In vitro sensitivity to acyclovir in genital herpes simplex viruses from acyclovir-treated patients

Current and Emerging Therapies for Ocular Herpes Simplex Virus Type-1 Infections

Herpes simplex virus type-1 (HSV-1) is a neurotropic, double-stranded DNA virus that can cause a wide variety of diseases, including many ocular pathologies. It is one of the leading causes of infectious blindness in the United States. Because of its ubiquitous nature and its potential to cause serious ocular maladies, there is a significant need for more effective antiviral therapies against ocular HSV-1. In this review, we discuss the lifecycle of HSV-1 as it pertains to corneal infections and the clinically approved as well as emerging treatments to combat HSV-1 infections. We also highlight some newly identified host targets for the antiviral drug development.

A direct quantitative PCR-based measurement of herpes simplex virus susceptibility to antiviral drugs and neutralizing antibodies

Herpes simplex viruses (HSV) are common human pathogens that can cause painful but benign manifestations and recurrent complaints, but can also cause significant morbidity and mortality on infection of the eye or brain and with disseminated infection of an immunosuppressed patient or a neonate. HSV growth inhibition measurement by plaque or yield reduction is a key task in the development of novel antiviral compounds but the manual methods are very labour intensive. The sensitive and specific PCR technology could be an effective method for quantitation of HSV DNA related to virus replication; however the currently described PCR approaches have a major limitation, namely the requirement of purification of DNA from the infected cells. This limitation makes this approach unfeasible for high-throughput screenings. The monitoring of HSV specific antibody titre is essential in vaccination trials and in the improvement of HSV-based oncolytic virotherapy. Usually, conventional cytopathic effect-based and plaque reduction neutralization tests are applied to measure the neutralization titre, but these methods are also time-consuming. To overcome this, we developed a quantitative PCR (qPCR) method for the detection of HSV-2 DNA directly from the infected cells (direct qPCR) and the method was further adapted to measure the titre of HSV specific neutralizing antibody in human sera. The conditions of direct qPCR assay were optimized to measure the antiviral activity of known and novel antiviral substances. Using HSV-2 seronegative and seropositive patients' sera, the validity of the direct qPCR neutralization test was compared to traditional cytopathic effect-based assay. The direct qPCR method was able to detect the HSV-2 DNA quantitatively between multiplicity of infection 1/64 and 1/4194304, indicating that the dynamic range of the detection was approximately 65,500 fold with high correlation between the biological and technical replicates. As a proof of the adaptability of the method, we applied the direct qPCR for antiviral inhibitory concentration 50 (IC₅₀) measurements of known and novel antiviral compounds. The measured IC₅₀ of acyclovir was ∼0.28μg/ml, similar to the previously published IC₅₀ value. The IC₅₀ of novel antiviral candidates was between 1.6-3.1μg/ml. The direct qPCR-based neutralization titres of HSV positive sera were 1:32-1:64, identical to the neutralization titres determined using a traditional neutralization assay. The negative sera did not inhibit the HSV-2 replication in either of the tests. Our direct qPCR method for the HSV-2 growth determination of antiviral IC₅₀ and neutralization titre is less time-consuming, less subjective and a more accurate alternative to the traditional plaque titration and growth reduction assays.

Pharmacokinetics and pharmacodynamics of ASP2151, a helicase-primase inhibitor, in a murine model of herpes simplex virus infection

ASP2151 (amenamevir) is a helicase-primase inhibitor against herpes simplex virus 1 (HSV-1), HSV-2, and varicella zoster virus. Here, to determine and analyze the correlation between the pharmacodynamic (PD) and pharmacokinetic (PK) parameters of ASP2151, we examined the PD profile of ASP2151 using in vitro plaque reduction assay and a murine model of HSV-1 infection. ASP2151 inhibited the in vitro replication of HSV-1 with a mean 50% effective concentration (EC(50)) of 14 ng/ml. In the cutaneously HSV-1-infected mouse model, ASP2151 dose dependently suppressed intradermal HSV-1 growth, with the effect reaching a plateau at a dose of 30 mg/kg of body weight/day. The dose fractionation study showed that intradermal HSV-1 titers were below the detection limit in mice treated with ASP2151 at 100 mg/kg/day divided into two daily doses and at 30 or 100 mg/kg/day divided into three daily doses. The intradermal HSV-1 titer correlated with the maximum concentration of drug in serum (C(max)), the area under the concentration-time curve over 24 h (AUC(24h)), and the time during which the concentration of ASP2151 in plasma was above 100 ng/ml (T(>100)). The continuous infusion of ASP2151 effectively decreased intradermal HSV-1 titers below the limit of detection in mice in which the ASP2151 concentration in plasma reached 79 to 145 ng/ml. Our findings suggest that the antiviral efficacy of ASP2151 is most closely associated with the PK parameter T(>100) in HSV-1-infected mice. Based on these results, we propose that a plasma ASP2151 concentration exceeding 100 ng/ml for 21 to 24 h per day provides the maximum efficacy in HSV-1-infected mice.

