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

A trial of valganciclovir prophylaxis for cytomegalovirus prevention in lung transplant recipients

Cytomegalovirus (CMV) infection is common after lung transplantation. We performed a prospective trial of valganciclovir prophylaxis in lung recipients with outcomes compared to matched historical controls. The valganciclovir group (n = 40) (including D+/R- and R+ patients) was prospectively enrolled, and received oral valganciclovir 900 mg once daily for 12 weeks. Historical controls (n = 40) received 12 weeks of daily intravenous ganciclovir if D+/R- or 12 weeks of oral ganciclovir if R+. CMV viral load testing was done at two-week intervals until 6 months posttransplant. Baseline demographics and immunosuppression were comparable in the two groups. The incidence of CMV viremia was 16/40 (40.0%) in the valganciclovir arm versus 18/40 (45%) in the ganciclovir arm (p = NS). The incidence of symptomatic CMV disease was 8/40 (20%) versus 7/40 (17.5%), respectively (p = NS). In both groups viremia, while on prophylaxis, was uncommon (valganciclovir: 0/40 and ganciclovir: 2/40). Peak viral load and time to viremia were similar in the two arms. High rates of viremia and symptomatic disease occurred in the D+/R- patients after discontinuation of prophylaxis. Genotypic CMV sequence analysis demonstrated low rates of ganciclovir resistance in both groups. Valganciclovir prophylaxis had similar efficacy to either intravenous ganciclovir (D+/R- patients), or oral ganciclovir (R+ patients) in lung recipients.

Herpes simplex virus thymidine kinase mutations associated with resistance to acyclovir: a site-directed mutagenesis study

Mutations in the thymidine kinase (TK) gene of herpes simplex virus (HSV) may confer resistance to acyclovir (ACV). Because of the high genetic polymorphism of this gene, discriminating between mutations related to resistance and mutations related to gene polymorphism can be difficult, especially when no sensitive strain has been previously isolated from the same patient. To assess the role of the mutations located at codons 51, 77, 83, and 175, previously detected in HSV-1 clinical isolates (F. Morfin, G. Souillet, K. Bilger, T. Ooka, M. Aymard, and D. Thouvenot, J. Infect. Dis. 182:290-293, 2000), in the acquisition of resistance to ACV, four mutants with site-directed mutations at these respective codons were constructed. The enzymatic activity of the proteins, produced using both a reticulocyte lysate system and a bacterial system, was evaluated using [(3)H]thymidine as substrate. This site-directed mutagenesis revealed that mutations at codons 51, 83, and 175 induce a loss of HSV-1 TK activity and are thus clearly involved in the acquisition of resistance to ACV. On the other hand, the mutation at codon 77 does not affect enzyme activity.

Genotypic characterization of the DNA polymerase and sensitivity to antiviral compounds of foscarnet-resistant herpes simplex virus type 1 (HSV-1) derived from a foscarnet-sensitive HSV-1 strain

Foscarnet is widely used for the treatment of acyclovir-resistant herpesvirus infections, and foscarnet-resistant herpesvirus infections are a serious concern in immunocompromised patients. Twenty-seven single-plaque isolates of herpes simplex virus type 1 (HSV-1) resistant to foscarnet were selected from foscarnet- and acyclovir-sensitive HSV-1 strain TAS by exposure to foscarnet, and the DNA polymerase genes were analyzed. The sensitivities of these mutants to foscarnet, cidofovir, S2242, acyclovir, ganciclovir, and penciclovir were determined. A single amino acid substitution, double amino acid substitutions, and a combination of a single amino acid substitution with a deletion or insertion of amino acid residues in the viral DNA polymerase were demonstrated in 21, 4, and 2 isolates, respectively. Of the 27 isolates, an amino acid substitution of serine for asparagine at amino acid position 724 in the DNA polymerase (724 S-N) was detected in 8 isolates. An amino acid substitution in conserved region II was demonstrated in these eight isolates as well as four other isolates. The mutation in the DNA polymerase responsible for resistance to foscarnet was located between the pre-IV region and conserved region V, especially within conserved region II. All the isolates were sensitive or hypersensitive to cidofovir and ganciclovir. Seven, 5, and 15 of the 27 isolates were also sensitive to S2242, acyclovir, and penciclovir, respectively. Thus, most of the foscarnet-resistant HSV-1 isolates were sensitive or hypersensitive to cidofovir and ganciclovir.

