A comparative evaluation of three methods of antiviral susceptibility testing of clinical herpes simplex virus isolates

Antiherpetic Plants: A Review of Active Extracts, Isolated Compounds, and Bioassays

Herpes simplex is a disease that is widely distributed throughout the world. It is caused by herpes simplex virus type 1 (HSV-1) and simplex virus type 2 (HSV-2). The drugs of choice for treatment are acyclovir (ACV), Penciclovir (PCV) and other guanine analogues, which have the same mechanism of action. However, due to the constant increase of ACV-resistant strains in immunocompromised patients, it is necessary to find new treatment alternatives. It has been shown that natural products are a good alternative for the treatment of these diseases as well as being an excellent source of compounds with anti-herpetic activity, which may be useful for the development of new drugs and act through a mechanism of action different from ACV and PCV. This paper compiles reports on extracts and compounds isolated from plants that have anti-herpetic activity. We present an analysis of the solvents most widely used for extraction from plants as well as cells and commonly used methods for evaluating cytotoxic and anti-herpetic activity. Families that have a higher number of plants with anti-herpetic activity are evaluated, and we also highlight the importance of studies of mechanisms of action of extracts and compounds with anti-herpetic activity.

Frequency of acyclovir-resistant herpes simplex viruses isolated from the general immunocompetent population and patients with acquired immunodeficiency syndrome

BACKGROUND

Herpes simplex virus (HSV) infections are usually chronically recurrent in the normal population and represent a significant cause of morbidity in immunocompromised patients. Acyclovir (ACV) is widely used for the treatment and prophylaxis of HSV infections. The emergence of ACV-resistant strains has been frequently reported as a result of long-term ACV therapy.

AIM

Despite the widespread use of ACV, there are no data available in our area on the frequency of ACV-resistant HSVs. The purpose of this study was to evaluate the susceptibility of HSV isolated from normal subjects and patients with acquired immunodeficiency syndrome (AIDS) to ACV.

METHODS

HSVs were isolated from the orofacial region of normal individuals and patients with AIDS. The susceptibility of isolated HSV strains to various concentrations of ACV was determined by plaque reduction assay. The sensitivity of the viral strains was expressed as IC(50) (the concentration of drug reducing the viral plaque by 50%).

RESULTS

One hundred and thirty-three isolates from 102 normal subjects and 31 patients with AIDS were tested. One HSV-1 isolate from normal individuals had intermediate susceptibility. Two ACV-resistant isolates (one HSV-1 and one HSV-2), with IC(50) > or = 2 to < or = 3 microg/mL, and one highly resistant HSV-2 isolate, with IC(50) > or = 5 microg/mL, were detected in patients with AIDS.

CONCLUSIONS

Our data show that the prevalence of ACV-resistant strains is very low in the general immunocompetent population; however, in patients with AIDS, the prevalence of ACV-resistant strains is remarkable (P = 0.001). Alternative antiherpetic agents should be employed to control and reduce the emergence of ACV-resistant strains in patients with AIDS.

A cell line that secretes inducibly a reporter protein for monitoring herpes simplex virus infection and drug susceptibility

A cell line modified genetically (Vero-ICP10-SEAP) that responds to infection by herpes simplex virus (HSV) was established. The cell line was constructed by stable transfection of Vero cell with a plasmid encoding the secreted alkaline phosphatase (SEAP) driven by the promoter of the HSV-2 ICP10 gene. Following infection with HSV, the stable line secretes a high level of the SEAP in the supernatants as measured by a chemiluminescence-based assay. The detection system is sensitive to an HSV titer as low as a single plaque-forming unit (PFU), with a linear range up to the equivalent of 2.5 x 10(4) PFU inoculum after infection for 24 h. There was no detectable enhancement in SEAP activities following inoculations with several viruses other than HSV. The Vero-ICP10-SEAP cell line was also utilized to develop an assay for determination of antiviral susceptibility given that the induced SEAP activity appeared to reflect the numbers of plaque. Evaluations of the stable line with representative acyclovir (ACV)-sensitive and-resistant HSV isolates demonstrated that their drug susceptibilities were determined accurately. In summary, this novel SEAP reporter system is a sensitive means for rapid diagnosis, quantitation, and drug susceptibility testing for HSV, with potential to the development of an automated assay.

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.

Antiviral susceptibility of Herpes simplex viruses and its clinical correlates: a single center's experience

The in vitro susceptibility to acyclovir of 204 herpes simplex virus isolates from 165 immunocompromised patients treated at our hospital was determined by the cytopathic effect reduction assay. Approximately 95% of herpes simplex virus 1 and 73% of herpes simplex virus 2 isolates were inhibited by acyclovir at concentrations of <2 microgram/mL. From 8 patients (5%), an isolate with low susceptibility to acyclovir (50% inhibitory dose, >3 microgram/mL) was recovered. Medical records of 83 patients were reviewed. Lesions resolved in most of the patients, independent of treatment. Treatment failures were not always associated with isolation of an in vitro-resistant virus. On the contrary, when a virus with low susceptibility to acyclovir was isolated, resolution of the lesion was the rule. In 9 of 10 patients with subsequent recurrent episodes of disease, the susceptibility of the viruses isolated was similar to that of the first episode. Routine susceptibility testing in our geographic area is not encouraged because of the low incidence of acyclovir-resistant herpes simplex viruses.

