Novel pyrazolo[3,4-d]pyrimidine nucleoside analog with broad-spectrum antiviral activity

Synthesis and biological studies of pyrimidine derivatives targeting metabolic enzymes

Novel synthesized pyrimidine derivatives were investigated against carbonic anhydrase isoenzymes I and II (hCA I and II), acetylcholinesterase (AChE), butyrylcholinesterase (BChE), α-glycosidase, and aldose reductase (AR) enzymes associated with some common diseases such as epilepsy, glaucoma, Alzheimer's disease, diabetes, and neuropathy. When the results were examined, novel synthesized pyrimidine derivatives were found to have effective inhibition abilities toward the metabolic enzymes. IC50 values and Ki values were calculated for each pyrimidine derivative and compared to positive controls. The synthesized novel pyrimidine derivatives exhibited Ki values in the range of 39.16 ± 7.70-144.62 ± 26.98 nM against hCA I, 18.21 ± 3.66-136.35 ± 21.48 nM toward hCA II, which is associated with different pathological and physiological processes, 33.15 ± 4.85-52.98 ± 19.86 nM on AChE, and 31.96 ± 8.24-69.57 ± 21.27 nM on BChE. Also, Ki values were determined in the range of 17.37 ± 1.11-253.88 ± 39.91 nM against α-glycosidase and 648.82 ± 53.74-1902.58 ± 98.90 nM toward AR enzymes. Within the scope of the study, the inhibition types of the novel synthesized pyrimidine derivatives were evaluated.

SARS-CoV-2 tropism, entry, replication, and propagation: Considerations for drug discovery and development

Since the initial report of the novel Coronavirus Disease 2019 (COVID-19) emanating from Wuhan, China, Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) has spread globally. While the effects of SARS-CoV-2 infection are not completely understood, there appears to be a wide spectrum of disease ranging from mild symptoms to severe respiratory distress, hospitalization, and mortality. There are no Food and Drug Administration (FDA)-approved treatments for COVID-19 aside from remdesivir; early efforts to identify efficacious therapeutics for COVID-19 have mainly focused on drug repurposing screens to identify compounds with antiviral activity against SARS-CoV-2 in cellular infection systems. These screens have yielded intriguing hits, but the use of nonhuman immortalized cell lines derived from non-pulmonary or gastrointestinal origins poses any number of questions in predicting the physiological and pathological relevance of these potential interventions. While our knowledge of this novel virus continues to evolve, our current understanding of the key molecular and cellular interactions involved in SARS-CoV-2 infection is discussed in order to provide a framework for developing the most appropriate in vitro toolbox to support current and future drug discovery efforts.

Discovery of small molecule inhibitors of human uridine-cytidine kinase 2 by high-throughput screening

Clinically relevant inhibitors of dihydroorotate dehydrogenase (DHODH), a rate-limiting enzyme in mammalian de novo pyrimidine synthesis, have strong antiviral and anticancer activity in vitro. However, they are ineffective in vivo due to efficient uridine salvage by infected or rapidly dividing cells. The pyrimidine salvage enzyme uridine-cytidine kinase 2 (UCK2), a ∼29 kDa protein that forms a tetramer in its active state, is necessary for uridine salvage. Notwithstanding the pharmacological potential of this target, no medicinally tractable inhibitors of the human enzyme have been reported to date. We therefore established and miniaturized an in vitro assay for UCK2 activity and undertook a high-throughput screen against a ∼40,000-compound library to generate drug-like leads. The structures, activities, and modes of inhibition of the most promising hits are described. Notably, our screen yielded non-competitive UCK2 inhibitors which were able to suppress nucleoside salvage in cells both in the presence and absence of DHODH inhibitors.

Evaluation of cell viability dyes in antiviral assays with RNA viruses that exhibit different cytopathogenic properties

Studies were conducted to determine the performance of four dyes in assessing antiviral activities of compounds against three RNA viruses with differing cytopathogenic properties. Dyes included alamarBlue® measured by absorbance (ALB-A) and fluorescence (ALB-F), neutral red (NR), Viral ToxGlo™ (VTG), and WST-1. Viruses were chikungunya, dengue type 2, and Junin, which generally cause 100, 80-90, and 50% maximal cytopathic effect (CPE), respectively, in Vero or Vero 76 cells Compounds evaluated were 6-azauridine, BCX-4430, 3-deazaguanine, EICAR, favipiravir, infergen, mycophenolic acid (MPA), ribavirin, and tiazofurin. The 50% virus-inhibitory (EC50) values for each inhibitor and virus combination did not vary significantly based on the dye used. However, dyes varied in distinguishing the vitality of virus-infected cultures when not all cells were killed by virus infection. For example, VTG uptake into dengue-infected cells was nearly 50% when visual examination showed only 10-20% cell survival. ALB-A measured infected cell viability differently than ALB-F as follows: 16% versus 32% (dengue-infected), respectively, and 51% versus 72% (Junin-infected), respectively. Cytotoxicity (CC50) assays with dyes in uninfected proliferating cells produced similar CC50 values for EICAR (1.5-8.9μM) and MPA (0.8-2.5μM). 6-Azauridine toxicity was 6.1-17.5μM with NR, VTG, and WST-1, compared to 48-92μM with ALB-A and ALB-F (P<0.001). Curiously, the CC50 values for 3-deazaguanine were 83-93μM with ALB-F versus 2.4-7.0μM with all other dyes including ALB-A (P<0.001). Overall, ALB minimized the toxicities detected with these two inhibitors. Because the choice of dyes affected CC50 values, this impacted on the resulting in vitro selectivity indexes (calculated as CC50/EC50 ratio).

