Progressive esophagitis from acyclovir-resistant herpes simplex. Clinical roles for DNA polymerase mutants and viral heterogeneity?

Large Subunit of the Human Herpes Simplex Virus Terminase as a Promising Target in Design of Anti-Herpesvirus Agents

Herpes simplex virus type 1 (HSV-1) is an extremely widespread pathogen characterized by recurrent infections. HSV-1 most commonly causes painful blisters or sores around the mouth or on the genitals, but it can also cause keratitis or, rarely, encephalitis. First-line and second-line antiviral drugs used to treat HSV infections, acyclovir and related compounds, as well as foscarnet and cidofovir, selectively inhibit herpesvirus DNA polymerase (DNA-pol). It has been previously found that (S)-4-[6-(purin-6-yl)aminohexanoyl]-7,8-difluoro-3,4-dihydro-3-methyl-2H-[1,4]benzoxazine (compound 1) exhibits selective anti-herpesvirus activity against HSV-1 in cell culture, including acyclovir-resistant mutants, so we consider it as a lead compound. In this work, the selection of HSV-1 clones resistant to the lead compound was carried out. High-throughput sequencing of resistant clones and reference HSV-1/L2 parent strain was performed to identify the genetic determinants of the virus's resistance to the lead compound. We identified a candidate mutation presumably associated with resistance to the virus, namely the T321I mutation in the UL15 gene encoding the large terminase subunit. Molecular modeling was used to evaluate the affinity and dynamics of the lead compound binding to the putative terminase binding site. The results obtained suggest that the lead compound, by binding to pUL15, affects the terminase complex. pUL15, which is directly involved in the processing and packaging of viral DNA, is one of the crucial components of the HSV terminase complex. The loss of its functional activity leads to disruption of the formation of mature virions, so it represents a promising drug target. The discovery of anti-herpesvirus agents that affect biotargets other than DNA polymerase will expand our possibilities of targeting HSV infections, including those resistant to baseline drugs.

Absence of the lectin-like domain of thrombomodulin reduces HSV-1 lethality of mice with increased microglia responses

Background

Herpes simplex virus 1 (HSV-1) can induce fatal encephalitis. Cellular factors regulate the host immunity to affect the severity of HSV-1 encephalitis. Recent reports focus on the significance of thrombomodulin (TM), especially the domain 1, lectin-like domain (TM-LeD), which modulates the immune responses to bacterial infections and toxins and various diseases in murine models. Few studies have investigated the importance of TM-LeD in viral infections, which are also regulated by the host immunity.

Methods

In vivo studies comparing wild-type and TM-LeD knockout mice were performed to determine the role of TM-LeD on HSV-1 lethality. In vitro studies using brain microglia cultured from mice or a human microglia cell line to investigate whether and how TM-LeD affects microglia to reduce HSV-1 replication in brain neurons cultured from mice or in a human neuronal cell line.

Results

Absence of TM-LeD decreased the mortality, tissue viral loads, and brain neuron apoptosis of HSV-1-infected mice with increases in the number, proliferation, and phagocytic activity of brain microglia. Moreover, TM-LeD deficiency enhanced the phagocytic activity of brain microglia cultured from mice or of a human microglia cell line. Co-culture of mouse primary brain microglia and neurons or human microglia and neuronal cell lines revealed that TM-LeD deficiency augmented the capacity of microglia to reduce HSV-1 replication in neurons.

Conclusions

Overall, TM-LeD suppresses microglia responses to enhance HSV-1 infection.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02426-w.

2017 European guidelines for the management of genital herpes

Genital herpes is one of the commonest sexually transmitted infections worldwide. Using the best available evidence, this guideline recommends strategies for diagnosis, management, and follow-up of the condition as well as for minimising transmission. Early recognition and initiation of therapy is key and may reduce the duration of illness or avoid hospitalisation with complications, including urinary retention, meningism, or severe systemic illness. The guideline covers a range of common clinical scenarios, such as recurrent genital herpes, infection during pregnancy, and co-infection with human immunodeficiency virus.

Herpes Simplex Virus Isolates from an Immunocompromised Patient who Failed to Respond to Acyclovir Treatment Express Thymidine Kinase with Altered Substrate Specificity

Ten sequential post-treatment herpes simplex virus type 1 (HSV-1) isolates were obtained from an immunocompromised patient whose infection, during prolonged treatment, became unresponsive to acyclovir (ACV). Of the ten isolates, eight later isolates were resistant in vitro to ACV and ganciclovir (DHPG), but remained sensitive to 9-β-D-arabinofuranosyladenine (ara-A) and phosphonoformate (PFA). Biochemical characterization of plaque-purified clones of the resistant isolates revealed an altered thymidine kinase (TK) substrate specificity phenotype. The comparative nucleotide sequence analysis of polymerase chain reaction (PCR)-amplified DNA encoding the TK genes of one sensitive and two resistant clones showed a single mutation at nucleotide 527. This change would result in a substitution of arginine by glutamine at residue 176 of the polypeptide, a mutation previously observed in a laboratory isolated variant, SC16 Tr7 (Darby et al., 1986).

A Comparative Study of the in vitro and in vivo Antiviral Activities of Acyclovir and Penciclovir

The antiviral properties of the compounds acyclovir (ACV) and penciclovir (PCV) have been compared in a number of in vitro and in vivo assays. In vitro, both compounds had good activity against herpes simplex virus type 1 (HSV-1) and varicella-zoster virus (VZV), although ACV showed statistically significant superiority. In addition, ACV had greater activity against herpes simplex virus type 2 (HSV-2), human cytomegalovirus (HCMV) and Epstein-Barr virus (EBV). We examined the effect of time of addition and removal of ACV and PCV under a variety of conditions and found similar results with the two compounds under most conditions. However, at a high multiplicity of infection, when all of the cells would be expected to be synchronously expressing large amounts of the viral thymidine kinase, short exposures to PCV appeared to be superior to similar exposures to ACV. In the HSV-1 zosteriform mouse model there was no significant difference between the activities of ACV and PCV, or its prodrug famciclovir (FCV), in once- or twice-daily treatment. The possible significance of these results and those previously reported on the activity of the compounds in humans is discussed.

Penciclovir-Resistance Mutations in the Herpes Simplex Virus DNA Polymerase Gene

Penciclovir is the active form of the orally available prodrug famciclovir, which is entering clinical use for herpesvirus infections. Like aciclovir, penciclovir is an acyclic guanosine analogue that is phosphorylated by viral thymidine kinase and whose triphosphate can inhibit viral DNA polymerase. We tested several well-characterized herpes simplex virus mutants with aciclovir-resistance mutations in the viral DNA polymerase gene for altered sensitivity to penciclovir. The mutants varied in their susceptibilities to penciclovir with one exhibiting 2-fold hypersensitivity, one marginal resistance and three about 3-fold resistance. Marker rescue and DNA sequencing analyses mapped the penciclovir-resistance mutation of one mutant, AraA r7, to a single base change that alters a glycine to a cysteine at residue 841 within conserved region III of α-like DNA polymerases. The results have implications for the mechanism of selective action of penciclovir, for the potential for development of resistance in the clinic, and for the substrate recognition properties of herpes simplex virus DNA polymerase.

