A colorimetric assay for the measurement of the sensitivity of herpes simplex viruses to antiviral agents

Primary structural features, physicochemical and biological properties of two water-soluble polysaccharides extracted from the brown Tunisian seaweed Halopteris scoparia

Marine algae are the most abundant resource in the marine environment and are still a promising source of bioactive compounds including hydrocolloids. This study contributes to the evaluation of the biological and biotechnological potentials of two water soluble polysaccharides, namely alginates (AHS) and fucoidan (FHS), extracted and purified from Halopteris scoparia, an abundant Tunisian brown macroalgae collected in Tunisia (Tabarka region). The total sugars, neutral monosaccharides, uronic acids, proteins, polyphenols, and sulfate groups contents were quantified for both fractions, as well as their functional groups and primary structural features by Fourier transform infrared spectroscopy, ionic and/or gas chromatography and nuclear magnetic resonance analyses. AHS and FHS showed significant anti-inflammatory (IC50 ≈ 1 mg/mL), anticoagulant (e.g., 27-61.7 for the activated partial thromboplastin time), antihyperglycemic (0.1-40 μg/mL) and anti-trypsin (IC50 ≈ 0.3-0.4 mg/mL) effects. FHS and a hydrolyzed fraction showed a very promising potential against herpes viruses (HSV-1) (IC50 < 28 μg/mL). Besides, AHS and two hydrolyzed fractions were able to stimulate the natural defenses of tomato seedlings, assessing their elicitor capacity, by increasing the activity of phenylalanine ammonia-lyase (66-422 %) but also significantly changing the polyphenol content in the leaves (121-243 %) and roots (30-104 %) of tomato plants.

The Main Protease of SARS-CoV-2 as a Target for Phytochemicals against Coronavirus

In late December 2019, the first cases of COVID-19 emerged as an outbreak in Wuhan, China that later spread vastly around the world, evolving into a pandemic and one of the worst global health crises in modern history. The causative agent was identified as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Although several vaccines were authorized for emergency use, constantly emerging new viral mutants and limited treatment options for COVID-19 drastically highlighted the need for developing an efficient treatment for this disease. One of the most important viral components to target for this purpose is the main protease of the coronavirus (Mpro). This enzyme is an excellent target for a potential drug, as it is essential for viral replication and has no closely related homologues in humans, making its inhibitors unlikely to be toxic. Our review describes a variety of approaches that could be applied in search of potential inhibitors among plant-derived compounds, including virtual in silico screening (a data-driven approach), which could be structure-based or fragment-guided, the classical approach of high-throughput screening, and antiviral activity cell-based assays. We will focus on several classes of compounds reported to be potential inhibitors of Mpro, including phenols and polyphenols, alkaloids, and terpenoids.

Antiviral Activity of Zinc Oxide Nanoparticles Mediated by Plumbago indica L. Extract Against Herpes Simplex Virus Type 1 (HSV-1)

INTRODUCTION

Plumbago indica L. is considered a valuable source in the Plumbaginaceae family for various types of active compound such as alkaloids, phenolics and saponins. To promote the usage of P. indica in the bionanotechnology field, zinc oxide nanoparticles (ZnONPs) were biosynthesized by using its alcoholic extract. The inhibitory effects of ZnONPs and the plant extract were also evaluated against HSV-1.

METHODS

ZnONPs were described by the following techniques, UV-visible spectroscopy, Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS), zeta potential, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and x-ray diffraction (XRD). The phenolic and flavonoid contents of P. indica extract, which are accountable for bioreduction, formation and stabilization of the nanoparticles, were analyzed by HPLC technique. The antiviral assessment was implemented on both agents by using Vero cell lines.

RESULTS

DLS revealed that the average size of ZnONPs was 32.58 ± 7.98 nm and the zeta potential was -20.8 mV. The observation of TEM analysis revealed that the particle size of ZnONPs varied from 2.56 to 8.83 nm. The XRD analysis verified the existence of pure crystals of hexagonal shapes of nanoparticles of ZnO with a main average size of 35.28 nm that is approximating to the values of particle size acquired by SEM analysis (19.64 and 23.21 nm). The HPLC analysis of P. indica ethanolic extract showed that gallic acid, chlorogenic acid and rutin were the major compounds, with concentrations equal to 8203.99, 2965.95 and 1144.99 µg/g, respectively. Regarding the antiviral assessment, the synthesized uncalcinated ZnONPs were found to exhibit a promising activity against HSV-1, with CC50 and IC50 values equal to 43.96 ± 1.39 and 23.17 ± 2.29 µg/mL, respectively.

CONCLUSION

The green synthesized ZnONPs are considered promising adjuvants to enhance the efficacy of HSV-1 drugs.