Broad-spectrum inhibition of common respiratory RNA viruses by a pyrimidine synthesis inhibitor with involvement of the host antiviral response

Our previous screening of 50 240 structurally diverse compounds led to the identification of 39 influenza A virus infection inhibitors (Kao R.Y., Yang D., Lau L.S., Tsui W.H., Hu L. et al. Nat Biotechnol 2010;28:600-605). Further screening of these compounds against common respiratory viruses led to the discovery of compound FA-613. This inhibitor exhibited low micromolar antiviral activity against various influenza A and B virus strains, including the highly pathogenic influenza A strains H5N1 and H7N9, enterovirus A71, respiratory syncytial virus, human rhinovirus A, SARS- and MERS-coronavirus. No significant cellular toxicity was observed at the effective concentrations. Animal studies showed an improved survival rate in BALB/c mice that received intranasal FA-613 treatments against a lethal dose infection of A/HK/415742Md/2009 (H1N1). Further cell-based assays indicated that FA-613 interfer with the de novo pyrimidine biosynthesis pathway by targeting the dihydroorotate dehydrogenase. Surprisingly, FA-613 lost its antiviral potency in the interferon-deficient Vero cell line, while maintaining its inhibitory activity in an interferon-competent cell line which showed elevated expression of host antiviral genes when infected in the presence of FA-613. Further investigation of the specific connection between pyrimidine synthesis inhibition and the induction of host innate immunity might aid clinical development of this type of drug in antiviral therapies. Therefore, in acute cases of respiratory tract infections, when rapid diagnostics of the causative agent are not readily available, an antiviral drug with properties like FA-613 could prove to be very valuable.

Crystal structure of N,N-dimethyl-N′-(2-methyl-4-oxo-5,6,7,8-tetrahydrobenzo[4,5]thieno[2,3-d]pyrimidin-3(4H)-yl)formimidamide, C14H18N4OS

C14H18N4OS, monoclinic, P21/c (no. 14), a = 15.479(4) Å, b = 13.203(3) Å, c = 7.3147(17) Å, β = 99.385(7)°, V = 1474.9(6) Å3, Z = 4, R gt(F) = 0.070, wR ref(F 2) = 0.197, T = 296(2).

The Influenza Virus Polymerase Complex: An Update on Its Structure, Functions, and Significance for Antiviral Drug Design

Influenza viruses cause seasonal epidemics and pandemic outbreaks associated with significant morbidity and mortality, and a huge cost. Since resistance to the existing anti‐influenza drugs is rising, innovative inhibitors with a different mode of action are urgently needed. The influenza polymerase complex is widely recognized as a key drug target, given its critical role in virus replication and high degree of conservation among influenza A (of human or zoonotic origin) and B viruses. We here review the major progress that has been made in recent years in unravelling the structure and functions of this protein complex, enabling structure‐aided drug design toward the core regions of the PA endonuclease, PB1 polymerase, or cap‐binding PB2 subunit. Alternatively, inhibitors may target a protein–protein interaction site, a cellular factor involved in viral RNA synthesis, the viral RNA itself, or the nucleoprotein component of the viral ribonucleoprotein. The latest advances made for these diverse pharmacological targets have yielded agents in advanced (i.e., favipiravir and VX‐787) or early clinical testing, besides several experimental inhibitors in various stages of development, which are all covered here.

Development of a reverse genetics system to generate a recombinant Ebola virus Makona expressing a green fluorescent protein

Previous studies have demonstrated the potential application of reverse genetics technology in studying a broad range of aspects of viral biology, including gene regulation, protein function, cell entry, and pathogenesis. Here, we describe a highly efficient reverse genetics system used to generate recombinant Ebola virus (EBOV) based on a recent isolate from a human patient infected during the 2014-2015 outbreak in Western Africa. We also rescued a recombinant EBOV expressing a fluorescent reporter protein from a cleaved VP40 protein fusion. Using this virus and an inexpensive method to quantitate the expression of the foreign gene, we demonstrate its potential usefulness as a tool for screening antiviral compounds and measuring neutralizing antibodies.

Synthesis of orotidine by intramolecular nucleosidation

An intramolecular nucleosidation approach, using orotate as a leaving group, provides easy access to orotidine in high yields.

How to approach and treat viral infections in ICU patients

Patients with severe viral infections are often hospitalized in intensive care units (ICUs) and recent studies underline the frequency of viral detection in ICU patients. Viral infections in the ICU often involve the respiratory or the central nervous system and can cause significant morbidity and mortality especially in immunocompromised patients. The mainstay of therapy of viral infections is supportive care and antiviral therapy when available. Increased understanding of the molecular mechanisms of viral infection has provided great potential for the discovery of new antiviral agents that target viral proteins or host proteins that regulate immunity and are involved in the viral life cycle. These novel treatments need to be further validated in animal and human randomized controlled studies.

