. These compounds were characterised by IR, 1H, 13C, 31P NMR and mass spectral data found to possess higher antimicrobial activity then the standards.

Novel N-3 substituted TSAO-T derivatives: synthesis and anti-HIV-evaluation

Novel derivatives of the anti-HIV-1 agent, TSAO-T, bearing at the N-3 position alkylating groups or photoaffinity labels were prepared and evaluated for their anti-HIV activity. All of these compounds demonstrated pronounced anti-HIV-1 activity and inhibited HIV-1 RT; however, we were unable to detect stable covalent linkages between inhibitor and enzyme. In addition, compounds with an alcohol functional group connected to the N-3 position through a cis or trans double bond have been prepared. These compounds have been useful to study how the conformational restriction of the linker affects in the interaction between the N-3 substituent and the HIV-1 RT enzyme.

Virological pattern in plasma, peripheral blood mononuclear cells and liver tissue and clinical outcome in chronic hepatitis B and C virus coinfection

BACKGROUND

We aim to evaluate in chronic hepatitis B virus-hepatitis C virus (HBV-HCV) coinfection the interplay of these viruses in liver tissue, peripheral blood mononuclear cells (PBMC), and plasma and to analyze the effect on disease course and response to treatment.

METHODS

We enrolled 19 patients with chronic HBV-HCV coinfection, 20 with chronic HCV and 20 with chronic HBV infection at their first liver biopsy, all were naive for antiviral therapy. The patients' plasma, PBMC and liver biopsy samples were tested for HBV DNA and/or HCV RNA by real-time PCR, according to the presence/absence of hepatitis B surface antigen and antibodies against HCV in the serum.

RESULTS

Contemporary presence of HBV DNA and HCV RNA was rare in plasma (5.3% of cases) and PBMC (10.6%), but frequent in liver tissue (52.6%). Of 10 cases circulating only HCV RNA and treated with pegylated interferon (PEG-IFN) plus ribavirin for 12 months, two showed a sustained response, and eight cleared HCV RNA but became HBV-DNA-positive in plasma; these eight had detectable HBV DNA in liver at baseline. One patient, who was plasma HBV-DNA-positive/HCV-RNA-negative at baseline, showed a sustained response after 18 months of PEG-IFN treatment; another, who was plasma HBV-DNA/HCV-RNA-positive at baseline, cleared only HCV RNA during 12 months of PEG-IFN plus ribavirin treatment. Seven cases remained untreated.

CONCLUSION

Despite a reciprocal inhibition in plasma, HBV and HCV frequently coexist in liver tissue, a condition to be taken into consideration when deciding therapy.

Synthesis of Aminopropyl Phosphonate Nucleosides with Purine and Pyrimidine Bases

The synthesis and antiviral evaluation of new acyclic phosphonate nucleosides related to HPMPC (Cidofovir) has been described. These aminopropyl phosphonate nucleosides 1-3 have an amino function within either the acyclic chain (series 2 and 3) or as substituent (series 1). Both purine and pyrimidine nucleotide analogues have been synthesized. In contrast to HPMPC the oxygen analogue of 2c, only a weak antiherpes virus activity could be demonstrated for 2b and 2c.

Improving the Antiviral Efficacy and Selectivity of HIV-1 Reverse Transcriptase Inhibitor TSAO-T by the Introduction of Functional Groups at the N-3 Position

Novel derivatives of the anti-HIV-1 agent, TSAO-T, bearing at the N-3 position a variety of polar, lipophilic, or aromatic groups linked to that position through flexible polymethylene linkers of different length, were prepared and evaluated for their anti-HIV activity. Several compounds (within the series of polar bearing groups) exhibited a 2-6-fold improved antiviral potency with regard to the lead compound, TSAO-T. Moreover, some of the most active N-3 TSAO derivatives retain antiviral activity against the TSAO-T-resistant HIV-1 strain (Glu138 --> Lys). Interestingly, the N-methylcarboxamide derivative 17 was 5- to 6-fold more active (IC50: 0.56 microM) against recombinant HIV-1 reverse transcriptase than the lead compound, thus becoming the most active TSAO derivative synthesized so far. On the other hand, the N-3 methylcarboxamide TSAO derivative 12 turned out to have the highest selectivity index yet reported for this class of compounds (around > or =12 000).

Inhibition of Cyclooxygenase 2 Synthesis SuppressesHerpes simplexVirus Type 1 Reactivation

Recurrent herpes virus infection, in which the virus reactivates from the nervous system and causes painful lesions in peripheral tissues, is a significant clinical problem. Our recent studies showing that the amount of cyclooxygenase 2 (COX-2) in the trigeminal ganglia of heat-stressed untreated mice is higher than the amount in heat-stressed mice treated with the COX-2 inhibitor, celecoxib, have indicated that the prostaglandin synthesis pathway--and in particular COX-2--may be an intermediate in the pathway to herpes viral reactivation. To further study this process, we infected the corneas of mice using topical application to a lightly scratched epithelium and waited 30 days for Herpes simplex virus type 1 (HSV-1) latency to be established in the trigeminal ganglia. Prior to the induction of viral reactivation, the mice were treated orally with celecoxib. Treated and untreated mice were induced to undergo reactivation by immersion in 43 degrees C water for 10 min. The shedding of virus at the ocular surface was determined by culturing ocular swabs with indicator cells. The presence of infectious virus in the trigeminal ganglion was evaluated by incubating ganglion homogenates with indicator cells and observing for cytopathic effect. Celecoxib treatment significantly suppressed viral reactivation when given prophylactically by the gastrointestinal route. The numbers of corneas and ganglia containing infectious virus were significantly lower in the celecoxib-treated animals, compared to the placebo-treated mice. These experiments demonstrate that a selective COX-2 inhibitor can suppress hyperthermic stress-induced herpes viral reactivation in the nervous system. It may be possible to use COX-2 inhibitors to prevent viral reactivation in high-risk patients by drug prophylaxis.