Anti-Herpes simplex virus (HSV-1) activity and antioxidant capacity of carrageenan-rich enzymatic extracts from Solieria filiformis (Gigartinales, Rhodophyta)

Solieria filiformis has been reported to have molecules with various biological activities. In this study we used environmentally friendly extraction methods, such as enzyme-assisted extraction (EAE), as a first step to obtain bioactive compounds from this species. Five combinations of protease (PRO) and carbohydrase (AMG) were utilized (1:0, 0:1, 2:1, 1:1, 1:2 PRO:AMG) to obtain Water Soluble Enzymatic Hydrolysates (WSEHs). Extraction yields, biochemical and structural characterization, as well as in vitro activity against Herpes simplex virus type 1 (HSV-1) and antioxidant capacities were determined. All PRO:AMG combinations significantly improved yields. EAE yielded heterogeneous extracts rich in iota-carrageenan and phenols, as confirmed by FTIR spectra. The highest antiherpetic activity (EC₅₀ 4.5 ± 0.4 μg mL^-1) was found in the WSEHs obtained under 2:1 PRO:AMG. At this combination high antioxidant capacity was also obtained for ABTS (2,2'-Azino-Bis-3-ethylbenzoThiazoline-6-Sulfonic acid) radical scavenging activity and Ferric Reducing Antioxidant Power (FRAP). These could probably play a synergistic role associated to the strong antiviral activity obtained. These results suggest that 2:1 PRO:AMG could be effective in promoting the hydrolytic breakdown of high MW polysaccharides, contributing to the improvement of WSEHs bioactivity. Although Solieria filiformis WSEHs showed promising results, further research, including separation and purification techniques are needed.

Environmentally Friendly Valorization of Solieria filiformis (Gigartinales, Rhodophyta) from IMTA Using a Biorefinery Concept

Marine macroalgae (seaweed) are an excellent source of novel bioactive metabolites. The biorefinery concept applied to seaweed facilitates the extraction of many chemical constituents from the same biomass ensuring that the resource is used fully, generating few residues through a succession of extraction steps. In the present study, the biomass of the carragenophyte Solieria filiformis (Rhodophyta, Gigartinales) cultured in an integrated multi-trophic aquaculture (IMTA) system was evaluated to obtain valuable products by a biorefinery approach. Enzymatic-assisted extraction (EAE) and microwave-assisted extraction (MAE) were the eco-friendly technologies used to ensure an environmentally friendly valorization of the biomass. Three valuable products were successfully recovered: a water-soluble extract rich in proteins and sulfated polysaccharides suitable as a food supplement; a lipid fraction rich in polyunsaturated fatty acids (PUFAs) with potential to be used in the nutraceutical industry; and a pure ι-carrageenan with a powerful antiviral activity against Herpes simplex virus (EC₅₀ = 6.3 µg mL^-1) comparable to the commercial antiviral acyclovir (EC₅₀ = 3.2⁻5.4 µg mL^-1).

Total phenolic content and biological activities of enzymatic extracts from Sargassum muticum (Yendo) Fensholt

Seaweeds are potentially excellent sources of bioactive metabolites that could represent useful leads in the development of new functional ingredients in pharmaceutical and cosmetic industries. In the last decade, new marine bioprocess technologies have allowed the isolation of substances with biological properties. The brown alga Sargassum muticum (Yendo) Fensholt (Ochrophyta) was enzymatically hydrolyzed to prepare water-soluble extracts by using six different commercially available carbohydrate-degrading enzymes and two proteases. Evaluation of 2,2-diphenyl-1-picrylhydrazyl (DPPH) and ferric reducing antioxidant power (FRAP) antioxidant, tyrosinase, elastase, and biofilm inhibition, antibacterial and antiviral activities as well as evaluation of cytotoxicity were realized for each extract. Total phenolic content was measured for extract characterization, and solid-phase extraction was useful to purify the enzymatic extract. Soluble total phenolic content of S. muticum Viscozyme extract was highest with 6.4% of dry weight. Enzymatic Celluclast and Viscozyme extracts had the lowest value of DPPH IC₅₀ indicating a strong antiradical activity, 0.6 mg mL^-1, in comparison with other enzymes. The ferric reducing antioxidant power ranged between 48.7 μM Fe²⁺ Eq, digested with Viscozyme, and 60.8 μM Fe²⁺ Eq, digested with Amyloglucosidase. Tyrosinase inhibition activity of S. muticum Neutrase extract was 41.3% higher compared to other enzymes. Elastase inhibition activity of S. muticum Shearzyme extract had highest activity (32.8%). All enzymatic extracts showed no cytotoxic effect towards the kidney Vero cells. Meanwhile, only S. muticum Neutrase and Alcalase extracts exhibited potential antiviral activity. In addition, S. muticum Viscozyme and Shearzyme extracts showed promising activity in suppressing the biofilm formation against Pseudomonas aeruginosa and Escherichia coli, respectively. Purification of S. muticum Viscozyme extracts by solid-phase extraction managed to concentrate the phenolic content and improve the bioactivity. These results indicate the promising potential of enzyme-assisted followed by solid-phase extraction in recovering phenolic content and in improving its bioactivity.

