Potent inhibition of arenavirus infection by a novel fusion inhibitor.

Several arenaviruses, including Lassa and Lujo viruses in Africa and five New World arenavirus (NWA) species in the Americas, cause life-threatening viral hemorrhagic fevers. In the absence of licensed antiviral therapies, these viruses pose a significant public health risk. The envelope glycoprotein complex (GPC) mediates arenavirus entry through a pH-dependent fusion of the viral and host endosomal membranes. It thus is recognized as a viable target for small-molecule fusion inhibitors. Here, we report on the antiviral activity and pre-clinical development of the novel broad-spectrum arenavirus fusion inhibitors, ARN-75039 and ARN-75041. In Tacaribe virus (TCRV) pseudotyped and native virus assays, the ARN compounds were active in the low to sub-nanomolar range with selectivity indices exceeding 1000. Pharmacokinetic analysis of the orally administered compounds revealed an extended half-life in mice supporting once-daily dosing, and the compounds were well tolerated at the highest tested dose of 100 mg/kg. In a proof-of-concept prophylactic efficacy study, doses of 10 and 35 mg/kg of either compound dramatically improved survival outcome and potently inhibited TCRV replication in serum and various tissues. Additionally, in contrast to surviving mice that received ribavirin or placebo, animals treated with ARN-75039 or ARN-75041 were cured of TCRV infection. In a follow-up study with ARN-75039, impressive therapeutic efficacy was demonstrated under conditions where treatment was withheld until after the onset of disease. Taken together, the data strongly support the continued development of ARN-75039 as a candidate therapeutic for the treatment of severe arenaviral diseases.

Broad-spectrum antivirals of protoporphyrins inhibit the entry of highly pathogenic emerging viruses.

Severe emerging and re-emerging viral infections such as Lassa fever, Avian influenza (AI), and COVID-19 caused by SARS-CoV-2 urgently call for new strategies for the development of broad-spectrum antivirals targeting conserved components in the virus life cycle. Viral lipids are essential components, and viral-cell membrane fusion is the required entry step for most unrelated enveloped viruses. In this paper, we identified a porphyrin derivative of protoporphyrin IX (PPIX) that showed broad antiviral activities in vitro against a panel of enveloped pathogenic viruses including Lassa virus (LASV), Machupo virus (MACV), and SARS-CoV-2 as well as various subtypes of influenza A viral strains with IC50 values ranging from 0.91 ± 0.25 µM to 1.88 ± 0.34 µM. A mechanistic study using influenza A/Puerto Rico/8/34 (H1N1) as a testing strain showed that PPIX inhibits the infection in the early stage of virus entry through biophysically interacting with the hydrophobic lipids of enveloped virions, thereby inhibiting the entry of enveloped viruses into host cells. In addition, the preliminary antiviral activities of PPIX were further assessed by testing mice infected with the influenza A/Puerto Rico/8/34 (H1N1) virus. The results showed that compared with the control group without drug treatment, the survival rate and mean survival time of the mice treated with PPIX were apparently prolonged. These data encourage us to conduct further investigations using PPIX as a lead compound for the rational design of lipid-targeting antivirals for the treatment of infection with enveloped viruses.

Use of ribavirin in viruses other than hepatitis C. A review of the evidence. / Uso de ribavirina en virus distintos de la hepatitis C. Una revisión de la evidencia.

Ribavirin is a molecule with antiviral activity against different viruses. In clinical practice, it has made its niche almost exclusively for the treatment of the hepatitisC virus. However, there are other diseases in which it could be of benefit and it has the advantage of being suitable for oral, intravenous and inhaled administration. We conducted a review of the indications of the main drug agencies (Spanish, European and American) and other possible indications, mainly haemorrhagic fevers and coronavirus.

Screening and Identification of Lassa Virus Entry Inhibitors from an FDA-Approved Drug Library.

