Distribution and effects of amino acid changes in drug-resistant α and β herpesviruses DNA polymerase

Emergence of drug-resistance to all FDA-approved antiherpesvirus agents is an increasing concern in immunocompromised patients. Herpesvirus DNA polymerase (DNApol) is currently the target of nucleos(t)ide analogue-based therapy. Mutations in DNApol that confer resistance arose in immunocompromised patients infected with herpes simplex virus 1 (HSV-1) and human cytomegalovirus (HCMV), and to lesser extent in herpes simplex virus 2 (HSV-2), varicella zoster virus (VZV) and human herpesvirus 6 (HHV-6). In this review, we present distinct drug-resistant mutational profiles of herpesvirus DNApol. The impact of specific DNApol amino acid changes on drug-resistance is discussed. The pattern of genetic variability related to drug-resistance differs among the herpesviruses. Two mutational profiles appeared: one favoring amino acid changes in the Palm and Finger domains of DNApol (in α-herpesviruses HSV-1, HSV-2 and VZV), and another with mutations preferentially in the 3′-5′ exonuclease domain (in β-herpesvirus HCMV and HHV-6). The mutational profile was also related to the class of compound to which drug-resistance emerged.

Inhibition of Varicella-Zoster Virus by Penciclovir in Cell Culture and Mechanism of Action

The effect of penciclovir (BRL 39123) on the replication of varicella-zoster virus (VZV) in human embryonic lung fibroblasts (MRC-5 cells) was similar to aciclovir when the compounds were present continuously. However, when the compounds were withdrawn the antiviral activity of penciclovir was maintained more effectively than that of aciclovir. In the plaque reduction assay, median 50% effective concentrations (EC50s) were 3.8 μg ml−1 for penciclovir and 4.2 μg ml−1 for aciclovir ( n = 29 clinical isolates). Similarly, penciclovir and aciclovir were equally effective in reducing the numbers of VZV-infected MRC-5 cells and in reducing VZV DNA synthesis within infected cells following continuous treatment. Within VZV-infected cells (S)-penciclovir-triphosphate was formed from penciclovir with >95% enantiomeric purity, and the concentration of penciclovir-triphosphate was 360-fold greater than aciclovir-triphosphate immediately after treatment. This phosphorylation ratio compensates for the lower affinity of VZV DNA polymerase for penciclovir-triphosphate compared with aciclovir-triphosphate (Kis = 7.5 μM and 0.2 μM, respectively). When VZV-infected cultures were treated for 3 days, followed by withdrawal of the compound, inhibition of viral DNA synthesis by penciclovir was maintained for 24 h, whereas viral DNA synthesis resumed more readily after removal of aciclovir. Furthermore, following 8 h daily pulse treatment for 5 days, penciclovir was significantly more active than aciclovir in reducing VZV DNA synthesis ( p = 0.006, n = 10 clinical isolates). The long intracellular half-life of penciclovir-triphosphate (9.1 h) compared with that of aciclovir-triphosphate (0.8 h) accounts for the sustained inhibition of virus replication by penciclovir. This property may contribute to the clinical efficacy of famciclovir, the oral form of penciclovir.

Further Characterization of the Potent and Prolonged Inhibition of Herpes Simplex Virus Replication in Human Cell Lines by Penciclovir

The replication of herpes simplex virus type 1 (HSV-1) or HSV-2 in MRC-5 cells infected at 0.01 pfu cell−1 treated continuously for 72 h, was inhibited more efficiently by penciclovir than aciclovir ( p = 0.0001). However, multiple cycles of replication were required in order to distinguish the compounds. Virus from cultures treated for 72 h with either compound, at 3 or 10 μg ml−1 was resistant to penciclovir and aciclovir (50% effective concentrations > 10 μg ml−1), but infectivity titres of supernatants from these aciclovirtreated cultures were higher than for penciclovir. Increased production of resistant virus in aciclovirtreated cultures may be the consequence of the less potent inhibition of virus replication by aciclovir. Penciclovir caused prolonged inhibition of HSV-1 and HSV-2 replication in three human cell lines infected at 1 pfu cell−1 following treatment for 18 h, whereas virus replication resumed rapidly after withdrawal of aciclovir. Neither compound showed prolonged activity after 18 h treatment, when the multiplicity of infection was reduced to 0.01 pfu cell−1. This surprising observation prompted experiments testing the effect of repeated pulse treatment in cultures infected at low multiplicity. Penciclovir inhibited HSV-1 replication significantly more effectively than aciclovir in MRC-5 cells infected at 10−4 pfu cell−1 treated daily for 6 h ( p < 0.001, n = 5) but only a trend was observed for HSV-2 ( p = 0.06, n = 6).