A Generally Applicable, High-Throughput Screening–Compatible Assay to Identify, Evaluate, and Optimize Antimicrobial Agents for Drug Therapy

Efficacy and tolerability are the key criteria for a successful medication in the clinic. Therefore, a new test method to obtain selective and active lead molecules has been developed. Recently, this novel screening strategy enabled a breakthrough in drug discovery in the field of herpes viruses. Here the authors report that this assay is a generally applicable screening test, which allows not only for identifying tolerable and potent antimicrobial agents in compound libraries, but also covers all potential in vitro targets of both the pathogen and the host simultaneously. The test system mimics the smallest unit of a natural infection. Host cells are incubated in the presence of the test sample and are infected with microbes, such as viruses, bacteria, or fungi. Analogous to (lethal challenge) animal models, cell survival is determined. This assay maximizes the chances of success of anti-infective drug discovery, is sensitive, robust, time- and cost-efficient, and especially effective in optimizing screening hits to lead structures and development candidates. In addition to the minimal inhibitory concentration or dose, this test system simultaneously provides the selectivity index, a measure of tolerability in vitro. The authors propose the activity selectivity assay format as a new standard in anti-infective drug discovery and clinical development.

Antiviral drug discovery: broad-spectrum drugs from nature

Covering: up to April 2014. The development of drugs with broad-spectrum antiviral activities is a long pursued goal in drug discovery. It has been shown that blocking co-opted host-factors abrogates the replication of many viruses, yet the development of such host-targeting drugs has been met with scepticism mainly due to toxicity issues and poor translation to in vivo models. With the advent of new and more powerful screening assays and prediction tools, the idea of a drug that can efficiently treat a wide range of viral infections by blocking specific host functions has re-bloomed. Here we critically review the state-of-the-art in broad-spectrum antiviral drug discovery. We discuss putative targets and treatment strategies, with particular focus on natural products as promising starting points for antiviral lead development.

Enzyme-assisted extraction of bioactive material from Chondrus crispus and Codium fragile and its effect on herpes simplex virus (HSV-1)

Codium fragile and Chondrus crispus are, respectively, green and red seaweeds which are abundant along the North Atlantic coasts. We investigated the chemical composition and antiviral activity of enzymatic extracts of C. fragile (CF) and C. crispus (CC). On a dry weight basis, CF consisted of 11% protein, 31% neutral sugars, 0.8% sulfate, 0.6% uronic acids, and 49% ash, while CC contained 27% protein, 28% neutral sugars, 17% sulfate, 1.8% uronic acids, and 25% ash. Enzyme-assisted hydrolysis improved the extraction efficiency of bioactive materials. Commercial proteases and carbohydrases significantly improved (p ≤ 0.001) biomass yield (40%-70% dry matter) as compared to aqueous extraction (20%-25% dry matter). Moreover, enzymatic hydrolysis enhanced the recovery of protein, neutral sugars, uronic acids, and sulfates. The enzymatic hydrolysates exhibited significant activity against Herpes simplex virus (HSV-1) with EC50 of 77.6-126.8 μg/mL for CC and 36.5-41.3 μg/mL for CF, at a multiplicity of infection (MOI) of 0.001 ID50/cells without cytotoxity (1-200 μg/mL). The extracts obtained from proteases (P1) and carbohydrases (C3) were also effective at higher virus MOI of 0.01 ID50/cells without cytotoxity. Taken together, these results indicate the potential application of enzymatic hydrolysates of C. fragile and C. crispus in functional food and antiviral drug discovery.

Ostreid herpesvirus type 1 replication and host response in adult Pacific oysters, Crassostrea gigas

Since 2008, massive mortality outbreaks associated with OsHV-1 detection have been reported in Crassostrea gigas spat and juveniles in several countries. Nevertheless, adult oysters do not demonstrate mortality in the field related to OsHV-1 detection and were thus assumed to be more resistant to viral infection. Determining how virus and adult oyster interact is a major goal in understanding why mortality events are not reported among adult Pacific oysters. Dual transcriptomics of virus-host interactions were explored by real-time PCR in adult oysters after a virus injection. Thirty-nine viral genes and five host genes including MyD88, IFI44, IkB2, IAP and Gly were measured at 0.5, 10, 26, 72 and 144 hours post infection (hpi). No viral RNA among the 39 genes was detected at 144 hpi suggesting the adult oysters are able to inhibit viral replication. Moreover, the IAP gene (oyster gene) shows significant up-regulation in infected adults compared to control adults. This result suggests that over-expression of IAP could be a reaction to OsHV-1 infection, which may induce the apoptotic process. Apoptosis could be a main mechanism involved in disease resistance in adults. Antiviral activity of haemolymph against herpes simplex virus (HSV-1) was not significantly different between infected adults versus control.

