Meningoradiculoneuritis due to acyclovir-resistant varicella zoster virus in an acquired immune deficiency syndrome patient

Advances and Perspectives in the Management of Varicella-Zoster Virus Infections

Varicella-zoster virus (VZV), a common and ubiquitous human-restricted pathogen, causes a primary infection (varicella or chickenpox) followed by establishment of latency in sensory ganglia. The virus can reactivate, causing herpes zoster (HZ, shingles) and leading to significant morbidity but rarely mortality, although in immunocompromised hosts, VZV can cause severe disseminated and occasionally fatal disease. We discuss VZV diseases and the decrease in their incidence due to the introduction of live-attenuated vaccines to prevent varicella or HZ. We also focus on acyclovir, valacyclovir, and famciclovir (FDA approved drugs to treat VZV infections), brivudine (used in some European countries) and amenamevir (a helicase-primase inhibitor, approved in Japan) that augur the beginning of a new era of anti-VZV therapy. Valnivudine hydrochloride (FV-100) and valomaciclovir stearate (in advanced stage of development) and several new molecules potentially good as anti-VZV candidates described during the last year are examined. We reflect on the role of antiviral agents in the treatment of VZV-associated diseases, as a large percentage of the at-risk population is not immunized, and on the limitations of currently FDA-approved anti-VZV drugs. Their low efficacy in controlling HZ pain and post-herpetic neuralgia development, and the need of multiple dosing regimens requiring daily dose adaptation for patients with renal failure urges the development of novel anti-VZV drugs.

Differences in the Likelihood of Acyclovir Resistance-Associated Mutations in the Thymidine Kinase Genes of Herpes Simplex Virus 1 and Varicella-Zoster Virus

Acyclovir (ACV) resistance-associated mutations in two recombinant herpes simplex virus 1 (HSV-1) clones were compared. Recombinant HSV-1 lacking its thymidine kinase (TK) and expressing varicella-zoster virus (VZV) TK ectopically had no mutations in the VZV TK gene. In contrast, recombinant HSV-1 expressing HSV-1 TK ectopically harbored mutations in the HSV-1 TK gene. These results suggest that the relatively low frequency of ACV-resistant VZV is a consequence of the characteristics of the TK gene.

Pharmacokinetics and Safety of Amenamevir in Healthy Subjects: Analysis of Four Randomized Phase 1 Studies

Introduction

Amenamevir (ASP2151) is a nonnucleoside antiherpesvirus compound available for the treatment of varicella–zoster virus infections. In this article we summarize the findings of four phase 1 studies in healthy participants.

Methods

Four randomized phase 1 studies investigated the safety and pharmacokinetics of single and multiple doses of amenamevir, including the assessment of age group effect (nonelderly vs elderly), food effect, and the relative bioavailability of two formulations. Amenamevir was administered orally at various doses as a single dose (5–2400 mg) or daily (300 or 600 mg/day) for 7 days.

Results

Following single and multiple oral doses, amenamevir demonstrated a less than dose proportional increase in the pharmacokinetic parameters area under the plasma drug concentration versus time curve from time zero to infinity (AUC_inf) and C_max. After single and multiple oral 300-mg doses of amenamevir, no apparent differences in pharmacokinetics were observed between nonelderly and elderly participants. In contrast, with the amenamevir 600-mg dose both the area under the plasma drug concentration versus time curve from time zero to 24 h and C_max were slightly increased and renal clearance was decreased in elderly participants. The pharmacokinetics of amenamevir was affected by food, with AUC_inf increased by about 90%. In the bioavailability study, AUC_inf and C_max were slightly lower following tablet versus capsule administration (decreased by 14 and 12%, respectively), with relative bioavailability of 86%. The different amenamevir doses and formulations were safe and well tolerated; no deaths or serious adverse events were reported.

Conclusion

Amenamevir had less than dose proportional pharmacokinetic characteristics. Age may have an influence on amenamevir pharmacokinetics; however, the effect was considered minimal. The pharmacokinetics of amenamevir were affected by food, with AUC_inf almost doubling when amenamevir was administered with food. The concentration versus time profile of the tablet was slightly lower than that of the capsule; the relative bioavailability of the tablet versus the capsule was 86%. Amenamevir was safe and well tolerated in the dose range investigated.

Funding

Astellas Pharma.

Trial registration

ClinicalTrials.gov identifiers NCT02852876 (15L-CL-002) and NCT02796118 (15L-CL-003).

Electronic supplementary material

The online version of this article (10.1007/s12325-017-0642-4) contains supplementary material, which is available to authorized users.

