A screening dye-uptake assay to evaluate in vitro susceptibility of herpes simplex virus isolates to acyclovir

Phenotypic testing of patient herpes simplex virus type 1 and 2 isolates for acyclovir resistance by a novel method based on real-time cell analysis

BACKGROUND

Acyclovir (ACV) is the most commonly used drug for herpes simplex virus (HSV) infection therapy. Prolonged antiviral therapy or prophylaxis in immunocompromised patients may promote the development of drug-resistant strains. Due to the high polymorphism in genes involved in drug resistance, phenotypic methods, although work-intensive, are still required to test drug susceptibility. Real-time cell analysis (RTCA) based methods could offer a rapid and less labor-intensive alternative for phenotypic testing of ACV resistance.

OBJECTIVE

To investigate the utility of a new RTCA based assay (RTCAA) to test acyclovir susceptibility of HSV clinical isolates.

STUDY DESIGN

Four reference strains and 93 clinical isolates (60 HSV-1 and 33 HSV-2) were tested by RTCAA. In the presence of ACV concentrations from 2.2 to 140.8 μM, Vero cells were infected with different virus dilutions. IC₅₀ values were calculated by dose-response curve (DRC) with area-under-curve (AUC) method. The reference strains and 22 clinical isolates were additionally tested by dye-uptake assay, and IC₅₀ values of both methods were compared.

RESULTS

IC₅₀ values from RTCAA and dye-uptake assays were positively correlated (Spearman's rho = 0.897, p < 0.001) and quantitatively agreed (Bland-Altman plot). Based on a cut-off of 4 μM for HSV-1 and 13 μM for HSV-2, 87 isolates were classified as ACV-sensitive and 6 isolates as ACV-resistant. The reference strains showed the expected results of ACV susceptibility.

CONCLUSION

RTCAA agrees well with the dye-uptake assay. Compared with other phenotypic methods, RTCAA requires less manipulation, reduces the workload and the turnaround time, and appears to be an objective and reliable method to test ACV susceptibility.

Testing the susceptibility of human herpesviruses to antivirals

Herpesviruses cause chronic lifelong infections in humans and may cause life-threatening diseases in immunosuppressed patients. Antiviral drugs targeted to viral DNA polymerase, such as acyclovir, penciclovir, ganciclovir, foscarnet and cidofovir, are currently available and have been proven to be efficient against clinical symptoms of herpesvirus infections. The resistance of herpesviruses to these drugs is associated with specific mutations of viral genes encoding either DNA polymerase or enzymes phosphorylating nucleoside analogs. Resistance is detected and characterized by means of specific susceptibility assays, which can be classified as phenotypic, genetic and functional. These tests are used both to investigate novel antiviral compounds and look for the emergence of resistant viruses in treated patients in case of clinical failure. Although susceptibility assays are often time consuming and present some limitations regarding the interpretation of their results, their use in the monitoring of antiherpetic treatments should be promoted and improved, in parallel to the development of novel efficient drugs.

A Retrospective, Case-Control Study of Acyclovir Resistance in Herpes Simplex Virus

BACKGROUND

Occasional cases of acyclovir resistance have been documented in the treatment of herpes simplex virus (HSV) infection. Thirty-eight subjects with acyclovir-resistant infections were identified in an epidemiological surveillance program involving 1811 HSV-infected subjects in France.

METHODS

Twenty-three index cases from whom acyclovir-resistant HSV strains had been isolated were compared with 46 control subjects matched for immunological status. Sociodemographic characteristics, features of the acyclovir-resistant HSV episode, history of HSV infection, treatment, outcome, and immunological history were recorded.

RESULTS

Twenty-two index case patients presented with immunodepression. Sixty-five percent reported clinically manifest recurrences, compared with 33% of matched control subjects. Significantly more index case patients had used antiviral drugs, and they had used them more often than had control subjects. However, 26.1% of index case patients reported no antiviral exposure in the previous 2 years. Two-thirds of the strains recovered from the index case patients were isolated because of suspicion of clinical resistance to acyclovir.

CONCLUSIONS

Clinical treatment resistance is associated with acyclovir-resistant HSV strains, but acyclovir-resistant strains were also isolated from treatment-naive subjects.