Biological activities of purified marennine, the blue pigment responsible for the greening of oysters

Marennine, the blue pigment produced by the diatom Haslea ostrearia , exists in two different forms, the intra- and extracellular forms. We investigated the antibacterial, antiviral, and antiproliferative properties of both of these forms. Both forms of marennine inhibited the development of marine bacteria, in particular the pathogenic organism Vibrio aesturianus , at concentrations as low as 1 μg/mL, but they did not display any effect on a wide range of pathogenic bacteria that are relevant for food safety. Both forms of the pigment produced by H. ostrearia also exhibited antiviral activity against the HSV1 herpes virus, with intra- and extracellular marennine having EC(50) values of 24.0 and 27.0 μg/mL, respectively. These values are 2 orders of magnitude higher than the value for the reference drug, Zovirax. Moreover, both forms of marennine were effective in slowing or inhibiting the proliferation of cancer cells. This study confirms the potential of marennine as a biologically active organic molecule, which could have a protective effect on bivalves, which filter seawater and fix the pigment on their gills. Moreover, marennine could be used in food engineering and chemistry as a natural blue pigment. However, despite that it is eaten and possibly assimilated by green oyster consumers, it also deserves in depth evaluation before being considered for use as a nutraceutical.

Synthesis and Biological Evaluation of 4'-C,3'-O-Propylene-Linked Bicyclic Nucleosides

A set of pyrimidine nucleosides fused with a 4'-C,3'-O-propylene bridge was successfully synthesised in 12 steps from 1,2:5,6-di-O-isopropylidene-α-d-glucofuranose, an inexpensive starting material, based on a ring-closing metathesis (RCM) reaction followed by Vorbrüggen-type nucleobase coupling. Antiviral and cytotoxicity activities of the targeted modified nucleosides, as well as their phosphoramidate prodrugs, are described.

A dual reporter cell assay for identifying serotype and drug susceptibility of herpes simplex virus

A dual reporter cell assay (DRCA) that allows real-time detection of herpes simplex virus (HSV) infection was developed. This was achieved by stable transfection of cells with an expression cassette that contains the dual reporter genes, secreted alkaline phosphatase (SEAP) and enhanced green fluorescent protein (EGFP), under the control of an HSV early gene promoter. Baby hamster kidney (BHK) and Chinese hamster ovary (CHO) cell lines were used as parental cell lines because the former is permissive for both HSV serotypes, HSV-1 and HSV-2, whereas the latter is susceptible to infection only by HSV-2. The DRCA permitted differential detection of HSV-1 and HSV-2 by observation of EGFP-positive cells, as substantiated by screening a total of 35 samples. The BHK-based cell line is sensitive to a viral titer as low as a single plaque-forming unit with a robust assay window as measured by a chemiluminescent assay. Evaluations of the DRCA with representative acyclovir-sensitive and acyclovir-resistant HSV strains demonstrated that their drug susceptibilities were accurately determined by a 48-h format. In summary, this novel DRCA is a useful means for serotyping of HSV in real time as well as a rapid screening method for determining anti-HSV susceptibilities.

Expression of herpes simplex virus type 1 recombinant thymidine kinase and its application to a rapid antiviral sensitivity assay

Antiviral-resistant herpesvirus infection has become a great concern for immunocompromised patients. Herpes simplex virus type 1 (HSV-1) infections are treated with viral thymidine kinase (vTK)-associated drugs such as acyclovir (ACV), and most ACV-resistance (ACV(r)) is due to mutations in the vTK. The standard drug sensitivity test is usually carried out by the plaque reduction assay-based method, which requires over 10 days. To shorten the time required, a novel system was developed by the concept, in which 293T cells transiently expressing recombinant vTK derived from the test sample by transfection of the cells with an expression vector were infected with vTK-deficient and ACV(r) HSV-1 (TAR), and then cultured in a maintenance medium with or without designated concentrations of ACV, ganciclovir (GCV) and brivudine (BVdU). The replication of TAR was strongly inhibited by ACV, GCV and BVdU in 293T cells expressing recombinant vTK of the ACV-sensitive HSV-1, whereas replication was not or slightly inhibited in cells expressing the recombinant vTK of highly resistant or intermediately resistant HSV-1, respectively. An inverse correlation was demonstrated in the 50% effective concentrations (EC(50)s) and inhibitory effects of these compounds on the replication of TAR among ACV(s) and ACV(r) HSV-1 clones. These results indicate that the EC(50)s of the vTK-associated drugs including ACV can be assumed by measuring the inhibitory effect of drugs in 293T cells expressing recombinant vTK of the target virus. The newly developed antiviral sensitivity assay system for HSV-1 makes it possible to estimate EC(50) for vTK-associated drugs, when whole vTK gene is available for use by gene amplification directly from lesion's samples or from virus isolates.