Clinical potential of the acyclic nucleoside phosphonates cidofovir, adefovir, and tenofovir in treatment of DNA virus and retrovirus infections

The acyclic nucleoside phosphonates HPMPC (cidofovir), PMEA (adefovir), and PMPA (tenofovir) have proved to be effective in vitro (cell culture systems) and in vivo (animal models and clinical studies) against a wide variety of DNA virus and retrovirus infections: cidofovir against herpesvirus (herpes simplex virus types 1 and 2 varicella-zoster virus, cytomegalovirus [CMV], Epstein-Barr virus, and human herpesviruses 6, 7, and 8), polyomavirus, papillomavirus, adenovirus, and poxvirus (variola virus, cowpox virus, vaccinia virus, molluscum contagiosum virus, and orf virus) infections; adefovir against herpesvirus, hepadnavirus (human hepatitis B virus), and retrovirus (human immunodeficiency virus types 1 [HIV-1] and 2 [HIV-2], simian immunodeficiency virus, and feline immunodeficiency virus) infections; and tenofovir against both hepadnavirus and retrovirus infections. Cidofovir (Vistide) has been officially approved for the treatment of CMV retinitis in AIDS patients, tenofovir disoproxil fumarate (Viread) has been approved for the treatment of HIV infections (i.e., AIDS), and adefovir dipivoxil (Hepsera) has been approved for the treatment of chronic hepatitis B. Nephrotoxicity is the dose-limiting side effect for cidofovir (Vistide) when used intravenously (5 mg/kg); no toxic side effects have been described for adefovir dipivoxil and tenofovir disoproxil fumarate, at the approved doses (Hepsera at 10 mg orally daily and Viread at 300 mg orally daily).

Comparison of HSV-1 thymidine kinase-dependent and -independent inhibition of replication-competent adenoviral vectors by a panel of drugs

Replication-competent adenoviral vectors hold the promise to be more efficient gene delivery vehicles than their replication-deficient counterparts, but they are also associated with a higher risk for adverse effects, especially in light of the fact that there is no established effective therapy for serious, disseminated adenovirus infection. To assess whether the therapeutic options to inhibit adenoviral replication can be enhanced by expressing a suicide gene, we examined the antiadenoviral effects of 15 drugs against wild-type adenovirus type 5 (Ad5) and an Ad5-based replication-competent vector expressing herpes simplex virus-1 thymidine kinase (HSV-tk) (Ad.OW34) using a real-time polymerase chain reaction -based assay and flow cytometry. Ad5 and Ad.OW34 were highly susceptible to the fluorinated pyrimidine analogs 5-fluoro-2'-deoxyuridine (FUdR), 5-fluorouridine (FUR), and trifluorothymidine (TFT), with a mean 50% inhibitory concentration (IC(50)) ranging from 0.12 to 0.32 microM. The mean IC(50) of ribavirin and cidofovir (CDV) for Ad5, the most frequently used drugs to treat adenovirus disease, was 6.87 and 3.19 microM, respectively. In contrast to Ad5, the Ad.OW34 vector was susceptible to (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVdU, IC(50) 0.09 microM), ganciclovir (GCV, IC(50) 0.19 microM), and acyclovir (ACV, IC(50) 32.04 microM). Additionally, we demonstrated in an animal model that Ad.OW34 vector replication can be inhibited significantly by GCV, CDV, and TFT by 35.2, 7.7, and 3.7-fold, respectively, compared to untreated animals. The observed antiadenoviral effects were primarily not through cell killing, since the in vitro 50% cytotoxic concentrations (CC(50)) were more than 1000 times higher than the antiadenoviral IC(50) of the drugs examined, even in cells stably expressing HSV-tk. Since for HSV-tk-dependent inhibition of adenoviral vectors, stability of HSV-tk expression is crucial, we examined Ad.OW34 vector stability, by passaging the vector 10 times serially in the presence of 10 microM GCV. The HSV-tk/GCV system neither changed the susceptibility of Ad.OW34 to GCV significantly nor detectable vector rearrangements occurred, suggesting that the system might be suitable as a fail-safe mechanism to stop adenoviral vector replication.

Pharmacokinetics of intravenous acyclovir, zidovudine, and acyclovir-zidovudine in pregnant rats

The pharmacokinetics and placental transfer of acyclovir and zidovudine monotherapies and acyclovir-zidovudine combination therapy were compared in the pregnant rat. Timed-pregnancy Sprague-Dawley rats were used for the study. Doses of 60 mg of each drug/kg of body weight in monotherapy and in combination therapy were given by intravenous bolus, and samples of maternal plasma, amniotic fluid, fetal tissue, and placental tissue were collected over a period of 8 h postdose. Concentrations of each drug in the various matrices were measured by high-performance liquid chromatography. All data were analyzed by using WinNonlin. A one-compartment model with first-order elimination was used to fit the AZT plasma data from the combination therapy rats, but the plasma data from the other groups were fit to a two-compartment model. Tissue data were analyzed by noncompartmental analysis to generate area-under-the-concentration-time-curve values. Implementation of the combination therapy altered the pharmacokinetics of each drug compared to its monotherapy pharmacokinetics. The combination of these two drugs may potentiate fetal and amniotic fluid exposures to each drug.