High-throughput, luciferase-based reverse genetics systems for identifying inhibitors of Marburg and Ebola viruses

Marburg virus (MARV) and Ebola virus (EBOV), members of the family Filoviridae, represent a significant challenge to global public health. Currently, no licensed therapies exist to treat filovirus infections, which cause up to 90% mortality in human cases. To facilitate development of antivirals against these viruses, we established two distinct screening platforms based on MARV and EBOV reverse genetics systems that express secreted Gaussia luciferase (gLuc). The first platform is a mini-genome replicon to screen viral replication inhibitors using gLuc quantification in a BSL-2 setting. The second platform is complementary to the first and expresses gLuc as a reporter gene product encoded in recombinant infectious MARV and EBOV, thereby allowing for rapid quantification of viral growth during treatment with antiviral compounds. We characterized these viruses by comparing luciferase activity to virus production, and validated luciferase activity as an authentic real-time measure of viral growth. As proof of concept, we adapt both mini-genome and infectious virus platforms to high-throughput formats, and demonstrate efficacy of several antiviral compounds. We anticipate that both approaches will prove highly useful in the development of anti-filovirus therapies, as well as in basic research on the filovirus life cycle.

Design of inhibitors of ODCase

ODCase is a highly proficient enzyme responsible for the decarboxylation of orotidine monophosphate to generate uridine monophosphate. ODCase has attracted early attention due to its interesting mechanism of catalysis. In order to exploit therapeutic advantages due to the inhibition of ODCase, one must have selective inhibitors of this enzyme from the pathogen, or a dysregulated molecular mechanism involving ODCase. ODCase inhibitors have potential applications as anticancer agents, antiviral agents, antimalarial agents and potentially act against other parasitic diseases. A variety of C6-substituted uridine monophosphate derivatives have shown excellent inhibition of ODCase. 6-iodouridine is a potent inhibitor of the malaria parasite, and its monophosphate form covalently inhibits ODCase. A variety of inhibitors of ODCase with potential applications as therapeutic agents are discussed in this review.

A conformational mimetic approach for the synthesis of carbocyclic nucleosides as anti-HCV leads

Computer-aided approaches coupled with medicinal chemistry were used to explore novel carbocyclic nucleosides as potential anti-hepatitis C virus (HCV) agents. Conformational analyses were carried out on 6-amino-1H-pyrazolo[3,4-d]pyrimidine (6-APP)-based carbocyclic nucleoside analogues, which were considered as nucleoside mimetics to act as HCV RNA-dependent RNA polymerase (RdRp) inhibitors. Structural insight gained from the modeling studies revealed the molecular basis behind these nucleoside mimetics. The rationally chosen 6-APP analogues were prepared and evaluated for anti-HCV activity. RdRp SiteMap analysis revealed the presence of a hydrophobic cavity near C7 of the nucleosides; introduction of bulkier substituents at this position enhanced their activity. Herein we report the identification of an iodinated compound with an EC50 value of 6.6 μM as a preliminary anti-HCV lead.

Experimental therapies for yellow fever

A number of viruses in the family Flaviviridae are the focus of efforts to develop effective antiviral therapies. Success has been achieved with inhibitors for the treatment of hepatitis C, and there is interest in clinical trials of drugs against dengue fever. Antiviral therapies have also been evaluated in patients with Japanese encephalitis and West Nile encephalitis. However, no treatment has been developed against the prototype flavivirus, yellow fever virus (YFV). Despite the availability of the live, attenuated 17D vaccine, thousands of cases of YF continue to occur each year in Africa and South America, with a significant mortality rate. In addition, a small number of vaccinees develop severe systemic infections with the 17D virus. This paper reviews current efforts to develop antiviral therapies, either directly targeting the virus or blocking detrimental host responses to infection.

D282, a non-nucleoside inhibitor of influenza virus infection that interferes with de novo pyrimidine biosynthesis

BACKGROUND

The discovery of novel influenza virus inhibitors remains an important priority in light of the emergence of drug-resistant viruses. Toward this end, a library of over 6,000 compounds was tested for antiviral activity.

METHODS

Strains of influenza virus were evaluated by cytopathic effect (CPE) inhibition and virus yield reduction assays. Intracellular nucleoside triphosphate pools were analysed by strong anion exchange HPLC. Dihydroorotate dehydrogenase inhibition assays were conducted. Influenza virus-infected mice were treated for 5 days with D282.

RESULTS

A non-nucleoside, 4-[(4-butylphenyl)amino]-2-methylene-4-oxo-butanoic acid (D282), was discovered that inhibited influenza A and B virus CPE by 50% at 6-31 μM (giving selectivity indices of >13 to >67, based on cytotoxicity of >400 µM in stationary cell cultures). Ribavirin (positive control) was active at 14-44 µM (yielding selectivity indices of >9 to >29, with >400 µM toxicity). D282 and ribavirin inhibited virus yield by 90% at 9.5 ±3.3 and 10.8 ±3.2 µM, respectively. The antiviral activity of D282 in vitro was reversed by addition of uridine, cytidine and orotic acid. D282 exhibited an uncompetitive inhibition of mouse liver dihydroorotate dehydrogenase (inhibitor constant [Ki] of 2.3 ±0.9 µM, Michaelis constant [Km] of 150 ±16 µM). Because cellular pyrimidine biosynthesis was inhibited, D282-treated cells had decreased uridine triphosphate and cytidine triphosphate levels. D282 (≤100 mg/kg/day) failed to prevent death of mice infected with influenza.

CONCLUSIONS

D282 was active against influenza A and B viruses by inhibiting de novo pyrimidine biosynthesis. Although effective in vitro, the compound, like others in its class, was devoid of antiviral activity in infected mice.