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.

Isolation of a sulphated polysaccharide from a recently discovered sponge species (Celtodoryx girardae) and determination of its anti-herpetic activity

Exopolysaccharides (EPS) were extracted from a sponge, Celtodoryx girardae recently discovered in the Golfe du Morbihan in 2000. Sponge samples were collected monthly from November 2007 to May 2008. SEC analysis of EPS samples showed that they exhibit a unique molecular weight of approximately 800 kDa. However, infrared analysis revealed that structural seasonal variations occur. EPS fractions also exhibit significant sulphate contents and were screened in vitro for a potential antiviral activity against Herpes simplex virus type 1 (HSV-1). The best result was obtained with a sample collected in January which exhibits an EC(50) of 5.9 microg/mL without cytotoxicity on the Vero cell line. Experiments carried out to elucidate the mechanism of the EPS showed that the sulphated groups of EPS interact with the glycoproteins on the surface of the virus' membrane.

Cell-based Assays to Identify Inhibitors of Viral Disease

BACKGROUND: Antagonizing the production of infectious virus inside cells requires drugs that can cross the cell membrane without harming host cells. OBJECTIVE: It is therefore advantageous to establish intracellular potency of anti-viral drug candidates early in the drug-discovery pipeline. METHODS: To this end, cell-based assays are being developed and employed in high-throughput drug screening, ranging from assays that monitor replication of intact viruses to those that monitor activity of specific viral proteins. While numerous cell-based assays have been developed and investigated, rapid counter screens are also needed to define the specific viral targets of identified inhibitors and to eliminate nonspecific screening hits. RESULTS/CONCLUSIONS: Here, we describe the types of cell-based assays being used in antiviral drug screens and evaluate the equally important counter screens that are being employed to reach the full potential of cell-based high-throughput screening.

Antiviral activity of some Tunisian medicinal plants against Herpes simplex virus type 1

Fifteen species of Tunisian traditional medicinal plants, belonging to 10 families, were selected for this study. They were Inula viscosa (L.) Ait and Reichardia tingitana (L.) Roth ssp. discolor (Pom.) Batt. (Asteraceae), Mesembryanthemum cristallinum L. and M. nodiflorum L. (Aizoaceae), Arthrocnemum indicum (Willd.) Moq., Atriplex inflata Muell., A. parvifolia Lowe var. ifiniensis (Caball) Maire, and Salicornia fruticosa L. (Chenopodiaceae), Cistus monspeliensis L. (Cistaceae), Juniperus phoenicea L. (Cupressaceae), Erica multiflora L. (Ericaceae), Frankenia pulverulenta L. (Frankeniaceae), Hypericum crispum L. (Hypericaceae), Plantago coronopus L. ssp. eu-coronopus Pilger var. vulgaris G.G. (Plantaginaceae) and Zygophyllum album L. (Zygophyllaceae). Fifty extracts prepared from those plants were screened in order to assay their antiviral activity against Herpes simplex virus type 1 (HSV-1), using neutral red incorporation. Extracts from eight plants among these 15 showed some degree of antiviral activity, while the methanolic extract of E. multiflora was highly active with EC(50) of 132.6 microg mL(-1). These results corroborate that medicinal plants from Tunisia can be a rich source of potential antiviral compounds.

Antiviral activity of esterified alpha-lactalbumin and beta-lactoglobulin against herpes simplex virus type 1. Comparison with the effect of acyclovir and L-polylysines

The antiviral activity of methylated alpha-lactalbumin (Met-ALA), methylated and ethylated beta-lactoglobulins (Met- and Et-BLG) was evaluated against acyclovir (ACV)-sensitive and -resistant strains of herpes simplex virus type 1 (HSV-1) and compared to that of ACV and L-polylysines (4-15 kDa) using fixed or suspended Vero cell lines. Esterified whey proteins and their peptic hydrolyzates displayed protective action against HSV-1, which was relatively lower than that induced by ACV or L-polylysines. The higher activity of L-polylysines was maintained against an ACV-resistant strain of HSV-1, whereas ACV lost much of its activity. The mean 50% inhibitory concentration (IC50) was about 0.8-0.9 microg/mL for L-polylysines against ACV-sensitive and -resistant strains of HSV-1 when using two concentrations of virus (50% and 100% cytopathic effect, CPE). The IC50 values of ACV against the sensitive strain of HSV-1 were 3 and 15 microg/mL when using the low and high concentrations of virus, respectively. When using 50% CPE, IC50 values for esterified whey proteins ranged from 20 to 95 microg/mL, depending on the nature of the ester group, the degree of esterification, and the nature of the protein. Using the real-time PCR technique, it was shown that Met-ALA inhibited HSV-1 replication.