Advances in the treatment of varicella-zoster virus infections

Varicella-zoster virus (VZV) causes two distinct diseases, varicella (chickenpox) and shingles (herpes zoster). Chickenpox occurs subsequent to primary infection, while herpes zoster (usually associated with aging and immunosuppression) appears as a consequence of reactivation of latent virus. The major complication of shingles is postherpetic neuralgia. Vaccination strategies to prevent varicella or shingles and the current status of antivirals against VZV will be discussed in this chapter. Varivax®, a live-attenuated vaccine, is available for pediatric varicella. Zostavax® is used to boost VZV-specific cell-mediated immunity in adults older than 50 years, which results in a decrease in the burden of herpes zoster and pain related to postherpetic neuralgia. Regardless of the availability of a vaccine, new antiviral agents are necessary for treatment of VZV infections. Current drugs approved for therapy of VZV infections include nucleoside analogues that target the viral DNA polymerase and depend on the viral thymidine kinase for their activation. Novel anti-VZV drugs have recently been evaluated in clinical trials, including the bicyclic nucleoside analogue FV-100, the helicase-primase inhibitor ASP2151, and valomaciclovir (prodrug of the acyclic guanosine derivative H2G). Different candidate VZV drugs have been described in recent years. New anti-VZV drugs should be as safe as and more effective than current gold standards for the treatment of VZV, that is, acyclovir and its prodrug valacyclovir.

In vitro-selected drug-resistant varicella-zoster virus mutants in the thymidine kinase and DNA polymerase genes yield novel phenotype-genotype associations and highlight differences between antiherpesvirus drugs

Varicella zoster virus (VZV) is usually associated with mild to moderate illness in immunocompetent patients. However, older age and immune deficiency are the most important risk factors linked with virus reactivation and severe complications. Treatment of VZV infections is based on nucleoside analogues, such as acyclovir (ACV) and its valyl prodrug valacyclovir, penciclovir (PCV) as its prodrug famciclovir, and bromovinyldeoxyuridine (BVDU; brivudin) in some areas. The use of the pyrophosphate analogue foscarnet (PFA) is restricted to ACV-resistant (ACV(r)) VZV infections. Since antiviral drug resistance is an emerging problem, we attempt to describe the contributions of specific mutations in the viral thymidine kinase (TK) gene identified following selection with ACV, BVDU and its derivative BVaraU (sorivudine), and the bicyclic pyrimidine nucleoside analogues (BCNAs), a new class of potent and specific anti-VZV agents. The string of 6 Cs at nucleotides 493 to 498 of the VZV TK gene appeared to function as a hot spot for nucleotide insertions or deletions. Novel amino acid substitutions (G24R and T86A) in VZV TK were also linked to drug resistance. Six mutations were identified in the "palm domain" of VZV DNA polymerase in viruses selected for resistance to PFA, PCV, and the 2-phophonylmethoxyethyl (PME) purine derivatives. The investigation of the contributions of specific mutations in VZV TK or DNA polymerase to antiviral drug resistance and their impacts on the structures of the viral proteins indicated specific patterns of cross-resistance and highlighted important differences, not only between distinct classes of antivirals, but also between ACV and PCV.

Emerging drugs for varicella-zoster virus infections

INTRODUCTION

Varicella-zoster virus (VZV) is the etiological agent of two distinct diseases, varicella (chickenpox) and shingles (herpes zoster). Chickenpox occurs following primary infection, while herpes zoster (usually associated with ageing and immunosuppression) is the consequence of reactivation of the latent virus. Post-herpetic neuralgia is the major complication of shingles.

AREAS COVERED

This review will discuss vaccination strategies and the current status of antivirals against VZV. A live attenuated vaccine, Varivax, is available for pediatric varicella while Zostavax was developed to boost VZV-specific cell-mediated immunity in adults older than 60 years and, via this mechanism, to decrease the burden of herpes zoster and pain associated with post-herpetic neuralgia. Despite the availability of a vaccine, there is a need for new antiviral agents. Current drugs approved for the treatment of VZV infections include nucleoside analogs that target the viral DNA polymerase and depend on the viral thymidine kinase. Novel anti-VZV drugs have recently been evaluated in clinical trials, including the bicyclic nucleoside analog FV-100, the helicase-primase inhibitor ASP2151 and valomaciclovir (prodrug of the acyclic guanosine derivative H2G).

EXPERT OPINION

New anti-VZV drugs should be as safe as and more effective than acyclovir and its prodrug valacyclovir (current gold standard for the treatment of VZV).