Survey of acyclovir-resistant herpes simplex virus in the Netherlands: prevalence and characterization

BACKGROUND

Widespread and frequent use of acyclovir (ACV) for treatment, suppressive therapy and prophylaxis of herpes simplex virus (HSV) infections and its over the counter availability may be associated with emergence of HSV resistance.

OBJECTIVES

To determine the prevalence of ACV-resistant HSV isolates in different patient groups between 1999 and 2002 in the Netherlands.

STUDY DESIGN

A total of 542 isolates, 410 HSV-1 and 132 HSV-2, from 496 patients were screened for reduced susceptibility to ACV. A newly developed ELVIRA HSV screening assay was used that allowed a high throughput screening. The genotypic analysis of the HSV thymidine kinase gene was performed to identify resistance-associated mutations.

RESULTS

Thirteen isolates, 8 HSV-1 and 5 HSV-2, from 10 patients (2%) were found resistant to ACV. A single ACV-resistant strain was identified among isolates from 368 immunocompetent patients (0.27%; 95% confidence interval [CI], 0.007%-1.5%), whereas in nine isolates from 128 immunocompromised patients resistant HSV was identified (7%; 95% CI, 3.26%-12.93%). The highest frequency of ACV-resistant HSV was associated with bone marrow transplantation: four patients out of 28 (14.3%) shed resistant virus. In addition, resistant virus was obtained from two HIV-positive patients, one patient with a hematological malignancy and two patients on immunosuppressive drugs. Further testing showed that none of the isolates was resistant to foscarnet. Several new mutations were identified in the thymidine kinase gene of these resistant isolates, and their effect on ACV-resistance is discussed.

CONCLUSIONS

Our study shows that the prevalence of ACV resistance is low in immunocompetent patients (0.27%), whereas ACV-resistant HSV infections occur relatively frequently in immunocompromised patients (7%; P < 0.0001). This emphasizes the need for drug susceptibility monitoring of HSV infections in immunocompromised patients with persisting infections despite antiviral therapy.

Identification of a small molecule that inhibits herpes simplex virus DNA Polymerase subunit interactions and viral replication

The interaction between the catalytic subunit Pol and the processivity subunit UL42 of herpes simplex virus DNA polymerase has been characterized structurally and mutationally and is a potential target for novel antiviral drugs. We developed and validated an assay for small molecules that could disrupt the interaction of UL42 and a Pol-derived peptide and used it to screen approximately 16,000 compounds. Of 37 "hits" identified, four inhibited UL42-stimulated long-chain DNA synthesis by Pol in vitro, of which two exhibited little inhibition of polymerase activity by Pol alone. One of these specifically inhibited the physical interaction of Pol and UL42 and also inhibited viral replication at concentrations below those that caused cytotoxic effects. Thus, a small molecule can inhibit this protein-protein interaction, which provides a starting point for the discovery of new antiviral drugs.

Analysis of cytomegalovirus DNA polymerase (UL54) mutations in solid organ transplant patients receiving valganciclovir or ganciclovir prophylaxis

We previously reported the absence of CMV UL97 (kinase) gene resistance mutations up to 12 months post-transplant following 100 days of valganciclovir prophylaxis, and a low incidence of resistance mutations following 100 days of oral ganciclovir prophylaxis in a prospective multicenter study in solid organ transplant recipients excluding lung transplants. Herein, we report UL54 (DNA polymerase) gene sequencing results for all patients with previous UL97 PCR-positive samples (n = 99) in our study. One UL54 resistance mutation (L545S known to confer ganciclovir and cidofovir resistance) was detected in a routine day-100 sample from an asymptomatic patient who received oral ganciclovir. Notably, this CMV UL54 mutation occurred in the absence of a UL97 mutation. Additionally, new UL54 variants were observed. Thus, emergence of CMV UL54 mutations in the absence of UL97 mutations is a rare but possible event that is not necessarily associated with detrimental clinical outcome in solid organ transplant recipients.

Update on human herpesvirus 6 biology, clinical features, and therapy

Human herpesvirus 6 (HHV-6) is a betaherpesvirus that is closely related to human cytomegalovirus. It was discovered in 1986, and HHV-6 literature has expanded considerably in the past 10 years. We here present an up-to-date and complete overview of the recent developments concerning HHV-6 biological features, clinical associations, and therapeutic approaches. HHV-6 gene expression regulation and gene products have been systematically characterized, and the multiple interactions between HHV-6 and the host immune system have been explored. Moreover, the discovery of the cellular receptor for HHV-6, CD46, has shed a new light on HHV-6 cell tropism. Furthermore, the in vitro interactions between HHV-6 and other viruses, particularly human immunodeficiency virus, and their relevance for the in vivo situation are discussed, as well as the transactivating capacities of several HHV-6 proteins. The insight into the clinical spectrum of HHV-6 is still evolving and, apart from being recognized as a major pathogen in transplant recipients (as exemplified by the rising number of prospective clinical studies), its role in central nervous system disease has become increasingly apparent. Finally, we present an overview of therapeutic options for HHV-6 therapy (including modes of action and resistance mechanisms).