Lassa virus (LASV) belongs to the Mammarenavirus genus (family Arenaviridae) and causes severe hemorrhagic fever in humans. At present, there are no Food and Drug Administration (FDA)-approved drugs or vaccines specific for LASV. Here, high-throughput screening of an FDA-approved drug library was performed against LASV entry by using pseudotype virus bearing LASV envelope glycoprotein (GPC). Two hit compounds, lacidipine and phenothrin, were identified as LASV entry inhibitors in the micromolar range. A mechanistic study revealed that both compounds inhibited LASV entry by blocking low-pH-induced membrane fusion. Accordingly, lacidipine showed virucidal effects on the pseudotype virus of LASV. Adaptive mutant analyses demonstrated that replacement of T40, located in the ectodomain of the stable-signal peptide (SSP), with lysine (K) conferred LASV resistance to lacidipine. Furthermore, lacidipine showed antiviral activity against LASV, the closely related Mopeia virus (MOPV), and the New World arenavirus Guanarito virus (GTOV). Drug-resistant variants indicated that V36M in the ectodomain of the SSP mutant and V436A in the transmembrane domain of the GP2 mutant conferred GTOV resistance to lacidipine, suggesting the interface between SSP and GP2 is the target of lacidipine. This study shows that lacidipine is a candidate for LASV therapy, reinforcing the notion that the SSP-GP2 interface provides an entry-targeted platform for arenavirus inhibitor design.IMPORTANCE Currently, there is no approved therapy to treat Lassa fever; therefore, repurposing of approved drugs will accelerate the development of a therapeutic stratagem. In this study, we screened an FDA-approved library of drugs and identified two compounds, lacidipine and phenothrin, which inhibited Lassa virus entry by blocking low-pH-induced membrane fusion. Additionally, both compounds extended their inhibition against the entry of Guanarito virus, and the viral targets were identified as the SSP-GP2 interface.

The New World arenavirus Tacaribe virus induces caspase-dependent apoptosis in infected cells.

The Arenaviridae is a diverse and growing family of viruses that already includes more than 25 distinct species. While some of these viruses have a significant impact on public health, others appear to be non-pathogenic. At present little is known about the host cell responses to infection with different arenaviruses, particularly those found in the New World; however, apoptosis is known to play an important role in controlling infection of many viruses. Here we show that infection with Tacaribe virus (TCRV), which is widely considered the prototype for non-pathogenic arenaviruses, leads to stronger induction of apoptosis than does infection with its human-pathogenic relative Junín virus. TCRV-induced apoptosis occurred in several cell types during late stages of infection and was shown to be caspase-dependent, involving the activation of caspases 3, 7, 8 and 9. Further, UV-inactivated TCRV did not induce apoptosis, indicating that the activation of this process is dependent on active viral replication/transcription. Interestingly, when apoptosis was inhibited, growth of TCRV was not enhanced, indicating that apoptosis does not have a direct negative effect on TCRV infection in vitro. Taken together, our data identify and characterize an important virus-host cell interaction of the prototypic, non-pathogenic arenavirus TCRV, which provides important insight into the growing field of arenavirus research aimed at better understanding the diversity in responses to different arenavirus infections and their functional consequences.

Mission critical: mobilization of essential animal models for Ebola, Nipah, and Machupo virus infections.

The reports for Ebola virus Zaire (EBOV), Nipah virus, and Machupo virus (MACV) pathogenesis, in this issue of Veterinary Pathology, are timely considering recent events, both nationally and internationally. EBOV, Nipah virus, and MACV cause highly lethal infections for which no Food and Drug Administration (FDA) licensed vaccines or therapies exist. Not only are there concerns that these agents could be used by those with malicious intent, but shifts in ecological distribution of viral reservoirs due to climate change or globalization could lead to more frequent infections within remote regions than previously seen as well as outbreaks in more populous areas. The current EBOV epidemic shows no sign of abating across 3 West African nations (as of October 2014), including densely populated areas, far outpacing infection rates of previous outbreaks. A limited number of cases have also arisen in the United States and Europe. With few treatment options for these deadly viruses, development of animal models reflective of human disease is paramount to combat these diseases. As an example of this potential, a new treatment compound, ZMapp, that had demonstrated efficacy against EBOV infection in nonhuman primates (NHPs) received an emergency compassionate use exception from the FDA for the treatment of 2 American medical workers infected with EBOV, and they are currently virus free and recovering.

The clinically approved drugs amiodarone, dronedarone and verapamil inhibit filovirus cell entry.