Resistance testing of clinical varicella-zoster virus strains

Acyclovir resistance of varicella-zoster virus (VZV) has been reported in rare cases of immunocompromised patients. In this study, the natural polymorphism of the thymidine kinase (TK) and DNA polymerase (pol) genes was examined in 51 clinical VZV isolates sensitive to acyclovir (ACV). In addition, 16 VZV strains with clinical resistance to ACV were analyzed. None of the ACV-sensitive strains of the clades 1, 3 and 5 showed gene polymorphism of the TK. By contrast, the DNA pol gene exhibited polymorphism-related substitutions as a function of the VZV clade. The novel substitutions M286I, E824Q, R984H and H1089Y were detected in strains of clades 3 and 5. In the TK gene of 7 VZV strains with clinical ACV resistance, the novel substitutions L73I, A163stop, W225R, T256M, N334stop and the deletion of nucleotides 19-223 were found to be associated most likely with resistance. In one strain showing the substitution W225R, ACV resistance could be confirmed by the viral phenotype. In the DNA pol gene, the novel amino acid substitutions T237K and A955T could be detected, but their significance remains unclear. In conclusion, the characterization of resistance using genetic analysis of the TK and DNA pol genes has to be considered the method of choice for the determination of VZV resistance to antiviral drugs. In a considerable number of patients with clinical ACV-resistant VZV infections, resistance cannot be verified by virological methods.

Foscarnet salvage therapy for acyclovir-resistant varicella zoster: report of a novel thymidine kinase mutation and review of the literature

The authors describe an acyclovir-resistant varicella zoster virus infection in a pediatric patient after hematopoietic stem cell transplant, the use of foscarnet as salvage therapy, and review the literature to clarify the pediatric experience with foscarnet in this setting. A novel thymidine kinase mutation is described, along with a new phenotypic assay for characterizing acyclovir resistance in varicella zoster virus.

Susceptibilities of several clinical varicella-zoster virus (VZV) isolates and drug-resistant VZV strains to bicyclic furano pyrimidine nucleosides

Varicella-zoster virus (VZV) is responsible for primary infections as well as reactivations after latency in the dorsal root ganglia. The treatment of such infections is mandatory for immunocompromised patients and highly recommended for elderly patients with herpes zoster infections (also called zona or shingles). The treatment of choice is presently based on four molecules, acyclovir (ACV), valaciclovir, famciclovir, and (in Europe) brivudine (BVDU). We present here our data on the antiviral activity of a new class of potent and selective anti-VZV compounds, bicylic pyrimidine nucleoside analogues (BCNAs), against a broad variety of clinical isolates and different drug-resistant virus strains. The results show that the BCNAs are far more potent inhibitors than ACV and BVDU against clinical VZV isolates as well as the VZV reference strains Oka and YS. The BCNAs were not active against ACV- and BVDU-resistant VZV strains bearing mutations in the viral thymidine kinase gene but kept their inhibitory potential against virus strains with mutations in the VZV DNA polymerase gene. Mutant virus strains selected in the presence of the BCNAs were solely cross-resistant to drugs, such as ACV and BVDU, that depend for their antiviral action on metabolic activation by the viral thymidine kinase.

In vitro selection of drug-resistant varicella-zoster virus (VZV) mutants (OKA strain): differences between acyclovir and penciclovir?

Varicella-zoster virus (VZV) mutants were isolated under the pressure of different classes of antiviral compounds: (i) drugs that depend on the viral thymidine kinase (TK) for their activation, i.e. acyclovir (ACV), brivudin (BVDU), penciclovir (PCV) and sorivudine (BVaraU); (ii) drugs that are independent of the viral TK for their activation, i.e. 2-phosphonylmethoxyethyl (PME) derivatives of adenine (PMEA, adefovir) and 2,6-diaminopurine (PMEDAP); and (iii) drugs that do not require any metabolism to inhibit the viral DNA polymerase, i.e. foscarnet (PFA). Drug-resistant virus strains were obtained by serial passage of the OKA strain in human embryonic lung (HEL) fibroblasts and the different drug-resistant mutants were subsequently evaluated for their in vitro susceptibility to a broad range of antiviral drugs. Virus strains emerging under the pressure of ACV, BVDU and BVaraU were cross-resistant to all drugs that depend on the viral TK for activation, but remained susceptible to the acyclic nucleoside phosphonates (i.e. PMEA, PMEDAP and the 3-hydroxy-2-phosphonylmethoxypropyl derivatives of adenine (HPMPA) and cytosine (HPMPC, cidofovir)) and PFA. In contrast, the virus strains selected under pressure of PCV were resistant to PCV, ACV, PMEA and PFA; but not BVDU, BVaraU, GCV, HPMPC or HPMPA. Similar patterns of drug susceptibility were noted for the virus strains selected under the pressure of PMEA or PFA, pointing to an alteration in the viral DNA polymerase as basis for the resistant phenotype selected by PCV, as well as PMEA and PFA. In contrast, the resistant phenotype selected by ACV as well as BVDU and BVaraU may be attributed primarily to mutations in the viral TK gene. Our data thus indicate that ACV and PCV select in vitro for different drug-resistant VZV phenotypes; whether this is also the situation in vivo remains to be investigated.