Mathematical Modeling Predicts that Increased HSV-2 Shedding in HIV-1 Infected Persons Is Due to Poor Immunologic Control in Ganglia and Genital Mucosa

A signature feature of HIV infection is poor control of herpes virus infections, which reactivate from latency and cause opportunistic infections. While the general mechanism underlying this observation is deficient CD4+T-cell function, it is unknown whether increased severity of herpes virus infections is due primarily to poor immune control in latent or lytic sites of infection, or whether CD4+ immunodeficiency leads to more critical downstream deficits in humoral or cell-mediated immunologic responses. Here we compare genital shedding patterns of herpes simplex virus-2 (HSV-2) in 98 HIV infected and 98 HIV uninfected men matched on length of infection, HSV-1 serostatus and nationality. We demonstrate that high copy HSV-2 shedding is more frequent in HIV positive men, particularly in participants with CD4+ T-cell count <200/μL. Genital shedding is more frequent due to higher rate of shedding episodes, as well as a higher proportion of prolonged shedding episodes. Peak episode viral load was not found to differ between HIV infected and uninfected participants regardless of CD4+ T-cell count. We simulate a mathematical model which recapitulates these findings and identifies that rate of HSV-2 release from neural tissue increases, duration of mucosal cytolytic immune protection decreases, and cell-free viral lifespan increases in HIV infected participants. These results suggest that increased HSV-2 shedding in HIV infected persons may be caused by impaired immune function in both latent and lytic tissue compartments, with deficits in clearance of HSV-2 infected cells and extracellular virus.

2014 UK national guideline for the management of anogenital herpes

These guidelines concern the management of anogenital herpes simplex virus infections in adults and give advice on diagnosis, management, and counselling of patients. This guideline replaces the 2007 BASHH herpes guidelines and includes new sections on herpes proctitis, key points to cover with patients regarding transmission and removal of advice on the management of HSV in pregnancy which now has a separate joint BASHH/RCOG guideline.

Rapid genome assembly and comparison decode intrastrain variation in human alphaherpesviruses

Herpes simplex virus (HSV) is a widespread pathogen that causes epithelial lesions with recurrent disease that manifests over a lifetime. The lifelong aspect of infection results from latent viral infection of neurons, a reservoir from which the virus reactivates periodically. Recent work has demonstrated the breadth of genetic variation in globally distributed HSV strains. However, the amount of variation or capacity for mutation within one strain has not been well studied. Here we developed and applied a streamlined new approach for assembly and comparison of large DNA viral genomes such as HSV-1. This viral genome assembly (VirGA) workflow incorporates a combination of de novo assembly, alignment, and annotation strategies to automate the generation of draft genomes for large viruses. We applied this approach to quantify the amount of variation between clonal derivatives of a common parental virus stock. In addition, we examined the genetic basis for syncytial plaque phenotypes displayed by a subset of these strains. In each of the syncytial strains, we found an identical DNA change, affecting one residue in the gB (UL27) fusion protein. Since these identical mutations could have appeared after extensive in vitro passaging, we applied the VirGA sequencing and comparison approach to two clinical HSV-1 strains isolated from the same patient. One of these strains was syncytial upon first culturing; its sequence revealed the same gB mutation. These data provide insight into the extent and origin of genome-wide intrastrain HSV-1 variation and present useful methods for expansion to in vivo patient infection studies.

Herpes simplex virus (HSV) infects more than 70% of adults worldwide, causing epithelial lesions and recurrent disease that manifests over a lifetime. Prior work has demonstrated that HSV strains vary from country to country and between individuals. However, the amount of variation within one strain has not been well studied. To address this, we developed a new approach for viral genome assembly (VirGA) and analysis. We used this approach to quantify the amount of variation between sister clones of a common parental virus stock and to determine the basis of a unique fusion phenotype displayed by several variants. These data revealed that while sister clones of one HSV stock are more than 98% identical, these variants harbor enough genetic differences to change their observed characteristics. Comparative genomics approaches will allow us to explore the impacts of viral inter- and intrastrain diversity on drug and vaccine efficacy.

In vivo reactivation of latent herpes simplex virus 1 in mice can occur in the brain before occurring in the trigeminal ganglion

Herpes simplex virus 1 (HSV-1) establishes latency in neurons of the brains and sensory ganglia of humans and experimentally infected mice. The latent virus can reactivate to cause recurrent infection. Both primary and recurrent infections can induce diseases, such as encephalitis. In humans, the majority of encephalitis cases occur as a recurrent infection. However, in the past, numerous mouse studies documented that viral reactivation occurs efficiently in the ganglion, but extremely rarely in the brain, when assessed ex vivo by cultivating minced tissue explants. Here, we compare the brains and the trigeminal ganglia of mice latently infected with HSV-1 (strain 294.1 or McKrae) for levels of viral genomes and in vivo reactivation. The numbers of copies of 294.1 and McKrae genomes in the brain stem were significantly greater than those in the trigeminal ganglion. Most importantly, 294.1 and McKrae reactivation was detected in the brain stems earlier than in the trigeminal ganglia of mice treated with hyperthermia to reactivate latent virus in vivo. In addition, the brain stem yielded reactivated virus at a high frequency compared with the trigeminal ganglion, especially in mice latently infected with 294.1 after hyperthermia treatment. These results provide evidence that recurrent brain infection can be induced by the reactivation of latent virus in the brain in situ.

IMPORTANCE

Herpes simplex virus 1 (HSV-1) establishes latency in neurons of the brains and sensory ganglia of humans and experimentally infected mice. The latent virus can reactivate to cause recurrent infection. In the past, studies of viral reactivation focused on the ganglion, because efficient viral reactivation was detected in the ganglion but not in the brain when assessed ex vivo by cultivating mouse tissue explants. In this study, we report that the brain contains more viral genomes than the trigeminal ganglion in latently infected mice. Notably, the brain yields reactivated virus early and efficiently compared with the trigeminal ganglion after mice are stimulated to reactivate latent virus. Our findings raise the potential importance of HSV-1 latent infection and reactivation in the brain.

Single nucleotide polymorphisms of thymidine kinase and DNA polymerase genes in clinical herpes simplex virus type 1 isolates associated with different resistance phenotypes

The role of mutations in the thymidine kinase (TK, UL23) and DNA polymerase (pol, UL30) genes of herpes simplex virus (HSV) for development of different resistance phenotypes has to be exactly determined before genotypic resistance testing can be implemented in patient's care. Furthermore, the occurrence of cross-resistance is of utmost clinical importance. In this study, clinical HSV-1 isolates obtained between 2004 and 2011 from 26 patients after stem cell transplantation were examined in parallel by phenotypic and genotypic resistance testing. Thirteen isolates, which were phenotypically cross-resistant to acyclovir (ACV), penciclovir (PCV) and brivudin (BVDU), exhibited consistently frameshift or non-synonymous mutations in the TK gene known to confer resistance. One of these mutations (insertion of C at the nucleotide positions 1061-1065) has not been described before. Seven strains, phenotypically resistant to ACV and PCV and, except one each, sensitive to BVDU and resistant to foscarnet (FOS), carried uniformly resistance-related substitutions in the DNA pol gene. Finally, 3 isolates, resistant to ACV, PCV and 2 out of these also resistant to BVDU, had known but also unclear substitutions in the TK and DNA pol genes, and 3 isolates were completely sensitive. In conclusion, clinical ACV-resistant HSV-1 isolates, carrying resistance-associated mutations in the TK gene, can be regarded as cross-resistant to other nucleoside analogs such as BVDU. In contrast, clinical FOS-resistant HSV-1 strains which are cross-resistant to ACV may be sensitive to BVDU. This has to be considered for drug changes in antiviral treatment in case of ACV resistance.