Genotypic detection of acyclovir-resistant HSV-1: characterization of 67 ACV-sensitive and 14 ACV-resistant viruses

Infections due to herpes simplex virus (HSV) resistant to acyclovir (ACV) represent an important clinical concern in immunocompromised patients. In order to switch promptly to an appropriate treatment, rapid viral susceptibility assays are required. We developed herein a genotyping analysis focusing on thymidine kinase gene (TK) mutations in order to detect acyclovir-resistant HSV in clinical specimens. A total of 85 HSV-1 positive specimens collected from 69 patients were analyzed. TK gene could be sequenced directly for 81 clinical specimens (95%) and 68 HSV-1 specimens could be characterized as sensitive or resistant by genotyping (84%). Genetic characterization of 67 susceptible HSV-1 specimens revealed 10 polymorphisms never previously described. Genetic characterization of 14 resistant HSV-1 revealed 12 HSV-1 with either TK gene additions/deletions (8 strains) or substitutions (4 strains) and 2 HSV-1 with no mutation in the TK gene. DNA polymerase gene was afterwards explored. With this rapid PCR-based assay, ACV-resistant HSV could be detected directly in clinical specimens within 24 h.

Management of acyclovir-resistant herpes simplex virus

In immunocompetent patients, HSV is controlled rapidly by the human host's immune system, and recurrent lesions are small and short lived. When treated with antiviral agents, these patients rarely develop resistance to these drugs. In contrast immunocompromised patients might not be able to control HSV infection. Thus, frequent and severe reactivations are often seen and might lead to fatal herpetic encephalitis or disseminated HSV infection. Treatment in these patients is limited because immunocompromised hosts often develop severe herpes disease refractory to antiviral drug therapy. It is therefore imperative that physicians develop regimens to deal with both receptive and refractory HSV disease. The following treatment protocol (modified from Balfour and colleagues) might serve as a guide until further investigation of new drugs is performed. In all patients standard oral ACV therapy should be initiated at a dose of 200 mg orally, five times a day for the first 3 to 5 days. Prior to treatment, cultures the lesions should be obtained to verify HSV etiology. If the response is poor, the dose of oral ACV should be increased to 800 mg five times a day. If no response seen after 5 to 7 days, it is unlikely that the lesion will respond to intravenous ACV (or chemically and structurally related drugs such as VCV or famciclovir), so an alternative regimen must be assigned. First, repeat cultures for vital, fungal, and bacterial pathogens must be performed. In addition, ACV susceptibility studies should be ordered, if available. If the mucocutaneous lesion is accessible for topical treatment, TFT (as ophthalmic solution) should be applied to the area three to four times a day until the lesion is completely healed. If the lesion is inaccessible or if the response to TFT is poor, therapy with intravenous foscarnet should be given for 10 days or until complete resolution of the lesions. The dosage of foscarnet should be 40 milligrams per kilogram three times per day or 60 milligrams per kilogram twice daily. If foscarnet fails to achieve clinical clearing, consideration should be given to use of intravenous cidofovir (or application of compounded 1% to 3% topical cidofovir ointment). Vidarabine is reserved for situations in which all of these therapies fail. If lesions reoccur in the same location following clearing, the patient should started on high-dose oral ACV (800 mg, five times daily) or intravenous foscarnet (40 mg/kg tid or 60 mg/kg bid) as soon as possible. When lesions occur in a different location, the patient should be treated initially with standard doses of oral ACV (200 mg, five times daily) and the above protocol should be followed should there be clinical failure. In the future, new treatment options for patients with documented HSV resistance will be important in reducing the clinical impact of HSV.

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.