Crystal structure of varicella zoster virus thymidine kinase

Herpes virus thymidine kinases are responsible for the activation of nucleoside antiviral drugs including (E)-5-(2-bromovinyl)-2'-deoxyuridine. Such viral thymidine kinases (tk), beside having a broader substrate specificity compared with host cell enzymes, also show significant variation in nucleoside phosphorylation among themselves. We have determined the crystal structure of Varicella zoster virus (VZV, human herpes virus 3) thymidine kinase complexed with (E)-5-(2-bromovinyl)-2'-deoxyuridine 5'-monophosphate and ADP. Differences in the conformation of a loop region (residues 55-61) and the position of two alpha-helices at the subunit interface of VZV-tk compared with the herpes simplex virus type 1 (human herpes virus 1) enzyme give rise to changes in the positioning of residues such as tyrosine 66 and glutamine 90, which hydrogen bond to the substrate in the active site. Such changes in combination with the substitution in VZV-tk of two phenylalanine residues (in place of a tyrosine and methionine), which sandwich the substrate pyrimidine ring, cause an alteration in the positioning of the base. The interaction of the (E)-5-(2-bromovinyl)-2'-deoxyuridine deoxyribose ring with the protein is altered by substitution of tyrosine 21 and phenylalanine 139 (analagous to herpes simplex virus type 1 histidine 58 and tyrosine 172), which may explain some of the differences in nucleoside sugar selectivity between both enzymes. The altered active site architecture may also account for the differences in the substrate activity of ganciclovir for the two thymidine kinases. These data should be of use in the design of novel antiherpes and antitumor drugs.

Antiviral agents: Nonantiviral drugs

The current arsenal of antiviral agents available to the practitioner is expanding rapidly, such that by the time this article goes to press, new drugs may have already been added. Although the majority of approved drugs have been developed for use in only a few viral infections (eg, HIV, herpesviruses, and papillomavirus), discoveries made in the development of these drugs may lead to antiviral agents effective against other viruses. In addition, new uses for the currently available drugs are under evaluation. This review of antiviral agents discusses the treatments available for viral infections such as herpes simplex virus, varicella zoster virus, cytomegalovirus, human papillomavirus, chronic viral hepatitis, and others.

Clinical and virologic characterization of acyclovir-resistant varicella-zoster viruses isolated from 11 patients with acquired immunodeficiency syndrome

We studied the clinical resistance to acyclovir of infections with varicella-zoster viruses (VZV) in patients with acquired immunodeficiency syndrome, and we correlated it to virologic analyses. Eleven patients with VZV infections (treated with acyclovir, 30 mg/kg/day, given intravenously, or 4 g/day, given orally) were included in the study because of the failure of 10 days of acyclovir therapy. Susceptibility of VZV isolates to acyclovir was tested using a plaque reduction assay to determine the 50% inhibitory concentration (IC(50)) of acyclovir and the SI(50) (IC(50) of the patient isolate/IC(50) of the reference strain) to acyclovir. The thymidine kinase (TK) gene, which supports the resistance, was sequenced on amplified products. Only 3 patients had a significant increase in the IC(50), as compared with the IC(50) of the reference strain (SI(50) of > or =4), and a mutation in the TK gene. For the other 8 patients, the clinical resistance was not confirmed by the virologic results: the SI(50) was < 4, and no mutation was detected in the TK gene. Because no acyclovir-resistant strain appeared during a shorter period of time, we suggest an increase in the duration of the treatment to 21 days before acyclovir resistance is suspected.

Characterization of the DNA polymerase gene of varicella-zoster viruses resistant to acyclovir

The nucleotide changes of the DNA polymerase gene and the susceptibility of acyclovir (ACV)-resistant varicella-zoster virus (VZV) mutants to anti-herpetic drugs were determined and compared to those of herpes simplex virus type 1 (HSV-1) mutants. The seven ACV-resistant VZV mutants were classified into three groups, N(779)S, G(805)C and V(855)M, according to the sequences of their DNA polymerase genes. The amino acid substitutions N(779)S and G(805)C were identical in position to the N(815)S and G(814)C mutations in the HSV-1 DNA polymerase mutants, respectively, and the V(855)M amino acid substitution was similar to the HSV-1 V(892)M mutation. All three groups of VZV mutants were susceptible to ACV, phosphonoacetic acid, vidarabine and aphidicolin, at levels similar to those seen with the respective HSV-1 mutants, except for subtle differences that were due possibly to the non-conserved regions in their sequences. Although both the HSV-1 and the VZV DNA polymerase genes show 53% sequence similarity, both viruses essentially show a similar biochemical behaviour.

Disseminated vaccine strain varicella as the acquired immunodeficiency syndrome-defining illness in a previously undiagnosed child

The Food and Drug Administration licensed a live-virus varicella vaccine (Varivax; Merck & Co Inc, West Point, PA) in March 1995. Prelicensure adverse events were minimal; however, since licensure and increased vaccine use, rare previously undetected risks have arisen. Presented here is the clinical course of a previously undiagnosed, human immunodeficiency virus-infected boy who developed dissemination of the vaccine strain of varicella zoster after immunization. chickenpox, human immunodeficiency virus, pneumonia, encephalopathy, varicella vaccine, adverse events, dissemination.

