Standardisation of a microplate in situ ELISA (MISE-test) for the susceptibility testing of herpes simplex virus to acyclovir

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.

A small-molecule dengue virus entry inhibitor

The incidence of dengue fever epidemics has increased dramatically over the last few decades. However, no vaccine or antiviral therapies are available. Therefore, the need for safe and effective antiviral drugs has become imperative. The entry of dengue virus into a host cell is mediated by its major envelope (E) protein. The crystal structure of the E protein reveals a hydrophobic pocket that is presumably important for low-pH-mediated membrane fusion. High-throughput docking with this hydrophobic pocket was performed, and hits were evaluated in cell-based assays. Compound 6 was identified as one of the inhibitors and had an average 50% effective concentration of 119 nM against dengue virus serotype 2 in a human cell line. Mechanism-of-action studies demonstrated that compound 6 acts at an early stage during dengue virus infection. It arrests dengue virus in vesicles that colocalize with endocytosed dextran and inhibits NS3 expression. The inhibitors described in this report can serve as molecular probes for the study of the entry of flavivirus into host cells.

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.

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

The widespread use of acyclovir (ACV) could increase the prevalence of herpes simplex virus (HSV) ACV-resistant isolates, and a screening assay are thus important for routine surveillance of the ACV susceptibility of HSV. A screening dye-uptake assay was developed, based on the conventional dye-uptake assay [J. Biol. Stand. 14 (1986) 201]. The susceptibility of HSV was measured by testing two virus dilutions (10(-1) and 10(-2)) against two ACV concentrations (5 and 10 microM) on Vero cells and expressed as a reduced percentage of viral replication. The reproducibility was evaluated with HSV1 and HSV2 ACV-sensitive and ACV-resistant reference strains introduced as controls in successive series. The dye-uptake by Vero cells, the growth capacity of the HSV strains and the reduction of the viral replication in the presence of acyclovir varied by less than 14, 20 and 30%, respectively. This assay allowed the detection of a heterogenous population containing as few as 20% of ACV-resistant strain. The screening test was applied to 500 HSV isolates in a prospective study, and over 95% of the HSV isolates tested were characterised using a single test. This test appeared to be half the cost and much easier to carry out than the conventional dye-uptake assay, and consequently is well suited for large scale surveillance.

Application of real-time PCR for determination of antiviral drug susceptibility of herpes simplex virus

A quantitative real-time PCR (TaqMan) assay was developed for determination of antiviral drug susceptibility of herpes simplex virus (HSV). After short-time culture of the virus, the antiviral drug susceptibility of HSV isolates for acyclovir (ACV) was determined by measuring the reduction of the HSV type 1 (HSV-1) DNA levels in culture supernatants using real-time PCR. The 50% inhibitory concentration was reported as the concentration of antiviral drug that reduced the number of HSV-1 DNA copies by 50%. A total of 15 well-characterized ACV-sensitive or -resistant strains and clinical isolates were used for assay evaluation. The new assay with real-time PCR readout permitted rapid (3 days), objective, and reproducible determination of HSV-1 drug susceptibilities with no need for stringent control of initial multiplicity of infection. Furthermore, the real-time PCR assay results showed good correlation (r = 0.86) with those for the plaque reduction assay. In conclusion, the real-time PCR assay described here is a suitable quantitative method for determination of antiviral susceptibility of HSV-1, amenable for use in the routine diagnostic virology laboratory.

