Antiviral drug therapy of filovirus infections: S-adenosylhomocysteine hydrolase inhibitors inhibit Ebola virus in vitro and in a lethal mouse model

Other viral bioweapons: Ebola and Marburg hemorrhagic fever

The term viral hemorrhagic fever refers to a clinical syndrome characterized by acute onset of fever accompanied by nonspecific findings of malaise, prostration, diarrhea,and headache. Patients frequently show signs of increased vascular permeability, and many develop bleeding diatheses. The hemorrhagic fever viruses represent potential agents for biologic warfare because of capability of aerosol transmission, high morbidity,and mortality associated with infection, and ability to replicate in cell culture in high concentrations. Herein we discuss the Filoviridae, the agents of Ebola and Marburg hemorrhagic fevers.

Treatment of Ebola virus infection with a recombinant inhibitor of factor VIIa/tissue factor: a study in rhesus monkeys

BACKGROUND

Infection with the Ebola virus induces overexpression of the procoagulant tissue factor in primate monocytes and macrophages, suggesting that inhibition of the tissue-factor pathway could ameliorate the effects of Ebola haemorrhagic fever. Here, we tested the notion that blockade of fVIIa/tissue factor is beneficial after infection with Ebola virus.

METHODS

We used a rhesus macaque model of Ebola haemorrhagic fever, which produces near 100% mortality. We administered recombinant nematode anticoagulant protein c2 (rNAPc2), a potent inhibitor of tissue factor-initiated blood coagulation, to the macaques either 10 min (n=6) or 24 h (n=3) after a high-dose lethal injection of Ebola virus. Three animals served as untreated Ebola virus-positive controls. Historical controls were also used in some analyses.

FINDINGS

Both treatment regimens prolonged survival time, with a 33% survival rate in each treatment group. Survivors are still alive and healthy after 9 months. All but one of the 17 controls died. The mean survival for the six rNAPc2-treated macaques that died was 11.7 days compared with 8.3 days for untreated controls (p=0.0184). rNAPc2 attenuated the coagulation response as evidenced by modulation of various important coagulation factors, including plasma D dimers, which were reduced in nearly all treated animals; less prominent fibrin deposits and intravascular thromboemboli were observed in tissues of some animals that succumbed to Ebola virus. Furthermore, rNAPc2 attenuated the proinflammatory response with lower plasma concentrations of interleukin 6 and monocyte chemoattractant protein-1 (MCP-1) noted in the treated than in the untreated macaques.

INTERPRETATION

Post-exposure protection with rNAPc2 against Ebola virus in primates provides a new foundation for therapeutic regimens that target the disease process rather than viral replication.

Preventive and therapeutic approaches to viral agents of bioterrorism

Certain viruses, such as those that cause smallpox and hemorrhagic fevers, have been identified as possible bioterrorism agents by the Centers for Disease Control and Prevention. They have been designated as potential threats because large quantities can be propagated in cell culture, they are transmissible as aerosols and, for the most part, there are only limited vaccine and pharmaceutical strategies for either prevention or treatment of established infection. An additional concern is the potential to genetically modify these agents to enhance virulence or promote resistance to vaccines or identified antivirals. Although the major impact of these agents is human illness, the release of zoonotic agents, such as the Nipah virus, would have consequences for both humans and animals because infected and noninfected animals might need to be sacrificed to control the spread of infection. Continued research is necessary to develop effective strategies to limit the impact of these biological threats.

Defense against filoviruses used as biological weapons

The filoviruses, Marburg and Ebola, are classified as Category A biowarfare agents by the Centers for Disease Control. Most known human infections with these viruses have been fatal, and no vaccines or effective therapies are currently available. Filoviruses are highly infectious by the airborne route in the laboratory, but investigations of African outbreaks have shown that person-to-person spread requires direct contact with virus-containing material. In consequence, filovirus epidemics can be halted by isolating patients and instituting standard infection control and barrier nursing procedures. The filovirus disease syndrome resembles that caused by other hemorrhagic fever viruses, necessitating studies in a biocontainment laboratory to confirm the diagnosis. Some progress has been made in developing vaccines and antiviral drugs, but efforts are hindered by the limited number of maximum containment laboratories. Terrorists might have great difficulty acquiring a filovirus for use as a weapon, but my attempt to do so because of the agents' ability to inspire fear. Accurate information is the best tool to prevent panic in the event of an attack.