Penciclovir-Resistance Mutations in the Herpes Simplex Virus DNA Polymerase Gene

Penciclovir is the active form of the orally available prodrug famciclovir, which is entering clinical use for herpesvirus infections. Like aciclovir, penciclovir is an acyclic guanosine analogue that is phosphorylated by viral thymidine kinase and whose triphosphate can inhibit viral DNA polymerase. We tested several well-characterized herpes simplex virus mutants with aciclovir-resistance mutations in the viral DNA polymerase gene for altered sensitivity to penciclovir. The mutants varied in their susceptibilities to penciclovir with one exhibiting 2-fold hypersensitivity, one marginal resistance and three about 3-fold resistance. Marker rescue and DNA sequencing analyses mapped the penciclovir-resistance mutation of one mutant, AraA r7, to a single base change that alters a glycine to a cysteine at residue 841 within conserved region III of α-like DNA polymerases. The results have implications for the mechanism of selective action of penciclovir, for the potential for development of resistance in the clinic, and for the substrate recognition properties of herpes simplex virus DNA polymerase.

Famciclovir, from the bench to the patient--a comprehensive review of preclinical data

Famciclovir is converted rapidly and efficiently after oral administration to the selective antiviral compound, penciclovir. In cell culture, penciclovir is a potent inhibitor of herpes simplex virus (HSV) types 1 and 2, varicella-zoster virus (VZV), Epstein-Barr virus (EBV) and hepatitis B virus (HBV). Phosphorylation of penciclovir and aciclovir in uninfected cells is limited, and penciclovir, like aciclovir, has minimal effect on replicating cells in culture as expected for a selective antiviral agent. Mode of action studies with VZV and HSV have shown that the phosphorylation of penciclovir in infected cells is far more efficient than for aciclovir. This compensates for differences observed between penciclovir triphosphate and aciclovir triphosphate in the inhibition of HSV and VZV DNA polymerases. Because HBV is not known to encode a thymidine kinase, a different rationale for the selective inhibition of this virus by penciclovir is required. Recent data indicate that the DNA polymerase of HBV is far more sensitive to inhibition by penciclovir triphosphate than cellular DNA polymerases, suggesting that for this virus, selectivity operates at the DNA polymerase. Penciclovir triphosphate is more stable within infected cells than aciclovir triphosphate, and consequently penciclovir has more prolonged antiviral activity than aciclovir. Similarly, famciclovir is more effective than aciclovir or valaciclovir in suppressing HSV replication when given at a lower dosing frequency in certain animal models. These preclinical properties have helped to provide the foundation for the famciclovir clinical programme.

Famciclovir

Famciclovir (Famvir) is the oral prodrug of penciclovir, an agent that has demonstrated antiviral activity against herpes simplex viruses, type 1 (HSV-1) and 2 (HSV-2) [which cause orolabial and/or genital herpes simplex], and against varicella zoster virus (VZV) [a reactivation of which leads to herpes zoster]. Famciclovir has efficacy similar to that of aciclovir (in immunocompetent or immunocompromised patients) or valaciclovir (in immunocompetent patients) in the treatment of herpes zoster, and efficacy similar to aciclovir in the treatment of first or recurrent episodes of genital herpes (in immunocompetent or immunocompromised patients). Famciclovir also has efficacy in the suppression of recurrent episodes of genital herpes, and in the treatment of orolabial herpes, in immunocompetent patients. As such, famciclovir is a well tolerated first-line option for the treatment of herpes zoster and the treatment and suppression of genital herpes, and is approved for the treatment of recurrent orolabial herpes. Convenient patient-initiated single-day (for recurrent genital herpes) and single-dose (for orolabial herpes) dosage regimens may contribute to treatment compliance, patient acceptability and subsequent treatment outcomes.