Tranylcypromine reduces herpes simplex virus 1 infection in mice

Herpes simplex virus 1 (HSV-1) infects the majority of the human population and establishes latency by maintaining viral genomes in neurons of sensory ganglia. Latent virus can undergo reactivation to cause recurrent infection. Both primary and recurrent infections can cause devastating diseases, including encephalitis and corneal blindness. Acyclovir is used to treat patients, but virus resistance to acyclovir is frequently reported. Recent in vitro findings reveal that pretreatment of cells with tranylcypromine (TCP), a drug widely used in the clinic to treat neurological disorders, restrains HSV-1 gene transcription by inhibiting the histone-modifying enzyme lysine-specific demethylase 1. The present study was designed to examine the anti-HSV-1 efficacy of TCP in vivo because of the paucity of reports on this issue. Using the murine model, we found that TCP decreased the severity of wild-type-virus-induced encephalitis and corneal blindness, infection with the acyclovir-resistant (thymidine kinase-negative) HSV-1 mutant, and tissue viral loads. Additionally, TCP blocked in vivo viral reactivation in trigeminal ganglia. These results support the therapeutic potential of TCP for controlling HSV-1 infection.

Factors affecting herpes simplex virus reactivation from the explanted mouse brain

The majority of encephalitis induced by herpes simplex virus type I (HSV-1) is due to viral reactivation from latency, but few studies have investigated the factors influencing viral reactivation in the brain due to the lack of a sensitive assay. We have established an ex vivo explant assay, which induced efficient viral reactivation in the dissociated mouse brain. Applying this assay, we investigated the infection of four HSV-1 strains with varying degrees of neurovirulence in three mouse strains with different levels of susceptibility to HSV-1 infection. We found that virulent HSV-1 strains and susceptible mouse strains exhibited prolonged viral growth during acute infection, increased latent viral genomes, and efficient explant reactivation in the brain stem. Collectively, both viral neurovirulence and host susceptibility positively correlate with HSV-1 reactivation from the explanted mouse brain.

Longitudinal Characterization of Herpes Simplex Virus (HSV) Isolates Acquired From Different Sites in an Immune-Compromised Child: A New HSV Thymidine Kinase Mutation Associated With Resistance

BACKGROUND

Herpes simplex virus resistance to acyclovir is well described in immune-compromised patients. Management of prolonged infection and recurrences in such patients may be problematic.

METHODS

A patient with neuroblastoma developed likely primary herpes gingivostomatitis shortly after starting a course of chemotherapy, with spread to the eye during treatment with acyclovir. Viral isolates were serially obtained from separate sites after treatment was begun and tested for susceptibility to acyclovir and foscarnet by plaque reduction and plating efficiency assays. The thymidine kinase and DNA polymerase genes from each isolate were sequenced.

RESULTS

Initial isolates from a throat swab, an oral lesion, and conjunctiva were resistant to acyclovir within 13 days of treatment. Subsequent isolates while on foscarnet were initially acyclovir-susceptible, but reactivation of an acyclovir-resistant isolate was subsequently documented while on acyclovir suppression. Genotypic analysis identified a previously unreported UL23 mutation in some resistant isolates. None of the amino acid changes identified in UL30 were associated with resistance.

CONCLUSIONS

Phenotypic and genotypic antiviral resistance of herpes simplex isolates may vary from different compartments and over time in individual immune-compromised hosts, highlighting the importance of obtaining cultures from all sites. Phenotypic resistance testing should be considered for isolates obtained from at-risk patients not responding to first-line therapy. Empiric combination treatment with multiple antivirals could be considered in some situations.

Successful treatment of acyclovir-resistant herpes simplex virus type 2 proctitis with leflunomide in an HIV-infected man

Human herpes simplex virus infections are very common and represent significant morbidity in the immunocompromised host. Patients with acyclovir resistant strains of HSV based on viral thymidine kinase gene mutations need alternative therapeutic approaches. Leflunomide has been shown to possess antiviral activity against several viruses. Herein we describe a case of acyclovir resistant HSV-2 proctitis in an HIV patient successfully treated with leflunomide without significant side effects.

Slipping and sliding: frameshift mutations in herpes simplex virus thymidine kinase and drug-resistance

Some of the most successful antiviral agents currently available are effective against herpes simplex virus. However, resistance to these drugs is frequently associated with significant morbidity, particularly in immunocompromised patients. In addition to the clinical implications of drug resistance, the range of biological processes exploited by the virus to attain resistance while maintaining pathogenicity is proving to be surprising. These mechanisms, which include ribosomal frameshifting, induced infidelity of the DNA polymerase, and internal ribosome entry, are discussed.

Expression of herpes simplex virus type 1 recombinant thymidine kinase and its application to a rapid antiviral sensitivity assay

Antiviral-resistant herpesvirus infection has become a great concern for immunocompromised patients. Herpes simplex virus type 1 (HSV-1) infections are treated with viral thymidine kinase (vTK)-associated drugs such as acyclovir (ACV), and most ACV-resistance (ACV(r)) is due to mutations in the vTK. The standard drug sensitivity test is usually carried out by the plaque reduction assay-based method, which requires over 10 days. To shorten the time required, a novel system was developed by the concept, in which 293T cells transiently expressing recombinant vTK derived from the test sample by transfection of the cells with an expression vector were infected with vTK-deficient and ACV(r) HSV-1 (TAR), and then cultured in a maintenance medium with or without designated concentrations of ACV, ganciclovir (GCV) and brivudine (BVdU). The replication of TAR was strongly inhibited by ACV, GCV and BVdU in 293T cells expressing recombinant vTK of the ACV-sensitive HSV-1, whereas replication was not or slightly inhibited in cells expressing the recombinant vTK of highly resistant or intermediately resistant HSV-1, respectively. An inverse correlation was demonstrated in the 50% effective concentrations (EC(50)s) and inhibitory effects of these compounds on the replication of TAR among ACV(s) and ACV(r) HSV-1 clones. These results indicate that the EC(50)s of the vTK-associated drugs including ACV can be assumed by measuring the inhibitory effect of drugs in 293T cells expressing recombinant vTK of the target virus. The newly developed antiviral sensitivity assay system for HSV-1 makes it possible to estimate EC(50) for vTK-associated drugs, when whole vTK gene is available for use by gene amplification directly from lesion's samples or from virus isolates.

Lack of association of herpes simplex virus type 2 seropositivity with the progression of HIV infection in the HERS cohort

Many studies have chronicled the "epidemiologic synergy" between human immunodeficiency virus (HIV) and herpes simplex virus type 2 (HSV-2). HIV adversely affects the natural history of HSV-2 and results in more frequent and severe HSV-2 reactivation. Few longitudinal studies, however, have examined whether HSV-2 is associated with increased HIV plasma viral loads or decreased CD4 counts. The authors estimated the effect of HSV-2 seropositivity on HIV RNA viral load and on CD4 count over time among 777 HIV-seropositive US women not receiving suppressive HSV-2 therapy in the HIV Epidemiology Research Study (1993-2000). Linear mixed models were used to assess the effect of HSV-2 on log HIV viral load and CD4 count/mm(3) prior to widespread initiation of highly active antiretroviral therapy. Coinfection with HSV-2 was not associated with HIV RNA plasma viral loads during study follow-up. There was a statistically significant association between HSV-2 seropositivity and CD4 count over time, but this difference was small and counterintuitive at an increase of 8 cells/mm(3) (95% confidence interval: 2, 14) per year among HSV-2-seropositive women compared with HSV-2-seronegative women. These data do not support a clinically meaningful effect of baseline HSV-2 seropositivity on the trajectories of HIV plasma viral loads or CD4 counts.

