Association between sensitivity of viral thymidine kinase-associated acyclovir-resistant herpes simplex virus type 1 and virulence

Combined use of pritelivir with acyclovir or foscarnet suppresses evolution of HSV-1 drug resistance

The widespread use of antivirals in immunocompromised individuals has led to frequent occurrences of drug-resistant herpes simplex virus 1 (HSV-1) infections. Current antivirals target the viral DNA polymerase (DP), resulting in cross-resistance patterns that emphasize the need for novel treatment strategies. In this study, we assessed whether combining antivirals with different targets affects drug resistance emergence by passaging wild-type HSV-1 under increasing concentrations of acyclovir (ACV), foscarnet (phosphonoformic acid, PFA), or the helicase-primase inhibitor pritelivir (PTV), individually or in combination (ACV + PTV or PFA + PTV). The resistance selection procedure was initiated from two different drug concentrations for each condition. Deep sequencing and subsequent phenotyping showed the rapid acquisition of resistance mutations under monotherapy pressure, whereas combination therapy resulted in either no mutations or mutations conferring ACV and/or PFA resistance. Notably, mutations associated with PTV resistance were not detected after five passages under combination pressure. Strains resistant to both ACV and PTV were eventually obtained upon further passaging under ACV + PTV pressure initiated from lower drug concentrations. PFA + PTV dual treatment induced PFA resistance mutations in the DP, but PTV resistance mutations were not acquired, even after 15 passages. Our data suggest that combining the helicase-primase inhibitor PTV with a DP inhibitor may be an effective strategy to prevent drug resistance evolution in HSV-1.

Heterogeneity and viral replication fitness of HSV-1 clinical isolates with mutations in the thymidine kinase and DNA polymerase

BACKGROUND

Prolonged antiviral therapy in immunocompromised individuals can result in the emergence of (multi)drug-resistant herpes simplex virus 1 (HSV-1) infections, forming a therapeutic challenge.

OBJECTIVES

To evaluate spatial and temporal differences in drug resistance of HSV-1 samples from a HSCT recipient and to determine the effect of resistance mutations on viral replication fitness.

PATIENTS AND METHODS

Five HSV-1 isolates were recovered from a HSCT recipient who suffered from persistent HSV-1 lesions, consecutively treated with aciclovir, foscarnet, cidofovir and a combination of ganciclovir and cidofovir. Spatial and temporal differences in HSV-1 drug resistance were evaluated genotypically [Sanger sequencing and next-generation sequencing (NGS) of the viral thymidine kinase (TK) and DNA polymerase (DP)] and phenotypically (plaque reduction assay). Viral replication fitness was determined by dual infection competition assays.

RESULTS

Rapid evolution to aciclovir and foscarnet resistance was observed due to acquisition of TK (A189V and R222H) and DP (L778M and L802F) mutations. Virus isolates showed heterogeneous populations, spatial virus compartmentalization and minor viral variants in three out of five isolates (detectable by NGS but not by Sanger sequencing). Mutations in the TK and DP genes did not alter replication fitness without drug pressure. TK and/or DP mutants influenced replication fitness under antiviral pressure and showed increased fitness under pressure of the drug they showed resistance to.

CONCLUSIONS

The use of NGS and dual infection competition assays revealed rapid evolution of HSV-1 drug resistance in a HSCT recipient with spatial and temporal compartmentalization of viral variants that had altered replication fitness under antiviral pressure.

Putting drug resistant epithelial herpes keratitis in the spotlight: A case series

Purpose

To strengthen the sparse evidence on acyclovir (ACV) resistance, especially in recalcitrant herpetic keratitis (HK), by describing the clinical course of 3 genotypically proven ACV resistant HK cases. An overview of mechanisms of resistance and therapeutic options currently available to ophthalmologists is provided based upon recent literature search.

Observations

Resistance to ACV due to known mutations in the gene encoding the viral thymidine kinase was confirmed in 2 cases, and a novel mutation in the UL23 gene (N202K) conferring phenotypical resistance to ACV was discovered in 1 case. Three unique therapeutic strategies finally led to epithelial closure.

Conclusions

The novel thymidine kinase mutation (N202K) should be considered to infer resistance to all molecules requiring activation by the viral thymidine kinase. Current topical alternatives in the ophthalmologist's armamentarium include trifluridine 1%, foscarnet 1,2%-1,4% or cidofovir 0,2–0,5%. Epithelial debridement, high-frequency dosing and reduction of immunosuppression are useful adjuncts.

Importance

Clinicians should perform epithelial debridement in recalcitrant HK, allowing geno- and phenotypically guided therapy, even without a history of long-term anti-viral prophylaxis or recurrent HK. This report provides mandatory knowledge allowing the reader to comprehend how therapy should be altered based upon these results. To the best of our knowledge, successful treatment of proven ACV resistant HK with topical foscarnet has not yet previously been published.

Furthermore, this paper highlights a lack of controlled studies investigating alternative topical treatments in case of viral resistance, offering opportunities for future research.