Novel agents for the therapy of varicella-zoster virus infections

Varicella-zoster virus (VZV), a member of the herpesvirus family, is responsible for both primary (varicella or chickenpox) as well as recurrent (zoster or shingles) infections. Acyclovir has been the mainstay for treating VZV infections in both immunocompetent and immunocompromised patients. Recently, newer anti-VZV drugs, i.e., valaciclovir (the oral prodrug form of acyclovir) and famciclovir (the oral prodrug form of penciclovir) have been developed and have enlarged the therapeutic options to treat VZV infections. Both acyclovir and penciclovir are dependent on the virus-encoded thymidine kinase (TK) for their intracellular activation. Although emergence of drug-resistant strains does not occur in immunocompetent patients, several reports have documented the isolation of drug-resistant VZV strains following long-term acyclovir therapy in immunocompromised patients. Mutations at the level of the TK are responsible for development of resistance to drugs that depend on the viral TK for their phosphorylation (i.e., acyclovir and penciclovir). Foscarnet, a direct inhibitor of the viral DNA polymerase, which does not require activation by the viral TK, is the drug of choice for the treatment of TK-deficient VZV mutants emerging under acyclovir therapy. Recently, emergence of foscarnet-resistant strains has also been reported. Both TK-deficient strains and foscarnet-resistant mutants are sensitive to the acyclic nucleoside phosphonate cidofovir, CDV, HPMPC, (S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine. This agent does not depend on the virus-encoded TK, but on cellular enzymes for its conversion to the diphosphoryl derivative, which then inhibits the viral DNA polymerase.

A rapid phenotypic assay for detection of acyclovir-resistant varicella-zoster virus with mutations in the thymidine kinase open reading frame

Susceptibility assays by cell culture methods are time-consuming and are particularly difficult to perform with varicella-zoster virus (VZV). To overcome this limitation, we have adapted a functional test of the viral thymidine kinase (TK) in TK-deficient (tdk mutant) bacteria to detect ACV-resistant VZV in clinical samples. After PCR amplification, the complete viral TK open reading frame (ORF) is purified from PCR primers, digested with two restriction enzymes, and ligated in an oriented fashion into a bacterial expression vector. The ligation products are then used to transform tdk mutant bacteria. After transformation, an aliquot of the bacteria is plated onto a plate with minimal medium containing (i) ampicillin to select for plasmids carrying the viral TK ORF and (ii) isopropyl beta-D-thiogalactopyranoside (IPTG) to induce its expression. An identical aliquot of bacteria is also plated onto a medium containing, in addition to the components described above, 5-fluorodeoxyuridine (FUdR). Compared to the number of transformants on FUdR-free medium, the number of colonies carrying TK derived from susceptible strains was reduced by 86%, on average, in the presence of FUdR. In contrast, the number of transformants carrying TK from resistant strains with a mutant TK were reduced by only 4%, on average, on FUdR-containing plates. We have assessed the validity of this assay with cell culture isolates and several clinical samples including two cerebrospinal fluid samples from which no virus could be isolated. This colony reduction assay allowed the correct identification of the TK phenotype of each VZV isolate tested and can be completed within 3 days of receipt of the sample.

Phenotypic and genetic characterization of thymidine kinase from clinical strains of varicella-zoster virus resistant to acyclovir

Varicella-zoster virus (VZV) is a common herpesvirus responsible for disseminated or chronic infections in immunocompromised patients. Effective drugs such as acyclovir (ACV), famciclovir (prodrug of penciclovir), and foscarnet are available to treat these infections. Here we report the phenotypic and genetic characterization of four ACV-resistant VZV strains isolated from AIDS patients and transplant recipients. Sensitivity to six antiviral drugs was determined by an enzyme-linked immunosorbent assay, viral thymidine kinase (TK) activity was measured by comparing [(3)H]thymidine and 1-beta-D-arabinofuranosyl-[(3)H]thymine as substrates, and the TK gene open reading frame was sequenced. Three strains were found to be TK deficient, and the fourth was a mixed population composed of TK-positive and TK-deficient viruses. Each strain presented a unique TK gene mutation that could account for ACV resistance. In one strain, the deletion of two nucleotides at codon 215 induced a premature stop signal at codon 217. In another strain, a single nucleotide addition at codon 167 resulted in a premature stop signal at codon 206. In both other strains, we identified amino acid substitutions already described in other ACV-resistant VZV strains: either Glu-->Gly at residue 48 or Arg-->Gly at residue 143. According to our work and data previously reported on resistant VZV strains, there are three areas in the TK gene where 71% of the mutations described to date are located. These areas are putative candidates for a genotypic diagnosis of ACV resistance.