Resistance of herpes simplex viruses to nucleoside analogues: mechanisms, prevalence, and management

Herpes simplex viruses (HSV) type 1 and type 2 are responsible for recurrent orolabial and genital infections. The standard therapy for the management of HSV infections includes acyclovir (ACV) and penciclovir (PCV) with their respective prodrugs valacyclovir and famciclovir. These compounds are phosphorylated by the viral thymidine kinase (TK) and then by cellular kinases. The triphosphate forms selectively inhibit the viral DNA polymerase (DNA pol) activity. Drug-resistant HSV isolates are frequently recovered from immunocompromised patients but rarely found in immunocompetent subjects. The gold standard phenotypic method for evaluating the susceptibility of HSV isolates to antiviral drugs is the plaque reduction assay. Plaque autoradiography allows the associated phenotype to be distinguished (TK-wild-type, TK-negative, TK-low-producer, or TK-altered viruses or mixtures of wild-type and mutant viruses). Genotypic characterization of drug-resistant isolates can reveal mutations located in the viral TK and/or in the DNA pol genes. Recombinant HSV mutants can be generated to analyze the contribution of each specific mutation with regard to the drug resistance phenotype. Most ACV-resistant mutants exhibit some reduction in their capacity to establish latency and to reactivate, as well as in their degree of neurovirulence in animal models of HSV infection. For instance, TK-negative HSV mutants establish latency with a lower efficiency than wild-type strains and reactivate poorly. DNA pol HSV mutants exhibit different degrees of attenuation of neurovirulence. The management of ACV- or PCV-resistant HSV infections includes the use of the pyrophosphate analogue foscarnet and the nucleotide analogue cidofovir. There is a need to develop new antiherpetic compounds with different mechanisms of action.

Famciclovir, from the bench to the patient--a comprehensive review of preclinical data

Famciclovir is converted rapidly and efficiently after oral administration to the selective antiviral compound, penciclovir. In cell culture, penciclovir is a potent inhibitor of herpes simplex virus (HSV) types 1 and 2, varicella-zoster virus (VZV), Epstein-Barr virus (EBV) and hepatitis B virus (HBV). Phosphorylation of penciclovir and aciclovir in uninfected cells is limited, and penciclovir, like aciclovir, has minimal effect on replicating cells in culture as expected for a selective antiviral agent. Mode of action studies with VZV and HSV have shown that the phosphorylation of penciclovir in infected cells is far more efficient than for aciclovir. This compensates for differences observed between penciclovir triphosphate and aciclovir triphosphate in the inhibition of HSV and VZV DNA polymerases. Because HBV is not known to encode a thymidine kinase, a different rationale for the selective inhibition of this virus by penciclovir is required. Recent data indicate that the DNA polymerase of HBV is far more sensitive to inhibition by penciclovir triphosphate than cellular DNA polymerases, suggesting that for this virus, selectivity operates at the DNA polymerase. Penciclovir triphosphate is more stable within infected cells than aciclovir triphosphate, and consequently penciclovir has more prolonged antiviral activity than aciclovir. Similarly, famciclovir is more effective than aciclovir or valaciclovir in suppressing HSV replication when given at a lower dosing frequency in certain animal models. These preclinical properties have helped to provide the foundation for the famciclovir clinical programme.

Mutation hot spots in the canine herpesvirus thymidine kinase gene

The guanine and cytosine content (GC-content) of alpha-herpesvirus genes are highly variable despite similar genome structures. It is known that drug resistant HSV, which has the genome with a high GC-content (approximately 70%), commonly includes frameshift mutations in homopolymer stretches of guanine (G) and cytosine (C) within the thymidine kinase (TK) gene. However, whether such mutation hotspots exist in the TK gene of canine herpesvirus (CHV) which has a low GC-content was unknown. In this study, we investigated mutations in the TK gene of CHV. CHV was passaged in the presence of iodo-deoxyuridine (IDU), and IDU-resistant clones were isolated. In all IDU-resistant virus clones, mutations in the TK gene were observed. The majority of these mutations were frameshift mutations of an adenine (A) insertion or deletion within either of 2 stretches of eight A's in the TK gene. It was demonstrated that CHV TK mutations frequently occur at a limited number of hot spots within long homopolymer nucleotide stretches.

Management of HSV, VZV and EBV infections in patients with hematological malignancies and after SCT: guidelines from the Second European Conference on Infections in Leukemia

These guidelines on the management of HSV, VZV and EBV infection in patients with hematological malignancies and after SCT were prepared by the European Conference on Infections in Leukemia following a predefined methodology. A PubMed search was conducted using the appropriate key words to identify studies pertinent to management of HSV, VZV and EBV infections. References of relevant articles and abstracts from recent hematology and SCT scientific meetings were also reviewed. Prospective and retrospective studies identified from the data sources were evaluated, and all data deemed relevant were included in this analysis. The clinical and scientific background was described and discussed, and the quality of evidence and level of recommendation were graded according to the Centers for Disease Control criteria.

Suppression of transcription factor early growth response 1 reduces herpes simplex virus lethality in mice

Herpes simplex virus type 1 (HSV-1) infection is the most common cause of sporadic, fatal encephalitis, but current understanding of how the virus interacts with cellular factors to regulate disease progression is limited. Here, we show that HSV-1 infection induced the expression of the cellular transcription factor early growth response 1 (Egr-1) in a human neuronal cell line. Egr-1 increased viral replication by activating promoters of viral productive cycle genes through binding to its corresponding sequences in the viral promoters. Mouse studies confirmed that Egr-1 expression was enhanced in HSV-1-infected brains and that Egr-1 functions to promote viral replication in embryonic fibroblasts. Furthermore, Egr-1 deficiency or knockdown of Egr-1 by a DNA-based enzyme greatly reduced the mortality of HSV-1-infected mice by decreasing viral loads in tissues. This study provides what we believe is the first evidence that Egr-1 increases the mortality of HSV-1 encephalitis by enhancing viral replication. Moreover, blocking this cellular machinery exploited by the virus could prevent host mortality.

Genotypic detection of acyclovir-resistant HSV-1: characterization of 67 ACV-sensitive and 14 ACV-resistant viruses

Infections due to herpes simplex virus (HSV) resistant to acyclovir (ACV) represent an important clinical concern in immunocompromised patients. In order to switch promptly to an appropriate treatment, rapid viral susceptibility assays are required. We developed herein a genotyping analysis focusing on thymidine kinase gene (TK) mutations in order to detect acyclovir-resistant HSV in clinical specimens. A total of 85 HSV-1 positive specimens collected from 69 patients were analyzed. TK gene could be sequenced directly for 81 clinical specimens (95%) and 68 HSV-1 specimens could be characterized as sensitive or resistant by genotyping (84%). Genetic characterization of 67 susceptible HSV-1 specimens revealed 10 polymorphisms never previously described. Genetic characterization of 14 resistant HSV-1 revealed 12 HSV-1 with either TK gene additions/deletions (8 strains) or substitutions (4 strains) and 2 HSV-1 with no mutation in the TK gene. DNA polymerase gene was afterwards explored. With this rapid PCR-based assay, ACV-resistant HSV could be detected directly in clinical specimens within 24 h.

