Cidofovir inhibits genome encapsidation and affects morphogenesis during the replication of vaccinia virus

New challenges in human monkeypox outside Africa: A review and case report from Italy

BACKGROUND

Human monkeypox (MPX) is a neglected zoonotic disease caused by the MPX virus a double-stranded DNA virus which belongs to the Poxviridae family genus Orthopoxvirus. It is endemic in the rural rainforests of Central and Western Africa where it is responsible of human sporadic cases and outbreaks since 1970. Outside Africa MPXV caused an outbreak in 2003 in the United States linked to importation of infected rodents from Ghana and a few travel-related cases in the USA, United Kingdom, Israel and Singapore. Actually, a worldwide outbreak with more than 1200 confirmed cases mainly concentrated among men who have sex with men is ongoing.

CASE REPORT

We present the case of an Italian man living in Portugal that was diagnosed with MPX at our clinic in Milan, Italy. Monkeypox virus infection was confirmed by a specific homemade Real-Time PCR. Samples obtained from different sites (pharynx, skin lesions, anal ulcer, seminal fluid) turned all positive with different viral load.

CONCLUSIONS

Our report illustrates the challenge of a disease that seems to present in a different way from classic description with possible human-to-human transmission through sexual contact.

Why do poxviruses still matter?

Poxviruses comprise many members that infect both vertebrate and invertebrate animals, including humans. Despite the eradication of the historically notorious smallpox, poxviruses remain significant public health concerns and serious endemic diseases. This short review briefly summarizes the present, historical, and future threats posed by poxviruses to public health, wildlife and domestic animals, the role poxviruses have played in shaping modern medicine and biomedical sciences, the insight poxviruses have provided into complex life processes, and the utility of poxviruses in biotechniques and in fighting other infectious diseases and cancers. It is anticipated that readers will appreciate the great merit and need for continued strong support of poxvirus research; research which benefits not only the expansion of fundamental biological knowledge but also the battle against diverse diseases.

Drug Development against Smallpox: Present and Future

Forty years after the last endemic smallpox case, variola virus (VARV) is still considered a major threat to humans due to its possible use as a bioterrorism agent. For many years, the risk of disease reemergence was thought to solely be through deliberate misuse of VARV strains kept in clandestine laboratories. However, recent experiments using synthetic biology have proven the feasibility of recreating a poxvirus de novo, implying that VARV could, in theory, be resurrected. Because of this new perspective, the WHO Advisory Committee on VARV Research released new recommendations concerning research on poxviruses that strongly encourages pursuing the development of new antiviral drugs against orthopoxviruses. In 2018, the U.S. FDA advised in favor of two molecules for smallpox treatment, tecovirimat and brincidofovir. This review highlights the difficulties to develop new drugs targeting an eradicated disease, especially as it requires working under the FDA "animal efficacy rule" with the few, and imperfect, animal models available.

Vaccinia virus G1 protein: absence of autocatalytic self-processing

Vaccinia virus relies on a series of proteolytic cleavage events involving two viral proteins, I7 and G1, to complete its life cycle. Furthermore, G1 itself is cleaved during vaccinia virus infection. However, convincing evidence is lacking to show whether G1 participates in autoproteolysis or is a substrate of another protease. We employed both biochemical and cell-based approaches to investigate G1 cleavage. G1, when expressed in bacteria, rabbit reticulocyte lysates or HeLa cells, was not processed. Moreover, G1 was cleaved in infected cells, but only in the presence of virus late gene expression; cleavage was strongly inhibited by proteasome inhibitors. Thus, these results imply a more complex G1 cleavage reaction than previously envisaged.

Vaccinia Virus B1 Kinase Is Required for Postreplicative Stages of the Viral Life Cycle in a BAF-Independent Manner in U2OS Cells

The vaccinia virus B1R gene encodes a highly conserved protein kinase that is essential for the poxviral life cycle. As demonstrated in many cell types, B1 plays a critical role during viral DNA replication when it inactivates the cellular host defense effector barrier to autointegration factor (BAF or BANF1). To better understand the role of B1 during infection, we have characterized the growth of a B1-deficient temperature-sensitive mutant virus (Cts2 virus) in U2OS osteosarcoma cells. In contrast to all other cell lines tested to date, we found that in U2OS cells, Cts2 viral DNA replication is unimpaired at the nonpermissive temperature. However, the Cts2 viral yield in these cells was reduced more than 10-fold, thus indicating that B1 is required at another stage of the vaccinia virus life cycle. Our results further suggest that the host defense function of endogenous BAF may be absent in U2OS cells but can be recovered through either overexpression of BAF or fusion of U2OS cells with mouse cells in which the antiviral function of BAF is active. Interestingly, examination of late viral proteins during Cts2 virus infection demonstrated that B1 is required for optimal processing of the L4 protein. Finally, execution point analyses as well as electron microscopy studies uncovered a role for B1 during maturation of poxviral virions. Overall, this work demonstrates that U2OS cells are a novel model system for studying the cell type-specific regulation of BAF and reveals a role for B1 beyond DNA replication during the late stages of the viral life cycle.