Construction of recombinant herpes simplex virus type I expressing green fluorescent protein without loss of any viral genes

For use in various applications in research on herpes simplex virus type 1, we attempted to generate recombinant HSV-1 expressing green fluorescent protein (GFP) without any loss of viral genes. Our results were as follows. (i) A recombinant HSV-1 (YK333), in which a GFP expression cassette driven by the Egr-1 promoter was inserted into the intergenic region between UL3 and UL4, was constructed. (ii) YK333 replicated as well as wild-type HSV-1 F strain in Vero cells. (iii) As one application of the recombinant YK333 for research on HSV-1, we developed a system to detect anti-herpetic activity, termed a fluorescence-based anti-viral assay. The 50% inhibitory concentration of ganciclovir for YK333 determined using our newly developed assay was comparable to that determined using a plaque reduction assay. YK333 will be a convenient tool for herpes simplex virus research, including such applications as monitoring of viral replication in vitro and in vivo, and rapid screening of potential anti-herpetic agents.

Cytomegalovirus UL97 mutations associated with ganciclovir resistance in immunocompromised patients from Argentina

BACKGROUND

Prolonged therapy with ganciclovir (GCV) can result in the development of GCV-resistant strains due to mutations in the viral phosphotransferase (UL97 gene) and/or in the viral DNA polymerase (UL54 gene).

OBJECTIVES

The purpose of this study was to detect by molecular methods the most prevalent UL97 mutants which confer ganciclovir-resistance in immunocompromised populations.

STUDY DESIGN

Patients from two populations were selected: (a) renal transplant patients with active cytomegalovirus (CMV) infection and more than one cycle of GCV; (b) HIV-infected patients with retinitis due to CMV, who were under GCV induction, maintenance therapy or withdrawal. Patients were followed up by pp65 antigenemia and by viral isolation from blood or/and urine samples. Two fragments (133 and 255pb) of the UL97 gene were amplified by polymerase chain reaction (PCR) from CMV isolates.

RESULTS

Nine from 12 isolates obtained were sequenced, three from two renal transplant patients and six from five HIV-infected patients. A UL97 mutation, known to confer GCV resistance, was found in two isolates from a renal transplant patient. A methionine to valine mutation at codon 460 (M460V) was detected. These isolates exhibited another mutation at codon 605, whose amino acid changed from aspartic acid (D) to glutamic acid (E). These findings were observed after treatment with IV-GCV/ O-GCV/ IV-GCV for 151 days. The 605 mutation was also detected in leukocytes from the same patient previous to the beginning of the treatment with GCV.

CONCLUSIONS

Although a known resistant mutation appeared in a renal transplant patient, it was not associated with CMV disease. We suggest that the D605E mutation could "partially or totally compensate" for the effect of GCV resistance conferred by the 460 mutation. Further studies should be performed to confirm this hypothesis.

Genotypic and Phenotypic Characterization of Acyclovir-Resistant Herpes Simplex Viruses Isolated from Haematopoietic Stem Cell Transplant Recipients

Thirty-one herpes simplex virus type one (HSV-1) isolates from 12 haematopoietic stem cell transplant recipients with persistent HSV infections despite acyclovir (ACV) prophylaxis or treatment, were genotypically and phenotypically characterized. The relationship between drug susceptibility of the isolates and mutations in thymidine kinase (TK) and DNA polymerase (DNA pol) genes was examined. In all 12 patients, HSV infections were due to ACV-resistant, foscarnet-sensitive viruses. Out of 31 isolates examined, 23 were resistant and eight were sensitive to ACV; eight patients carried viruses with frameshift mutations in the TK gene (due to addition or deletion of single nucleotides in homopolymeric repeats). These mutations were found at codon 61 (G deletion, one patient), 146 (G insertion, five patients) and 153 or 185 (C deletion, one patient each). In four patients, viruses were selected during ACV therapy that contained novel amino acid substitutions in the TK gene (H58R, G129D, A189V, R216H, R220C). Their possible role in ACV resistance was further confirmed phenotypically and by the absence of any resistance-associated mutations in the DNA pol gene. These substitutions were located in ATP- or nucleoside-binding sites or in conserved regions of the TK gene. In addition, a single mutation, Q570R, in the δ-region C of the DNA pol gene, was identified in an isolate from a single patient with resistance to ACV. Our study confirms and expands previous data on genotypic changes associated with ACV resistance of HSV-1 clinical isolates.