Inhibition of inosine monophosphate dehydrogenase (IMPDH) by the antiviral compound, 2-vinylinosine monophosphate

A new enzyme-mediated synthesis of 2-vinylinosine, a compound with broad-spectrum RNA antiviral activity, is described. In order to understand the mechanism of action of this compound, we synthesized its monophosphate and investigated the behavior of that compound toward the enzyme, inosine monophosphate dehydrogenase (IMPDH), a key enzyme involved in the biosynthesis of nucleotides. 2-Vinylinosine monophosphate is a potent inhibitor of IMPDH with a K(i) of 3.98 microM (k(inact)=2.94 x 10(-2) s(-1)). The antiviral activity of 2-vinylinosine may be explained by its cellular conversion to the monophosphate through the sequential action of PNP and HGPRT and subsequent inhibition of IMPDH by the cellularly produced 2-vinylinosine 5'-monophosphate.

A new class of acyclic nucleoside phosphonates: synthesis and biological activity of 9-[[(phosphonomethyl)aziridin-1-yl]methyl]guanine (PMAMG) and analogues

A new class of acyclic nucleoside phosphonates PMAMG, PMAMA, PMAMC, and PMAMT (compounds 1, 2, 3 and 4) have been synthesized and tested in vitro against a wide variety of viruses, fungi and bacteria. PMAMG (1) was synthesized by the alkylation reaction of acetylguanine with the phosphonate side-chain, diisopropyl [[2-(bromomethyl)aziridin-1-yl]]methylphosphonate (9), followed by deesterification reaction in the presence of TMSBr. In similar way, PMAMA, PMAMC, and PMAMT were prepared.

Separation methods for acyclovir and related antiviral compounds

Acyclovir (ACV) is an antiviral drug, which selectively inhibits replication of members of the herpes group of DNA viruses with low cell toxicity. Valaciclovir (VACV), a prodrug of ACV is usually preferred in the oral treatment of viral infections, mainly herpes simplex virus (HSV). Also other analogues such as ganciclovir and penciclovir are discussed here. The former acts against cytomegalovirus (CMV) in general and the latter against CMV retinitis. The action mechanism of these antiviral drugs is presented briefly here, mainly via phosphorylation and inhibition of the viral DNA polymerase. The therapeutic use and the pharmacokinetics are also outlined. The measurement of the concentration of acyclovir and related compounds in biological samples poses a particularly significant challenge because these drugs tend to be structurally similar to endogenous substances. The analysis requires the use of highly selective analytical techniques and chromatography methods are a first choice to determine drug content in pharmaceuticals and to measure them in body fluids. Chromatography can be considered the procedure of choice for the bio-analysis of this class of antiviral compounds, as this methodology is characterised by good specificity and accuracy and it is particularly useful when metabolites need to be monitored. Among chromatographic techniques, the reversed-phase (RP) HPLC is widely used for the analysis. C18 Silica columns from 7.5 to 30 cm in length are used, the separation is carried out mainly at room temperature and less than 10 min is sufficient for the analysis at 1.0-1.5 ml/min of flow-rate. The separation methods require an isocratic system, and various authors have proposed a variety of mobile phases. The detection requires absorbance or fluorescence measurements carried out at 250-254 nm and at lambdaex=260-285 nm, lambdaem=375-380 nm, respectively. The detection limit is about 0.3-10 ng/ml but the most important aspect is related to the sample treatment, mainly when body fluids are under examination. The plasma samples obtained from human blood are pre-treated with an acid or acetonitrile deproteinization and the supernatant after centrifugation is successively extracted before RP-HPLC injection. Capillary Electrophoresis methods are also discussed. This new analytical approach might be the expected evolution, in fact the analyses are improved with regard to time and performance, in particular coated capillary as well as addition of stabilisers have been employed. The time of analysis is shortened arriving at less than half a minute. Furthermore by using an electrochemical detection, and having a calibration linearity in the range of 0.2-20.0 ng/ml, the detection limit is 0.15 microg/ml. The measurements of acyclovir and penciclovir have been presented but in the future other related drugs will probably be available using CE methods.

Novel carbocyclic nucleosides containing a cyclobutyl ring: adenosine analogues

(1S,1'R)-cis-1-(3'-aminomethyl-2',2'-dimethylcyclobutyl)ethanol (1) and (1S,1'R)-cis-1-[3'-(2-aminoethyl)-2',2'-dimethylcyclobutyl]ethanol (2) were used as precursors in the synthesis of cyclobutyl nucleoside analogues containing adenine and 8-azaadenine moieties, which were tested as antiviral and antitumoral agents in a variety of assay systems. Compounds 8 and 9 displayed significant activity against respiratory syncytial virus, and compounds 14 and 15 were moderately active against vaccinia virus.