IMPORTANCE

The most well characterized role for the vaccinia virus B1 kinase is to facilitate viral DNA replication by phosphorylating and inactivating BAF, a cellular host defense responsive to foreign DNA. Additional roles for B1 later in the viral life cycle have been postulated for decades but are difficult to examine directly due to the importance of B1 during DNA replication. Here, we demonstrate that in U2OS cells, a B1 mutant virus escapes the block in DNA replication observed in other cell types and, instead, this mutant virus exhibits impaired late protein accumulation and incomplete maturation of new virions. These data provide the clearest evidence to date that B1 is needed for multiple critical junctures in the poxviral life cycle in a manner that is both dependent on and independent of BAF.

Brincidofovir (CMX001) inhibits BK polyomavirus replication in primary human urothelial cells

BK polyomavirus (BKPyV)-associated hemorrhagic cystitis (PyVHC) complicates 5 to 15% of allogeneic hematopoietic stem cell transplantations. Targeted antivirals are still unavailable. Brincidofovir (BCV; previously CMX001) has shown inhibitory activity against diverse viruses, including BKPyV in a primary human renal tubule cell culture model of polyomavirus-associated nephropathy. We investigated the effects of BCV in BKPyV-infected and uninfected primary human urothelial cells (HUCs), the target cells of BKPyV in PyVHC. The BCV concentrations causing 50 and 90% reductions (EC50 and EC90) in the number of intracellular BKPyV genome equivalents per cell (icBKPyV) were 0.27 μM and 0.59 μM, respectively. At 0.63 μM, BCV reduced viral late gene expression by 90% and halted progeny release. Preinfection treatment for only 24 h reduced icBKPyV similarly to treatment from 2 to 72 h postinfection, while combined pre- and postinfection treatment suppressed icBKPyV completely. After investigating BCV's effects on HUC viability, mean selectivity indices at 50 and 90% inhibition (SI50 and SI90) calculated for cellular DNA replication were 2.7 and 2.9, respectively, those for mitochondrial activity were 8.9 and 10.4, those for total ATP were 8.6 and 8.2, and those for membrane integrity were 25.9 and 16.7. The antiviral and cytostatic effects, but less so the cytotoxic effects, were inversely related to cell density. The cytotoxic effects at concentrations of ≥10 μM were rapid and likely related to BCV's lipid moiety. After carefully defining the antiviral, cytostatic, and cytotoxic properties of BCV in HUCs, we conclude that a preemptive or prophylactic approach in PyVHC is likely to give the best results.

Increased susceptibility of Cantagalo virus to the antiviral effect of ST-246®

Cantagalo virus (CTGV) is the etiologic agent of a pustular disease in dairy cows and dairy workers in Brazil with important economical and occupational impacts. Nevertheless, no antiviral therapy is currently available. ST-246 is a potent inhibitor of orthopoxvirus egress from cells and has proved its efficacy in cell culture and in animal models. In this work, we evaluated the effect of ST-246 on CTGV replication. Plaque reduction assays indicated that CTGV is 6-38 times more susceptible to the drug than VACV-WR and cowpox virus, respectively, with an EC50 of 0.0086μM and a selective index of >11,600. The analysis of β-gal activity expressed by recombinant viruses in the presence of ST-246 confirmed these results. In addition, ST-246 had a greater effect on the reduction of CTGV spread in comet tail assays and on the production of extracellular virus relative to VACV-WR. Infection of mice with CTGV by tail scarification generated primary lesions at the site of scarification that appeared less severe than those induced by VACV-WR. Animals infected with CTGV and treated with ST-246 at 100mg/kg for 5days did not develop primary lesions and virus yields were inhibited by nearly 98%. In contrast, primary lesions induced by VACV-WR were not affected by ST-246. The analysis of F13 (p37) protein from CTGV revealed a unique substitution in residue 217 (D217N) not found in other orthopoxviruses. Construction of recombinant VACV-WR containing the D217N polymorphism did not lead to an increase in the susceptibility to ST-246. Therefore, it is still unknown why CTGV is more susceptible to the antiviral effects of ST-246 compared to VACV-WR. Nonetheless, our data demonstrates that ST-246 is a potent inhibitor of CTGV replication that should be further evaluated as a promising anti-CTGV therapy.