Susceptibilities of herpes simplex viruses to penciclovir and acyclovir in eight cell lines

The commonly used antiviral drugs acyclovir (ACV) and penciclovir (PCV) possess similarly potent antiviral activities in vivo against herpes simplex virus (HSV). Assay methods for sensitivity to ACV are not necessarily transferable to PCV, even though the two drugs have similar in vivo potencies and mechanisms of action. We determined by plaque reduction assay the relative activities of ACV and PCV against five laboratory-adapted strains of HSV types 1 and 2 (including sensitive and resistant strains) in seven human cell lines and one nonhuman primate cell line. Seven characteristics were used to evaluate the cell lines. All cell lines were similar in their plating efficiencies and abilities to discriminate between sensitive and resistant HSV isolates. Vero and MRC-5 cells yielded the most discordant 50% inhibitory concentrations (IC50s) for the two HSV types, while Vero and WI-38 VA-13 cells yielded large differences in the IC50s of ACV and PCV. The limited life spans and poor plaque morphologies of the fibroblast lines were undesirable characteristics. Among the transformed cell lines producing well-defined plaques, A549 cells provided the best concordance between IC50s for the two HSV types and two antiherpes drugs. Comparison experiments with a yield reduction format indicated that the use of assays of this type might allow some of the cell-specific properties observed in plaque reduction assays to be avoided.

Comparison of methods for identifying resistant herpes simplex virus and measuring antiviral susceptibility

BACKGROUND

A number of in vitro assays are used to determine susceptibility of HSV to antiviral agents, but results from these in vitro assays do not necessarily correlate with treatment outcome.

OBJECTIVES

A method with improved capability for identifying an isolate as acyclovir (ACV) or penciclovir (PCV) resistant when resistance is borderline could greatly improve the management of HSV disease.

STUDY DESIGN

A comparative evaluation of four in vitro assays, plaque reduction (PRA), DNA hybridization, plating efficiency (PEA) and plaque autoradiography (PAR) was performed to accurately identify and measure resistance of a TK-altered clinical HSV isolate (HSV-1 N4) from a patient who was non-responsive to ACV treatment. Two established criteria for the prediction of antiviral resistance, IC(50)> or =2.0 microg/ml or an IC(50) greater than 10x above a sensitive virus IC(50), as well as testing in human (MRC-5) and nonhuman (Vero and CV-1 monkey kidney) cell lines were evaluated.

RESULTS

The PRA and DNA hybridization assays accurately identified HSV-1 N4 as ACV(r) in human cells when using the 10x above sensitive virus IC(50) resistance criterion. Moreover, the PEA and PAR assays failed to classify HSV-1 N4 as drug resistant and indicate that these technologies alone are inadequate for identifying resistant virus.

CONCLUSIONS

The data presented herein indicate that the PRA and DNA hybridization assays most accurately identified an otherwise borderline-resistant isolate as drug resistant: (i) when a sensitive virus is used within each individual assay as a control, (ii) when ACV and PCV susceptibility is evaluated in human cells, and (iii) when the 10x above sensitive IC(50) criterion is used to classify a virus as drug-resistant. Testing of additional clinical samples is warranted to further confirm these findings.

Biochemical characterization of a virus isolate, recovered from a patient with herpes keratitis, that was clinically resistant to acyclovir

In vitro susceptibility assays of herpes simplex virus (HSV) do not necessarily correlate with treatment outcome. An HSV type 1 (HSV-1) isolate, N4, recovered from a patient who presented with herpes keratitis with localized immunosuppression, was characterized for susceptibility. Although the 50% inhibitory concentration (IC(50)) for this isolate was less than the accepted breakpoint for defining resistance to acyclovir (>2.0 microg/mL), the following lines of evidence suggest that the isolate was acyclovir resistant: (1) the clinical history confirmed that the infection was nonresponsive to acyclovir; (2) the in vitro susceptibility was similar to that of a thymidine kinase (TK)-negative, acyclovir-resistant virus SLU360; (3) the IC(50) of acyclovir was more than 10 times the IC(50) for an acyclovir-susceptible control strain; (4) plaque-purified clonal isolates were resistant to acyclovir (IC(50)s, >2.0 microg/mL); and (5) biochemical studies indicated that the HSV-1 N4 TK was partially impaired for acyclovir phosphorylation. Although residue changes were found in both the viral tk and pol coding regions of HSV-1 N4, characterization of a recombinant virus expressing the HSV-1 N4 polymerase suggested that the TK and Pol together conferred the acyclovir-resistance phenotype.

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.

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.