Effects of antiviral usage on transmission dynamics of herpes simplex virus type 1 and on antiviral resistance: predictions of mathematical models

Herpes simplex virus type 1 (HSV-1) causes recurrent herpes labialis (RHL), a common disease afflicting up to 40% of adults worldwide. Mathematical models are used to analyze the effect of antiviral treatment on the transmission of, and the prevalence of drug resistance in, HSV-1 in the United States. Three scenarios are analyzed: no antiviral use, the current level of use, and a substantial increase in nucleoside analogue use, such as might occur if topical penciclovir were available over-the-counter for the treatment of RHL. A basic model predicts that present level of nucleoside analogue use has a negligible effect on HSV-1 transmission and that even if use of topical penciclovir for (RHL) increased substantially, the overall prevalence of infectious HSV-1 is unlikely to be reduced by more than 5%. An expanded model, which allows for acquired resistance and includes immunocompromised hosts and other more realistic features, predicts that current antiviral use is unlikely to lead to any noticeable increase in resistance. If antiviral use increases, the resulting rise in resistance in the population will depend primarily on the probability that immunocompetent hosts will acquire permanent resistance upon treatment. This probability is known to be small, but its exact value remains uncertain. If acquired resistance occurs less than once per 2,500 treated episodes, then in the community at large, the frequency of HSV-1 resistance is predicted to increase slowly, if at all (remaining below 0.5% for >50 years), even with extensive nucleoside analogue use. If acquired resistance emerges in 1 of 625 treated episodes (the maximum of an approximate 95% confidence interval derived from the results of several studies of resistance in treated hosts), then the prevalence of infection with resistant HSV-1 could rise from about 0.2% to 1.5 to 3% within 50 years. The limitations of existing data on acquired resistance and the potential impact of acquired resistance if it occurs are discussed, and strategies are suggested for enhancing information on acquired resistance. The predictions of this model contrast with the more rapid increases in antimicrobial resistance anticipated by models and observed for other pathogenic bacteria and viruses. The reasons for these contrasting predictions are discussed.

Intravenous immunoglobulins suppress the recurrences of genital herpes simplex virus: a clinical and immunological study

Effective treatment is not currently available for suppressing the recurrence of genital herpes simplex virus (HSV) infections. Since intravenous immunoglobulins (IVIG) proved useful against HSV in experimental models, we treated patients with very high frequency of HSV genital recurrences (more than 15 episodes per year) with IVIG (400 mg/Kg every fourth week). The control group was treated with intermittent oral acyclovir (800 mg twice a day for one week every month). Both groups were treated for six months and, then, patients were followed-up to further six months. Both IVIG and acyclovir were effective in reducing the frequency of HSV genital recurrences as compared to base-line. However, patients treated with IVIG had a more striking reduction in the frequency of recurrences as well as both a shorter mean duration and a minor severity of the lesions as compared to acyclovir-treated patients. Furthermore, we found a trend indicating IVIG as more effective in reducing the viral load. Since in IVIG-recipients we found a strong increase of peripheral blood lymphocytes with natural killer (NK) surface phenotype, we suggest that the clinical effectiveness of IVIG treatment is probably mediated via the expansion of NK cell populations. Our study indicates that the treatment with IVIG is an effective and safe tool for suppressing the recurrences of genital HSV infections.

Herpes simplex virus and oral mucositis in children with cancer

The relationship between herpes simplex virus (HSV) and oral mucositis was investigated in children undergoing antineoplastic chemotherapy. HSV culture was performed in 20 children with stomatitis developing after antineoplastic chemotherapy. Viral isolates were typed and susceptibility to acyclovir was investigated. The virus was isolated from oral lesions in 10 of 20 children with severe oral mucositis. Viral reactivation was the most likely explanation in most cases, since HSV was isolated in 9 of 13 seropositive patients (and in 1 patient with unknown anti-HSV serology), but in no seronegative patient. HSV type 1 was isolated more frequently than HSV type 2 (8 versus 2). Acyclovir showed standard in vitro activity against all isolates. Our results suggest that oral mucositis in children receiving antineoplastic treatment is probably multifactorial in origin and that HSV can be an important cofactor, especially in children who are seropositive for HSV. In our Centre, acyclovir remains active in vitro against this opportunistic pathogen and could be employed in prophylaxis and therapy.