Anti-HSV activity of digitoxin and its possible mechanisms

Herpes simplex virus type 1 (HSV-1) can establish latent infection in the nervous system and usually leads to life-threatening diseases in immunocompromised individuals upon reactivation. Treatment with conventional nucleoside analogue such as acyclovir is effective in most cases, but drug-resistance may arise due to prolonged treatment in immunocompromised individuals. In this study, we identified an in-use medication, digitoxin, which actively inhibited HSV-1 replication with a 50% effective concentration (EC(50)) of 0.05 microM. The 50% cytotoxicity concentration (CC(50)) of digitoxin is 10.66 microM and the derived selective index is 213. Several structural analogues of digitoxin such as digoxin, ouabain octahydrate and G-strophanthin also showed anti-HSV activity. The inhibitory effects of digitoxin are likely to be introduced at the early stage of HSV-1 replication and the virus release stage. The observation that digitoxin can inhibit acyclovir-resistant viruses further implicates that digitoxin represents a novel drug class with distinct antiviral mechanisms from traditional drugs.

Genital herpes due to acyclovir-sensitive herpes simplex virus caused secondary and recurrent herpetic whitlows due to thymidine kinase-deficient/temperature-sensitive virus

Herpes simplex virus (HSV)-2 caused a genital ulcer in a 40-year-old allogenic stem cell recipient, and a secondary herpetic whitlow appeared during 2 months of acyclovir (ACV) therapy. Both genital ulcer, and whitlow were cured 3 months later, but 6 months after recovery the whitlow alone recurred. DNA of the genital, first, and recurrent whitlow isolates showed similar endonuclease digestion fragment profiles. The genital virus was ACV-sensitive, and the two whitlow isolates were ACV-resistant/thymidine kinase (TK)-deficient. The TK gene of the whitlow isolates had the same frame shift from the 274th amino acid and termination at the 347th amino acid due to the deletion of a cytosine at the 819th nucleotide. Because the temperature of the thumb is 33/34 degrees C or lower, the temperature sensitivity of the isolates were compared, and both whitlow isolates were significantly more temperature-sensitive (ts) at 39 degrees C than the genital isolate. The two whitlow isolates showed cutaneous pathogenicity in mouse ear pinna but not midflank, while the genital isolate was pathogenic at both sites, suggesting that temperature adaptation was an important element of pathogenicity in the whitlow. The virus populations of isolates of the genital, and first whitlow were examined by 31, and 82 clones, respectively, and the clones from genital, and whitlow isolates were ACV-sensitive, and -resistant, respectively, showing their homogeneity. The acyclovir-sensitive genital lesion had spread as a TK-deficient/ts herpetic whitlow during ACV treatment, and an apparently TK-deficient virus adapted to the local temperature might have caused the whitlow recurrence.

[Genotyping diagnosis of acyclovir resistant herpes simplex virus]

Herpes simplex virus resistant to acyclovir (ACV) is a major concern among immunocompromised patients. ACV resistance might be due to mutations located in one of the two genes involved in ACV mechanism of action, the thymidine kinase gene (TK, involved in 95% of the cases) and the DNA polymerase gene. TK gene mutations consist, in half of the cases, in nucleotide insertion or deletion, occurring most of the time in G or C homopolymers considered as hot spots. Half of the other cases involves nucleotide substitutions leading to amino acids substitutions. Studies of sensitive strains revealed a high degree of TK polymorphism, many mutations being not implied in ACV resistance. At the present time, resistance detection can be performed by phenotypic tests that require virus culture and results cannot be given to the physician before 7 to 10 days. Genotyping diagnosis performed directly from clinical samples would allow to detect resistance more rapidly, in order to switch quickly to an appropriate treatment by foscarnet or cidofovir.

DNA Polymerase Mutations in Drug-Resistant Herpes Simplex Virus Mutants Determine In Vivo Neurovirulence and Drug-Enzyme Interactions

Mutations in the thymidine kinase and DNA polymerase genes of herpes simplex virus (HSV) might confer resistance to antiviral drugs, particularly in immunocompromised patients who suffer from chronic and/or disseminated lesions. The patterns of cross-resistance and neurovirulence in mice of several DNA polymerase mutants selected under pressure of foscarnet (PFA) and different acyclic nucleoside phosphonates (ANPs), including ( S)-3-hydroxy-2-phosphonylmethoxypropyl (HPMP) derivatives of adenine (HPMPA) and cytosine (HPMPC, cidofovir) and 2-phosphonylmethoxyethyl (PME) derivatives of adenine (PMEA) and 2,6-diaminopurine (PMEDAP), were investigated. The mutants were derived from the HSV-1 strain KOS following either single or multiple steps of selection with PFA (V714M, A719V, S724N and T821M), PMEA (S724N, L802F and R959H), PMEDAP (Q618H, S724N, S724N+D1070N), HPMPC (V573M, R700M and K960R) or HPMPA (W998L, L1007M and I1028T). These amino acid substitutions were located in different subdomains of the HSV-1 DNA polymerase, either in conserved or non-conserved regions. The sensitivity of the mutants to a new class of ANPs, the 6-(2-[phosphonomethoxy]alkoxy)pyrimidines HPMPO-DAPy and PMEO-DAPy, was investigated. Cross-resistance between the HPMP derivatives and HPMPO-DAPy, on the one hand, and between the PME derivatives and PMEO-DAPy, on the other hand, was observed. Different degrees of cross-resistance between PME derivatives, PMEO-DAPy, PFA and acyclovir were noticed. The mutants ranged from exhibiting near wild-type neurovirulence (V714M, A719V, S724N and L1007M) to significant attenuation (Q618H, S724N+D1070N, L802F, R700M, K960R, W998L and I1028T) or higher levels of attenuation (V573M). It appears that drug-resistant mutants arising under the pressure of HPMP derivatives have the lowest levels of neurovirulence.

Thymidine kinase mutations conferring acyclovir resistance in herpes simplex type 1 recombinant viruses

Contributions of thymidine kinase (TK) mutations to acyclovir (ACV) resistance were evaluated in herpes simplex virus type 1 recombinant viruses generated using a set of overlapping cosmids and plasmids. Alterations in both conserved and nonconserved regions of the TK gene were shown to confer high levels of resistance to ACV.

Low-level expression and reversion both contribute to reactivation of herpes simplex virus drug-resistant mutants with mutations on homopolymeric sequences in thymidine kinase

Many acyclovir-resistant herpes simplex virus isolates from patients contain insertions or deletions in homopolymeric sequences in the thymidine kinase (TK) gene (tk). Viruses that have one (G8) or two (G9) base insertions in a run of seven G's (G string) synthesize low levels of active TK (TK-low phenotype), evidently via ribosomal frameshifting. These levels of TK can suffice to permit reactivation from latently infected mouse ganglia, but in a majority of ganglia, especially with the G9 virus, reactivation of virus that has reverted to the TK-positive phenotype predominates. To help address the relative contributions of translational mechanisms and reversion in reactivation, we generated viruses with a base either inserted or deleted just downstream of the G string. Both of these viruses had a TK-low phenotype similar to that of the G8 and G9 viruses but with less reversion. Both of these viruses reactivated from latently infected trigeminal ganglia, albeit inefficiently, and most viruses that reactivated had a uniformly TK-low phenotype. We also generated viruses that have one insertion in a run of six C's or one deletion in a run of five C's. These viruses lack measurable TK activity. However, they reactivated from latently infected ganglia, albeit inefficiently, with the reactivating viruses having reverted to the wild-type TK phenotype. Therefore, for G-string mutants, levels of active TK as low as 0.25% generated by translational mechanisms can suffice for reactivation, but reversion can also contribute. For viruses that lack TK activity due to mutations on other homopolymeric sequences, reactivation can occur via reversion.