Determination of the amino acid sequence requirements for catalysis by the highly proficient orotidine monophosphate decarboxylase

Orotidine 5'-monophosphate decarboxylase (ODCase) catalyzes the decarboxylation of orotidine 5'-monophosphate to uridine 5'-monophosphate during pyrimidine nucleotide biosynthesis. This enzyme is one of the most proficient known, exhibiting a rate enhancement of over 17 orders of magnitude over the uncatalyzed rate. An interesting question is whether the high proficiency of ODCase is associated with a highly optimized sequence of active site residues. This question was addressed by randomizing 24 residue positions in and around the active site of the E. coli ODCase (pyrF) by site-directed mutagenesis. The libraries of mutants were selected for function from a multicopy plasmid or by single-copy replacement at the pyrF locus on the E. coli chromosome. Stringent sequence requirements for function were found for the mutants expressed from the chromosomal pyrF locus. Six positions were not tolerant of substitutions and several others accepted very limited substitutions. In contrast, all positions could be substituted to some extent when the library mutants were expressed from a multicopy plasmid. For the conserved quartet of charged residues Lys44-Asp71-Lys73-Asp76, a cysteine substitution was found to provide function at positions 71 and 76. A lower pK(a) for both cysteine mutants supports a mechanism whereby the thiolate group of cysteine substitutes for the negatively charged aspartate side chain. The partial function mutants such as D71C and D76C exhibit reduced catalytic efficiency relative to wild type but nevertheless provide a rate enhancement of 15 orders of magnitude over the uncatalyzed rate indicating the catalytic proficiency of the enzyme is robust and tolerant of mutation.

Identification of broad-spectrum antiviral compounds and assessment of the druggability of their target for efficacy against respiratory syncytial virus (RSV)

The search for novel therapeutic interventions for viral disease is a challenging pursuit, hallmarked by the paucity of antiviral agents currently prescribed. Targeting of viral proteins has the inextricable challenge of rise of resistance. Safe and effective vaccines are not possible for many viral pathogens. New approaches are required to address the unmet medical need in this area. We undertook a cell-based high-throughput screen to identify leads for development of drugs to treat respiratory syncytial virus (RSV), a serious pediatric pathogen. We identified compounds that are potent (nanomolar) inhibitors of RSV in vitro in HEp-2 cells and in primary human bronchial epithelial cells and were shown to act postentry. Interestingly, two scaffolds exhibited broad-spectrum activity among multiple RNA viruses. Using the chemical matter as a probe, we identified the targets and identified a common cellular pathway: the de novo pyrimidine biosynthesis pathway. Both targets were validated in vitro and showed no significant cell cytotoxicity except for activity against proliferative B- and T-type lymphoid cells. Corollary to this finding was to understand the consequences of inhibition of the target to the host. An in vivo assessment for antiviral efficacy failed to demonstrate reduced viral load, but revealed microscopic changes and a trend toward reduced pyrimidine pools and findings in histopathology. We present here a discovery program that includes screen, target identification, validation, and druggability that can be broadly applied to identify and interrogate other host factors for antiviral effect starting from chemical matter of unknown target/mechanism of action.

A facile synthesis and anti-avian influenza virus (H5N1) screening of some novel pyrazolopyrimidine nucleoside derivatives

Treatment of 5-amino-1-(9-methyl-5,6-dihydronaphtho[1',2':4,5]thieno[2,3-d]pyrimidin-11-yl)-1H-pyrazole-4-carbonitrile (1) with formic acid afforded pyrazolo[3,4-d]pyrimidin-4-one derivative 2. The sodium salt of the latter compound (generated in situ) was treated with some alkyl halides to afford the corresponding N-substituted compounds 3-7. The siloxy derivative 8 (generated also in situ from 2) was ribosylated and glycosylated to yield compounds 9 and 11, respectively. Deprotection of compounds 9 and 11 in methanolic ammonia produced the free nucleosides 10 and 12, respectively. Moreover, the prepared compounds were tested for antiviral activity against H5N1 virus [A/chicken/Egypt/1/2006] and some of them revealed moderate results compared with the other tested compounds.

Structural determinants for the inhibitory ligands of orotidine-5'-monophosphate decarboxylase

In recent years, orotidine-5'-monophosphate decarboxylase (ODCase) has gained renewed attention as a drug target. As a part of continuing efforts to design novel inhibitors of ODCase, we undertook a comprehensive study of potent, structurally diverse ligands of ODCase and analyzed their structural interactions in the active site of ODCase. These ligands comprise of pyrazole or pyrimidine nucleotides including the mononucleotide derivatives of pyrazofurin, barbiturate ribonucleoside, and 5-cyanouridine, as well as, in a computational approach, 1,4-dihydropyridine-based non-nucleoside inhibitors such as nifedipine and nimodipine. All these ligands bind in the active site of ODCase exhibiting distinct interactions paving the way to design novel inhibitors against this interesting enzyme. We propose an empirical model for the ligand structure for rational modifications in new drug design and potentially new lead structures.

Novel antiviral agent DTriP-22 targets RNA-dependent RNA polymerase of enterovirus 71

Enterovirus 71 (EV71) has emerged as an important virulent neurotropic enterovirus in young children. DTriP-22 (4{4-[(2-bromo-phenyl)-(3-methyl-thiophen-2-yl)-methyl]-piperazin-1-yl}-1-pheny-1H-pyrazolo[3,4-d]pyrimidine) was found to be a novel and potent inhibitor of EV71. The molecular target of this compound was identified by analyzing DTriP-22-resistant viruses. A substitution of lysine for Arg163 in EV71 3D polymerase rendered the virus drug resistant. DTriP-22 exhibited the ability to inhibit viral replication by reducing viral RNA accumulation. The compound suppressed the accumulated levels of both positive- and negative-stranded viral RNA during virus infection. An in vitro polymerase assay indicated that DTriP-22 inhibited the poly(U) elongation activity, but not the VPg uridylylation activity, of EV71 polymerase. These findings demonstrate that the nonnucleoside analogue DTriP-22 acts as a novel inhibitor of EV71 polymerase. DTriP-22 also exhibited a broad spectrum of antiviral activity against other picornaviruses, which highlights its potential in the development of antiviral agents.

