Ineffectiveness of adenine arabinoside and adenine arabinoside 5'-monophosphate in simian varicella infection

Animal models of varicella zoster virus infection

Primary infection with varicella zoster virus (VZV) results in varicella (chickenpox) followed by the establishment of latency in sensory ganglia. Declining T cell immunity due to aging or immune suppressive treatments can lead to VZV reactivation and the development of herpes zoster (HZ, shingles). HZ is often associated with significant morbidity and occasionally mortality in elderly and immune compromised patients. There are currently two FDA-approved vaccines for the prevention of VZV: Varivax® (for varicella) and Zostavax® (for HZ). Both vaccines contain the live-attenuated Oka strain of VZV. Although highly immunogenic, a two-dose regimen is required to achieve a 99% seroconversion rate. Zostavax vaccination reduces the incidence of HZ by 51% within a 3-year period, but a significant reduction in vaccine-induced immunity is observed within the first year after vaccination. Developing more efficacious vaccines and therapeutics requires a better understanding of the host response to VZV. These studies have been hampered by the scarcity of animal models that recapitulate all aspects of VZV infections in humans. In this review, we describe different animal models of VZV infection as well as an alternative animal model that leverages the infection of Old World macaques with the highly related simian varicella virus (SVV) and discuss their contributions to our understanding of pathogenesis and immunity during VZV infection.

Evaluation of new antiviral agents: II. The use of animal models

Antiviral chemotherapy has become a reality in the 1980s. Since the use of animal models in the testing of new antiviral agents is an inevitable step prior to clinical trial in human patients, it is important to understand the basic principles of using model systems. Briefly reviewed in this paper are the heterologous and homologous animal models which have been used for studies of various herpesvirus infections in humans. Discussions of the use of the guinea pig models mainly, for members of the Herpesviridae are presented in more detail. Precautions needed for the development of new animal models, and suggestions proposed for the use of animal models for testing new antiviral agents are outlined. It is hoped that new animal models will be developed in the foreseeable future for evaluating the much needed effective but less toxic antiviral agents for a variety of human viral diseases.

Investigation of antiviral activity of 1-beta-D-arabinofuranosylthymine (ara-T) and 1-beta-D-arabinofuranosyl-E-5-(2-bromovinyl)uracil (BV-ara-U) in monkeys infected with simian varicella virus

1-beta-D-Arabinofuranosylthymine (ara-T) and 1-beta-D-arabinofuranosyl-E-5-(2-bromovinyl)uracil (BV-ara-U) were shown to have antiviral activity in vitro and in vivo against simian varicella virus. Both compounds successfully prevented clinical disease caused by inoculation of African green monkeys with simian varicella virus, eliminating the development of rash and substantially suppressing viremia. Ara-T treatment was effective by either intraperitoneal or oral routes of administration and BV-ara-U was active by both oral and intramuscular routes. Ara-T, however, was associated with the appearance of marked signs of neurotoxicity. Histologic examination of brain tissue demonstrated chromatolysis and pyknosis of neurons and pyknotic nuclei in glial cells. The neurologic impairment persisted in affected monkeys. This observation of central nervous system toxicity in monkeys is in contrast to studies in mice and rats where high doses of ara-T by multiple routes of administration were nontoxic. No apparent toxicity was observed in monkeys treated with BV-ara-U.

Antiherpes drugs: promises and pitfalls

In recent years several selective antiherpes drugs have been developed which all show great promise for the systemic and topical treatment of herpes simplex virus and varicella-zoster virus infections. These new antiherpes agents include acyclovir, bromovinyldeoxyuridine, fluoroiodoaracytosine and phosphonoformate. Acyclovir has already been licensed for both topical and systemic use, and it is expected that other compounds will follow soon. Although this new generation of antiherpes drugs suffer from some drawbacks, i.e. narrow spectrum of activity, inefficacy during virus latency, and the possible emergence of drug-resistant virus strains, these limitations by no means outweigh the potentials of these drugs in the therapy and prophylaxis of herpesvirus infections in humans.

Oral (E)-5-(2-bromovinyl)-2'-deoxyuridine treatment of severe herpes zoster in cancer patients

BVDU [(E)-5-(2-bromovinyl)-2'-deoxyuridine] is a highly potent and selective anti-herpes drug. It is particularly active against Varicella zoster virus, as demonstrated in cell culture and animals (monkeys). BVDU has been administered orally, at a dose of 7.5 mg/kg/day for 5 days, to 20 patients with severe localised or disseminated Herpes zoster. All patients had a malignant disorder for which they had been given intensive chemo- or radiotherapy. Upon BVDU treatment a rapid cessation of the acute Herpes zoster episode was noted in all but one patient. In the majority of patients progression of the infection was arrested within 1 day of starting treatment. No toxic side-effects could be attributed to the drug at the dosage used.

Activation and antiviral effect of acyclovir in cells infected with a varicella-like simian virus

Acyclovir inhibited the replication of a varicella-like simian virus (DHV-1) in cell culture (Vero cells) with an ED50 of 38 +/- 2 microM. The activation of acyclovir in this cell culture system was compared with that in the cell system with human varicella zoster virus (VZV). Extracts of cells infected with DHV-1 catalyzed the phosphorylation of acyclovir. The phosphorylation was inhibited by dThd, suggesting the catalyst was a dThd kinase. Electrophoresis of cytosol fractions on polyacrylamide gels corroborated the existence of a virus-associated dThd kinase. This enzyme copurified with an acyclovir-phosphorylating activity. The enzyme catalyzed the phosphorylation of acyclovir at a greater relative rate than that with the VZV enzyme, but with a higher apparent Km value for acyclovir. The relative efficiencies for the two enzymes with acyclovir were similar. Anabolic studies with cells infected with DHV-1 and incubated with [14C]acyclovir indicated that triphosphate of acyclovir did accumulate. The results indicate that acyclovir is activated in cells infected with DHV-1 in a manner similar to that in cells infected with VZV.

Acyclovir treatment of experimental simian varicella infection of monkeys

Replication of simian varicella virus (SVV) in Vero cell cultures was inhibited by acyclovir, 9-(2-hydroxyethoxymethyl)guanine (ACV), at a concentration of 10 micrograms/ml in culture medium. Intravenous administration of ACV at 10 mg/kg twice a day for 10 days or 15 mg/kg three times a day for 5 days to patas monkeys (Erythrocebus patas) beginning 48 h after SVV inoculation blocked the appearance of rash and other clinical symptoms but did not affect viremia. ACV treatment of African green monkeys (Cercopithecus aethiops) at 10 mg/kg twice a day by intravenous injection beginning 24 or 72 h after SVV inoculation and continuing for 10 days had no effect on clinical symptoms, including the development of rash, or on the appearance of viremia. The minimal therapeutic results could be due to the observation that doses of 10 or 15 mg/kg produced plasma levels of ACV which were lower than 5 micrograms/ml, the concentration that inhibited SVV multiplication in vitro, and decayed rapidly.