HIV/AIDS: Management of Herpes Simplex Virus Type 2 Infection in HIV Type 1–Infected Persons

Human immunodeficiency virus type 1 (HIV-1)-infected persons have high rates of herpes simplex virus type 2 (HSV-2) infection, ranging from 50% to 90% in studies of HIV-infected populations from different parts of the world. Genital herpes in persons with HIV type 1 (HIV-1) infection is associated with more-severe and chronic lesions, as well as increased rates of asymptomatic genital shedding of HSV-2. Nucleoside analogues (acyclovir, valacyclovir, and famciclovir) decrease the frequency and severity of HSV-2 recurrences and asymptomatic HSV-2 reactivation and are effective, safe, well-tolerated drugs in patients with HIV-1 infection. These anti-HSV drugs may result in additional clinical and public health benefits for persons with HIV-1 and HSV-2 coinfection by decreasing HIV-1 levels in the blood and genital tract. Given these benefits, HIV-1-infected persons should be routinely tested for HSV-2 infection using type-specific serologic tests. Persons with HSV-2 infection should be offered HSV-2 education and treatment options. Studies to quantify the potential clinical and public health benefits of treating individuals who have HIV-1 and HSV-2 coinfection with anti-HSV therapy are underway.

Oral and perioral herpes simplex virus type 1 (HSV-1) infection: review of its management

Herpes simplex virus type 1 (HSV-1) gives rise to a variety of clinical disorders and is a major cause of morbidity and mortality worldwide. HSV-1 infections are common in oral and perioral area. The aim of the present report was to critically examine the published literature to evaluate the advantages and limitations of therapy of HSV-1 infection in both immunocompetent and immunocompromised patients. Systemic antiviral therapy has been widely accepted as effective for primary herpetic gingivostomatitis. Aciclovir (ACV) 5% cream seems to be the accepted standard topical therapy for herpes labialis, being both effective and well tolerated, although penciclovir 1% cream has been proposed as a potentially useful treatment. Systemic ACV may be effective in reducing the duration of symptoms of recurrent HSV-1 infection, but the optimal timing and dose of the treatment are uncertain. Aciclovir and famciclovir may be of benefit in the acute treatment of severe HSV-1 disease in immunocompromised patients. There is also evidence that prophylactic oral ACV may reduce the frequency and severity of recurrent attack of herpetic infection in immunocompromised patients, but the optimal timing and duration of treatment is uncertain and can vary in different situations.

The effects of herpes simplex virus-2 on HIV-1 acquisition and transmission: a review of two overlapping epidemics

Increasing evidence demonstrates a substantial link between the epidemics of sexually transmitted HIV-1 and herpes simplex virus (HSV)-2 infection. More than 30 epidemiologic studies have demonstrated that prevalent HSV-2 is associated with a 2- to 4-fold increased risk of HIV-1 acquisition. Per-sexual contact transmission rates among couples from Rakai, Uganda indicate that at all levels of plasma HIV-1 RNA in the source partner, HSV-2-seropositive HIV-1-susceptible persons have a 5-fold greater risk of acquiring HIV-1 compared with HSV-2-negative persons. In vitro and in vivo studies suggest that mucosal HIV-1 shedding is more frequent and in greater amounts during mucocutaneous HSV-2 replication, including subclinical mucosal reactivations. Most HIV-1-infected persons are coinfected with HSV-2, and most experience frequent subclinical and clinical reactivations of HSV-2. Subclinical HSV reactivation elevates serum HIV-1 RNA levels, and daily therapy with acyclovir appears to reduce plasma HIV-1 RNA. These data show that greater attention to the diagnosis and treatment of HSV-2 among HIV-1-infected persons is warranted, especially those who continue to be sexually active, those not on antiretroviral therapy, or those whose disease is not well suppressed by antiretrovirals.

An unusual internal ribosome entry site in the herpes simplex virus thymidine kinase gene

We have investigated a herpes simplex virus mutant that expresses low levels of thymidine kinase (TK), a phenotype associated with drug resistance and pathogenicity, despite a single-base deletion in the gene. Using a dual-reporter system, a 39-nt sequence including the mutation was shown to direct expression of the downstream reporter gene in reticulocyte lysate. Translation of the downstream reporter was not impaired when the mRNA lacked a 5' cap or had a stable stem loop 5' of the upstream reporter and was relatively resistant to edeine, an antibiotic that prevents AUG codon recognition by the 40S-eIF2-GTP/Met-tRNAi complex. Twelve nucleotides were as active as the original sequence for translation of the downstream reporter. Surprisingly, this sequence lacks an AUG codon. Analysis of point mutations showed that a CUG codon in the sequence was important. However, many single-base changes had only limited effects, and introduction of AUG codons did not increase translation. A mutant virus containing both the single-base deletion and a mutation that reduced downstream translation in vitro had significantly less TK activity than a virus with the single-base deletion alone. Thus, a remarkably short internal ribosome entry site (IRES) that lacks an AUG codon resides in the viral tk gene. The IRES appears to be responsible for TK expression from a drug-resistant mutant that would otherwise express no TK, which may contribute to pathogenicity. Because we found numerous short sequences with IRES activity, there might be many hitherto unrecognized polypeptides expressed at low levels from eukaryotic mRNAs.

Characterization of herpes simplex virus type 1 thymidine kinase mutants selected under a single round of high-dose brivudin

A broad variety of herpes simplex virus type 1 clones was selected under a single round of high-dose selection with brivudin. Mutations in the thymidine kinase (TK) genes consisted of 42% frameshift mutations within homopolymer repeats of G's and C's and single nucleotide substitutions (58%) that produced stop codons (Q261 and R281) or a new codon at the site of the substitution (A168T, R51W, G59W, G206R, R220H, Y239S, and T287 M). The A168T change, associated with an altered TK phenotype, proved to be the most commonly selected substitution. For the different mutants, a correlation between phenotype, genotype, and in vivo neurovirulence was observed.

Resistance of Herpes Simplex Virus Infections to Nucleoside Analogues in HIV‐Infected Patients

Antiviral treatment of herpes simplex virus (HSV) infections with nucleoside analogues has been well established for >2 decades, but isolation of drug-resistant HSV from immunocompetent patients has remained infrequent (0.1%-0.7% of isolates) during this period. Even when drug-resistant HSV is isolated from an immunocompetent patient, this virus, with rare exceptions, is cleared normally without adverse clinical outcome. Although drug-resistant HSV is more commonly isolated from immunocompromised patients (4%-7% of isolates) and is more likely to be clinically significant, the prevalence of drug-resistant HSV even among these patients, has been stable over the past 2 decades. Despite this stable prevalence, disease due to drug-resistant HSV remains an important problem for many immunocompromised patients, including those with HIV infection. This article reviews the prevalence, pathogenesis, and implications of drug-resistant HSV infections in HIV-infected patients.

Cutaneous markers of HIV infection

Despite the development of laboratory methods, dermatological symptoms are a basic index of the presence and physical course of HIV infection. HIV infection usually undergoes a long latent period, proceeds to a period of immunodeficiency-related symptoms, and ends in an advanced immunodeficiency state characterized by opportunistic infections and neoplasms. Occasionally, dermatological manifestations can be the first signs of asymptomatic disease, indices of advanced immunodeficiency, or symptoms of opportunistic infections or neoplasms. The variety of symptoms and signs for the skin during the course of HIV infection is a consequence of the progressing immunodeficiency and therefore indicates the underlying disorder. The use of these manifestations is a challenge for clinical praxis.