Correlation between response to acyclovir and foscarnet therapy and in vitro susceptibility result for isolates of herpes simplex virus from human immunodeficiency virus-infected patients

In vitro susceptibility testing of herpes simplex virus (HSV) isolates will play an increasingly important role in guiding the clinical management of immunocompromised hosts who have lesions that are poorly responsive to therapy with standard antiviral agents. We assessed the correlation between the in vitro susceptibility result using a plaque reduction assay in Vero cells and the response to antiviral therapy with acyclovir or foscarnet for 243 clinical isolates of HSV collected from 115 human immunodeficiency virus-infected patients. The in vitro results and clinical responses were highly associated for both acyclovir and foscarnet (P < 0.001 and P < 0.001, respectively). The predictive values of a susceptible result (50% effective concentrations, < 2 micrograms/ml for acyclovir and < 100 micrograms/ml for foscarnet) for complete healing of lesions were 62% for acyclovir and 82% for foscarnet; the predictive values of a resistant result for failure to heal were 95% for acyclovir and 88% for foscarnet. Thus, in vitro testing has clinical utility in guiding therapy, although the 1 to 2 weeks required to derive a definitive result by the plaque reduction assay is a major limitation.

In vitro and in vivo characterization of herpes simplex virus clinical isolates recovered from patients infected with human immunodeficiency virus

A total of 100 herpes simplex viruses isolated from lesions not responding to acyclovir (ACV) therapy were recovered from 51 patients infected with human immunodeficiency virus. In vitro analysis of these isolates included testing their susceptibility to ACV and determining their thymidine kinase (TK) phenotypes. Of the 100 isolates evaluated, 23 were ACV sensitive and 77 were ACV resistant. Seventy-four of these ACV-resistant isolates were of the TK-deficient or low-TK-producer phenotype and three were of the TK-altered phenotype. The TKs isolates that represented each of the different autoradiographic phenotypes were further characterized by enzyme kinetics. The ability of selected isolates to cause disease in vivo was evaluated by using several mouse virulence models. Cutaneous virulence in normal and immunocompromised mice was evaluated, and neurovirulence in normal mice was determined. Latent infections were assayed by the cocultivation of trigeminal ganglia recovered from mice that had survived acute infection. These reactivated viruses were evaluated in vitro and compared with the original infecting isolate. The mechanisms of resistance and pathogenicity of these herpes simplex virus isolates recovered from patients positive for human immunodeficiency virus are similar to those reported for isolates recovered from normal and immunocompromised patients without AIDS.

Herpes simplex virus in immunocompromised patients: growing evidence of drug resistance

Antiviral drugs have proven effective in treatment of herpesvirus infections and in prevention of reactivation of latent virus. The prototype drug is acyclovir. Herpes simplex virus (HSV) is susceptible to acyclovir. There is increasing evidence in immunocompromised patients of acyclovir-resistant HSV causing clinical disease. Mechanisms of HSV resistance to acyclovir are known. These findings have implications for the future development and clinical use of antiviral drugs.

An application of tetrazolium (MTT) colorimetric assay for the screening of anti-herpes simplex virus compounds

A rapid and sensitive procedure was developed for in vitro evaluation of anti-herpes simplex virus (HSV) agents. The procedure is based on spectrophotometrical assessment for viability of virus- and mock-infected cells via in situ reduction of a tetrazolium dye MTT, which has already been used for the detection of anti-human immunodeficiency virus (HIV) agents (Pauwels et al., 1988). Monolayer cells such as human embryonic fibroblast, VERO, or HeLa cells were not suitable for this purpose. Among the non-adherent cell lines examined for susceptibility to HSV type 1 (HSV-1), a B-lymphoblastoid cell line NC-37 was found to be the most sensitive. The cell line was found to have a good correlation between the viable cell number and the reduction of MTT. In addition, centrifugation of the virus-infected cells resulted in further increase of the sensitivity of NC-37 cells to HSV-1. After optimization, the method proved to be as sensitive as plaque reduction. The system simplifies significantly the assay procedures and thus permits the evaluation of larger numbers of compounds for anti-HSV-1 activity.

Simplified test for detecting the resistance of herpes simplex virus to acyclovir

The detection of herpes simplex virus (HSV) antigen by means of an enzyme amplified ELISA was investigated for rapid screening of acyclovir (ACV) resistance. Vero cell monolayers were inoculated in the presence of different concentrations of ACV. When cytopathic effect was present, the culture supernatants were tested by ELISA. The absorbance values were found to correlate with the results of virus yield and plaque reduction assays. The comparison between absorbance values obtained in the presence of 10 microM ACV and in the absence of drug provided the basis for a simplified sensitivity test. The use of a single ACV concentration allowed discrimination between ACV-resistant and ACV-sensitive reference strains, the detection of ACV-resistant virus mixed in the proportion of 10% with ACV-sensitive virus, and a study of the emergence of an ACV-resistant virus population in serial samples taken from experimental rabbit keratitis. The simplified susceptibility assay is a sensitive and convenient method for rapid screening of HSV resistance to ACV.