Structure-activity relationships of orotidine-5'-monophosphate decarboxylase inhibitors as anticancer agents

A series of 6-substituted and 5-fluoro-6-substituted uridine derivatives were synthesized and evaluated for their potential as anticancer agents. The designed molecules were synthesized from either fully protected uridine or the corresponding 5-fluorouridine derivatives. The mononucleotide derivatives were used for enzyme inhibition investigations against ODCase. Anticancer activities of all the synthesized derivatives were evaluated using the nucleoside forms of the inhibitors. 5-Fluoro-UMP was a very weak inhibitor of ODCase. 6-Azido-5-fluoro and 5-fluoro-6-iodo derivatives are covalent inhibitors of ODCase, and the active site Lys145 residue covalently binds to the ligand after the elimination of the 6-substitution. Among the synthesized nucleoside derivatives, 6-azido-5-fluoro, 6-amino-5-fluoro, and 6-carbaldehyde-5-fluoro derivatives showed potent anticancer activities in cell-based assays against various leukemia cell lines. On the basis of the overall profile, 6-azido-5-fluoro and 6-amino-5-fluoro uridine derivatives exhibited potential for further investigations.

Pyrazolo[3,4-d]pyrimidine ribonucleosides related to 2-aminoadenosine and isoguanosine: synthesis, deamination and tautomerism

The syntheses and properties of 8-aza-7-deazapurine (pyrazolo[3,4-d]pyrimidine) ribonucleosides related to 2-aminoadenosine and isoguanosine are described. Glycosylation of 8-aza-7-deazapurine-2,6-diamine 5 with 1-O-acetyl-2,3,5-tri-O-benzoyl-beta-D-ribofuranose (12) in the presence of BF(3) x Et(2)O as a catalyst gave the N(8) isomer 14 (73%) with a trace amount of the N(9) isomer 13a (4.8%). Under the same reaction conditions, the 7-halogenated 8-aza-7-deazapurine-2,6-diamines 6-8 afforded the thermodynamically more stable N(9) nucleosides 13b-d as the only products (53-70%). Thus, a halogen in position 7 shifts the glycosylation from N(8) to N(9). The 8-aza-7-deazapurine-4,6-diamine ribonucleosides 1a-d were converted to the isoguanosine derivatives 3a-d by diazotization of the 2-amino group. Although compounds 1a,b do not contain a nitrogen at position 7 (the enzyme binding site), they were deaminated by adenosine deaminase; however, their deamination occurred with a much slower velocity than that of the related purines. The pK(a) values indicate that the 7-non-functionalized nucleosides 1a (pK(a) 5.8) and 15 (pK(a) 6.4) are possibly protonated in neutral conditions when incorporated into RNA. The nucleosides 3a-d exist predominantly in the keto (lactam) form with K(TAUT) (keto/enol) values of 400-1200 compared to 10(3)-10(4) for pyrrolo[2,3-d]pyrimidine isoguanosine derivatives 4a-c and 10 for isoguanosine itself, which will reduce RNA mispairing with U.

Activities of oseltamivir and ribavirin used alone and in combination against infections in mice with recent isolates of influenza A (H1N1) and B viruses

Mouse models have been widely used for evaluating potential influenza virus inhibitors. However, the viral strains traditionally used in these models are fairly old and do not represent currently circulating viruses in nature. We developed two new lethal infection models in mice using mouse-adapted influenza A/New Caledonia/20/99 (H1N1) and influenza B/Sichuan/379/99 viruses. Both virus infections were used to study oral treatment with oseltamivir and ribavirin, both alone and in combination. Oral treatments were given twice daily for 5 days starting 4 h before infection in initial studies. Against influenza A, oseltamivir was active at 10, 20, and 40 mg/kg/day, protected 80-100% of mice from death and reduced lung consolidation - ribavirin was similarly effective at 20, 40, and 80 mg/kg/day. When treatments were initiated after virus challenge, delaying treatment with oseltamivir even 1 day caused it to be ineffective. Ribavirin prevented mortality by 50-80% when treatments were delayed 1-4 days after infection. The combination of the two drugs (oseltamivir at 20 mg/kg/day and ribavirin at 40 mg/kg/day) was no better than ribavirin alone. In contrast to what we observed with influenza A virus infections, oseltamivir and ribavirin showed similar dose-related antiviral activities against influenza B virus infections. The compounds both significantly increased survival when treatments started up to 4 days after infection, but ribavirin was more active than oseltamivir (50-80% survival compared to 30-40% survival, respectively, when starting treatments on days 2-4 after infection). By varying the doses of each drug that were used in combination (oseltamivir at 1.25, 2.5 and 5 mg/kg/day; ribavirin at 5, 10 and 20 mg/kg/day) certain dosage combinations were superior to either compound used alone as assessed by decreased mortality, lung virus titre, lung score and lung weight parameters. These activities differed from published results with older, more established virus strains as oseltamivir was less effective and ribavirin was more active than previously reported.