OBJECTIVES: Filoviruses such as Ebola virus and Marburg virus cause a severe haemorrhagic fever syndrome in humans for which there is no specific treatment. Since filoviruses use a complex route of cell entry that depends on numerous cellular factors, we hypothesized that there may be drugs already approved for human use for other indications that interfere with signal transduction or other cellular processes required for their entry and hence have anti-filoviral properties. METHODS: We used authentic filoviruses and lentiviral particles pseudotyped with filoviral glycoproteins to identify and characterize such compounds. RESULTS: We discovered that amiodarone, a multi-ion channel inhibitor and adrenoceptor antagonist, is a potent inhibitor of filovirus cell entry at concentrations that are routinely reached in human serum during anti-arrhythmic therapy. A similar effect was observed with the amiodarone-related agent dronedarone and the L-type calcium channel blocker verapamil. Inhibition by amiodarone was concentration dependent and similarly affected pseudoviruses as well as authentic filoviruses. Inhibition of filovirus entry was observed with most but not all cell types tested and was accentuated by the pre-treatment of cells, indicating a host cell-directed mechanism of action. The New World arenavirus Guanarito was also inhibited by amiodarone while the Old World arenavirus Lassa and members of the Rhabdoviridae (vesicular stomatitis virus) and Bunyaviridae (Hantaan) families were largely resistant. CONCLUSIONS: The ion channel blockers amiodarone, dronedarone and verapamil inhibit filoviral cell entry.

Activity of a phenolic dibenzylsulfide against New World arenavirus infections.

BACKGROUND: Junín virus (JUNV) and several other clade B New World arenaviruses cause human disease ranging from mild febrile illness to severe viral haemorrhagic fever. These viruses pose a significant threat to national security and safe and effective therapies are limited except in Argentina, where immune plasma is the standard of care for treating JUNV infection in cases of Argentine haemorrhagic fever. METHODS: An in vitro screen of the Chemtura library identified several compounds with activity against Tacaribe virus (TCRV), a clade B arenavirus closely related to JUNV. Of these compounds, D746, a phenolic dibenzylsulfide, was further pursued for additional in vitro studies and evaluated in the AG129 mouse TCRV infection model. RESULTS: D746 was found to act during an early to intermediate stage of the TCRV replication cycle and µM range activity was confirmed by virus yield reduction assays with both TCRV and JUNV. Although intraperitoneal twice daily treatment regimens were found to be highly effective when started 2 h prior to TCRV challenge in AG129 mice, post-exposure treatment initiated 3 days after infection was not efficacious. Interestingly, despite the pre-exposure treatment success, D746 did not reduce serum or tissue virus titres during the acute infection. Moreover, D746 elicited ascites fluid accumulation in mice during, as well as independent of, infection. CONCLUSIONS: Our findings suggest that D746 may be altering the host response to TCRV infection in AG129 mice in a way that limits pathogenesis and thereby protects mice from otherwise lethal infection in the absence of measurable reductions in viral burden.

Synthesis of a novel carbocyclic analog of bredinin.

The natural nucleoside antibiotic, bredinin, exhibits antiviral and other biological activities. While various nucleosides related to bredinin have been synthesized, its carbocyclic analog has remained unknown. Synthesis of this heretofore unknown analog of bredinin is described. The key precursor, (3aS,4R,6R,6aR)-6-((methoxy-methoxy)methyl)-2,2-dimethyltetrahydro-3aH-cyclopenta[d][1,3]dioxol-4-amine (5), was prepared from the commercially available compound, (1R,4S)-2-azabicyclo[2.2.1] hept-5-en-3-one (4). Our initial approach used intermediate 6, derived in three transformations from 5, for the key photolytic step to produce the desired ring-opened precursor to the target compound. This photochemical transformation was unsuccessful. However, an appropriately protected and related precursor was synthesized from 5 through the following side-chain functional group transformations: elaboration of the amino group through malonyl ester formation, oximation at the central carbon, conversion of ester to amide and catalytic reduction of the oxime group. This precursor, on treatment with triethylorthoformate and catalytic acetic acid in ethanol, underwent cyclization to produce the desired 4-carbamoyl-imidazolium-5-olate ring. Deprotection of the latter product proceeded smoothly to give the carbocyclic analog of bredinin. This target molecule exhibits antiviral activity, albeit low, against a number of RNA viruses. Further biological evaluations are in progress.

Drug discovery technologies and strategies for Machupo virus and other New World arenaviruses.