Genetic risks of antiviral nucleoside analogues--a survey

The available informations on the genotoxic effects in experimental systems of the antiherpesvirus nucleosides aciclovir, penciclovir, ganciclovir, brivudine and cidofovir as well as of the antiretrovirals zidovudine (AZT), lamivudine, zalcitabine (ddC), didanosine and stavudine are reviewed. Furthermore, data on carcinogenic activity of these drugs in laboratory rodents are compiled. Most nucleoside analogue antivirals induce chromosomal aberrations but are inactive in gene mutation assays. Carcinogenicity findings in mice and rats are variable but clearly positive for AZT and ddC. The possible mechanisms by which these agents may cause damage in the genetic information are still largely hypothetical, and experimental findings do not permit relevant extrapolations to the situation in man. There is no conclusive evidence that any of the drugs caused tumours in humans. The use of nucleoside analogues in antiviral therapy remains a pragmatic option that seems justified by risk/benefit assessment.

Thienothiadiazine 2,2-dioxide acyclonucleosides: synthesis and antiviral activity

The synthesis of acyclonucleosides derived from thieno[3,2-c] and thieno[2,3-c][1,2,6]thiadiazine 2,2-dioxides was achieved following the silylation method. Lipase-mediated methodology was employed for deprotection of the acyclic moieties. The antiviral effects were determined against a broad spectrum of viruses, including cytomegalovirus (CMV) and varicella zoster virus (VZV). Only minor antiviral activity against VZV was observed for those acyclonucleosides carrying a benzyl group.

Broad-spectrum antiviral activity of the IMP dehydrogenase inhibitor VX-497: a comparison with ribavirin and demonstration of antiviral additivity with alpha interferon

The enzyme IMP dehydrogenase (IMPDH) catalyzes an essential step in the de novo biosynthesis of guanine nucleotides, namely, the conversion of IMP to XMP. The major event occurring in cells exposed to competitive IMPDH inhibitors such as ribavirin or uncompetitive inhibitors such as mycophenolic acid (MPA) is a depletion of the intracellular GTP and dGTP pools. Ribavirin is approved as an inhaled antiviral agent for treatment of respiratory syncytial virus (RSV) infection and orally, in combination with alpha interferon (IFN-alpha), for the treatment of chronic hepatitis C virus (HCV) infection. VX-497 is a potent, reversible uncompetitive IMPDH inhibitor which is structurally unrelated to other known IMPDH inhibitors. Studies were performed to compare VX-497 and ribavirin in terms of their cytotoxicities and their efficacies against a variety of viruses. They included DNA viruses (hepatitis B virus [HBV], human cytomegalovirus [HCMV], and herpes simplex virus type 1 [HSV-1]) and RNA viruses (respiratory syncytial virus [RSV], parainfluenza-3 virus, bovine viral diarrhea virus, Venezuelan equine encephalomyelitis virus [VEEV], dengue virus, yellow fever virus, coxsackie B3 virus, encephalomyocarditis virus [EMCV], and influenza A virus). VX-497 was 17- to 186-fold more potent than ribavirin against HBV, HCMV, RSV, HSV-1, parainfluenza-3 virus, EMCV, and VEEV infections in cultured cells. The therapeutic index of VX-497 was significantly better than that of ribavirin for HBV and HCMV (14- and 39-fold, respectively). Finally, the antiviral effect of VX-497 in combination with IFN-alpha was compared to that of ribavirin with IFN-alpha in the EMCV replication system. Both VX-497 and ribavirin demonstrated additivity when coapplied with IFN-alpha, with VX-497 again being the more potent in this combination. These data are supportive of the hypothesis that VX-497, like ribavirin, is a broad-spectrum antiviral agent.

Inhibitory effect of cycloSaligenyl-nucleoside monophosphates (cycloSal-NMP) of acyclic nucleoside analogues on HSV-1 and EBV

The in vitro antiviral activity of a new series of cycloSal-pro-nucleotides derived from the acyclic nucleoside analogues aciclovir and penciclovir against herpes simplex virus type 1 (HSV-1), thymidine kinase deficient (TK-) HSV-1, and Epstein-Barr virus (EBV) was evaluated. Using the XTT-based tetrazolium reduction assay EZ4U, the cycloSal derivatives were examined for their antiviral and cytotoxic effects in HSV-1 as well as HSV-1-TK--infected Vero cells. The anti-EBV activity was assessed by means of an EBV DNA hybridization assay using a digoxigenin-labeled probe specific for the Bam H1-W-fragment of the EBV genome and by measuring viral capsid antigen (VCA) expression in P3HR-1 cells by indirect immunofluorescence. Among the new cycloSal-phosphotriesters the three aciclovir monophosphates proved to be potent and selective inhibitors of HSV-1 replication, EBV DNA synthesis and EB-VCA expression. Of interest is the retention of activity of the aciclovir monophosphates in HSV-1-TK--infected cells. Particularly 3-methyl-cycloSal-aciclovir monophosphate retained the same effectiveness, as compared to the wild type virus strain. In contrast to the aciclovir pro-nucleotides the penciclovir cycloSal-phosphotriesters exhibited at best only a marginal antiviral effect on HSV and EBV replication.