Long-term suppression of severe recurrent genital herpes simplex infections with oral acyclovir: a dose-titration study

Twenty immunocompetent patients, four females and 16 males, with severe recurrent genital herpes (median number of recurrences the previous year 16, range (8-24] entered an open continuous long-term suppressive treatment with oral acyclovir (ACV) for 12 months. The study included a dose-titration schedule: (ACV, 200 mg x 4/1-3 months, ACV, 400 mg x 2/4-6 months, ACV, 200 mg x 2/7-9 months, and ACV, 400 mg x 1/10-12 months). Patients with recurrences on steps two and three received an alternative dose of ACV, 200 mg x 3. Otherwise patients entered the previous dose-step. Five (20%) of patients were completely free of symptoms (recurrences and abortive lesions) during the four dose-reduction periods. A further nine patients (50%) could be dose-reduced to 200 mg x 3 without symptoms. Isolates from three patients showed a decrease in virus sensitivity after ceasing treatment. In conclusion, 14/20 of treated patients could be dose reduced to 200 mg x 2-3 without selection of HSV strains showing clinically important decreases in sensitivity towards ACV.

Antiviral Activity of Mixtures of Compounds against Herpesviruses

A simple method for investigating the antiviral activity of mixtures of two antiviral compounds against herpes simplex virus (HSV) is described; the technique involves a modification of the dye-uptake technique for measuring antiviral activity, and the results are analysed by two methods for evidence of synergy or additive or antagonist effects. The antiviral activity of combinations of four antiviral compounds against HSV-1 and HSV-2 were investigated. The results indicate that interferon-alpha (IFN-α) with acyclovir or ara-A shows synergy, but IFN-α with ribavirin shows an additive effect. Ribavirin shows no synergism with any of the three compounds tested, and ara-A shows marginal synergy with acyclovir. The pattern of these results does not allow any prediction of which mixtures may show synergy. With the present limited number of antiviral compounds, all new combinations should be tested; and these further results may allow future predictions.

Persistent herpes simplex virus infection and mechanisms of virus drug resistance

Herpes simplex virus (HSV) is susceptible to a variety of antiviral compounds, most of which are nucleoside analogues that interfere with DNA metabolism involving the virus enzymes DNA-polymerase and thymidine kinase. Single mutations in the virus genome give rise to resistant mutants following selection in vitro in the presence of a particular drug, and in this respect HSV is similar to several other viruses. Such mutants have been invaluable research tools. HSV is responsible for a variety of lesions which tend to be recurrent, owing to the special ability of the virus to remain latent in and reactivate from neural tissue. The consequences of this upon clinical resistance are discussed in the present review. In fact, clinical resistance in HSV infections has not yet become widespread but does appear to be especially important in immunocompromised patients, including those suffering from AIDS. HSV is proposed as an important model for the investigation of drug resistance in other, more complex organisms, and with respect to antiviral strategies against the human immunodeficiency virus.

Acyclovir-resistant, foscarnet-sensitive oral herpes simplex type 2 lesion in a patient with AIDS

We report the case of an immunocompromised patient with AIDS in whom developed a perioral and several intraoral HSV 2 lesions that persisted for more than 1 year. The virus was resistant to acyclovir but was sensitive to foscarnet. Viral isolates were thymidine kinase negative. The lesions resolved with intravenous foscarnet therapy given over a 15-week period, and when last seen, 8 months after foscarnet was discontinued, the patient had not had a recurrence.

Evaluation of herpes simplex virus susceptibility to acyclovir using an enzyme-linked immunosorbent assay

An in situ ELISA was performed directly on the adherent cell monolayer in order to determine the susceptibility of herpes simplex virus isolates to acyclovir. Various fixation procedures and antisera conjugated to different enzymes were tested. The use of glutaraldehyde for fixation and beta-galactosidase as a labelling enzyme was shown to give the best results. As with other currently used assays, 50% inhibitory doses were subject to an inoculum effect. The data obtained indicate that this assay is suitable for routine determination of herpes simplex virus susceptibility to antiviral drugs.