Potent in vivo antiviral activity of the herpes simplex virus primase-helicase inhibitor BAY 57-1293

BAY 57-1293 belongs to a new class of antiviral compounds and inhibits replication of herpes simplex virus (HSV) type 1 and type 2 in the nanomolar range in vitro by abrogating the enzymatic activity of the viral primase-helicase complex. In various rodent models of HSV infection the antiviral activity of BAY 57-1293 in vivo was found to be superior compared to all compounds currently used to treat HSV infections. The compound shows profound antiviral activity in murine and rat lethal challenge models of disseminated herpes, in a murine zosteriform spread model of cutaneous disease, and in a murine ocular herpes model. It is active in parenteral, oral, and topical formulations. BAY 57-1293 continued to demonstrate efficacy when the onset of treatment was initiated after symptoms of herpetic disease were already apparent.

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.

Valaciclovir: a review of its use in the management of herpes zoster

Varicella zoster virus (VZV), the pathogen responsible for herpes zoster, belongs to the herpesvirus family and is sensitive to the antiviral drug aciclovir. However, the low oral bioavailability of aciclovir has to some extent limited its efficacy in the treatment of herpes zoster and has prompted the development of the more readily absorbed oral prodrug valaciclovir. In a large comparative study valaciclovir, (1000 mg 3 times daily for 7 days) was at least as effective as aciclovir (800 mg 5 times daily for 7 days) in controlling the symptoms of acute herpes zoster. Importantly, valaciclovir alleviated zoster-associated pain and postherpetic neuralgia significantly faster than aciclovir. A 14-day regimen of valaciclovir showed no significant advantage over the 7-day regimen. A smaller trial in Japanese patients focusing primarily on the cutaneous (rash) signs of herpes zoster confirmed the similar efficacy of valaciclovir and aciclovir in the 7-day regimen. This study did not follow all patients for a formal analysis of postherpetic neuralgia. Valaciclovir and aciclovir demonstrated similar efficacy for the control of cutaneous lesions and ocular complications in patients with zoster ophthalmicus. Preliminary results of a large controlled trial indicate that valaciclovir 1000 mg 3 times daily and famciclovir (the prodrug of penciclovir) 500 mg 3 times daily are of similar efficacy in speeding resolution of acute herpes zoster rash and shortening the duration of postherpetic neuralgia. Starting treatment later than 72 hours after rash onset did not significantly reduce the beneficial effect of valaciclovir on duration of zoster-associated pain (a continuum of pain that encompasses both acute pain and postherpetic neuralgia) in a large observational study, suggesting that valaciclovir might be effective when given later than previously thought. However, valaciclovir should ideally be given as soon as possible after symptoms appear. With the recommended regimen for the treatment of herpes zoster (1000 mg 3 times daily for 7 days) valaciclovir was well tolerated, with nausea and headache being the most commonly reported adverse events. The adverse events profile of the agent was similar to that seen with aciclovir or famciclovir.

CONCLUSION

The efficacy of valaciclovir for the treatment of herpes zoster has been confirmed and extended by follow-up studies in herpes zoster ophthalmicus, in Japanese patients, and in the wider primary care setting. Valaciclovir is at least equivalent to, and better in certain parameters than, aciclovir and appears to have similar efficacy to famciclovir 500 mg 3 times daily. Valaciclovir is a well tolerated first-line therapy with an established place in the treatment of immunocompetent patients with herpes zoster.

Acyclovir is phosphorylated by the human cytomegalovirus UL97 protein

Acyclovir (ACV) has shown efficacy in the prophylactic suppression of human cytomegalovirus (HCMV) reactivation in immunocompromised renal transplant patients without the toxicity associated with ganciclovir (GCV). The HCMV UL97 gene product, a protein kinase, is responsible for the phosphorylation of GCV in HCMV-infected cells. This report provides evidence for the phosphorylation of ACV by UL97. Anabolism studies with the HCMV wild-type strain AD169 and with recombinant mutants derived from marker transfer experiments performed by using mutant UL97 DNA from both clinical isolates and a laboratory-derived strain resistant to GCV showed that mutations in the UL97 gene cripple the ability of recombinant virus-infected cells to anabolize both GCV and ACV. These mutant UL97 recombinant viruses were less susceptible to both GCV and ACV than was the wild-type strain. A recombinant herpes simplex virus type 1 strain, in which the thymidine kinase gene is deleted and the UL13 gene is replaced with the HCMV UL97 gene, was able to induce the phosphorylation of ACV in infected cells. Finally, purified UL97 phosphorylated both GCV and ACV to their monophosphates. Our results indicate that UL97 promotes the selective activity of ACV against HCMV.

