Mode of action of phosphonoformate as an anti-herpes simplex virus agent

A spliced latency-associated VZV transcript maps antisense to the viral transactivator gene 61

Varicella-zoster virus (VZV), an alphaherpesvirus, establishes lifelong latent infection in the neurons of >90% humans worldwide, reactivating in one-third to cause shingles, debilitating pain and stroke. How VZV maintains latency remains unclear. Here, using ultra-deep virus-enriched RNA sequencing of latently infected human trigeminal ganglia (TG), we demonstrate the consistent expression of a spliced VZV mRNA, antisense to VZV open reading frame 61 (ORF61). The spliced VZV latency-associated transcript (VLT) is expressed in human TG neurons and encodes a protein with late kinetics in productively infected cells in vitro and in shingles skin lesions. Whereas multiple alternatively spliced VLT isoforms (VLT_ly) are expressed during lytic infection, a single unique VLT isoform, which specifically suppresses ORF61 gene expression in co-transfected cells, predominates in latently VZV-infected human TG. The discovery of VLT links VZV with the other better characterized human and animal neurotropic alphaherpesviruses and provides insights into VZV latency.

Varicella-zoster virus (VZV) establishes lifelong infection in the majority of the population, but mechanisms underlying latency remain unclear. Here, the authors use ultra-deep RNA sequencing, enriched for viral RNAs, of latently infected human trigeminal ganglia and identify a spliced, latency-associated VZV mRNA.

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.

Alpha-carboxy nucleoside phosphonates as universal nucleoside triphosphate mimics

Polymerases have a structurally highly conserved negatively charged amino acid motif that is strictly required for Mg(2+) cation-dependent catalytic incorporation of (d)NTP nucleotides into nucleic acids. Based on these characteristics, a nucleoside monophosphonate scaffold, α-carboxy nucleoside phosphonate (α-CNP), was designed that is recognized by a variety of polymerases. Kinetic, biochemical, and crystallographic studies with HIV-1 reverse transcriptase revealed that α-CNPs mimic the dNTP binding through a carboxylate oxygen, two phosphonate oxygens, and base-pairing with the template. In particular, the carboxyl oxygen of the α-CNP acts as the potential equivalent of the α-phosphate oxygen of dNTPs and two oxygens of the phosphonate group of the α-CNP chelate Mg(2+), mimicking the chelation by the β- and γ-phosphate oxygens of dNTPs. α-CNPs (i) do not require metabolic activation (phosphorylation), (ii) bind directly to the substrate-binding site, (iii) chelate one of the two active site Mg(2+) ions, and (iv) reversibly inhibit the polymerase catalytic activity without being incorporated into nucleic acids. In addition, α-CNPs were also found to selectively interact with regulatory (i.e., allosteric) Mg(2+)-dNTP-binding sites of nucleos(t)ide-metabolizing enzymes susceptible to metabolic regulation. α-CNPs represent an entirely novel and broad technological platform for the development of specific substrate active- or regulatory-site inhibitors with therapeutic potential.

Chlamydia trachomatis enters a viable but non-cultivable (persistent) state within herpes simplex virus type 2 (HSV-2) co-infected host cells

Epidemiological and clinical studies have shown that double infection with herpes simplex virus type 2 (HSV-2) and Chlamydia trachomatis occurs in vivo. We hypothesized that co-infection would alter replication of these agents. To test this hypothesis, HeLa cells were infected with C. trachomatis serovar E, followed 24 h later by HSV-2 strain 333. Transmission electron microscopic (TEM) analyses indicated that, by 10 h after HSV addition, reticulate bodies (RBs) in co-infected cells were swollen, aberrantly shaped and electron-lucent. In infectious titre assays, HSV-2 co-infection abrogated production of infectious chlamydial progeny. Western blot analyses indicated that accumulation of chlamydial major outer membrane protein (MOMP) was decreased by HSV co-infection while accumulation of chlamydial heat-shock protein 60-1 (HSP60-1) was increased. Polymerase chain reaction (PCR) experiments indicated that chlamydial genome copy number was unaltered by HSV-2 superinfection. Semi-quantitative, reverse transcription PCR (RT-PCR) experiments demonstrated that levels of chlamydial groEL, ftsK, ftsW, dnaA and unprocessed 16S rRNA transcripts were not changed by HSV-2 super-infection. These data indicate that HSV-2 superinfection drives chlamydia into a viable but non-cultivable state, which is the hallmark of persistence. Because chlamydial HSP60-1 has been associated with immunopathology in vivo, these results also suggest that disease severity might be increased in co-infected individuals.