Significance of resistance of herpes simplex virus to acyclovir

The genome of herpes simplex virus codes for several enzymes, including viral thymidine kinase and viral deoxyribonucleic acid (DNA) polymerase. When viral resistance develops, it does so by changes in these two enzymes. Three possible mechanisms of viral resistance to acyclovir include (1) selection of viral mutants that make little or no thymidine kinase and do not phosphorylate acyclovir adequately, (2) selection of mutants that can phosphorylate thymidine but cannot phosphorylate acyclovir (i.e., these viruses have thymidine kinases with altered substrate specificity), and (3) selection of viruses that have altered DNA polymerases that replicate viral DNA in the presence of acyclovir triphosphate. Thymidine kinase-deficient virus has been isolated from clinical isolates frequently, but few strains appear to be virulent for animals or humans and only a few seem to have caused clinical disease. Viruses with altered substrate specificity have been reported but viruses with an altered DNA polymerase have not occurred in clinical practice. Antiviral drugs should be used only when necessary to minimize the appearance of resistant strains of virus.

Clinical isolate of herpes simplex virus type 2 that induces a thymidine kinase with altered substrate specificity

In vitro and in vivo studies were done on a herpes simplex virus type 2 strain recovered from a patient on acyclovir (ACV) which was ACV resistant but expressed thymidine (dThd) kinase (EC 2.7.1.21) activity. Plaque-purified clones derived from the original clinical sample were heterogeneous with respect to plaque size and drug susceptibility. The heterogeneity of this viral mixture was also evident from varied 125I-labeled 5-iodo-2'-deoxycytidine autoradiographic patterns and from varied expression of dThd kinase-associated phosphorylating activities. Four clones from this mixture were 1-beta-D-arabinofuranosylthymine (ara-T) susceptible and ACV resistant. Extracts of cells infected with these clones catalyzed the phosphorylation of ara-T but little of ACV. The virus-coded dThd kinase was purified from one of these clones to determine whether its substrate specificity was altered. The amount of virus-coded dThd phosphorylating activity with the cell extracts was estimated to be sevenfold lower with the resistant clone than with the MS strain of herpes simplex virus type 2. The dThd kinase eluted from a dThd-agarose affinity column under the same conditions with extracts from both sources and substrate saturations of both enzymes by acyclic nucleoside analog phosphate acceptors were classical hyperbolic functions. However, there were significant differences in the kinetic parameters of substrates between the two enzymes. Apparent Km (Km') values for dThd, deoxycytidine, ara-T, ACV, and the acyclic guanosine analog 9-[[2-hydroxyl-1-(hydroxymethyl)ethoxy]methyl]guaine (BW B759U) were 2- to 60-fold higher with the variant enzyme than with the enzyme from laboratory strain MS. Comparing these two enzymes, relative maximal phosphorylation rates (Vm) were eightfold lower for ACV but unchanged for BW B759U. In contrast, the relative rates for deoxycytidine and ara-T were eight- and twofold higher, respectively. The surprisingly good substrate activity with BW B759U compared with that of ACV (Vm/Km' = 0.39 versus 0.01) coincided with susceptibility of the ACV-resistant virus to BW B759U. This clinical variant retained its pathogenicity for mice and was only moderately less neurovirulent than wild-type virus. Although such mutants have the potential to induce illness less responsive to therapy, the recurrence from which the isolate was obtained was typical for this patient in severity and duration. Since this episode, the patient has been treated successfully with ACV.

Acyclovir prophylaxis for herpes simplex virus infection

ACV is an effective agent for the treatment and prophylaxis of HSV infections in both IC and immunologically normal individuals. The drug is well tolerated in both populations and is not significantly associated with clinical or laboratory toxicities. Because of the great potential benefit and low risk, organ transplant recipients and patients with hematologic malignancies undergoing induction chemotherapy should be screened routinely for HSV antibodies; seropositive individuals should receive prophylactic ACV during the period of most profound immunosuppression. Immunologically normal individuals with frequently recurring genital HSV or serious complications associated with outbreaks are candidates for long-term suppression with ACV.