Synthesis of Acyclic Adenine 8,N-Anhydronucleosides

9-(4-Hydroxybutyl)adenine (10) was obtained by reaction of adenine with 4-[(2-tetrahydropyran-2-yl)oxy]butyl chloride (7) in the presence of DBU. 8-Bromo-9-(4-hydroxybutyl)adenine (13) was prepared by bromination of 10 or by alkylation of 8-bromoadenine (11) with 4-bromoethyl acetate followed by methanolysis. Tosylation of compound 13 afforded the 4-tosyloxy derivative 15 which gave on heating with methylamine or cyclopropylamine 6-methyl- (17a) or 6-cyclopropyl-7,8,9,10-tetrahydro-6H-[1,3]diazepino[1,2-e]purin-4-amine (17b), while the reaction with hydrazine afforded 7,8,9,10-tetrahydro-6H-[1,3]diazepino[1,2-e]purine-4,6-diamine (17d). Treatment of compound 13 with thionyl chloride gave 9-(4-chlorobutyl)-8-chloroadenine (18) as the main product which was transformed to 17b, 6-propyl-7,8,9,10-tetrahydro-6H-[1,3]diazepino[1,2-e]purin-4-amine (17c) or 7,8,9,10-tetrahydro-6H-[1,3]diazepino[1,2-e]purin-4-amine (17e) by reaction with cyclopropylamine, propylamine or ammonia, respectively. Compound 17e was quite stable both in acid and alkaline solutions, at room temperature or at 90 °C. Compound 13 was converted to 9-(4-hydroxybutyl)-8-methylaminoadenine (19) by reaction with methylamine. Compound 19 failed to undergo intramolecular cyclization to diazepine 17a on treatment with diphenyl carbonate, bis(4-nitrophenyl) carbonate or 1,1'-carbonyldiimidazole.

Hydroxyurea potentiates the antiherpesvirus activities of purine and pyrimidine nucleoside and nucleoside phosphonate analogs

Hydroxyurea has been shown to potentiate the anti-human immunodeficiency virus activities of 2',3'-dideoxynucleoside analogs such as didanosine. We have now evaluated in vitro the effect of hydroxyurea on the antiherpesvirus activities of several nucleoside analogs (acyclovir [ACV], ganciclovir [GCV], penciclovir [PCV], lobucavir [LBV], (R)-9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine [H2G], and brivudin and nucleoside phosphonate analogs (cidofovir [CDV] and adefovir [ADV]). When evaluated in cytopathic effect (CPE) reduction assays, hydroxyurea by itself had little effect on CPE progression and potentiated in a subsynergistic (herpes simplex virus type 1 [HSV-1]) to synergistic (HSV-2) fashion the antiviral activities of ACV, GCV, PCV, LBV, H2G, ADV, and CDV. Hydroxyurea also caused marked increases in the activities of ACV, GCV, PCV, LBV, and H2G (compounds that depend for their activation on a virus-encoded thymidine kinase [TK]) against TK-deficient (TK(-)) HSV-1. In fact, in combination with hydroxyurea the 50% effective concentrations of these compounds for inhibition of TK(-) HSV-1-induced CPE decreased from values of 20 to > or = 100 microg/ml (in the absence of hydroxyurea) to values of 1 to 5 microg/ml (in the presence of hydroxyurea at 25 to 100 microg/ml). When evaluated in a single-cycle virus yield reduction assay, hydroxyurea at a concentration of 100 microg/ml inhibited progeny virus production by 60 to 90% but had little effect on virus yield at a concentration of 25 microg/ml. Under these assay conditions hydroxyurea still elicited a marked potentiating effect on the antiherpesvirus activities of GCV and CDV, but this effect was less pronounced than that in the CPE reduction assay. It is conceivable that the potentiating effect of hydroxyurea stems from a depletion of the intracellular deoxynucleoside triphosphate pools, thus favoring the triphosphates of the nucleoside analogues (or the diphosphates of the nucleoside phosphonate analogues) in their competition with the natural nucleotides at the viral DNA polymerase level. The possible clinical implications of these findings are discussed.

Imidazothiadiazine dioxides: synthesis and antiviral activity

A new series of imidazothiadiazine dioxides, including the first acyclonucleosides derived from this heterocycle moiety, has been synthesized. A wide-spectrum antiviral screening was performed. Some of the N-1 benzyl imidazothiadiazines and the new acyclonucleosides showed interesting anti-CMV or anti-HIV activity. These structures could be considered as new lead compounds for antiviral drug research.

The immunosuppressive agent mycophenolate mofetil markedly potentiates the activity of lobucavir [1R(1alpha,2beta,3alpha)]-9-[2,3-bis(hydroxymethyl)cyclobutyl]guanine against different herpes viruses

BACKGROUND

Mycophenolate mofetil (MMF) has been approved as an immunosuppressive agent in kidney transplant recipients and may thus be used concomitantly with antiherpetic agents, the latter for the treatment of intercurrent herpesvirus infections. The parent compound of MMF, mycophenolic acid (MPA), is a potent inhibitor of inosine monophosphate dehydrogenase and causes depletion of the intracellular (deoxy)guanosine triphosphate [(d)GTP] pools. Lobucavir [1R(1alpha,2beta,3alpha)]-9-[2,3-bis(hydroxymethyl)cyc lobutyl]guanine (LBV) is a novel antiviral agent with activity against ganciclovir-resistant cytomegalovirus (CMV) strains, that is in phase II clinical trials for the treatment of CMV infections. LBV triphosphate inhibits the viral DNA polymerase competitively with dGTP. We present the results of our studies on the antiviral effects of the combinations LBV + MMF and LBV + MPA.