Chronic verrucous varicella zoster virus skin lesions: clinical, histological, molecular and therapeutic aspects

The outbreak of HIV infection introduced a new phenomenon in varicella zoster virus (VZV) pathology, namely the long-standing wart-like skin lesions that are frequently associated with resistance to thymidine kinase (TK)-dependent antiviral agents. This paper reviews the clinical, histological, and molecular aspects and the therapeutic management of these verrucous lesions. The majority of lesions are characterized by chronically evolving, unique or multiple wart-like cutaneous lesions. The main histopathological features include hyperkeratosis, verruciform acanthosis and VZV-induced cytopathic changes with scant or absent cytolysis of infected keratinocytes. The mechanism that establishes the chronic nature of the lesions appears to be associated with a particular pattern of VZV gene expression exhibiting reduced or nondetectable gE and gB synthesis. Drug resistance to TK-dependent antiviral agents is a result of nonfunctional or deficient viral TK. This necessitates alternative therapeutic management using antiviral agents that target the viral DNA polymerase.

Current pharmacological approaches to the therapy of varicella zoster virus infections: a guide to treatment

Varicella zoster virus (VZV), a member of the herpesvirus family, is responsible for both primary (varicella, chickenpox) as well as reactivation (zoster, shingles) infections. In immunocompetent patients, the course of varicella is generally benign. For varicella zoster, post-herpetic neuralgia is the most common complication. In immunocompromised patients (particularly those with AIDS), transplant recipients and cancer patients, VZV infections can be life-threatening. For these patients and also for immunocompetent patients at risk such as pregnant women or premature infants, the current treatment of choice is based on either intravenous or oral aciclovir (acyclovir). The low oral bioavailability of aciclovir, as well as the emergence of drug-resistant virus strains, have stimulated efforts towards the development of new compounds for the treatment of individuals with VZV infections. Among these new compounds, penciclovir, its oral prodrug form famciclovir and the oral pro-drug form of aciclovir (valaciclovir), rank among the most promising. As with aciclovir itself, all of these drugs are dependent on the virus-encoded thymidine kinase (TK) for their intracellular activation (phosphorylation), and, upon conversion to their triphosphate form, they act as inhibitors/alternative substrate of the viral DNA polymerase. Therefore, cross-resistance to these drugs may be expected for those virus mutants that are TK-deficient and thus resistant to aciclovir. Other classes of nucleoside analogues dependent for their phosphorylation on the viral TK that have been pursued for the treatment of VZV infections include sorivudine, brivudine, fialuridine, fiacitabine and netivudine. Among oxetanocins, which are partially dependent on viral TK, lobucavir is now under clinical evaluation. Foscarnet, which does not require any previous metabolism to interact with the viral DNA polymerase, is used in the clinic when TK-deficient VZV mutants emerge during aciclovir treatment. TK-deficient mutants are also sensitive to the acyclic nucleoside phosphonates (i.e. [s]-1-[3-hydroxy-2-phosphonylmethoxypropyl]cytosine; HPMPC); these agents do not depend on the virus-encoded TK for their phosphorylation but depend on cellular enzymes for conversion to their diphosphoryl derivatives which then inhibit viral DNA synthesis. Vaccination for VZV has now come of age. It is recommended for healthy children, patients with leukaemia, and patients receiving immunosuppressive therapy or those with chronic diseases. The protection induced by the vaccine seems, to some extent, to include zoster and associated neuralgia. Passive immuniatin based on specific immunoglobulins does not effectively prevent VZV infection and is therefore restricted to high risk individuals (i.e. immunocompromised children and pregnant women).

Emergence of resistance to acyclovir and penciclovir in varicella-zoster virus and genetic analysis of acyclovir-resistant variants

We have characterized the differential actions of acyclovir and penciclovir against varicella-zoster virus (VZV) in cell culture by comparing the frequency of appearance of resistant viruses followed by their characterization. Cells were infected with cell-free virus and the cultures were successively treated with increasing concentrations of acyclovir or penciclovir. Drug-resistant viruses were selected in the presence of 6 microg/ml of acyclovir or penciclovir. The emergence frequency of resistant viruses was significantly higher following acyclovir exposure than following penciclovir exposure (Fisher's exact test, P<0.0001), possibly reflecting virus growth differences under these experimental conditions. Based on antiviral drug susceptibility and thymidine kinase (TK) activity assays, 11 acyclovir-resistant variants from seven experiments using three virus strains (Kawaguchi strain, Oka varicella vaccine strain and a clinical isolate from a zoster patient) were found to be TK-deficient. Sequence analysis of TK-deficient variants of the Kawaguchi strain revealed deletions that caused frameshifts, resulting in premature termination in the TK gene.