The use of nerve growth factor in herpetic keratitis: a case report

Background

We evaluate the role of nerve growth factor (NGF) eye drops to treat a herpetic corneal ulcer resistant to systemic and local acyclovir treatment in an HIV-positive patient.

Case Presentation

A 68 year old HIV-positive male presented with a herpetic corneal ulcer which was treated unsuccessfully with acyclovir. Acyclovir sensitivity of herpes simplex virus was tested with a dye uptake assay and we found that the herpes simplex virus isolated was resistant to acyclovir. We started eye drop therapy with NGF and the corneal herpetic ulcer healed in 23 treatment days.

Conclusion

The case presented here is the first described in the literature in which a herpetic corneal ulcer was successfully treated with NGF. We recommend that trials of NGF therapy in herpetic keratitis should be carried out on a larger number of acyclovir resistant cases.

Sulforhodamine B colorimetric assay for cytotoxicity screening

The sulforhodamine B (SRB) assay is used for cell density determination, based on the measurement of cellular protein content. The method described here has been optimized for the toxicity screening of compounds to adherent cells in a 96-well format. After an incubation period, cell monolayers are fixed with 10% (wt/vol) trichloroacetic acid and stained for 30 min, after which the excess dye is removed by washing repeatedly with 1% (vol/vol) acetic acid. The protein-bound dye is dissolved in 10 mM Tris base solution for OD determination at 510 nm using a microplate reader. The results are linear over a 20-fold range of cell numbers and the sensitivity is comparable to those of fluorometric methods. The method not only allows a large number of samples to be tested within a few days, but also requires only simple equipment and inexpensive reagents. The SRB assay is therefore an efficient and highly cost-effective method for screening.

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.

Syntheses, in vitro antibacterial and cytotoxic activities of a series of 3-substituted succinimides

We have synthesized a series of 3-substituted succinimides and their in vitro antibacterial activities have been tested towards Gram-positive and Gram-negative bacteria from the ATCC collection. Some of them possess significant antibacterial activity against Gram-positive organisms (Staphylococcus aureus ATCC 25923 and Enterococcus faecalis ATCC 29212) but all are poorly active or inactive against Gram-negative organisms (Escherichia coli ATCC 25922 and Pseudomonas aeruginosa ATCC 27853). The compounds with the lowest minimal inhibitory concentrations (esters of 3-hydroxy succinimides) are also the most cytotoxic against green monkey Vero cell line (ATCC CCL-81) and could explain that perhaps apoptosis should be implicated in eukaryotic cell cytotoxicity of succinimides.

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.

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.

Use of a single monoclonal antibody to determine the susceptibilities of herpes simplex virus type 1 and type 2 clinical isolates to acyclovir

This report describes a flow cytometry drug susceptibility assay that uses a single fluorochrome-labeled monoclonal antibody to determine the acyclovir susceptibilities of herpes simplex virus (HSV) type 1 or type 2 clinical isolates. This assay yields 50% effective doses (drug concentrations that reduce the number of antigen-positive cells by 50%) for HSV clinical isolates that are equivalent to those obtained with the plaque reduction 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.

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

Nucleoside analogues such as acyclovir and ganciclovir have been the mainstay of therapy for alphaherpesviruses (herpes simplex virus (HSV) and varicella-zoster virus (VZV)) and cytomegalovirus (CMV) infections, respectively. Drug-resistant herpesviruses are found relatively frequently in the clinic, almost exclusively among severely immunocompromised patients receiving prolonged antiviral therapy. For instance, close to 10% of patients with AIDS receiving intravenous ganciclovir for 3 months excrete a drug-resistant CMV isolate in their blood or urine and this percentage increases with cumulative drug exposure. Many studies have reported that at least some of the drug-resistant herpesviruses retain their pathogenicity and can be associated with progressive or relapsing disease. Viral mutations conferring resistance to nucleoside analogues have been found in either the drug activating/phosphorylating genes (HSV or VZV thymidine kinase, CMV UL97 kinase) and/or in conserved regions of the viral DNA polymerase. Currently available second line agents for the treatment of herpesvirus infections--the pyrophosphate analogue foscarnet and the acyclic nucleoside phosphonate derivative cidofovir--also inhibit the viral DNA polymerase but are not dependent on prior viral-specific activation. Hence, viral DNA polymerase mutations may lead to a variety of drug resistance patterns which are not totally predictable at the moment due to insufficient information on specific drug binding sites on the polymerase. Although some CMV and HSV DNA polymerase mutants have been found to replicate less efficiently in cell cultures, further research is needed to correlate viral fitness and clinical outcome.

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.