Chapter 22. Non-HIV antiviral agents

This chapter focuses on non-HIV antiviral agents. The development of antiviral agents to treat non-HIV infections is largely focused on therapies for the treatment of chronic hepatitis infections B and C. Nucleoside analog continue to be the mainstay of Hepatitis B Virus (HBV) therapeutics. The first small molecule inhibitor of Hepatitis C Virus (HCV), the NS3 protease inhibitor BILN-2061, entered phase 2 clinical trials, producing a striking reduction in viral load in treated individuals. The development of the HCV replicon system and its application to screening for antiviral agents provided tangible benefit with the disclosure of mechanistically and structurally diverse HCV inhibitors. Adefovir dipivoxil has been approved in the United States and the European Union for the treatment of HBV, providing a second small molecule antiviral to add to lamivudine (3TC) and the injectable protein IFNα as the only approved agents for treating HBV infection. The chapter also provides details of the inhibitors of hepatitis B and C virus, the inhibitors of simplex virus and human cytomegalovirus, the inhibitors of respiratory viruses and the inhibitors of West Nile virus and Papilloma virus.

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.

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.

Local treatment of viral disease using photodynamic therapy

Although reports of the photodynamic inactivation of viruses appeared in 1928, long before chemotherapeutic antiviral drugs, the first clinical trial in humans-the topical treatment of herpes genitalis-did not take place until the early 1970s. Trials were discontinued due to the transformation of healthy cells and concomitant incidence of Bowen's disease in some patients, probably due to the migration of infective sections of photodamaged viral nucleic acid. With the modern development of photodynamic therapy as a cancer treatment and the use of photosensitisers in the photodecontamination of blood products, a great deal of experience has been gained, both in the minimisation of side effects in humans and in the targeting and eradication of viruses. This suggests that the photodynamic approach to a range of virus-associated infections, lesions and cancer might now be revisited with greater success.

Novel agents and strategies to treat herpes simplex virus infections

The quiet pandemic of herpes simplex virus (HSV) infection has plagued humanity since ancient times, causing mucocutaneous infection, such as herpes labialis and herpes genitalis. Disease symptoms often interfere with everyday activities and occasionally HSV infections are the cause of life-threatening or sight-impairing disease, especially in neonates and the immunocompromised patient population. After primary or initial infection the virus persists for life in a latent form in neurons of the host, periodically reactivating and often resulting in significant psychosocial distress for the patient. Currently, no cure is available. In the mid-1950s the first antiviral, idoxuridine, was developed for topical treatment of herpes disease and, in 1978, vidarabine was licensed for systemic use to treat HSV encephalitis. Acyclovir (Zovirax), a potent, specific and tolerable nucleosidic inhibitor of the herpes DNA polymerase, was a milestone in the development of antiviral drugs in the late 1970s. In the mid-1990s, when acyclovir became a generic drug, valacyclovir (Valtrex) and famciclovir (Famvir), prodrugs of the gold standard and penciclovir (Denavir), Vectavir), a close analogue, were launched. Though numerous approaches and strategies were tested and considerable effort was expended in the search of the next generation of an antiherpetic therapy, it proved difficult to outperform acyclovir. Notable in this regard was the award of a Nobel Prize in 1988 for the elucidation of mechanistic principles which resulted in the development of new drugs such as acyclovir. Vaccines, interleukins, interferons, therapeutic proteins, antibodies, immunomodulators and small-molecule drugs with specific or nonspecific modes of action lacked either efficacy or the required safety profile to replace the nucleosidic drugs acyclovir, valacyclovir, penciclovir and famciclovir as the first choice of treatment. Recently though, new inhibitors of the HSV helicase-primase with potent in vitro antiherpes activity, novel mechanisms of action, low resistance rates and superior efficacy against HSV in animal models have been discovered. This review summarises the current therapeutic options, discusses the potential of preclinical or investigational drugs and provides an up-to-date interpretation of the challenge to establish novel treatments for herpes simplex disease.