METHODS

The antiviral effects of LBV either alone or in combination with MMF or MPA on the replication of CMV, herpes simplex virus type- (HSV) 1 (HSV-1), HSV-2, and thymidine kinase-deficient HSV-1 were studied by means of plaque or CPE reduction assays.

RESULTS

When combined with LBV, MPA (at concentrations ranging from 0.25 to 10 microg/ml, which are readily attainable in human plasma) markedly potentiated the antiviral efficacy of LBV against HSV-1 and HSV-2, that is a 10- to 100-fold decrease in EC50. Moreover, the EC50 of LBV against TK- HSV-1 decreased up to 1400-fold upon combination with MPA. MPA by itself had little or no effect on the replication of these viruses. Moreover, MPA and MMF resulted in a marked increase in the anti-CMV activity of LBV minimal FIC (FICmin: 0.24 and 0.26, respectively). Exogenously added guanosine reversed the potentiating effect of MPA on the antiviral activity of LBV, which indicates that this potentiating effect results from a depletion of the endogenous dGTP pools, thus favoring the inhibitory action of the LBV-triphosphate on the viral DNA polymerase. Ribavirin, another inhibitor of inosine monophosphate-dehydrogenase, also caused a marked enhancement of the antiviral activity of LBV against HSV-1 (12-fold), HSV-2 (20-fold), and TK- HSV-1 (25-fold).

CONCLUSION

MMF markedly potentiates the activity of LBV against HSV-1, HSV-2, TK- HSV-1, and CMV. This drug interaction may have important implications when using LBV in the treatment of intercurrent herpesvirus infections in transplant recipients under MMF therapy.

Comparative study of the anti-human cytomegalovirus activities and toxicities of a tetrahydrofuran phosphonate analogue of guanosine and cidofovir

Cidofovir is the first nucleoside monophosphate analogue currently being used for the treatment of human cytomegalovirus (HCMV) retinitis in individuals with AIDS. Unfortunately, the period of therapy with the use of this compound may be limited due to the possible emergence of serious irreversible nephrotoxic effects. New drugs with improved toxicity profiles are needed. The goal of this study was to investigate the anticytomegaloviral properties and drug-induced toxicity of a novel phosphonate analogue, namely, (-)-2-(R)-dihydroxyphosphinoyl-5-(S)-(guanin-9'-yl-methyl) tetrahydrofuran (compound 1), in comparison with those of cidofovir. The inhibitory activities of both compounds on HCMV propagation in vitro were similar against the AD 169 and Towne strains, with 50% inhibitory concentrations ranging from 0.02 to 0.17 microgram/ml for cidofovir and < 0.05 to 0.09 microgram/ml for compound 1. A clinical HCMV isolate that was resistant to ganciclovir and that had a known mutation within the UL54 DNA polymerase gene and a cidofovir-resistant laboratory strain derived from strain AD 169 remained sensitive to compound 1, whereas their susceptibilities to ganciclovir and cidofovir were reduced by 33- and 10-fold, respectively. Both compound 1 and cidofovir exhibited equal potencies in an experimentally induced murine cytomegalovirus (MCMV) infection in mice, with a prevention or prolongation of mean day to death at dosages of 1.0, 3.2, and 10.0 mg/kg of body weight/day. In cytotoxicity experiments, compound 1 was found to be generally more toxic than cidofovir in cell lines Hs68, HFF, and 3T3-L1 (which are permissive for HCMV or MCMV replication) but less toxic than cidofovir in MRC-5 cells (which are permissive for HCMV replication). Drug-induced toxic side effects were noticed for both compounds in rats and guinea pigs in a 5-day repeated-dose study. In guinea pigs, a greater weight loss was noticed with cidofovir than with compound 1 at dosages of 3.0 and 10.0 mg/kg/day. An opposite effect was detected in rats, which were treated with the compounds at relatively high dosages (up to 100 mg/kg/day). Compound 1 and cidofovir were nephrotoxic in both rats and guinea pigs, with the epithelium lining the proximal convoluted tubules in the renal cortex being the primary target site. The incidence and the severity of the lesions were found to be dose dependent. The lesions observed were characterized by cytoplasm degeneration and nuclear modifications such as karyomegaly, the presence of pseudoinclusions, apoptosis, and degenerative changes. In the guinea pig model, a greater incidence and severity of lesions were observed for cidofovir than for compound 1 (P < 0.001) with a drug regimen of 10 mg/kg/day.