Review of varicella zoster seroepidemiology in India and Southeast Asia

Varicella zoster virus (VZV) predominantly affects children in temperate countries, with near-universal seroconversion occurring by late childhood. However, in tropical regions, VZV infection is common in adolescents and adults. This review identifies age-related VZV seroprevalence patterns in a number of Asian countries which indicate that seroconversion in tropical countries occurs at a later age than in temperate countries. Seasonal and regional variations in acute disease within some Asian countries suggest that temperate climates might favour transmission of the varicella virus, with incidence peaking during cooler months and in cooler, more temperate regions. VZV infection is often more severe in adults than in children, suggesting that tropical countries may be at risk of greater morbidity and mortality as a result of later-age seroconversion. Susceptibility of pregnant women and their infants, and of people infected with HIV/AIDS is also cause for concern. Vaccination may be beneficial in reducing the impact of VZV in Asian populations.

Infections of the nervous system

Epidemiologic trends causing infections of the nervous system remain a significant source of morbidity and mortality one half-century after the introduction of penicillin. This article outlines common causes of bacterial meningitis, aseptic meningitis syndrome, encephalitis, abscess, spinal cord syndromes, and cranial and peripheral nerve problems. Recommendations for diagnostic evaluation and both empiric and definitive antimicrobial therapy are offered; controversial management issues are also discussed. The protean manifestations of varicella-zoster virus and Lyme diseases are outlined. In addition, special considerations in the immunocompromised host, including organ transplant recipients, cancer patients, and HIV-positive persons are explained, and antimicrobial therapy is discussed.

In search of a selective antiviral chemotherapy

This article describes several approaches to a selective therapy of virus infections: (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU [brivudin]) for the therapy of herpes simplex virus type 1 and varicella-zoster virus infections: (S)-9-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine (HPMPC [cidofovir]) for the therapy of various DNA virus (i.e., herpesvirus, adenovirus, papillomavirus, polyomavirus, and poxvirus) infections; 9-(2-phosphonylmethoxyethyl)adenine (PMEA [adefovir]) for the therapy of retrovirus, hepadnavirus, and herpesvirus infections; (R)-9-(2-phosphonylmethoxypropyl)adenine (PMPA) for the therapy and prophylaxis of retrovirus and hepadnavirus infections; and nonnucleoside reverse transcriptase inhibitors (NNRTIs), such as tetrahydroimidazo[4,5,1-jk][1,4]-benzodiazepin-2(IH)-one and -thione (TIBO), 1-[(2-hydroxyethoxy)methyl]-6-(phenylthio)thymine (HEPT), alpha-anilinophenylacetamide (alpha-APA), and 2',5'bis-O-(tert-butyldimethylsilyl)-3'-spiro-5"-(4"-amino-1",2"-oxat hiole- 2",2"-dioxide)pyrimidine (TSAO) derivatives, and thiocarboxanilides for the treatment of human immunodeficiency virus type 1 (HIV-1) infections. For the clinical use of NNRTIs, some guidelines have been elaborated, such as starting treatment with combinations of different compounds at sufficiently high concentrations to effect a pronounced and sustained suppression of the virus. Despite the diversity of the compounds described here and the different viruses at which they are targeted, they have a number of characteristics in common. As they interact with specific viral proteins, the compounds achieve a selective inhibition of the replication of the virus, which, in turn, should be able to develop resistance to the compounds. However, as has been established for the NNRTIs, the problem of viral resistance may be overcome if the compounds are used from the start at sufficiently high doses, which could be reduced if different compounds are combined. For HIV infections, drug treatment regimens should be aimed at reducing the viral load to such an extent that the risk for progression to AIDS will be minimized, if not avoided entirely. This may result in a real "cure" of the disease but not necessarily of the virus infection, and in this sense, HIV disease may be reduced to a dormant infection, reminiscent of the latent herpesvirus infections. Should virus replication resume after a certain time, the armamentarium of effective anti-HIV and anti-herpesvirus compounds now available, if applied at the appropriate dosage regimens, should make the virus return to its dormant state before it has any chance to damage the host. It is unlikely that this strategy would eradicate the virus and thus "cure" the viral infection, but it definitely qualifies as a cure of the disease.

Specific diagnostic methods for herpesvirus infections of the central nervous system: a consensus review by the European Union Concerted Action on Virus Meningitis and Encephalitis

BACKGROUND

Herpesvirus infections of the central nervous system are often severe but are fortunately rare. The incidence of these infections has however, increased in recent years as a consequence of an increase in the number of immune-compromised individuals. New diagnostic procedures have improved our ability to diagnose these infections and herpesviruses may yet be implicated as the cause of further neurological diseases with no known aetiology. Methodological standards for selection and evaluation of patient materials are essential to the provision of reliable diagnosis, yet few studies have addressed this important issue.

OBJECTIVES

To describe and define methodological standards and reference methodology for diagnosis of herpesvirus infections of the CNS.

STUDY DESIGN

Information gathered by literature review.

RESULTS

Only for herpes simplex encephalitis is there sufficient data to allow the definition of reference methodology. Good methodological standards exist but few studies have adhered to these standards. As methods for the detection of specific intrathecal antibody synthesis are well established yet under-used in diagnostic virology, the principle of these measurements is reviewed in some detail.