Activity and mechanism of action of N-methanocarbathymidine against herpesvirus and orthopoxvirus infections

N-Methanocarbathymidine [(N)-MCT] is a conformationally locked nucleoside analog that is active against some herpesviruses and orthopoxviruses in vitro. The antiviral activity of this molecule is dependent on the type I thymidine kinase (TK) in herpes simplex virus and also appears to be dependent on the type II TK expressed by cowpox and vaccinia viruses, suggesting that it is a substrate for both of these divergent forms of the enzyme. The drug is also a good inhibitor of viral DNA synthesis in both viruses and is consistent with inhibition of the viral DNA polymerase once it is activated by the viral TK homologs. This mechanism of action explains the rather unusual spectrum of activity, which is limited to orthopoxviruses, alphaherpesviruses, and Epstein-Barr virus, since these viruses express molecules with TK activity that can phosphorylate and thus activate the drug. The compound is also effective in vivo and reduces the mortality of mice infected with orthopoxviruses, as well as those infected with herpes simplex virus type 1 when treatment is initiated 24 h after infection. These results indicate that (N)-MCT is active in vitro and in vivo, and its mechanism of action suggests that the molecule may be an effective therapeutic for orthopoxvirus and herpesvirus infections, thus warranting further development.

Identification of active antiviral compounds against a New York isolate of West Nile virus

The recent West Nile virus (WNV) outbreak in the United States has increased the need to identify effective therapies for this disease. A chemotherapeutic approach may be a reasonable strategy because the virus infection is typically not chronic and antiviral drugs have been identified to be effective in vitro against other flaviviruses. A panel of 34 substances was tested against infection of a recent New York isolate of WNV in Vero cells and active compounds were also evaluated in MA-104 cells. Some of these compounds were also evaluated in Vero cells against the 1937 Uganda isolate of the WNV. Six compounds were identified to be effective against virus-induced CPE with 50% effective concentrations (EC50) less than 10 microg/ml and with a selectivity index (SI) of greater than 10. Known inhibitors of orotidine monophosphate decarboxylase and inosine monophosphate dehydrogenase involved in the synthesis of GTP, UTP, and TTP were most effective. The compounds 6-azauridine, 6-azauridine triacetate, cyclopententylcytosine (CPE-C), mycophenolic acid and pyrazofurin appeared to have the greatest activities against the New York isolate, followed by 2-thio-6-azauridine. Anti-WNV activity of 6-azauridine was confirmed by virus yield reduction assay when the assay was performed 2 days after initial infection in Vero cells. The neutral red assay mean EC50 of ribavirin was only 106 microg/ml with a mean SI of 9.4 against the New York isolate and only slightly more effective against the Uganda isolate. There were some differences in the drug sensitivities of the New York and Uganda isolates, but when comparisons were made by categorizing drugs according to their modes of action, similarities of activities between the two isolates were identified.

Antiviral activity and mode of action studies of ribavirin and mycophenolic acid against orthopoxviruses in vitro

Two inhibitors of cellular inosine monophosphate dehydrogenase, mycophenolic acid (MPA) and ribavirin, were evaluated for inhibitory activity against orthopoxviruses. Unrelated antipoxvirus agents tested for comparison included 6-azauridine, cidofovir (HPMPC) and cyclic HPMPC. MPA inhibited camelpox, cowpox, monkeypox and vaccinia viruses by 50% in plaque reduction assays at 0.2-3 microM in African green monkey kidney (Vero 76) and mouse 3T3 cells. Ribavirin was considerably more active in 3T3 cells (50% inhibition at 2-12 microM) than in Vero 76 cells (inhibitory at 30-250 microM) against these viruses. In cytotoxicity assays, MPA and ribavirin were more toxic to replicating cells than to stationary cell monolayers, with greater toxicity seen in 3T3 than in Vero 76 cells. The superior antiviral potency and increased toxicity of ribavirin in 3T3 cells was related to greater accumulation of mono-, di- and triphosphate forms of the drug compared with Vero 76 cells. For both MPA and ribavirin, virus inhibition was closely correlated to the extent of suppression of intracellular guanosine triphosphate (GTP) pools. Treatment with extracellular guanosine (which restored intracellular GTP levels) did not lead to complete reversal of the anticowpox virus activity of ribavirin. This suggests that other modes of virus inhibition also appear to contribute to the anti-orthopoxvirus activity of ribavirin. Biological differences in mode of action and immunosuppressive potential between ribavirin and MPA may account for why the former compound is active against orthopoxvirus infections in animals and the latter inhibitor is not.

Selective inhibition of arthropod-borne and arenaviruses in vitro by 3′-fluoro-3′-deoxyadenosine