The antiherpesvirus activity of H2G [(R)-9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine] is markedly enhanced by the novel immunosuppressive agent mycophenolate mofetil

Mycophenolate mofetil (MMF) has been approved as an immunosuppressive agent in kidney transplant recipients and may thus be used concomitantly with antiherpetic agents, which are used for the treatment of intercurrent herpesvirus infections. We have recently demonstrated that MMF and its parent compound mycophenolic acid (MPA), which is a potent inhibitor of IMP dehydrogenase, potentiate the antiherpesvirus activity of acyclovir, ganciclovir, and penciclovir. We have now evaluated the antiviral efficacy of the combination of MPA and the novel antiherpesvirus agent H2G [(R)-9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine]. When combined with H2G, MPA (at concentrations ranging from 0.25 to 10 microgram/ml, which are readily attainable in human plasma) markedly potentiated the antiviral efficacy of H2G against herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2), as reflected by a 10- to 150-fold decrease in the 50% effective concentration. Moreover, the activity of H2G against a thymidine kinase-deficient strain of HSV-1 (TK- HSV-1) was increased more than 2,500-fold when combined with MPA. MPA by itself had little or no effect on the replication of these viruses. Similar observations were made for varicella-zoster virus. Also, ribavirin (another inhibitor of IMP dehydrogenase) caused a marked enhancement of the activity of H2G against HSV-1 (10-fold), HSV-2 (10-fold), and TK- HSV-1 (>185-fold). Exogenously added guanosine reversed the potentiating effects of MPA on the antiviral activity of H2G, indicating that this potentiating effect resulted from a depletion of the endogenous dGTP pools, thus favoring the inhibitory action of the H2G triphosphate on the viral DNA polymerase.

Virus infections of the eye

In reviewing the clinical features, diagnostic evaluations and therapies of the most common ocular viral infections we attempt to whet your appetite for attacking the numerous challenges in diagnosis and treatment of viral eye disease. The herpes viruses, HSV, VZV and CMV are the cause of significant ocular morbidity. HSV most commonly affects the cornea producing keratitis that can be recurrent and may lead to corneal clouding and neovascularisation. Manifestations can be purely infectious or immunological and treatment options must be tailored to the underlying pathophysiology. Herpes zoster ophthalmicus, caused by VZV infection of the first branch of the trigeminal nerve, produces a characteristic rash and can progress to keratitis and uveitis. HSV and VZV can cause retinitis in both immunocompetent and immunocompromised individuals. There has been a significant increase in the incidence of CMV retinitis since the beginning of the AIDS epidemic. We review the numerous new treatments, diagnostic tests and treatment strategies which have been developed in response to this potentially blinding retinal infection. Adenovirus produces an epidemic conjunctivitis and epidemic keratoconjunctivitis which are severe and extremely contagious conjunctival infections. HIV, molluscum contagiosum, EBV and rubeola also cause ocular diseases which are described.Copyright 1998 John Wiley & Sons, Ltd.

Advances in the treatment of chronic hepatitis B virus infection

Recent advances in our understanding of the replicative mechanism of HBV, and the development of potent nucleoside analogues as clinically effective inhibitors of the HIV reverse transcriptase or herpesvirus polymerases has opened a new era in the treatment of chronic HBV infection. Single agent therapies, such as famciclovir, lamivudine or lobucavir, have had some success. There is now a logical basis for combination therapy, because of the clear need for prolonged treatment and the associated possibility that drug resistant strains will emerge with monotherapy, but the choice of agents to combine and the regimens in which they should be employed remain uncertain.Copyright 1998 John Wiley & Sons, Ltd.

Novel carbocyclic nucleosides containing a cyclobutyl ring. Guanosine and adenosine analogues

(1R,cis)-2-(3-Amino-2,2-dimethylcyclobutyl)ethanol (4) was used as a precursor in the synthesis of cyclobutyl nucleoside analogues containing guanine, 8-azaguanine, adenine or 8-azaadenine. All the compounds were evaluated as antiviral agents in a variety of assay systems. Some activity was noted for compound 13, 17, 19 and 20 against vaccinia virus and for compounds 11, 12, 13, 17, 19 and 20 against herpes simplex virus, at concentrations that were up to 10-fold below the cytotoxic concentrations for the host cells.

The novel immunosuppressive agent mycophenolate mofetil markedly potentiates the antiherpesvirus activities of acyclovir, ganciclovir, and penciclovir in vitro and in vivo

The immunosuppressive agent mycophenolate mofetil (MMF) has been approved for use in kidney transplant recipients and may thus be used concomitantly for the treatment of intercurrent herpesvirus infections with drugs such as acyclovir (ACV), ganciclovir (GCV), and penciclovir (PCV). We found that MMF and its parent compound mycophenolic acid (at concentrations that are attainable in plasma) strongly potentiate the antiherpesvirus (herpes simplex virus [HSV] type 1 [HSV-1], HSV-2, thymidine kinase-deficient [TK-] HSV-1, both wild-type and TK- varicella-zoster virus, and human cytomegalovirus) activities of ACV, PCV, and GCV (up to 350-fold increases in their activities). The mechanism of potentiation was found to reside in the depletion of endogenous dGTP pools, which favored the inhibitory effect of the triphosphate of ACV, GCV, or PCV on the viral DNA polymerase. The combination of topically applied 5% MMF with 0.1% ACV strongly protected against HSV-1-induced cutaneous lesions in hairless mice, whereas therapy with either compound used singly had no protective effect. Interestingly, the combination of topically applied 5% MMF with 5% ACV was also highly effective in protecting against TK- HSV-2-induced cutaneous lesions (that were refractory to ACV treatment) in athymic nude mice. Topical therapy with MMF was very well tolerated, and no signs of irritation were observed. When given perorally at 200 mg/kg of body weight/day, MMF potentiated to some extent the growth retardation induced by GCV in young NMRI mice. These observations may have clinical implications (i) for those transplant recipients who receive both MMF and either ACV, GCV, or PCV and (ii) for the treatment of ACV-resistant mucocutaneous HSV infections.