Flow cytometric evaluation of antiviral agents against human herpesvirus 6

Antiviral activities of acyclovir (9-[(2-hydroxyethoxy) methyl] guanine, ACV), penciclovir (9-[4-hydroxy-3-(hydroxymethyl) butyl] guanine, PCV), ganciclovir ([9-(1,3-dihydroxy-2-propoxy) methyl] guanine, GCV), and foscarnet (phosphonoformic acid, PFA) were determined against Human Herpesvirus 6 (HHV-6) by flow cytometric technique. The technique is based on the detection of gp116 antigen expression in virus infected cells. Susceptibility was defined in terms of drug concentration which reduced the number of cells expressing HHV-6 gp116 antigen with a mean fluorescent intensity (MFI) by 50% as compared to virus infected untreated cells. GCV was found to be most effective against HHV-6 followed by PFA, PCV and ACV. For HHV-6A, the mean 50% inhibitory concentrations (IC50) of GCV and PFA were found to be 3.4 microM and 34.7 microM respectively, whereas the IC50 of ACV and PCV were found to be 53.7 microM and 37.9 microM respectively. For HHV-6B, the IC50 of GCV and PFA were found to be 5.7 microM and 71.4 microM respectively, whereas the IC50 of ACV and PCV were found to be 119.0 microM and 77.8 microM respectively. Flow cytometry is a valuable technique for the evaluation of antiviral compounds against viruses including HHV-6.

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.

Rapid phenotypic characterization method for herpes simplex virus and Varicella-Zoster virus thymidine kinases to screen for acyclovir-resistant viral infection

A rapid phenotypic screening method for herpes simplex virus (HSV) and varicella-zoster virus (VZV) thymidine kinase (TK) genes was developed for monitoring acyclovir-resistant viruses. This method determines the biochemical phenotype of the TK polypeptide, which is synthesized in vitro from viral DNA using a procedure as follows. The TK gene of each sample virus strain is amplified and isolated under the control of a T7 promoter by PCR. The PCR products are transcribed with T7 RNA polymerase and translated in a rabbit reticulocyte lysate. Using this method, enzymatic characteristics and the size of the TK polypeptides encoding HSV and VZV DNA were defined in less than 2 days without virus isolation. The assay should be a powerful tool in monitoring drug-resistant viruses, especially in cases in which virus isolation is difficult.

A high throughput colorimetric cell proliferation assay for the identification of human cytomegalovirus inhibitors

A colorimetric assay based on the cleavage of the tetrazolium salt WST-1 has been developed for human cytomegalovirus (HCMV) antiviral susceptibility testing and adapted to a microtiter plate format. Optimal conditions were determined and the standard routine assay was calibrated with a viral input of 0.05-0.10 plaque forming unit (p.f.u.)/cell with a density of 2000 cells/well in a 96-well microtiter plate for an incubation period of 7 days. Ganciclovir (9-(2-hydroxy-1(hydroxymethyl) ethyoxymethyl) guanine; DHPG), and cidofovir ((S)-1-(3-hydroxy-2-phosphonylmethoxypropyl) cytosine; HPMPC) were used as positive control test compounds to validate the assay. The effective EC50 concentration values obtained with the two antiviral compounds in the present assay were in good agreement with plaque reduction assay results performed in parallel experiments. This method presents the advantage of being easy and rapid to perform, reliable, reproducible, and convenient for use in a high throughput screening capacity.

Polymorphisms of thymidine kinase gene in herpes simplex virus type 1: analysis of clinical isolates from herpetic keratitis patients and laboratory strains

Drug-resistance of herpes simplex virus (HSV) is caused most frequently by mutation of the viral thymidine kinase (TK) gene. To elucidate the significance of detecting nucleotide changes of the TK gene for screening drug-resistant viruses, the frequency and variation of the genetic polymorphisms in the whole coding region of the TK gene were studied in 14 acyclovir-susceptible HSV type 1 (HSV-1) clinical isolates from 14 patients with epithelial herpetic keratitis. Two reference HSV-1 laboratory strains, McKrae and PH, and two acyclovir-resistant variants of the PH strain were also studied as controls. Polymerase chain reaction-single-strand conformation polymorphism (PCR-SSCP) and direct sequencing detected nucleotide differences at 24 positions, and amino acid substitutions at 12 codons in the TK gene of the examined viruses. Nucleotide diversity of 0.0029 per base (the average number of nucleotide substitutions of 3.3 per 1,131 base pairs) in the TK gene in the clinical isolates was comparable to 0.0037 per base of the whole HSV-1 genome in Japanese isolates reported previously. PCR-SSCP analysis of the acyclovir-resistant strains easily detected aberrantly shifted bands by comparing them with those of the parental strain, followed by the quick determination of mutated sequences. These results suggest that detection of nucleotide changes of the TK gene is useful for serial observation of persistent or recurrent HSV infection as observed in immunocompromised hosts, but that it is not useful for screening drug-resistant viruses from nonepidemic clinical isolates because of the comparable genetic polymorphisms in the TK gene as in the whole HSV-1 genome.