Molecular analysis of clinical isolates of acyclovir resistant herpes simplex virus

We characterised the antiviral phenotype and genotype of 41 herpes simplex virus (HSV) strains from patients clinically resistant to acyclovir (ACV). Our results confirm recognised mutational sites as being major determinants of thymidine kinase (tk)-associated ACV resistance, in particular insertions and/or deletions at homopolymer stretches of Gs and Cs (59% of all isolates). Previously described amino acid substitutions in functional sites of the tk were also identified (7% of all isolates). In addition, we identified several stop codons in novel locations on the amino acid sequence (7% of all isolates) and amino acid substitutions (15% of all isolates) likely to be directly responsible for conferring resistance to ACV. When there were no mutations detected in the tk gene (12% of all isolates), mutations in the DNA polymerase gene likely to be important in the generation of resistant virus were identified.

Sequential Switching of Dna Polymerase and Thymidine Kinase-Mediated Hsv-1 Drug Resistance in An Immunocompromised Child

Sequential herpes simplex virus type 1 (HSV-1) isolates were obtained from a paediatric haematopoietic stem cell transplant (HSCT) patient who received prolonged therapy with acyclovir (ACV) followed by foscarnet (PFA) and topical cidofovir (HPMPC) for severe persistent mucocutaneous HSV-1 infection. The isolates were retrospectively studied for drug resistance. The first resistant isolate associated with clinical failure of antiviral therapy emerged 44 days post-ACV treatment initiation. Susceptibility testing revealed an ACV-resistant HSV strain that demonstrated cross resistance to PFA in the absence of any previous PFA treatment. The observed cross resistance was conferred by a single amino acid substitution, Ser724Asn, in the HSV DNA polymerase (DNA pol) gene. During the subsequent course of ACV therapy, the ACV/PFA-cross-resistant isolates were replaced by ACV-resistant, PFA-sensitive isolates. These isolates carried no DNA pol mutations, but had an Arg163His substitution in the thymidine kinase gene. Upon subsequent switching of antiviral therapy from ACV to PFA, the original ACV/PFA-cross-resistant DNA pol mutant re-appeared. Our study shows the emergence of different drug-resistant HSV variants during ongoing, unchanged ACV therapy. Furthermore, a rapid re-selection of the original resistant variant was observed after switch. For optimal antiviral management of HSV infections in HSCT recipients, therapeutic decisions should be guided by drug susceptibility results whenever therapeutic failure is observed and/or when changes in antiviral treatment are considered.

Current and potential therapies for the treatment of herpes-virus infections

Human herpesviruses are found worldwide and are among the most frequent causes of viral infections in immunocompetent as well as in immunocompromised patients. During the past decade and a half a better understanding of the replication and disease-causing state of herpes simplex virus types 1 and 2 (HSV-1 and HSV-2), varicella zoster virus (VZV), and human cytomegalovirus (HCMV) has been achieved due in part to the development of potent antiviral compounds that target these viruses. While some of these antiviral therapies are considered safe and efficacious (acyclovir, penciclovir), some have toxicities associated with them (ganciclovir and foscarnet). In addition, the increased and prolonged use of these compounds in the clinical setting, especially for the treatment of immunocompromised patients, has led to the emergence of viral resistance against most of these drugs. While resistance is not a serious issue for immunocompetent individuals, it is a real concern for immunocompromised patients, especially those with AIDS and the ones that have undergone organ transplantation. All the currently approved treatments target the viral DNA polymerase. It is clear that new drugs that are more efficacious than the present ones, are not toxic, and target a different viral function would be of great use especially for immunocompromised patients. Here, an overview is provided of the diseases caused by the herpesviruses as well as the replication strategy of the better studied members of this family for which treatments are available. We also discuss the various drugs that have been approved for the treatment of some herpesviruses in terms of structure, mechanism of action, and development of resistance. Finally, we present a discussion of viral targets other than the DNA polymerase, for which new antiviral compounds are being considered.

Drug resistance patterns of recombinant herpes simplex virus DNA polymerase mutants generated with a set of overlapping cosmids and plasmids

Herpes simplex virus (HSV) DNA polymerase (Pol) mutations can confer resistance to all currently available antiherpetic drugs. However, discrimination between mutations responsible for drug resistance and those that are part of viral polymorphism can be difficult with current methodologies. A new system is reported for rapid generation of recombinant HSV type 1 (HSV-1) DNA Pol mutants based on transfection of a set of overlapping viral cosmids and plasmids. With this approach, twenty HSV-1 recombinants with single or dual mutations within the DNA pol gene were successfully generated and subsequently evaluated for their susceptibilities to acyclovir (ACV), foscarnet (FOS), cidofovir (CDV), and adefovir (ADV). Mutations within DNA Pol conserved regions II (A719T and S724N), VI (L778M, D780N, and L782I), and I (F891C) were shown to induce cross-resistance to ACV, FOS, and ADV, with two of these mutations (S724N and L778M) also conferring significant reduction in CDV susceptibility. Mutant F891C was associated with the highest levels of resistance towards ACV and FOS and was strongly impaired in its replication capacity. One mutation (D907V) lying outside of the conserved regions was also associated with this ACV-, FOS-, and ADV-resistant phenotype. Some mutations (K522E and Y577H) within the delta-region C were lethal, whereas others (P561S and V573M) induced no resistance to any of the drugs tested. Recombinants harboring mutations within conserved regions V (N961K) and VII (Y941H) were resistant to ACV but susceptible to FOS. Finally, mutations within conserved region III were associated with various susceptibility profiles. This new system allows a rapid and accurate evaluation of the functional role of various DNA Pol mutations, which should translate into improved management of drug-resistant HSV infections.

Oral bioavailability and in vivo efficacy of the helicase-primase inhibitor BILS 45 BS against acyclovir-resistant herpes simplex virus type 1

This study investigated the oral bioavailability and efficacy of BILS 45 BS, a selective herpes simplex virus (HSV) helicase-primase inhibitor, against acyclovir (ACV)-resistant (ACV(r)) infections mediated by the HSV type 1 (HSV-1) dlsptk and PAA(r)5 mutant strains. In vitro, the compound was more potent than ACV against wild-type clinical and laboratory HSV-1 strains and ACV(r) HSV isolates, as determined by a standard plaque reduction assay, with a mean 50% effective concentration of about 0.15 microM. The oral bioavailability of BILS 45 BS in hairless mice was 49%, with a peak concentration in plasma of 31.5 microM after administration of a single dose of 25 mg/kg. Following cutaneous infection of nude mice, both the HSV-1 dlsptk and PAA(r)5 mutant strains induced significant, reproducible, and persistent cutaneous lesions that lasted for more than 2 weeks. Oral treatment with ACV (100 or 125 mg/kg/day, three times a day by gavage) did not affect either mutant-induced infection. In contrast, BILS 45 BS at an oral dose of 100 mg/kg/day almost completely abolished cutaneous lesions mediated by both ACV(r) HSV-1 mutants. The 50% effective doses of BILS 45 BS were 56.7 and 61 mg/kg/day against dlsptk- and PAA(r)5-induced infections, respectively. Taken together, our results demonstrate very effective oral therapy of experimental ACV(r) HSV-1 infections in nude mice and support the potential use of HSV helicase-primase inhibitors for the treatment of nucleoside-resistant HSV disease in humans.