Enzymatic synthesis of 2'-O-acyl prodrugs of 1-(beta-D-arabinofuranosyl)-5(E)-(2-bromovinyl)uracil and of 2'-O-acyl-araU, -araC and -araA

Pig liver esterase (EC 3.1.1.1) catalysed regioselective hydrolysis of 1-(2,3,5-tri-O-acyl-beta-D-arabinofuranosyl)uracil, -cytosine and -adenine to give the corresponding 2'-monoesters effectively and in high yield. This methodology enabled the preparation of 1-(2-O-acyl-beta-D-arabinofuranosyl)-5-[(E)-(2-bromovinyl)]uracil prodrugs which, although slightly less active than the parent 1-(beta-D-arabinofuranosyl)-5-(E)-(2 bromovinyl)uracil (sorivudine; BV-araU), were strongly active in vitro against varicella-zoster virus (ED50 2.4-45 ng/ml). The retarded rates of enzymatic hydrolysis of the 2'-esters imply that they might function as lipophilic prodrugs, leading to increased plasma and cellular concentrations. In view of the marked in vitro activity, they represent an interesting approach to arabinofuranosyl nucleoside prodrugs with improved pharmacokinetics and enzymatic stability.

In search of a selective antiviral chemotherapy

This article describes several approaches to a selective therapy of virus infections: (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU [brivudin]) for the therapy of herpes simplex virus type 1 and varicella-zoster virus infections: (S)-9-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine (HPMPC [cidofovir]) for the therapy of various DNA virus (i.e., herpesvirus, adenovirus, papillomavirus, polyomavirus, and poxvirus) infections; 9-(2-phosphonylmethoxyethyl)adenine (PMEA [adefovir]) for the therapy of retrovirus, hepadnavirus, and herpesvirus infections; (R)-9-(2-phosphonylmethoxypropyl)adenine (PMPA) for the therapy and prophylaxis of retrovirus and hepadnavirus infections; and nonnucleoside reverse transcriptase inhibitors (NNRTIs), such as tetrahydroimidazo[4,5,1-jk][1,4]-benzodiazepin-2(IH)-one and -thione (TIBO), 1-[(2-hydroxyethoxy)methyl]-6-(phenylthio)thymine (HEPT), alpha-anilinophenylacetamide (alpha-APA), and 2',5'bis-O-(tert-butyldimethylsilyl)-3'-spiro-5"-(4"-amino-1",2"-oxat hiole- 2",2"-dioxide)pyrimidine (TSAO) derivatives, and thiocarboxanilides for the treatment of human immunodeficiency virus type 1 (HIV-1) infections. For the clinical use of NNRTIs, some guidelines have been elaborated, such as starting treatment with combinations of different compounds at sufficiently high concentrations to effect a pronounced and sustained suppression of the virus. Despite the diversity of the compounds described here and the different viruses at which they are targeted, they have a number of characteristics in common. As they interact with specific viral proteins, the compounds achieve a selective inhibition of the replication of the virus, which, in turn, should be able to develop resistance to the compounds. However, as has been established for the NNRTIs, the problem of viral resistance may be overcome if the compounds are used from the start at sufficiently high doses, which could be reduced if different compounds are combined. For HIV infections, drug treatment regimens should be aimed at reducing the viral load to such an extent that the risk for progression to AIDS will be minimized, if not avoided entirely. This may result in a real "cure" of the disease but not necessarily of the virus infection, and in this sense, HIV disease may be reduced to a dormant infection, reminiscent of the latent herpesvirus infections. Should virus replication resume after a certain time, the armamentarium of effective anti-HIV and anti-herpesvirus compounds now available, if applied at the appropriate dosage regimens, should make the virus return to its dormant state before it has any chance to damage the host. It is unlikely that this strategy would eradicate the virus and thus "cure" the viral infection, but it definitely qualifies as a cure of the disease.

Cytogenetic genotoxicity of antiherpes virostatics in Chinese hamster V79-E cells. I. Purine nucleoside analogues

The antiherpes virostatics acyclovir (ACV), valaciclovir (VACV), penciclovir (PCV), famciclovir (FCV) and ganciclovir (GCV), which belong to the group of purine acyclic nucleoside analogues, were tested for clastogenic and sister chromatid exchange (SCE)-inducing activity in Chinese hamster V79-E cells upon chronic application with and without a recovery period. ACV induced borderline effects in both cytogenetic assays, a dose-dependent reduction of the mitotic index and an increasing cell cycle delay. With VACV and PCV only a decrease of the mitotic index and an increase of cell cycle delay were observed. FCV was negative with respect to the four parameters studied, presumably due to the incapacity of the target cells of metabolizing FCV to PCV. GCV was a very potent genotoxin in both assays. It induced a statistically significant SCE response even in the range of the cytomegalovirus IC50 of < 10 microM. By variation of the experimental protocol it was shown that SCEs are induced in the second cell cycle following exposure to GCV but not in the first one. It is assumed that the drugs under study are metabolized to their respective triphosphates and then inhibit DNA replication as detected by decreasing mitotic index and increasing cell cycle delay. In the case of GCV it is suggested that GCV-TP is incorporated into the target cell DNA and that chromosomal aberrations and SCEs are secondary lesions due to repair processes at the substituted template.