Susceptibility of vaccine strains of varicella-zoster virus to antiviral compounds

Varicella zoster vaccines and their implications for development of HSV vaccines

Live attenuated vaccines to prevent varicella and zoster have been available in the US for the past 17 years, with a resultant dramatic decrease in varicella incidence and a predicted future decrease in the incidence of zoster. The pathogenesis and immune responses to varicella zoster virus (VZV) as well as the safety and effectiveness of VZV vaccines are reviewed. The lack of sterilizing immunity provided by VZV vaccines has not prevented them from being safe and effective. Virological and pathological information concerning parallels and differences between VZV and herpes simplex virus (HSV) are highlighted. Although VZV and HSV are distinct pathogens, they appear to have similarities in target organs and immunity that provide an expectation of a high likelihood for the success of vaccination against HSV, and predicted to be similar to that of VZV.

Susceptibility of Oka varicella vaccine strain to antiviral drugs

Susceptibility of Oka varicella vaccine virus to antiherpetic drugs was determined by the effective dose for 50% plaque reduction (ED50) using cell-free virus preparation. ED50 values were 3.02 microM for acyclovir, 3.72 microM for vidarabine, 0.0035 microM for sorivudine, and 4.67 microM for penciclovir. Oka varicella vaccine virus was as susceptible to these drugs as wild-type viruses. Sensitivity of thymidine kinase (TK)-deficient virus to penciclovir and of some DNA polymerase (DPase) mutants to sorivudine suggested that these drugs might be used for the treatment of vaccine recipients, even if Oka varicella vaccine became acyclovir-resistant by mutations in the TK or DPase genes, respectively. This result encourages the wider use of Oka varicella vaccine even for immunocompromised hosts because of its attenuation and susceptibility to chemotherapeutic drugs.

Comparison of antiviral assay methods using cell-free and cell-associated varicella-zoster virus

Assay methods for varicella-zoster virus (VZV) susceptibility to acyclovir (ACV) of VZV were compared by using cell-free (CF) and cell-associated (CA) virus of 6 x plaque-purified VZV. The 50% effective doses (ED50) of ACV, as required to reduce virus plaque formation by 50%, were about 8 times higher for CA virus than for CF virus. Also, the ED50 of 1-beta-D-arabinofuranosyl-(E)-5-(2-bromovinyl)uracil (BVaraU) for CA-VZV was higher than for CF-VZV, and fresh clinical isolates of VZV gave higher ACV ED50 values than CF virus. CA virus prepared at various times after CF virus infection showed a gradual increase of the ACV ED50 with time, ranging from the ED50 for CF virus to that for CA virus.

Current status and prospects of live varicella vaccine

Since its development in 1974 the Oka strain live attenuated varicella vaccine has been tested in healthy and immunocompromised adults and children. Its safety and efficacy have been established and it is now licensed for general use in Japan and Korea, and for immunocompromised patients in several other countries. Possibilities for the future include its use to prevent zoster in the elderly, its incorporation in a multivalent vaccine and its use as a vehicle to express foreign genes in recombinant vaccines.

Acyclovir. An updated review of its antiviral activity, pharmacokinetic properties and therapeutic efficacy

Acyclovir (aciclovir) is a nucleoside antiviral drug with antiviral activity in vitro against members of the herpes group of DNA viruses. As an established treatment of herpes simplex infection, intravenous, oral and to a lesser extent topical formulations of acyclovir provide significant therapeutic benefit in genital herpes simplex and recurrent orofacial herpes simplex. The effect of acyclovir therapy is maximised by early initiation of treatment, especially in non-primary infection which tends to have a less protracted course than the primary episode. Long term prophylactic oral acyclovir, in patients with frequent episodes of genital herpes simplex, totally suppresses recurrences in the majority of subjects; as with other infections responding to acyclovir, viral latency is not eradicated and pretreatment frequencies of recurrence return after discontinuation of treatment. Caution should accompany the prophylactic use of acyclovir in the general population, due to the theoretical risk of the emergence of viral strains resistant to acyclovir and other agents whose mechanism of action is dependent on viral thymidine kinase. Intravenous acyclovir is the treatment of choice in biopsy-proven herpes simplex encephalitis in adults, and has also been successful in the treatment of disseminated herpes simplex in pregnancy and herpes neonatorium. Intravenous and oral acyclovir protect against dissemination and progression of varicella zoster virus infection, but do not protect against post-herpetic neuralgia. In immunocompromised patients, intravenous, oral and topical acyclovir shorten the clinical course of herpes simplex infections while prophylaxis with oral or intravenous dosage forms suppresses reactivation of infection during the period of drug administration. Ophthalmic application of 3% acyclovir ointment rapidly heals herpetic dendritic corneal ulcers and superficial herpetic keratitis. Thus, despite an inability to eradicate latent virus, acyclovir administered in therapeutic or prophylactic fashion is now the standard antiviral therapy in several manifestations of herpes simplex virus infection, and indeed represents a major advance in this regard. With the exception of varicella zoster virus infections, early optimism concerning the use of the drug in diseases due to other herpes viruses has generally not been supported in clinical investigations.

Antiviral effects of phosphonoformate (PFA, foscarnet sodium)

This article describes the antiviral properties of foscarnet (trisodium phosphonoformate) at the enzyme level as well as in cell cultures and in vivo. The mechanism of action against herpesvirus DNA polymerases and reverse transcriptases is outlined. Clinical studies using topical foscarnet against mucocutaneous herpes simplex virus infections are presented. The clinical use of intravenous foscarnet against severe viral infections caused by cytomegalovirus, hepatitis B virus and human immunodeficiency virus is discussed.

Evaluation of varicella-zoster antiviral drugs by a nucleic acid hybridization assay

A simple nucleic acid hybridization assay was developed for the evaluation of antiviral compounds with activity against varicella-zoster virus. Antiviral-induced reduction in varicella-zoster virus DNA, as measured by hybridization with 32P-labeled probes, correlated with drug-induced inhibition of viral cytopathic effect in cell culture. Three compounds were shown to be more potent than acyclovir at inhibiting varicella-zoster virus in vitro.

Use of an enzyme-linked immunosorbent assay performed directly on fixed infected cell monolayers for evaluating drugs against varicella-zoster virus

An in situ enzyme-linked immunosorbent assay (ELISA), performed directly on fixed varicella-zoster virus-infected monolayers, was used to quantitate viral antigen, and by color reduction it was used to evaluate the activity of drugs against varicella-zoster virus. Color production in the ELISA (optical density) was directly related to the dose of input virus. Antigen representing 5 to 10 plaques could be detected 3 days after infection. The ELISA was specific and reproducible, as shown by absorption and repeat experiments, respectively. A color-reduction test by ELISA was compared with the conventional plaque-reduction assay for its ability to measure the antiviral activity of acyclovir, bromovinyldeoxyuridine, trifluorothymidine, and vidarabine against four strains of varicella-zoster virus. In all cases but one the 50% inhibitory doses were lower when measured by ELISA than by the plaque-reduction assay. This in situ ELISA color reduction method had the following advantages over the conventional plaque-reduction assay: the endpoint was an objective measurement; there was less well-to-well variation; the assay was sensitive to changes in plaque size as well as plaque number; it was less labor intensive.