Rapid screening tests for determining in vitro susceptibility of herpes simplex virus clinical isolates

The susceptibility of human herpes simplex virus (HSV) to acyclovir (ACV) was determined with the use of a single dose of the drug (1 and 2 micrograms of ACV per ml for HSV-1 and HSV-2, respectively) in two rapid assays: a rapid cytopathic effect inhibitory assay (Rapid CIA) and a rapid dye uptake assay (Rapid DUA). These tests allow the simultaneous determination of virus titer and susceptibility to ACV at a determined viral concentration (100 50% tissue culture infective doses and 100 50% dye uptake units). These tests were compared with a conventional susceptibility assay (dye uptake assay) and showed similar results. Indeterminate results with the Rapid CIA appeared in 3 of 30 samples. With the use of both Rapid CIA and Rapid DUA, we were able to determine the susceptibility of 100% of the isolates. The rapid tests, unlike conventional assays, are able to provide susceptibility results within 3 days after the virus has been isolated from a clinical specimen and could thus play a direct role in therapeutic decisions.

Antiviral activities of methylated nordihydroguaiaretic acids. 2. Targeting herpes simplex virus replication by the mutation insensitive transcription inhibitor tetra-O-methyl-NDGA

We had previously reported that tetramethyl-O-NGDA (M4N), a synthetic derivative of the naturally occurring nordihydroguaiaretic acid (NDGA), is able to inhibit HIV Tat transactivation by blocking host Sp1 protein at the Sp1 cognate binding site on the HIV LTR promoter. The present studies were undertaken to examine whether M4N is able to inhibit the replication of herpes simplex virus (HSV), another Sp1-regulated virus. The results showed that in Vero cells, M4N inhibits at micromolar levels (IC50 = 43.5 microM) the expression of the herpes immediate early gene (alpha-ICP4), which is essential for HSV replication. An electrophoretic mobility shift assay, examining Sp1 binding to the alpha-ICP4 promoter, showed a significant inhibition of the control bands: 88% inhibition of the fast moving band (FMB) and 45% of the slow moving band (SMB), at 100 microM of drug concentration. Comparative studies between M4N and acycloguanosine (acyclovir, ACV) in cultured Vero cells revealed an interesting pattern in the drug sensitivity (IC50) and cytotoxicity (TC50) parameters. For M4N, the IC50 varied between 11.7 and 4 microM in 10 passages of HSV-1 and 4 passages of HSV-2 with no indication for a requirement of higher drug concentration. In contrast, for acyclovir, the IC50 increased from 7 microM in the first passage to 444 microM in the tenth passage of HSV-1, and >88 microM for the fourth passage of HSV-2, indicating a rapid build-up of drug resistance against acyclovir. While the selective index (SI), defined as the ratio: TC50/IC50, remained relatively constant for M4N; it dropped 60-fold for acyclovir in the endpoints of viral passages. Drug sensitivity for M4N toward the acyclovir-sensitive strain (sm44) and the acyclovir-resistant strain (ACV-10) of HSV-1 was similar, indicating no cross-resistance between M4N and acyclovir in their anti-HSV effects. These results may have an important clinical relevance since HSV has been shown to be a factor for spreading of HIV.

Flow cytometric analysis of herpes simplex virus type 1 susceptibility to acyclovir, ganciclovir, and foscarnet

We established a quantitative flow cytometric method for determination of herpes simplex virus type 1 (HSV-1) susceptibility to acyclovir (ACV), ganciclovir, and foscarnet in vitro. Susceptibility was defined in terms of the drug concentration which reduced the number of cells expressing HSV-1 glycoprotein C (gpC) with a fluorescence intensity of > or =10(2) by 50% (IC50). Flow cytometry allowed us to use a high (1.0) as well as a low (0.005) multiplicity of infection, and determination of the IC50 was possible after one or more viral replicative cycles. IC50s were dependent on virus input and on time postinfection. In mixture experiments, 1 to 2% resistant viruses added to a sensitive strain could be detected. The results obtained by flow cytometry showed a good qualitative correlation with those achieved by cytopathic effect inhibitory assay. However, flow cytometry might detect more quantitative differences in drug susceptibility, especially among resistant strains, as confirmed also by determination of intracellular drug phosphorylation. The mean IC50s for ACV-sensitive strains were 0.45 to 1.47 microM, and those for ACV-resistant strains were between 140 and 3,134 microM. Flow cytometric analysis was fast and accurate, automatizable, and highly reproducible. Flow cytometry may be a more powerful tool than standard cytopathic effect-based assays and could have advantages for the detection of low levels of drug resistance or mixtures of sensitive and resistant virus strains.