The antiviral activity and mechanism of action of (S)-[3-hydroxy-2-(phosphonomethoxy)propyl] (HPMP) nucleosides

One class of compounds that has shown promise as antiviral agents are the (S)-[3-hydroxy-2-(phosphonomethoxy)propyl] (HPMP) nucleosides, members of the broader class of acyclic nucleoside phosphonates. These HPMP nucleosides are nucleotide analogs and have been shown to be effective inhibitors of a wide range of DNA viruses. Prodrugs of these compounds, which achieve higher levels of the active metabolites within the cell, have an expanded activity spectrum that also includes RNA viruses and retroviruses. Because they are analogs of natural nucleotide substrates, HPMP nucleosides are predicted to target polymerases (DNA polymerases, RNA polymerases and reverse transcriptases), resulting in the inhibition of viral genome replication. Previous work using the replicative enzymes of different viruses including human cytomegalovirus (HCMV) and vaccinia virus DNA polymerases and human immunodeficiency virus type 1 (HIV-1) reverse transcriptase has shown that the activated forms of these compounds are substrates for viral polymerases and that incorporation of these compounds into either the primer strand or the template strand inhibits, but does not necessarily terminate, further nucleic acid synthesis. The activity of these compounds against other viruses that do not encode their own polymerases, like polyoma viruses and papilloma viruses, suggests that host cell DNA polymerases are also targeted. This complex mechanism of action and broad activity spectrum has implications for the development of resistance and host cell genome replication, and suggests these compounds may be effective against other viruses such as influenza virus, respiratory syncytial virus and Dengue virus. This class of nucleotide analogs also points to a potential avenue for the development of newer antivirals.

A small molecule screen in yeast identifies inhibitors targeting protein-protein interactions within the vaccinia virus replication complex

Genetic and biochemical data have identified at least four viral proteins essential for vaccinia virus (VACV) DNA synthesis: the DNA polymerase E9, its processivity factor (the heterodimer A20/D4) and the primase/helicase D5. These proteins are part of the VACV replication complex in which A20 is a central subunit interacting with E9, D4 and D5. We hypothesised that molecules able to modulate protein-protein interactions within the replication complex may represent a new class of compounds with anti-orthopoxvirus activities. In this study, we adapted a forward duplex yeast two-hybrid assay to screen more than 27,000 molecules in order to identify inhibitors of A20/D4 and/or A20/D5 interactions. We identified two molecules that specifically inhibited both interactions in yeast. Interestingly, we observed that these compounds displayed a similar antiviral activity to cidofovir (CDV) against VACV in cell culture. We further showed that these molecules were able to inhibit the replication of another orthopoxvirus (i.e. cowpox virus), but not the herpes simplex virus type 1 (HSV-1), an unrelated DNA virus. We also demonstrated that the antiviral activity of both compounds correlated with an inhibition of VACV DNA synthesis. Hence, these molecules may represent a starting point for the development of new anti-orthopoxvirus drugs.

Orthopoxvirus targets for the development of new antiviral agents

Investments in the development of new drugs for orthopoxvirus infections have fostered new avenues of research, provided an improved understanding of orthopoxvirus biology and yielded new therapies that are currently progressing through clinical trials. These broad-based efforts have also resulted in the identification of new inhibitors of orthopoxvirus replication that target many different stages of viral replication cycle. This review will discuss progress in the development of new anti-poxvirus drugs and the identification of new molecular targets that can be exploited for the development of new inhibitors. The prototype of the orthopoxvirus group is vaccinia virus and its replication cycle will be discussed in detail noting specific viral functions and their associated gene products that have the potential to serve as new targets for drug development. Progress that has been achieved in recent years should yield new drugs for the treatment of these infections and might also reveal new approaches for antiviral drug development with other viruses.