S-adenosyl methionine decarboxylase activity is required for the outcome of herpes simplex virus type 1 infection and represents a new potential therapeutic target

All the available antiherpetic drugs are directed against viral proteins. Their extensive clinical use has led to the emergence of resistant viral strains. There is a need for the treatment of herpes infections due to resistant strains, especially for immunocompromised patients. To design new kinds of drugs, we have developed a strategy to identify cellular targets. Herpes simplex virus type 1 (HSV-1) infection is concomitant to a repression of most host protein synthesis. However, some cellular proteins continue to be efficiently synthesized. We speculated that some of them could determine the outcome of infection. Since two polyamines, spermidine and spermine, are components of the HSV-1 virions, we investigated whether enzymes involved in their synthesis could be required for viral infection. We show that inhibition of S-adenosyl methionine decarboxylase, a key enzyme of the polyamine metabolic pathway, prevents HSV-1 infection. Inhibition of polyamine synthesis prevents infection of culture cells with HSV-1 laboratory strains as well as clinical isolates that are resistant to the conventional antiviral drugs acyclovir and foscarnet. Our data provide the opportunity to develop molecules with a novel mechanism of action for the treatment of herpes infection.

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.

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.

Surveillance network for herpes simplex virus resistance to antiviral drugs: 3-year follow-up

Herpes simplex virus (HSV) infections are very common in the general population and among immunocompromised patients. Acyclovir (ACV) is an effective treatment which is widely used. We deemed it essential to conduct a wide and coordinated survey of the emergence of ACV-resistant HSV strains. We have formed a network of 15 virology laboratories which have isolated and identified, between May 1999 and April 2002, HSV type 1 (HSV-1) and HSV-2 strains among hospitalized subjects. The sensitivity of each isolate to ACV was evaluated by a colorimetric test (C. Danve, F. Morfin, D. Thouvenot, and M. Aymard, J. Virol. Methods 105:207-217, 2002). During this study, 3900 isolated strains among 3357 patients were collected; 55% of the patients were immunocompetent. Only six immunocompetent patients excreted ACV-resistant HSV strains (0.32%), including one female patient not treated with ACV who was infected primary by an ACV-resistant strain. Among the 54 immunocompromised patients from whom ACV-resistant HSV strains were isolated (3.5%), the bone marrow transplantation patients showed the highest prevalence of resistance (10.9%), whereas among patients infected by human immunodeficiency virus, the prevalence was 4.2%. In 38% of the cases, the patients who excreted the ACV-resistant strains were treated with foscarnet (PFA), and 61% of them developed resistance to PFA. The collection of a large number of isolates enabled an evaluation of the prevalence of resistance of HSV strains to antiviral drugs to be made. This prevalence has remained stable over the last 10 years, as much among immunocompetent patients as among immunocompromised patients.

Herpes simplex virus resistance to antiviral drugs

Herpes simplex virus (HSV) infections are efficiently treated with antiviral drugs such as acyclovir (ACV). However, resistance has been reported, mainly among immunocompromised patients (prevalence around 5%) and particularly allogeneic bone marrow transplant patients (prevalence reaching 30%). Resistance to ACV is associated with mutations on one of the two viral enzymes involved in the ACV mechanism of action: thymidine kinase (TK) and DNA polymerase. In 95% of the cases, ACV resistance is associated with a mutation in the TK gene as this enzyme is not essential for viral replication, unlike viral DNA polymerase, which is rarely involved in resistance. Strains resistant to ACV are almost always cross-resistant to other TK-dependent drugs such as penciclovir and famciclovir. Resistant infections can be managed by foscarnet or cidofovir but both are more toxic than ACV. These drugs also inhibit viral DNA polymerase but they are active on most ACV-resistant HSV as they do not depend on TK; nevertheless virus resistant to ACV because of a mutation in the DNA polymerase may be cross-resistant to these molecules. Published data on genetic characterization of resistant clinical isolates point out hot spots in viral TK and DNA polymerase genes. TK mutations associated with resistance are either insertion or deletion (codons 92 and 146 of TK gene) or substitution (codon 176-177, 336 of TK gene). DNA polymerase mutations are mainly located in conserved sites of the enzyme. A high level of gene polymorphism has also been reported for these genes, especially for TK. These results are useful for the development of rapid genotypic assays for the detection of mutations associated with resistance.