Potential antiviral therapeutics for smallpox, monkeypox and other orthopoxvirus infections

We assessed the activities of 24 different antiviral compounds against smallpox (two strains of variola major and one of variola minor), monkeypox, vaccinia and cowpox viruses by a neutral red uptake assay. To establish assay parameters, we examined viral replication and its inhibition at various times postinfection and at several multiplicities of infection. Drugs were selected to target a range of functions involved in viral replication. Eight compounds (cidofovir, cyclic HPMPC (cHPMPC), HPMPA, ribavirin, tiazofurin, carbocyclic 3-deazaadenosine, 3-deazaneplanocin A and DFBA (1-(2,4-difluorobenzyloxy)adenosine perchlorate)-a derivative of adenosine N1-oxide) inhibited the replication of all three variola strains and the other orthopoxviruses at drug concentrations within a pharmacologically achievable range. Two others (methisazone and bis-POM-PMEA) showed a lesser degree of antiviral effect, while the remainder were inactive. To examine possible naturally occurring drug resistance among a large number of variola isolates obtained from different geographical regions and at different times, we examined the sensitivity of 35 different strains of variola as well as other orthopoxviruses to a subset of three of the most active compounds: cidofovir, cHPMPC, and ribavirin. Preliminary data indicate that nearly all isolates appear to have similar drug sensitivities. These findings are currently being verified and expanded.

3-deazaneplanocin A induces massively increased interferon-alpha production in Ebola virus-infected mice

3-deazaneplanocin A, an analog of adenosine, is a potent inhibitor of Ebola virus replication. A single dose early in infection prevents illness and death in Ebola virus-infected mice. The ability of this and similar compounds to block both RNA and DNA viruses has been attributed to the inhibition of a cellular enzyme, S-adenosylhomocysteine hydrolase (SAH), indirectly resulting in reduced methylation of the 5' cap of viral messenger RNA. However, we found that the protective effect of the drug resulted from massively increased production of interferon-alpha in Ebola-infected, but not uninfected mice. Peak interferon levels increased with the extent of disease at the time of treatment, indicating that production was boosted only in virus-infected cells. Ebola virus has been shown to suppress innate antiviral mechanisms of the type I interferon response. 3-deazaneplanocin A appears to reverse such suppression, restricting viral dissemination. Further development should focus on identifying adenosine analogues that produce a similar effect in Ebola virus-infected primates.

Strategies in the design of antiviral drugs

A decade ago, just five drugs were licensed for the treatment of viral infections. Since then, greater understanding of viral life cycles, prompted in particular by the need to combat human immunodeficiency virus, has resulted in the discovery and validation of several targets for therapeutic intervention. Consequently, the current antiviral repertoire now includes more than 30 drugs. But we still lack effective therapies for several viral infections, and established treatments are not always effective or well tolerated, highlighting the need for further refinement of antiviral drug design and development. Here, I describe the rationale behind current and future drug-based strategies for combating viral infections.

Viral hemorrhagic fever hazards for travelers in Africa

This short review covers 6 viral hemorrhagic fevers (VHFs) that are known to occur in Africa: yellow fever, Rift Valley fever, Crimean-Congo hemorrhagic fever, Lassa fever, Marburg virus disease, and Ebola hemorrhagic fever. All of these have at one time or another affected travelers, often the adventurous kind who are "roughing it" in rural areas, who should therefore be made aware by their physicians or travel health clinics about their potential risk of exposure to any VHF along their travel route and how to minimize the risk. A significant proportion of VHF cases involving travelers have affected expatriate health care workers who were nosocomially exposed in African hospitals or clinics. The VHFs are associated with a high case-fatality rate but are readily prevented by well-known basic precautions.

Hamao Umezawa Memorial Award Lecture: "An Odyssey in the Viral Chemotherapy Field"

In the search of effective and selective chemotherapeutic agents for the treatment of viral infections, my "Odyssey" brought me to explore a variety of approaches, encompassing interferon and interferon inducers, suramin and other polyanionic substances, S-adenosylhomocysteine hydrolase inhibitors, inosine 5'-monophosphate dehydrogenase inhibitors, 5-substituted 2'-deoxyuridines such as (E)-5-(2-bromovinyl)-2'-deoxyuridine, acyclovir (esters) and other acyclic guanosine analogues, 2',3'-dideoxynucleoside analogues, non-nucleoside reverse transcriptase inhibitors (NNRTIs), bicyclams, and acyclic nucleoside phosphonates. This had led to the identification of a number of compounds, efficacious against such important viral pathogens as human immunodeficiency virus (HIV), hepatitis B virus (HBV), hepatitis C virus (HCV), herpes simplex virus (HSV), varicella-zoster virus (VZV), cytomegalovirus (CMV), and other herpesviruses, pox-, adeno-, polyoma-, and papillomaviruses, and hemorrhagic fever viruses.