Comparative methods for detection of thymidine kinase-deficient herpes simplex virus type 1 strains

Four methods for analyzing viral susceptibility to antiviral substances were compared. In two methods viral products were measured: late viral proteins were measured by an enzyme-linked immunosorbent assay and viral DNA was measured by DNA hybridization. Infectious virus was quantified in the other two assays as the number of plaques and the yield of virus. The enzyme-linked immunosorbent assay procedure in our hands detected the smallest amounts (lowest proportions) of thymidine kinase-deficient herpes simplex virus type 1 mixed with wild-type virus. The thymidine kinase-deficient proportion of the herpes simplex virus type 1 isolate increased rapidly in the presence of acyclovir in cell culture.

Susceptibility to other antiherpes drugs of pathogenic variants of herpes simplex virus selected for resistance to acyclovir

Cross-resistance data for a group of nine acyclovir-resistant variants of herpes simplex virus type 1 are reported. These mutants, which express either altered thymidine kinase (TK) or DNA polymerase, were all derived from the same wild-type (wt) strain after exposure to acyclovir in tissue culture. Furthermore, all variants have pathogenic properties similar to the wt parental strain as assessed using mouse model systems (G. Darby, H.J. Field, and S.A. Salisbury, Nature (London) 289:81-83, 1981; B.A. Larder and G. Darby, Virology 146:262-271, 1985). Two groups of antiherpes compounds were used: those requiring activation by TK and those whose action is independent of that enzyme. The TK substrate-specificity mutants were generally resistant to the TK-activated drugs but showed wt susceptibility to phosphonoacetic acid, 9-beta-D-arabinofuranosyladenine, and aphidicolin. The DNA polymerase mutants were relatively susceptible to most TK-activated drugs, although two were resistant to 5-(trifluoromethyl)-2'-deoxyuridine. The polymerase mutants showed a more complex pattern of susceptibility, however, to those compounds whose mode of action is independent of TK. In general, these variants showed similar responses to phosphonoacetic acid, phosphonoformate, and 9-beta-D-arabinofuranosyladenine, a particular variant being either resistant, susceptible, or hypertensive to all three. The response of each variant to aphidicolin, however, appeared to be the inverse of its response to the other three drugs. The cross-resistance patterns are discussed, and their implications for combined or successive therapies are considered.

Double-blind comparison of weekend and daily regimens of oral acyclovir for suppression of recurrent genital herpes

The potential utility of intermittent regimens of oral acyclovir for suppression of recurrent genital herpes depends on how long the suppressive effect of the drug persists during pauses in treatment. To study this question, we admitted 38 patients in a double-blind controlled trial comparing the results of daily acyclovir treatment (200 mg t.i.d.) with treatment on weekend days only (400 mg t.i.d. on Saturday and Sunday) for suppression of recurrent genital herpes. Of the 35 patients completing the study, significantly more failures occurred in the weekend group (13/17) than in the daily group (3/18, P less than 0.001). Failures on the weekend regimen were more frequent as the week progressed (P = 0.005). The findings suggest a short-term persistence of suppression by acyclovir and hence that intermittent regimens with more closely spaced periods of treatment may be more effective than the regimen we studied. Most virus isolates studied, including all of those isolated from the patients during treatment, were sensitive to acyclovir.

Drug resistance patterns of herpes simplex virus isolates from patients treated with acyclovir

A decrease in the in vitro sensitivity to acyclovir (ACV) was observed in successive isolates of herpes simplex virus type 1 from three immunocompromised patients during intravenous therapy with this drug. The ACV-resistant isolate from patient 1 was cross-resistant to dihydroxypropoxymethylguanine and bromovinyldeoxyuridine, but still susceptible to three fluoro-substituted pyrimidines, 2'-fluoro-5-iodo-1-beta-D-arabinofuranosylcytosine (FIAC), 2'-fluoro-5-iodo-1-beta-D-arabinofuranosyluracil (FIAU), and 2'-fluoro-5-iodo-1-beta-D-arabinofuranosylthymine (FMAU). The thymidine kinase (TK) from the resistant isolate showed a 50-fold or greater reduction in affinity for thymidine, FIAU, FMAU, and ACV, but the total enzyme activity was similar to that of the sensitive isolate. The ACV-resistant isolate from patient 2 was also resistant to dihydroxypropoxymethylguanine, bromovinyldeoxyuridine, and the fluoro-substituted compounds; TK activity for this isolate was less than 1% of the patient's pretherapy isolate. An isolate obtained during a subsequent recurrence in patient 2 was susceptible to ACV and the other TK-dependent agents. The ACV-resistant isolate from patient 3 was partially resistant to FIAC and FIAU but still susceptible to FMAU; the viral TK had a 10-fold-lower affinity for ACV, FIAU, and FMAU than did the sensitive pretherapy isolate, while the level of TK activity detected was reduced to 6%. In none of the isolates studied was a change in sensitivity to phosphonoformic acid observed. Compared with the corresponding pretherapy ACV-sensitive isolates, there was a 30-fold decrease in neurovirulence for mice of the two drug-resistant isolates with diminished levels of thymidine-phosphorylating activity and no change in virulence for the third isolate. These findings indicate that mixed patterns of drug-resistance to TK-dependent antiviral compounds can occur in clinical isolates, resulting from changes in either the amount or the affinity of viral TK activity.