Antiviral activity of phosphonoformate on rotavirus transcription and replication

The antiviral effect of foscarnet (PFA) on the replication of rotavirus, a member of the Reoviridae, was studied. The pyrophosphate analogue is an effective inhibitor of several viral polymerases acting on the enzyme pyrophosphate binding site. Replication of rotavirus in MA104 cells using different u.o.i. was inhibited by PFA in a concentration dependent manner, due to the inhibition of both plus- and minus-strand RNA synthesis. The addition of PFA to infected cells was specific for the inhibition of viral replication since uninfected cell incubated at the same PFA concentrations did not exhibit any cytotoxic effect. The 50% inhibitory effect of PFA on in vitro mRNA synthesis was obtained at a concentration of 150 microM. Over 80% of the in vitro minus-strand RNA synthesis was inhibited at a concentration of 320 microM, when PFA was assayed using replicase-enriched cell infected fraction. The results suggest that the effect may be due to an interaction of PFA with the viral polymerase, since this protein catalyses both plus- and minus-strand RNA synthesis. The results of experiments using the temperature-sensitive viral polymerase mutant show that the mutant is less sensitive to PFA, suggesting that this polypeptide is the target for PFA.

Phosphonoformate (foscarnet, PFA) versus trifluorthymidine in the treatment of keratitis dendritica in the human. A double-blind, randomized, preliminary trial

Twenty patients with dendritic keratitis were treated in a double-blind analysis study with either phosphonoformate 3% or trifluorthymidine 1% drops. Nineteen cases were virologically confirmed. Follow-up was performed until closure of the corneal lesion was registered and a pair comparison between both substances was made. Healing time was longer for phosphonoformate (7.16 days) than for trifluorthymidine (5.83 days). There was no significant difference between phosphonoformate and trifluorthymidine. One drug failure was registered in each series. No side effects were observed.

Inhibition of African swine fever virus in cultured swine monocytes by phosphonoacetic acid (PAA) and by phosphonoformic acid (PFA)

The use of phosphonoacetic (PAA) and phosphonoformic acid (PFA) as inhibitors of African swine fever virus (ASFV) replication in porcine monocytes/macrophages (MO) was investigated. At concentrations sufficient to inhibit replication, hemadsorption, and cytopathogenic damage by high inocula of ASFV, both antiviral agents were cytostatic and suppressed the DNA-synthetic growth response of porcine MO to the MO-specific colony-stimulating factor-1 (CSF-1). PAA and PFA inhibited ASFV-associated DNA-synthesis in the cytoplasm of infected swine MO. Using ASFV-specific monoclonal antibodies in immunebinding assays and in immunoprecipitation analysis of radiolabeled proteins of infected MO, PAA and PFA inhibited the synthesis of ASFV proteins of 13, 73, and 150/220 kDa, and caused a variable inhibition in the synthesis of a 12 kDa ASFV protein. These antiviral drugs, however, did not prevent the appearance of an early 32 kDa ASFV protein. The cytostatic and virus-suppressive effects of PAA and PFA could be reversed. ASFV resumed growth in infected MO cultures, if the cells maintained in medium with CSF-1 were removed from the antivirals before 1 week of drug exposure. With prolonged exposure to PAA or PFA (beyond 1 week), ASFV could not be recovered from infected MO cultures.

Antiviral effects of phosphonoformate (PFA, foscarnet sodium)

This article describes the antiviral properties of foscarnet (trisodium phosphonoformate) at the enzyme level as well as in cell cultures and in vivo. The mechanism of action against herpesvirus DNA polymerases and reverse transcriptases is outlined. Clinical studies using topical foscarnet against mucocutaneous herpes simplex virus infections are presented. The clinical use of intravenous foscarnet against severe viral infections caused by cytomegalovirus, hepatitis B virus and human immunodeficiency virus is discussed.

Metal chelators as potential antiviral agents

Metal-chelating compounds can inhibit virus-induced enzymes in infected cells by coordinating with metal ions at their active sites. Consideration of the coordinating properties of ligands can explain the antiviral activity of these compounds. The antiviral actions of a number of compounds (e.g., thiosemicarbazones, pyrophosphate analogues, beta-diketones, cyclic polyethers and flavanoids) are discussed in the light of their metal-chelating properties.

Cytomegalovirus DNA polymerase inhibition and kinetics

Cytomegalovirus (CMV) DNA polymerase has immunologic specificity in relation to the virus specific polymerases of other herpesviruses. During the early phase of CMV infection in vitro, the virus DNA polymerase is rapidly induced. Due to the lack of virus specific thymidine kinase, cytomegalovirus is resistant to nucleosides which require herpesvirus thymidine kinases for activation. Cytomegalovirus DNA polymerase is therefore the known possible target for antiviral drugs. Several pyrophosphate analogs have been assayed for their inhibitory effects on this enzyme. The most active compound is phosphonoformate (PFA). PFA also effectively inhibits other partially purified herpesvirus DNA polymerases as well as the multiplication of all human herpesviruses, at concentrations which do not affect cellular DNA polymerases or normal cell growth. The mechanism of inhibition is a noncompetitive inhibition of CMV DNA polymerase activity with respect to the four deoxyribonucleoside triphosphates, and an uncompetitive inhibition with respect to the template used. In cell culture, PFA inhibits the formation of late CMV polypeptides, but not the synthesis of early CMV polypeptides. The CMV specific polymerase persists in the presence of PFA, as measured by immunological methods, and enzyme activity can be demonstrated after removal of PFA. The inhibition of CMV replication is reversible even after long exposure to PFA. Our interpretation is that the CMV genome is highly resistant to the cellular metabolism also in non-producing cells. A new rapid CMV neutralization test was established, based on the appearance of early CMV-induced antigens.