Translational compensation of a frameshift mutation affecting herpes simplex virus thymidine kinase is sufficient to permit reactivation from latency

Herpes simplex virus thymidine kinase is important for reactivation of virus from its latent state and is a target for the antiviral drug acyclovir. Most acyclovir-resistant isolates have mutations in the thymidine kinase gene; however, how these mutations confer clinically relevant resistance is unclear. Reactivation from explanted mouse ganglia was previously observed with a patient-derived drug-resistant isolate carrying a single guanine insertion within a run of guanines in the thymidine kinase gene. Despite this mutation, low levels of active enzyme were synthesized following an unusual ribosomal frameshift. Here we report that a virus, generated from a pretherapy isolate from the same patient, engineered to lack thymidine kinase activity, was competent for reactivation. This suggested that the clinical isolate contains alleles of other genes that permit reactivation in the absence of thymidine kinase. Therefore, to establish whether thymidine kinase synthesized via a ribosomal frameshift was sufficient for reactivation under conditions where reactivation requires this enzyme, we introduced the mutation into the well-characterized strain KOS. This mutant virus reactivated from latency, albeit less efficiently than KOS. Plaque autoradiography revealed three phenotypes of reactivating viruses: uniformly low thymidine kinase activity, mixed high and low activity, and uniformly high activity. We generated a recombinant thymidine kinase-null virus from a reactivating virus expressing uniformly low activity. This virus did not reactivate, confirming that mutations in other genes that would influence reactivation had not arisen. Therefore, in strains that require thymidine kinase for reactivation from latency, low levels of enzyme synthesized via a ribosomal frameshift can suffice.

Management of acyclovir-resistant herpes simplex virus

In immunocompetent patients, HSV is controlled rapidly by the human host's immune system, and recurrent lesions are small and short lived. When treated with antiviral agents, these patients rarely develop resistance to these drugs. In contrast immunocompromised patients might not be able to control HSV infection. Thus, frequent and severe reactivations are often seen and might lead to fatal herpetic encephalitis or disseminated HSV infection. Treatment in these patients is limited because immunocompromised hosts often develop severe herpes disease refractory to antiviral drug therapy. It is therefore imperative that physicians develop regimens to deal with both receptive and refractory HSV disease. The following treatment protocol (modified from Balfour and colleagues) might serve as a guide until further investigation of new drugs is performed. In all patients standard oral ACV therapy should be initiated at a dose of 200 mg orally, five times a day for the first 3 to 5 days. Prior to treatment, cultures the lesions should be obtained to verify HSV etiology. If the response is poor, the dose of oral ACV should be increased to 800 mg five times a day. If no response seen after 5 to 7 days, it is unlikely that the lesion will respond to intravenous ACV (or chemically and structurally related drugs such as VCV or famciclovir), so an alternative regimen must be assigned. First, repeat cultures for vital, fungal, and bacterial pathogens must be performed. In addition, ACV susceptibility studies should be ordered, if available. If the mucocutaneous lesion is accessible for topical treatment, TFT (as ophthalmic solution) should be applied to the area three to four times a day until the lesion is completely healed. If the lesion is inaccessible or if the response to TFT is poor, therapy with intravenous foscarnet should be given for 10 days or until complete resolution of the lesions. The dosage of foscarnet should be 40 milligrams per kilogram three times per day or 60 milligrams per kilogram twice daily. If foscarnet fails to achieve clinical clearing, consideration should be given to use of intravenous cidofovir (or application of compounded 1% to 3% topical cidofovir ointment). Vidarabine is reserved for situations in which all of these therapies fail. If lesions reoccur in the same location following clearing, the patient should started on high-dose oral ACV (800 mg, five times daily) or intravenous foscarnet (40 mg/kg tid or 60 mg/kg bid) as soon as possible. When lesions occur in a different location, the patient should be treated initially with standard doses of oral ACV (200 mg, five times daily) and the above protocol should be followed should there be clinical failure. In the future, new treatment options for patients with documented HSV resistance will be important in reducing the clinical impact of HSV.

Herpes simplex virus resistance to acyclovir and penciclovir after two decades of antiviral therapy

Acyclovir, penciclovir, and their prodrugs have been widely used during the past two decades for the treatment of herpesvirus infections. In spite of the distribution of over 2.3 x 10(6) kg of these nucleoside analogues, the prevalence of acyclovir resistance in herpes simplex virus isolates from immunocompetent hosts has remained stable at approximately 0.3%. In immuncompromised patients, in whom the risk for developing resistance is much greater, the prevalence of resistant virus has also remained stable but at a higher level, typically 4 to 7%. These observations are examined in the light of characteristics of the virus, the drugs, and host factors.

Valacyclovir for herpes simplex virus infection: long-term safety and sustained efficacy after 20 years' experience with acyclovir

An extensive clinical trial program combined with 5 years' postmarketing experience with valacyclovir provides evidence of favorable safety and efficacy in herpes simplex virus (HSV) management. Valacyclovir enhances acyclovir bioavailability compared with orally administered acyclovir. Long-term use of acyclovir for up to 10 years for HSV suppression is effective and well tolerated. Acyclovir is also approved for use in children, is available in some countries over the counter in cream formulation for herpes labialis, and has been monitored in over 1000 pregnancies. Safety monitoring data from clinical trials of valacyclovir, involving over 3000 immunocompetent and immunocompromised persons receiving long-term therapy for HSV suppression, were analyzed. Safety profiles of valacyclovir (</=1000 mg/day), acyclovir (800 mg/day), and placebo were similar. Extensive sensitivity monitoring of HSV isolates confirmed a very low rate of acyclovir resistance among immunocompetent subjects (<0.5%). The incidence of resistance among immunocompromised patients remains low at about 5%.

Resistance of herpesviruses to antiviral drugs: clinical impacts and molecular mechanisms

Nucleoside analogues such as acyclovir and ganciclovir have been the mainstay of therapy for alphaherpesviruses (herpes simplex virus (HSV) and varicella-zoster virus (VZV)) and cytomegalovirus (CMV) infections, respectively. Drug-resistant herpesviruses are found relatively frequently in the clinic, almost exclusively among severely immunocompromised patients receiving prolonged antiviral therapy. For instance, close to 10% of patients with AIDS receiving intravenous ganciclovir for 3 months excrete a drug-resistant CMV isolate in their blood or urine and this percentage increases with cumulative drug exposure. Many studies have reported that at least some of the drug-resistant herpesviruses retain their pathogenicity and can be associated with progressive or relapsing disease. Viral mutations conferring resistance to nucleoside analogues have been found in either the drug activating/phosphorylating genes (HSV or VZV thymidine kinase, CMV UL97 kinase) and/or in conserved regions of the viral DNA polymerase. Currently available second line agents for the treatment of herpesvirus infections--the pyrophosphate analogue foscarnet and the acyclic nucleoside phosphonate derivative cidofovir--also inhibit the viral DNA polymerase but are not dependent on prior viral-specific activation. Hence, viral DNA polymerase mutations may lead to a variety of drug resistance patterns which are not totally predictable at the moment due to insufficient information on specific drug binding sites on the polymerase. Although some CMV and HSV DNA polymerase mutants have been found to replicate less efficiently in cell cultures, further research is needed to correlate viral fitness and clinical outcome.