INTRODUCTION: Seven arenaviruses cause viral hemorrhagic fever in humans: the Old World arenaviruses Lassa and Lujo, and the New World Clade B arenaviruses Machupo (MACV), Junín (JUNV), Guanarito (GTOV), Sabiá (SABV), and Chapare (CHPV). All of these viruses are Risk Group 4 biosafety pathogens. MACV causes human disease outbreak with high case-fatality rates. To date, at least 1,200 cases with ≈200 fatalities have been recorded. AREAS COVERED: This review summarizes available systems and technologies for the identification of antivirals against MACV. Furthermore, the article summarizes animal models that have been used for the in vivo evaluation of novel inhibitors. The article highlights present treatments for arenaviral diseases and provides an overview of efficacious small molecules and other therapeutics reported to date. Finally, the article summarizes strategies to identify novel inhibitors for anti-arenaviral therapy. EXPERT OPINION: New high-throughput approaches to quantitate infection rates of arenaviruses, as well as viruses modified to carry reporter genes, will accelerate compound screens and drug discovery efforts. RNAi, gene expression profiling and proteomics studies will identify host targets for therapeutic intervention. New discoveries in the cell entry mechanism of MACV and other arenaviruses as well as extensive structural studies of arenaviral L and NP could facilitate the rational design of antivirals effective against all pathogenic New World arenaviruses.

Development of peptide-conjugated morpholino oligomers as pan-arenavirus inhibitors.

Members of the Arenaviridae family are a threat to public health and can cause meningitis and hemorrhagic fever, and yet treatment options remain limited by a lack of effective antivirals. In this study, we found that peptide-conjugated phosphorodiamidate morpholino oligomers (PPMO) complementary to viral genomic RNA were effective in reducing arenavirus replication in cell cultures and in vivo. PPMO complementary to the Junín virus genome were designed to interfere with viral RNA synthesis or translation or both. However, only PPMO designed to potentially interfere with translation were effective in reducing virus replication. PPMO complementary to sequences that are highly conserved across the arenaviruses and located at the 5' termini of both genomic segments were effective against Junín virus, Tacaribe virus, Pichinde virus, and lymphocytic choriomeningitis virus (LCMV)-infected cell cultures and suppressed viral titers in the livers of LCMV-infected mice. These results suggest that arenavirus 5' genomic termini represent promising targets for pan-arenavirus antiviral therapeutic development.

T-705 (favipiravir) inhibition of arenavirus replication in cell culture.

A number of New World arenaviruses (Junín [JUNV], Machupo [MACV], and Guanarito [GTOV] viruses) can cause human disease ranging from mild febrile illness to a severe and often fatal hemorrhagic fever syndrome. These highly pathogenic viruses and the Old World Lassa fever virus pose a significant threat to public health and national security. The only licensed antiviral agent with activity against these viruses, ribavirin, has had mixed success in treating severe arenaviral disease and is associated with significant toxicities. A novel pyrazine derivative currently in clinical trials for the treatment of influenza virus infections, T-705 (favipiravir), has demonstrated broad-spectrum activity against a number of RNA viruses, including arenaviruses. T-705 has also been shown to be effective against Pichinde arenavirus infection in a hamster model. Here, we demonstrate the robust antiviral activity of T-705 against authentic highly pathogenic arenaviruses in cell culture. We show that T-705 disrupts an early or intermediate stage in viral replication, distinct from absorption or release, and that its antiviral activity in cell culture is reversed by the addition of purine bases and nucleosides, but not with pyrimidines. Specific inhibition of viral replication/transcription by T-705 was demonstrated using a lymphocytic choriomeningitis arenavirus replicon system. Our findings indicate that T-705 acts to inhibit arenavirus replication/transcription and may directly target the viral RNA-dependent RNA polymerase.

Development of a new tacaribe arenavirus infection model and its use to explore antiviral activity of a novel aristeromycin analog.