CONCLUSIONS

Herpesvirus infections of the CNS are of increasing importance. High quality, multi-centre studies are needed to establish the value of the new diagnostic test procedures if further improvement in the diagnostic sensitivity and specificity of these procedures is to be achieved.

Carbocyclic oxetanocins lacking the C-3' methylene

Using the observation that the side effects of aristeromycin (carbocyclic adenosine) were reduced by removing the methylene at the center in aristeromycin where phosphorylation occurs, derivatives of carbocyclic oxetanocin A (4a), oxetanocin G (4b), and 2-aminooxetanocin A (16) lacking the 3'-methylene have been prepared in racemic form. The only viruses for which an appreciable inhibitory effect of the compounds (minimum inhibitory concentration ranging from 1 to 40 microg/mL) was noted were herpes simplex virus type 1 (HSV-1) and varicella-zoster virus (VZV). However, when directly compared for their antiviral potency against HSV-1 with their parents oxetanocin A and oxetanocin G, compounds 4a and 4b proved clearly less active.

Acyclic nucleosides as antiviral compounds

Acyclovir is an effective drug for the treatment of HSV and VZV infections, which after phosphorylation to the triphosphate, inhibits viral DNA polymerase. Acyclovir has low oral bioavailability, therefore prodrugs have been developed, and the L-valyl ester, valaciclovir, recently has been licensed for the treatment of shingles. Ganciclovir is used against CMV, and famciclovir, a lipophilic prodrug of penciclovir, is marketed for shingles. The acyclic nucleoside phosphonates are active against thymidine kinase-resistant viral strains. Promising analogs are PMEA (in clinical trial for the treatment of AIDS) and (S)-HPMPC (good in vivo activity against HSV, VZV, CMV, and EBV). Oligonucleotides incorporating acyclic nucleosides at the 3'-and 5'-ends, or constituted of amino acyclic nucleosides, are resistant to cleavage by nucleases and may be useful in antisense and/or antigene therapy. HEPT is active against HIV-1: It binds in a hydrophic pocket on reverse transcriptase, rather than in the polymerase active site. Some acyclic nucleosides are potent inhibitors of purine and pyrimidine nucleoside phosphorylase. These compounds may have a therapeutic niche in combination therapy with antiviral and anticancer nucleosides, and in the treatment of diseases involving the T-cell.

Comparative activity of selected antiviral compounds against clinical isolates of varicella-zoster virus

Sixteen freshly isolated varicella-zoster virus (VZV) strains were evaluated in vitro, in parallel with two reference strains expressing a functional thymidine kinase (TK+) (Oka and YS) and two thymidine kinase-deficient mutants (TK-) (07-1 and YS-R), for their susceptibility to a broad range of antiviral compounds. The following compounds were included: acyclovir (ACV), brivudine (BVDU), sorivudine (BVaraU), other BVDU congeners such as BTDU, CTDU, CVDC and CVDU, ganciclovir (GCV), FIAC, araT, araA, araC, foscarnet (PFA), phosphonoacetic acid (PAA), the acyclic nucleoside phosphonates HPMPC, cHPMPC, HPMPA, cHPMPA, HPMPc3A, PMEA and PMEDAP, the N7-isomeric acyclic nucleoside analogue N7AP, penciclovir (PCV), compounds 882C87 and H2G and two oxetanocin derivatives OXT-A and OXT-G. Fourteen of the 16 clinical isolates displayed the following order of decreasing selectivity against VZV: BVaraU > BVDU > CVDU approximately CVDC > H2G > N7AP approximately CTDU approximately BTDU approximately OXT-G approximately 882C87 > ACV > FIAC approximately araT > HPMPC approximately cHPMPC approximately HPMPA approximately HPMPc3A approximately cHPMPA > PCV approximately GCV approximately OXT-A > PMEDAP approximately PMEA > PFA approximately PAA approximately araA > araC. Two VZV strains (isolated from the cerebrospinal fluid of an AIDS patient) that were shown to have a truncated TK were clearly resistant to all the compounds that need the viral TK for their phosphorylation, while sensitivity to the acyclic nucleoside phosphonates, PFA, PAA, OXT-A and araA, remained unchanged. A slight (5- and 10-fold) increase was noted in the 50% inhibitory concentration of N7AP and OXT-G, respectively, for the TK- VZV strains as compared to the TK+ VZV strains. Ganciclovir and FIAC also showed a marked decrease in their activity against these two strains, but this was not as pronounced as for the other viral TK-dependent drugs. From our results, it appears that although acyclic nucleoside phosphonates may not have as favourable a therapeutic index as drugs requiring the viral TK, they should be considered for the treatment of TK- VZV life-threatening infections that are resistant to the viral TK-dependent drugs.