A rapid assay for evaluation of antiviral activity against coxsackie virus B3, influenza virus A, and herpes simplex virus type 1

In order to identify new potential antiviral drugs, small amounts of extracts or compounds have to be examined for cytotoxicity and antiviral activity in primary screening using a rapid, easy, inexpensive, and highly standardised test system. In this study, high-throughput cytopathic effect (CPE) inhibitory assays were established for coxsackie virus B3 on HeLa Ohio cells, influenza virus A on Madin-Darby canine kidney cells, and herpes simplex virus type 1 (HSV-1) on green monkey kidney cells that meet these requirements. The cytotoxic and the antiviral effects were quantified using a crystal violet uptake assay allowing automated handling of large numbers of candidate agents. To ensure comparable results with plaque reduction assays, the 50 and 90% plaque inhibitory concentrations of guanidine, amantadine, and phosphonoformic acid were used to standardise the anti-coxsackie virus B3, anti-influenza virus A, and anti-HSV-1 tests, respectively. The strong correlation between the antiviral activity determined by CPE-inhibitory assays and plaque reduction assay was further proved for other antivirals. In summary, low amounts of large numbers of compounds may be tested inexpensively and standardised within 24 h (coxsackie virus B3 and influenza virus A) or 48 h (herpes simplex virus type 1) post-infection using CPE inhibitory assays.

Optimization of in situ cellular ELISA performed on influenza A virus-infected monolayers for screening of antiviral agents

Viral susceptibility testing has been traditionally performed by the plaque reduction assay (PRA) which is laborious, time consuming, relatively expensive, and requires subjective input by the reader. An in situ cellular enzyme-linked immunosorbent assay (ELISA) has been developed with the potential to overcome many of the limitations of PRA and has been applied to a variety of viruses. This study establishes the specific conditions necessary for susceptibility testing of influenza A virus to antiviral agents such as amount of inoculum size, duration of incubation, fixative type, and cell number; factors which are critical to the performance of the in situ cellular ELISA. In situ cellular ELISA was found to correlate strongly with the plaque assay (PA) (R2 = 0.997, P < 0.002). Both assays were applied to test the susceptibility of influenza A virus to a new antiviral emulsion agent and yielded comparable data. The optimized in situ cellular ELISA can serve as a reliable assay for the rapid screening of large numbers of antiviral agents.

New antiherpetic nucleoside from a Basidiomycete

Antiviral activity was characterized from the culture broth of the Basidiomycete Macrocystidia cucumis (Pers. ex Fr.) Heim. When the stationary phase was reached (21 d), the culture broth was shown through an ELISA assay to contain antiviral activity against herpes simplex virus type 1 (HSV-1), as assessed in baby hamster kidney cells (BHK-21). Once the presence of the anti-HSV-1 activity in the culture broth was demonstrated, we proceeded with the purification and isolation of the active principle using a semi-preparative HPLC technique. The activity was associated with a purine nucleoside designated McA. This compound displayed no cytotoxicity at antivirally effective concentrations and proved to be a novel nucleoside analogue.

Comparative evaluation of microplate enzyme-linked immunosorbent assay versus plaque reduction assay for antiviral susceptibility testing of herpes simplex virus isolates

We tested the antiviral susceptibilities of 30 clinical isolates of herpes simplex virus using the microplate in situ enzyme-linked immunosorbent assay (MISE) and the plaque reduction assay (PRA). There was concordance for 26 of 30 acyclovir results and all 30 foscarnet results. MISE and PRA results each predicted the response to acyclovir in 12 of 14 instances and the response to foscarnet in 8 instances. MISE is more rapid than PRA, has an objective endpoint, and correlates well with the clinical response to therapy.

A blinded comparison of two methods of viral susceptibility testing: plaque reduction assay versus microplate in situ ELISA

Viral susceptibility testing has been shown to have a role in the management of patients with herpes simplex infections. In this study, 25 isolates of herpes simplex virus representing a broad spectrum of acyclovir-susceptible and -resistant phenotypes were tested using a microplate in situ enzyme-linked immunosorbent assay (MISE). This method is objective and more rapid than the traditional plaque reduction assay (PRA). The previously derived PRA results were not known at the time of testing with the MISE method. The correlation coefficient between PRA and MISE was 0.85. Agreement on sensitive or resistant was reached for 21 of 25 isolates. The standardised microplate in situ ELISA was found to be an acceptable alternative to the plaque reduction assay.