Virulence of herpes simplex virus 1 harbouring a UAG stop codon between the first and second initiation codon in the thymidine kinase gene

Herpes simplex virus 1 (HSV-1)-TK(8UAG) expresses a truncated thymidine kinase (TK) translated from the second initiation codon due to a stop UAG codon at the 8th position (counted from the first initiation codon). Here, we showed that the sensitivity of HSV-1-TK(8UAG) to acyclovir (ACV) is similar to that of control HSV-1 wild-type (WT), which expresses an intact TK protein. However, HSV-1-TK(44UAG), which expresses a truncated TK due to a UAG codon at position 44, showed lower sensitivity to ACV. A mouse infection model was used to compare the virulence of HSV-1-TK(8UAG) and HSV-1-TK(44UAG) with that of HSV-1 wild-type (WT). The 50% lethal dose (LD₅₀) value of HSV-1-TK(44UAG) was 7.8-fold higher than that of HSV-1-TK(8UAG), whereas the LD₅₀ value of HSV-1-TK(8UAG) was the same as that of the parental HSV-1 WT. There were no statistically significant differences between HSV-1-TK(44UAG), HSV-1-TK(8UAG), and HSV-1 WT with respect to replication capacity and viral TK mRNA expression in mouse brain. Thus, the virulence of HSV-1 expressing a truncated viral TK translated from the second initiation codon might depend on the position of the UAG stop codon.

Herpesvirus Infections of the Nervous System

PURPOSE OF REVIEW

This article reviews the spectrum of neurologic disease associated with human herpesvirus infections.

RECENT FINDINGS

As more patients are becoming therapeutically immunosuppressed, human herpesvirus infections are increasingly common. Historically, infections with human herpesviruses were described as temporal lobe encephalitis caused by herpes simplex virus type 1 or type 2. More recently, however, additional pathogens, such as varicella-zoster virus, Epstein-Barr virus, cytomegalovirus, and human herpesvirus 6 have been identified to cause serious neurologic infections. As literature emerges, clinical presentations of herpesvirus infections have taken on many new forms, becoming heterogeneous and involving nearly every location along the neuraxis. Advanced diagnostic methods are now available for each specific pathogen in the herpesvirus family. As data emerge on viral resistance to conventional therapies, newer antiviral medications must be considered.

SUMMARY

Infections from the herpesvirus family can have devastating neurologic outcomes without prompt and appropriate treatment. Clinical recognition of symptoms and appropriate advanced testing are necessary to correctly identify the infectious etiology. Knowledge of secondary neurologic complications of disease is equally important to prevent additional morbidity and mortality. This article discusses infections of the central and peripheral nervous systems caused by herpes simplex virus type 1 and type 2, varicella-zoster virus, Epstein-Barr virus, cytomegalovirus, and human herpesvirus 6. The pathophysiology, epidemiology, clinical presentations of disease, diagnostic investigations, imaging characteristics, and treatment for each infectious etiology are discussed in detail.

Alpha-Herpesvirus Thymidine Kinase Genes Mediate Viral Virulence and Are Potential Therapeutic Targets

Alpha-herpesvirus thymidine kinase (TK) genes are virulence-related genes and are nonessential for viral replication; they are often preferred target genes for the construction of gene-deleted attenuated vaccines and genetically engineered vectors for inserting and expressing foreign genes. The enzymes encoded by TK genes are key kinases in the nucleoside salvage pathway and have significant substrate diversity, especially the herpes simplex virus 1 (HSV-1) TK enzyme, which phosphorylates four nucleosides and various nucleoside analogues. Hence, the HSV-1 TK gene is exploited for the treatment of viral infections, as a suicide gene in antitumor therapy, and even for the regulation of stem cell transplantation and treatment of parasitic infection. This review introduces the effects of α-herpesvirus TK genes on viral virulence and infection in the host and classifies and summarizes the current main application domains and potential uses of these genes. In particular, mechanisms of action, clinical limitations, and antiviral and antitumor therapy development strategies are discussed.

Targeting the Viral Polymerase of Diarrhea-Causing Viruses as a Strategy to Develop a Single Broad-Spectrum Antiviral Therapy

Viral gastroenteritis is an important cause of morbidity and mortality worldwide, being particularly severe for children under the age of five. The most common viral agents of gastroenteritis are noroviruses, rotaviruses, sapoviruses, astroviruses and adenoviruses, however, no specific antiviral treatment exists today against any of these pathogens. We here discuss the feasibility of developing a broad-spectrum antiviral treatment against these diarrhea-causing viruses. This review focuses on the viral polymerase as an antiviral target, as this is the most conserved viral protein among the diverse viral families to which these viruses belong to. We describe the functional and structural similarities of the different viral polymerases, the antiviral effect of reported polymerase inhibitors and highlight common features that might be exploited in an attempt of designing such pan-polymerase inhibitor.

Comparison of antiviral resistance across acute and chronic viral infections

Antiviral therapy can lead to drug resistance, but multiple factors determine the frequency of drug resistance mutations and the clinical consequences. When chronic infections caused by Human Immunodeficiency Virus (HIV), Hepatitis C Virus (HCV) and Hepatitis B Virus (HBV) are compared with acute infections such as influenza virus, respiratory syncytial virus (RSV), and other respiratory viruses, there are similarities in how and why antiviral resistance substitutions occur, but the clinical significance can be quite different. Emergence of resistant variants has implications for design of new therapeutics, treatment guidelines, clinical trial design, resistance monitoring, reporting, and interpretation. In this discussion paper, we consider the molecular factors contributing to antiviral drug resistance substitutions, and a comparison is made between chronic and acute infections. The implications of resistance are considered for clinical trial endpoints and public health, as well as the requirements for therapeutic monitoring in clinical practice with acute viral infections.