Selection and characterisation of acyclovir-resistant herpes simplex virus type 1 mutants inducing altered DNA polymerase activities

A collection of TK+, ACV-resistant mutants of herpes simplex virus type 1 (HSV-1) has been derived using a selection system based on biochemically transformed cells. Evidence is presented suggesting that most of these mutants induce resistant DNA polymerase activities and are thus likely to express variant DNA polymerases. Preliminary data on the pathogenesis of these mutants show that most are similar to wild type virus in the majority of their characteristics, although they may be reduced in their ability to kill mice.

Antiviral chemotherapy and chemoprophylaxis

Antiviral compounds have been developed for use in chemoprophylaxis and chemotherapy of a variety of infections in humans, including those caused by influenza viruses, respiratory syncytial virus, and herpesviruses. The efficacy of several of these compounds has been demonstrated in rigorously controlled trials. Advances in molecular virology have led to the identification of biochemically defined, virus-specific functions that serve as appropriate targets for the future development of antiviral compounds. Clinical investigators and practicing physicians are now confronting questions previously raised with the use of antibacterial antibiotics. These questions concern appropriate routes of administration for antiviral compounds, optimal dosage regimens, risks of long-term prophylaxis, and the emergence of resistant organisms.

Cutaneous herpes simplex virus infection of the guinea pig: lack of resistance to acyclovir and phosphonoformic acid after topical treatment

Guinea pigs were infected cutaneously with HSV1 and treated topically with acyclovir or phosphonoformic acid (0.1% to 0.5% solution), or with a combination of both. The therapy was clinically effective and the virus content in the skin diminished. Virus harvests from skin areas in no case showed increased drug resistance, as tested by plaque reduction.

Suppression of frequently recurring genital herpes. A placebo-controlled double-blind trial of oral acyclovir

We studied 35 otherwise healthy adults with frequently recurring genital herpes (greater than or equal to 1 episode per month), in a double-blind trial comparing oral acyclovir with placebo capsules for suppression of recurrent infection. The patients were treated for 125 days unless herpes recurred. Among 32 evaluable patients, there were significantly fewer recurrences during acyclovir treatment (4 of 16) than during placebo treatment (16 of 16, P less than 0.001). The mean duration of therapy was significantly longer for patients receiving acyclovir than for those receiving placebo (114.9 vs. 24.8 days, P less than 0.001). Of 19 patients who had recurrences in the blind trial, only 2 had recurrences when given acyclovir in a second, open-study phase. All patients had recurrences after completing acyclovir treatment. The therapy was well tolerated, with minimal gastrointestinal upset and one hypersensitivity reaction. Studies of the viral isolates demonstrated that lesions developing in patients receiving acyclovir contained drug-resistant virus. Later recurrences in these patients were associated with drug-sensitive virus. We conclude that oral acyclovir suppresses genital herpes in patients with frequent recurrences, but the potential for problems with drug resistance and the long-term safety need to be more fully explored.

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

A quantitative colorimetric method for measuring the inhibition of viral cytopathic effects has been adapted to the assay of antiviral compounds. Drug-treated, virus-infected cultures in microtiter plates were stained with the vital dye neutral red and the amount of dye incorporated was determined in a multichannel spectrophotometer. The technique required smaller volumes of reagents, was more easily automated than the standard plaque reduction assay and had good reproducibility. Standard conditions of 30 infectious units of challenge virus and 72-h incubation were judged to be optimal. Median inhibitory concentrations (ID50) for a number of compounds were approximately tenfold higher in the dye-uptake assay compared with the plaque reduction assay, possibly related to the higher multiplicity of infection required to give the desired level of cytopathic effect in the microtiter method.