BACKGROUND: A growing number of arenaviruses can cause a devastating viral hemorrhagic fever (VHF) syndrome. They pose a public health threat as emerging viruses and because of their potential use as bioterror agents. All of the highly pathogenic New World arenaviruses (NWA) phylogenetically segregate into clade B and require maximum biosafety containment facilities for their study. Tacaribe virus (TCRV) is a nonpathogenic member of clade B that is closely related to the VHF arenaviruses at the amino acid level. Despite this relatedness, TCRV lacks the ability to antagonize the host interferon (IFN) response, which likely contributes to its inability to cause disease in animals other than newborn mice. METHODOLOGY/PRINCIPAL FINDINGS: Here we describe a new mouse model based on TCRV challenge of AG129 IFN-α/ß and -γ receptor-deficient mice. Titration of the virus by intraperitoneal (i.p.) challenge of AG129 mice resulted in an LD(50) of ∼100 fifty percent cell culture infectious doses. Virus replication was evident in the serum, liver, lung, spleen, and brain 4-8 days after inoculation. MY-24, an aristeromycin derivative active against TCRV in cell culture at 0.9 µM, administered i.p. once daily for 7 days, offered highly significant (P<0.001) protection against mortality in the AG129 mouse TCRV infection model, without appreciably reducing viral burden. In contrast, in a hamster model of arenaviral hemorrhagic fever based on challenge with clade A Pichinde arenavirus, MY-24 did not offer significant protection against mortality. CONCLUSIONS/SIGNIFICANCE: MY-24 is believed to act as an inhibitor of S-adenosyl-L-homocysteine hydrolase, but our findings suggest that it may ameliorate disease by blunting the effects of the host response that play a role in disease pathogenesis. The new AG129 mouse TCRV infection model provides a safe and cost-effective means to conduct early-stage pre-clinical evaluations of candidate antiviral therapies that target clade B arenaviruses.

[Chemotherapy for Bolivian hemorrhagic fever in experimentally infected guinea pigs].

The efficacy of the antiviral agents nucleoside analogues manufactured in Russia and foreign countries against Bolivian hemorrhagic fever in the treatment and rapid prevention regimens was studied in guinea pigs. In rapid prevention and treatment of guinea pigs, the intraperitoneal injection of 1/10 of the equivalent dose of vero-ribavirin for this animal species was ascertained to protect 70% of the animals infected with Machupo virus strain Carvallo in doses 8 and 10 LD50.

Transferrin receptor 1 is a cellular receptor for New World haemorrhagic fever arenaviruses.

At least five arenaviruses cause viral haemorrhagic fevers in humans. Lassa virus, an Old World arenavirus, uses the cellular receptor alpha-dystroglycan to infect cells. Machupo, Guanarito, Junin and Sabia viruses are New World haemorrhagic fever viruses that do not use alpha-dystroglycan. Here we show a specific, high-affinity association between transferrin receptor 1 (TfR1) and the entry glycoprotein (GP) of Machupo virus. Expression of human TfR1, but not human transferrin receptor 2, in hamster cell lines markedly enhanced the infection of viruses pseudotyped with the GP of Machupo, Guanarito and Junin viruses, but not with those of Lassa or lymphocytic choriomeningitis viruses. An anti-TfR1 antibody efficiently inhibited the replication of Machupo, Guanarito, Junin and Sabia viruses, but not that of Lassa virus. Iron depletion of culture medium enhanced, and iron supplementation decreased, the efficiency of infection by Junin and Machupo but not Lassa pseudoviruses. These data indicate that TfR1 is a cellular receptor for New World haemorrhagic fever arenaviruses.

Identification and characterization of potent small molecule inhibitor of hemorrhagic fever New World arenaviruses.

Category A arenaviruses as defined by the National Institute of Allergy and Infectious Diseases (NIAID) are human pathogens that could be weaponized by bioterrorists. Many of these deadly viruses require biosafety level-4 (BSL-4) containment for all laboratory work, which limits traditional laboratory high-throughput screening (HTS) for identification of small molecule inhibitors. For those reasons, a related BSL-2 New World arenavirus, Tacaribe virus, 67-78% identical to Junín virus at the amino acid level, was used in a HTS campaign where approximately 400,000 small molecule compounds were screened in a Tacaribe virus-induced cytopathic effect (CPE) assay. Compounds identified in this screen showed antiviral activity and specificity against not only Tacaribe virus, but also the Category A New World arenaviruses (Junín, Machupo, and Guanarito). Drug resistant variants were isolated, suggesting that these compounds act through inhibition of a viral protein, the viral glycoprotein (GP2), and not through cellular toxicity mechanisms. A lead compound, ST-294, has been chosen for drug development. This potent and selective compound, with good bioavailability, demonstrated protective anti-viral efficacy in a Tacaribe mouse challenge model. This series of compounds represent a new class of inhibitors that may warrant further development for potential inclusion in a strategic stockpile.

Interferon alfacon-1 protects hamsters from lethal pichinde virus infection.