A novel nucleoside analog, 3'-fluoro-3'-deoxyadenosine (3'F3'dAdo), was evaluated for antiviral activity against several arthropod-borne and arenaviruses in Vero cell culture. The following 50% inhibitory concentrations (EC50) of virus plaque formation were obtained against the test viruses: Semliki Forest (10.3 microM) and Venezuelan equine encephalitis (5.3 microM) alphaviruses, lymphocytic choriomeningitis (7.7 microM) and Pichinde (greater than 32 microM) arenaviruses, Punta Toro (greater than 32 microM) and San Angelo (1.6 microM) bunyaviruses, banzi flavivirus (4.0 microM), and Colorado tick fever orbivirus (0.6 microM). By comparison, the broad-spectrum antiviral agent ribavirin was active against lymphocytic choriomeningitis (18 microM), Pichinde (24 microM), Punta Toro (114 microM), and San Angelo (99 microM) viruses, but was less active against the other 4 viruses (greater than 200 microM). Vero cell proliferation and thymidine and uridine incorporation into replicating Vero cells were inhibited by 50% with 3'F3'dAdo concentrations of 36, 45, and 32 microM, respectively. In virus yield reduction assays, increasing the multiplicity of infections of Semliki Forest and Venezuelan equine encephalitis viruses reduced the inhibitory activity of 3'F3'dAdo. Using the same assay, 3'F3'dAdo was found to enhance Punta Toro virus replication up to 5-fold relative to the untreated control. By adding the nucleoside transport inhibitor nitrobenzylthioinosine (100 microM) to the culture medium, antiviral activity against the two alphaviruses was eliminated, indicating that 3'F3'dAdo uses the nucleoside transport system for cell entry. When actinomycin D (5 microM) was used to greatly suppress cellular RNA synthesis in Semliki Forest virus-infected and uninfected cells, 3'F3'dAdo preferentially inhibited viral RNA synthesis. The results of these studies indicate 3'F3'dAdo is a selective inhibitor of most of the viruses tested and should be a promising candidate for in vivo evaluations.

Immunoenhancing properties and antiviral activity of 7-deazaguanosine in mice

The nucleotide analog 7-deazaguanosine has not previously been reported to possess biological (antiviral or antitumor) properties in cell culture or in vivo. Up to 10(5) U of interferon per ml was detected in mouse sera 1 to 4 h following oral (200-mg/kg of body weight) and intraperitoneal (50-mg/kg) doses of the compound. 7-Deazaguanosine also caused significant activation of natural killer and phagocytic cells but did not augment T- and B-cell blastogenesis. Intraperitoneal treatments of 50, 100, and 200 mg/kg/day administered 24 and 18 h before virus inoculation were highly protective in mice inoculated with lethal doses of Semliki Forest or San Angelo viruses. Less but still significant survivor increases were evident in treated mice infected with banzi or encephalomyocarditis viruses. In most cases, the degree of antiviral activity was similar to that exhibited by the biological response modifier 7-thia-8-oxoguanosine. 7-Thia-8-oxoguanosine was more potent than 7-deazaguanosine against encephalomyocarditis virus in mice, however. Oral efficacy was achieved with 7-deazaguanosine treatments of greater than or equal to 100 mg/kg against all virus infections, whereas 7-thia-8-oxoguanosine is reported to be devoid of oral activity in rodents. Thus, 7-deazaguanosine represents the first reported orally active nucleoside biological response modifier exhibiting broad-spectrum antiviral activity against particular types of RNA viruses.

Roles of interferon and natural killer cells in the antiviral activity of 7-thia-8-oxoguanosine against Semliki Forest virus infections in mice

7-Thia-8-oxoguanosine is a novel biological response modifier with broad-spectrum antiviral activity against many DNA and RNA viruses in vivo. Since two of its properties are to induce interferon and to activate natural killer (NK) cells, we investigated the roles of the lymphokine and NK cells in the antiviral activity of the compound against Semliki Forest virus. Antibody to interferon alpha/beta could completely abolish the protective activity of the nucleoside against virus infection in mice, whereas antibodies to interferons beta and gamma could not, indicating that interferon alpha was of major importance to confer protection to the animals. Reduced activation of NK cells was also observed in mice treated with 7-thia-8-oxoguanosine and antibody to interferon alpha/beta. The role of NK cells in the protective activity of the compound was directly assessed in beige mice or in Swiss Webster mice treated with asialo GM1 antibody. In both experiments, the animals were protected from lethal virus infection by treatment with nucleoside. Spleen cells primed by 7-thia-8-oxoguanosine and adoptively transferred to untreated mice could not save them from virus-induced mortality. These three results provide evidence that natural killer cells activated by 7-thia-8-oxoguanosine play a minimal role in protection from acute Semliki Forest virus infections in mice.

4,6-dibenzamidopyrazolo[3,4-d]pyrimidine is a highly selective inhibitor of cytomegalovirus adsorption to cells

The heterocycle, 4,6-dibenzamidopyrazolo[3,4-d]pyrimidine (DBAPP), inhibited cytopathology induced by human, mouse, and vervet monkey cytomegaloviruses (CMV) in vitro at 0.2 to 0.5 microM, but did not inhibit cell replication at less than or equal to 30 microM. Herpes simplex viruses were unaffected by the inhibitor. The antiviral agent ganciclovir was effective against these CMVs at 3-10 microM in parallel assays. DBAPP and ganciclovir were synergistic inhibitors when used in combination. The heterocycle was only active if applied to cells before virus replication, indicating that it inhibited virus adsorption. Cells pre-treated 1 h with 30 microM DBAPP, then extensively rinsed, were resistant to infection by mouse CMV even 3 days after removal of the inhibitor. Human and monkey CMVs were able to infect cells and replicate within 24 h of drug removal. When virus and DBAPP were combined together then dialyzed to remove the compound, mouse CMV infectivity was decreased 1.7 logs, whereas human CMV and monkey CMV infectivity titers were relatively unaffected. Treatment of mice with DBAPP twice a day for 7 days starting 6 h after mouse CMV inoculation caused a moderate increase in number of survivors at 30 mg/kg. Cell to cell spread of the virus may account for poor efficacy of the compound when added after virus infection. DBAPP may serve as a tool to explore aspects of CMV adsorption or to characterize the cellular component of the CMV receptor.