Intracellular phosphorylation of carbocyclic 3-deazaadenosine, an anti-Ebola virus agent

Carbocyclic 3-deazaadenosine (C-c3Ado) is a potent inhibitor of Ebola virus in mice by infrequent dosing, even though its half life in plasma is only 23-28 min. This prompted studies to determine whether C-c3Ado undergoes intracellular metabolism to derivatives that may promote in vivo activity. In cells, radiolabelled compound readily underwent metabolism to monophosphate, diphosphate and triphosphate (C-c3ATP) forms, with C-c3ATP being the major metabolite detected. A non-polar metabolite was also detected both inside and outside treated cells. The retention time of C-c3ATP was similar but not identical to ATP on a strong anion exchange high performance liquid chromatography (HPLC) column or on a DEAE-Sephadex open column. C-c3ATP and ATP were susceptible to degradation to their respective nucleosides by bovine alkaline phosphatase. Intracellular formation of C-c3ATP reached a plateau by about 4 h after treatment of monkey (Vero 76) and mouse (Balb/3T3 clone A31) cells with 10 or 100 microM extracellular compound. Phosphorylation was linearly dose responsive at 1, 3 and 10 microM. However, the extent of phosphorylation decreased with increasingly higher concentrations (30, 100 and 300 microM). When compound was removed from the medium, the nucleoside cleared the cells within 1 min, whereas C-c3ATP had a half life of decay of 2-3 h in five cell lines. Phosphorylation of C-c3Ado to C-c3ATP was not inhibited by cotreatment of cells (at a 20:1 ratio) with adenosine, guanosine, inosine, xanthosine, cytidine or uridine. There was no evidence of incorporation of C-c3Ado (10 microM) into macromolecules of cells over 72 h, whereas adenosine was readily incorporated. C-c3ATP may represent a form of C-c3Ado that might contribute to extending its intracellular half life or otherwise exhibit antiviral activity and/or toxicity.

Perspectives for the treatment of infections with Flaviviridae

The family Flaviviridae contains three genera: Hepacivirus, Flavivirus, and Pestivirus. Worldwide, more than 170 million people are chronically infected with Hepatitis C virus and are at risk of developing cirrhosis and/or liver cancer. In addition, infections with arthropod-borne flaviviruses (such as dengue fever, Japanese encephalitis, tick-borne encephalitis, St. Louis encephalitis, Murray Valley encephalitis, West Nile, and yellow fever viruses) are emerging throughout the world. The pestiviruses have a serious impact on livestock. Unfortunately, no specific antiviral therapy is available for the treatment or the prevention of infections with members of the Flaviviridae. Ongoing research has identified possible targets for inhibition, including binding of the virus to the cell, uptake of the virus into the cell, the internal ribosome entry site of hepaciviruses and pestiviruses, the capping mechanism of flaviviruses, the viral proteases, the viral RNA-dependent RNA polymerase, and the viral helicase. In light of recent developments, the prevalence of infections caused by these viruses, the disease spectrum, and the impact of infections, different strategies that could be pursued to specifically inhibit viral targets and animal models that are available to study the pathogenesis and antiviral strategies are reviewed.

Treatment of lethal Ebola virus infection in mice with a single dose of an S-adenosyl-L-homocysteine hydrolase inhibitor

Ebola Zaire virus causes lethal hemorrhagic fever in humans, for which there is no effective treatment. A variety of adenosine analogues inhibit the replication of Ebola virus in vitro, probably by blocking the cellular enzyme, S-adenosyl-L-homocysteine hydrolase, thereby indirectly limiting methylation of the 5' cap of viral messenger RNA. We previously observed that adult, immunocompetent mice treated thrice daily for 9 days with 2.2-20 mg/kg of an adenosine analogue, carbocyclic 3-deazaadenosine, were protected against lethal Ebola virus challenge. We now report that a single inoculation of 80 mg/kg or less of the same substance, or of 1 mg/kg or less of another analogue, 3-deazaneplanocin A, provides equal or better protection, without causing acute toxicity. One dose of drug given on the first or second day after virus infection reduced peak viremia more than 1000-fold, compared with mock-treated controls, and resulted in survival of most or all animals. Therapy was less effective when administered on the day of challenge, or on the third day postinfection. Single or multiple doses of the same medications suppressed Ebola replication in severe combined immunodeficient mice, but even daily treatment for 15 consecutive days did not eliminate the infection.

Perspectives for the treatment of infections with Flaviviridae

The family Flaviviridae contains three genera: Hepacivirus, Flavivirus, and Pestivirus. Worldwide, more than 170 million people are chronically infected with Hepatitis C virus and are at risk of developing cirrhosis and/or liver cancer. In addition, infections with arthropod-borne flaviviruses (such as dengue fever, Japanese encephalitis, tick-borne encephalitis, St. Louis encephalitis, Murray Valley encephalitis, West Nile, and yellow fever viruses) are emerging throughout the world. The pestiviruses have a serious impact on livestock. Unfortunately, no specific antiviral therapy is available for the treatment or the prevention of infections with members of the Flaviviridae. Ongoing research has identified possible targets for inhibition, including binding of the virus to the cell, uptake of the virus into the cell, the internal ribosome entry site of hepaciviruses and pestiviruses, the capping mechanism of flaviviruses, the viral proteases, the viral RNA-dependent RNA polymerase, and the viral helicase. In light of recent developments, the prevalence of infections caused by these viruses, the disease spectrum, and the impact of infections, different strategies that could be pursued to specifically inhibit viral targets and animal models that are available to study the pathogenesis and antiviral strategies are reviewed.