Hemorrhagic fever of arenaviral origin is a frequently fatal infectious disease of considerable priority to the biodefense mission. Historically, the treatment of arenaviral infections with alpha interferons has not yielded favorable results. Here we present evidence that interferon alfacon-1, a nonnaturally occurring bioengineered alpha interferon approved for the treatment of chronic hepatitis C, is active against Pichinde and Tacaribe arenaviruses in cell culture. In the hamster model of Pichinde virus (PCV) infection, interferon alfacon-1 treatment significantly protected animals from death, prolonged the survival of those that eventually died, reduced virus titers, and limited liver damage characteristic of PCV-induced disease. Moreover, interferon alfacon-1 also demonstrated therapeutic activity, to a lesser degree, when the initiation of treatment was delayed up to 2 days post-virus challenge. Despite the observed advantages of interferon alfacon-1 therapy, efforts to stimulate the immune system with the known interferon inducer poly(I:C12U) (Ampligen) offered only limited protection against lethal PCV challenge. Taken together, these data suggest that the increased potency of the bio-optimized interferon alfacon-1 molecule may be critical to the observed antiviral effects. These data are the first report demonstrating efficacious treatment of acute arenaviral disease with alpha interferon therapy, and further study is warranted.

Differential inhibitory action of two azoic compounds against arenaviruses.

The action of five azo-based compounds against the arenaviruses Junin (JUNV) and Tacaribe (TCRV) was evaluated in vitro by a virus yield inhibition assay in Vero cells and a cell-free virion inactivation assay. The compound 2-azo-(1'-(2'-nitroso)naphthyl)-benzoate (ANNB) was the most effective inhibitor of arenavirus production in Vero cells with EC(50) (effective concentration 50%) values in the range 6.5-26.2 microM and without inactivating properties. By contrast, the azodicarbonamide (ADA) was very effective in inactivating both arenaviruses with IC(50) (inactivating concentration 50%) values of 7.6 and 5.3 microM against JUNV and TCRV, respectively. The virucidal activity of ADA was time- and temperature-dependent. ANNB had no inhibitory action on virus binding or penetration of target cells and did not affect the synthesis of viral proteins. The most likely event susceptible to ANNB would be the process of intracellular virion assembly.

Mode of inactivation of arenaviruses by disulfide-based compounds.

Several disulfide-based compounds, including intermolecular aromatic disulfides of the type Ph-S-S-Ph and dithianes with the sulfur atoms tethered in a ring structure, have shown effective inhibitory activity against the arenaviruses Junin (JUNV), agent of Argentine hemorrhagic fever, and Tacaribe (TCRV). These compounds showed a strong virucidal effect with inactivating concentration 50% (IC(50)) values in the range 0.6-5.0 microM, and also were effective to reduce virus yields from infected cells. The mode of inactivating action of two active compounds, the aromatic bis disulfide NSC20625 and the dithiane NSC624152, was further studied. Both compounds were able to inactivate arenaviruses after a few minutes of direct contact with virions, in a concentration- and time-dependent manner. The ability of drug-treated virus to perform several steps of the replication cycle was analyzed. The killed virus particles were found to bind and enter to Vero cells with the same efficacy as infectious native virions, but the ability of inactivated virions to synthesize viral proteins in Vero cells was abolished. Thus, treatment of JUNV and TCRV with these compounds destroyed virion infectivity, generating particles which entered the host cell but were unable to complete the viral biosynthetic processes.

New neplanocin analogues. 6. Synthesis and potent antiviral activity of 6'-homoneplanocin A1.

The design, synthesis, and antiviral activities of 6'-homoneplanocin A (HNPA, 3) and its congeners having nucleobases other than adenine, such as 3-deazaadenine (4), guanine (5), thymine (6), and cytosine (7), were described. Starting from the known cyclopentenone derivative 8, the optically active (mesyloxy)cyclopentene derivative 15 was prepared, which was condensed with nucleobases then deprotected to give target compounds 3-7. Of these compounds, HNPA showed an antiviral activity spectrum that was comparable to, and an antiviral specificity that was higher than, that of neplanocin A. HNPA proved particularly active against human cytomegalovirus, vaccinia virus, parainfluenza virus, vesicular stomatitis virus, and arenaviruses, which is compatible with an antiviral action targeted at S-adenosylhomocysteine hydrolase. HNPA appears to be a promising candidate drug for the treatment of these viruses.