Broad-spectrum in vivo antiviral activity of 7-thia-8-oxoguanosine, a novel immunopotentiating agent

A novel immunopotentiating agent, 5-amino-3-beta-D-ribofuranosylthiazolo [4,5-d]pyrimidine-2,7(3H,6H)-dione (7-thia-8-oxoguanosine), lacks virus-inhibitory properties in vitro but induces interferon and potentiates immune functions, such as natural killer cell activity. It was evaluated in rodent models to determine the spectrum of antiviral activity and effective treatment regimens. At 50 to 200 mg/kg given as single or divided intraperitoneal (i.p.) doses 1 day before virus inoculation, significant protection was afforded to mice infected i.p. with Semliki Forest, San Angelo, banzi, and encephalomyocarditis viruses. Similarly, suckling rats were protected from an intranasal challenge with rat coronavirus. Against San Angelo virus, treatments could be delayed to 1 day post-virus inoculation and still show a beneficial effect. The compound was moderately effective in mice infected i.p. with herpes simplex virus type 2 or intranasally with vesicular stomatitis virus. No activity was seen against influenza B virus in mice when the analog was administered one time pre-virus inoculation or in multiple doses given before and after the virus inoculation. Nor was there a prophylactic effect against herpetic skin lesions on mice. This immune modulator may have promise for the treatment of a variety of virus infections.

Evaluation of continuous cell lines in antiviral studies with murine cytomegalovirus

Cell culture systems were developed for rapid antiviral drug screening, using murine cytomegalovirus (MCMV) as an alternative to the slower growing human CMV. Since previous assay methods with MCMV employed mouse embryo fibroblasts (MEF cells), which are labor intensive to prepare and die off after 3-4 passages from primary culture, identification of virus-susceptible continuous cell lines was desirable. Three cell lines were found useful for assaying MCMV: C127I, SC-1, and 3T3. The antiviral agents acyclovir, ganciclovir, 5-fluoroarabinofuranosylcytosine, and 2'-fluoro-2'-deoxy-5-iodoarabinofuranosylcytosine were evaluated in the 3 continuous cell lines and in MEF cells. The 50% virus- or cell-inhibitory concentration values determined for each compound did not vary much from cell to cell. MEF cells were 10-fold more sensitive than the other cell lines to quantify virus from mouse organs, however. Virus propagated in 3T3 and SC-1 cells were as virulent to mice as salivary gland virus, whereas virus from MEF and C127I cells was more attenuated. Overall, C127I cells were judged to be the best for large scale antiviral screening in vitro, but MEF was the cell type of choice for titration of viruses from mouse organs and tissues.

Inhibition of orotidylate decarboxylase by 4(5H)-oxo-1-beta-D-ribofuranosylpyrazolo[3,4-d] pyrimidine-3-thiocarboxamide (APR-TC) in B lymphoblasts. Activation by adenosine kinase

The nucleoside allopurinol riboside-3-thiocarboxamide (APR-TC; 4-(5H)oxo-1-beta-D-ribofuranosylpyrazolo[3,4,d]pyrimidine-3-thioca rboxamide) demonstrates potent in vitro antiviral activity against various DNA and RNA viruses and cytostatic activity against a variety of cell lines in culture. The IC50 for APR-TC in the splenic derived B lymphoblast cell line, WI-L2, was 0.3 microM. Adenosine kinase-deficient WI-L2 cells were resistant to growth inhibition by APR-TC, indicating that adenosine kinase (EC 2.7.1.20) is responsible for phosphorylation of APR-TC to form the monophosphate derivative (APR-TC-5'P). A 4-hr incubation of cells with 50 microM APR-TC resulted in severe depletion of intracellular pyrimidine nucleotide pools and the accumulation of 3 microM APR-TC-5'P. The cytotoxicity of APR-TC was reversed by uridine, indicating that the active form of this compound inhibits the de novo pyrimidine biosynthetic pathway. Further, APR-TC-treated cells could not utilize the pyrimidine nucleotide precursor [6-14C]orotic acid, suggesting that the UMP synthase complex is the major cellular site of inhibition. In studies utilizing cell-free lysates of WI-L2, chemically prepared APR-TC-5'P provided potent inhibition of the orotidylate decarboxylase activity (ODCase, EC 4.1.1.23) of the UMP synthase complex. APR-TC-5'P was competitive with OMP, and a Ki value of 0.35 nM was determined.

Antiviral activity and mode of action of ribavirin 5'-sulfamate against Semliki Forest virus

Ribavirin 5'-sulfamate, a nucleotide analog, inhibited Semliki Forest virus cytopathology by 50% at 10 microM, whereas ribavirin was inactive at less than or equal to 1 mM. Actinomycin D did not reverse (antagonize) the effect of ribavirin 5'-sulfamate against the virus. The compound inhibited amino acid incorporation into macromolecules of uninfected cells but had no appreciable effect on uridine incorporation. Infected cells treated with actinomycin D and nucleotide analog were inhibited in amino acid and uridine incorporation. The compound blocked the formation of the viral RNA polymerase protein in cells, which could account for the inhibited synthesis of new viral RNA. By electrophoresis, inhibition of the synthesis of viral proteins was more pronounced than the inhibition of cellular polypeptides. The analog inhibited the translation of mRNA to protein. Most animals treated intraperitoneally for 7 days with ribavirin 5'-sulfamate at 20 and 40 mg/kg/day starting 2 h before intraperitoneal Semliki Forest virus inoculation survived the otherwise lethal infection.