Evaluation of a novel, anti-herpes simplex virus compound, acyclovir elaidate (P-4010), in the female guinea pig model of genital herpes

The antiviral effect of acyclovir elaidate in the female guinea pig model of genital herpes was investigated in a series of experiments. The antiherpesvirus effects of this novel compound, 9-(2'-[trans-9"-octadecenoyloxyl]ethoxymethyl)guanine (code no. P-4010), were studied in both primary and recurrent genital herpes in the female guinea pig, following oral gavage or intraperitoneal injection, with different formulations of the compound, and in comparison with acyclovir (ACV) or penciclovir (PCV). The results indicate that compound P-4010 has a greater capability than either ACV or PCV in reducing the clinical symptoms of primary genital herpes induced following the inoculation of herpes simplex virus type 2 (HSV-2) intravaginally into guinea pigs. In addition, the administration of P-4010 twice daily over a 10-day period by the intraperitoneal route (15 to 40 mg/kg of body weight/day) or by oral gavage (50 to 200 mg/kg/day), commencing 4 h subsequent to intravaginal HSV-2 infection, resulted in a degree of reduction in the incidence and severity of spontaneous, recurrent genital herpes in these animals. The findings are discussed in the light of the value and relevance of the female guinea pig model of genital herpes for the assessment of anti-herpes simplex virus compounds.

Antiherpesvirus activities of (1'S,2'R)-9-[[1',2'-bis(hydroxymethyl)cycloprop-1'-yl]methyl]guanine (A-5021) in cell culture

Antiherpetic activity of (1'S,2'R)-9-([1',2'-bis(hydroxymethyl)cycloprop-1'yl]methyl)guanine (A-5021) was compared with those of acyclovir (ACV) and penciclovir (PCV) in cell cultures. In a plaque reduction assay using a selection of human cells, A-5021 showed the most potent activity in all cells. Against clinical isolates of herpes simplex virus type 1 (HSV-1, n = 5) and type 2 (HSV-2, n = 6), mean 50% inhibitory concentrations (IC50s) for A-5021 were 0.013 and 0.15 microgram/ml, respectively, in MRC-5 cells. Corresponding IC50s for ACV were 0.22 and 0.30 microgram/ml, and those for PCV were 0.84 and 1.5 micrograms/ml, respectively. Against clinical isolates of varicella-zoster virus (VZV, n = 5), mean IC50s for A-5021, ACV, and PCV were 0.77, 5.2, and 14 micrograms/ml, respectively, in human embryonic lung (HEL) cells. A-5021 showed considerably more prolonged antiviral activity than ACV when infected cells were treated for a short time. The selectivity index, the ratio of 50% cytotoxic concentration to IC50, of A-5021 was superior to those of ACV and PCV for HSV-1 and almost comparable for HSV-2 and VZV. In a growth inhibition assay of murine granulocyte-macrophage progenitor cells, A-5021 showed the least inhibitory effect of the three compounds. These results show that A-5021 is a potent and selective antiviral agent against HSV-1, HSV-2, and VZV.

Penciclovir and pathogenesis phenotypes of drug-resistant Herpes simplex virus mutants

We compared the penciclovir susceptibilities and pathogenesis phenotypes of mutants of Herpes simplex virus type 1 that are resistant to acyclovir and/or foscarnet. The mutants, which were derived from laboratory strain KOS, included six DNA polymerase mutants, a thymidine kinase negative mutant, a thymidine kinase partial mutant, and a double mutant. Two of four polymerase mutants not previously examined for penciclovir susceptibility exhibited modest resistance to this drug. A thymidine kinase negative mutant exhibited approximately 20-fold resistance while a thymidine kinase partial mutant was penciclovir-sensitive. Following intracerebral inoculation of 7-week old CD1 mice, the mutants ranged from exhibiting near wild-type neurovirulence (thymidine kinase partial) to modest attenuation (e.g. thymidine kinase negative) to more severe attenuation. Following corneal inoculation, three polymerase mutants exhibited modest deficits (relative to those of thymidine kinase negative mutants) in their abilities to replicate acutely in the ganglion and reactivate from latency. For mutant AraA(r)13, the deficit in ganglionic replication was shown to be due to its polymerase mutation by analysis of recombinant viruses derived by marker rescue. These results may have implications for issues of penciclovir action and resistance, for drug resistance in the clinic, and for the interactions of herpes viruses with the peripheral and central nervous systems.