SP600125 inhibits Orthopoxviruses replication in a JNK1/2 -independent manner: Implication as a potential antipoxviral

The pharmacological inhibitor SP600125 [anthra(1,9-cd)pyrazol-6(2H)-one 1,9-pyrazoloanthrone] has been largely employed as a c-JUN N-terminal kinase (JNK1/2) inhibitor. In this study, we evaluated whether pretreatment with SP600125 was able to prevent Orthopoxviruses Vaccinia virus (VACV), Cowpox virus (CPXV) and modified Vaccinia virus Ankara (MVA) replication. We found that incubation with SP600125 not only blocked virus-stimulated JNK phosphorylation, but also, significantly reduced virus production. We observed 1-3 log decline in viral yield depending on the cell line infected (A31, BSC-40 or BHK-21). The reduction in viral yield correlated with a dramatic impact on virus morphogenesis progress, intracellular mature viruses (IMV) were barely detected. Despite the fact that SP600125 can act as an efficient anti-orthopoxviral compound, we also provide evidence that this antiviral effect is not specifically exerted through JNK1/2 inhibition. This conclusion is supported by the fact that viral titers measured after infections of JNK1/2 knockout cells were not altered as compared to those of wild-type cells. In contrast, a decline in viral titers was verified when the infection of KO cells was carried out in the presence of the pharmacological inhibitor. SP600125 has been the focus of recent studies that have evaluated its action on diverse viral infections including DNA viruses. Our data support the notion that SP600125 can be regarded as a potential antipoxviral compound.

Developments in antivirals against influenza, smallpox and hemorrhagic fever viruses

INTRODUCTION

the search for effective inhibitors to multiple infectious agents including influenza, smallpox and hemorrhagic fever viruses is an area of active research as many of these agents pose dramatic health and economic challenges to the human population. Many of these infectious agents are not only endemic threats in different parts of the globe, but are also considered to have the potential of being used as bioterrorism agents.

AREAS COVERED

this review focuses on inhibitors that are currently in use in the research community against specific emerging infectious agents and those that have bioterrorism potential. The paper provides information about the availability of FDA approved drugs, whenever applicable, and insights into the specific aspect of the agent life cycle that is affected by drug treatment, when known.

EXPERT OPINION

the key message that is conveyed in this review is that a combination of pathogen and host-based inhibitors may have to be used for successful control of viral replication to limit the development of drug resistance.

Solution structure of a DNA duplex containing the potent anti-poxvirus agent cidofovir

Cidofovir (1(S)-[3-hydroxy-2-(phosphonomethoxy)propyl]cytosine, CDV) is a potent inhibitor of orthopoxvirus DNA replication. Prior studies have shown that, when CDV is incorporated into a growing primer strand, it can inhibit both the 3'-to-5' exonuclease and the 5'-to-3' chain extension activities of vaccinia virus DNA polymerase. This drug can also be incorporated into DNA, creating a significant impediment to trans-lesion DNA synthesis in a manner resembling DNA damage. CDV and deoxycytidine share a common nucleobase, but CDV lacks the deoxyribose sugar. The acyclic phosphonate bears a hydroxyl moiety that is equivalent to the 3'-hydroxyl of dCMP and permits CDV incorporation into duplex DNA. To study the structural consequences of inserting CDV into DNA, we have used (1)H NMR to solve the solution structures of a dodecamer DNA duplex containing a CDV molecule at position 7 and of a control DNA duplex. The overall structures of both DNA duplexes were found to be very similar. We observed a decrease of intensity (>50%) for the imino protons neighboring the CDV (G6, T8) and the cognate base G18 and a large chemical shift change for G18. This indicates higher proton exchange rates for this region, which were confirmed using NMR-monitored melting experiments. DNA duplex melting experiments monitored by circular dichroism revealed a lower T(m) for the CDV DNA duplex (46 °C) compared to the control (58 °C) in 0.2 M salt. Our results suggest that the CDV drug is well accommodated and stable within the dodecamer DNA duplex, but the stability of the complex is less than that of the control, suggesting increased dynamics around the CDV.

Cidofovir Activity against Poxvirus Infections

Cidofovir [(S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine, HPMPC] is an acyclic nucleoside analog approved since 1996 for clinical use in the treatment of cytomegalovirus (CMV) retinitis in AIDS patients. Cidofovir (CDV) has broad-spectrum activity against DNA viruses, including herpes-, adeno-, polyoma-, papilloma- and poxviruses. Among poxviruses, cidofovir has shown in vitro activity against orthopox [vaccinia, variola (smallpox), cowpox, monkeypox, camelpox, ectromelia], molluscipox [molluscum contagiosum] and parapox [orf] viruses. The anti-poxvirus activity of cidofovir in vivo has been shown in different models of infection when the compound was administered either intraperitoneal, intranasal (aerosolized) or topically. In humans, cidofovir has been successfully used for the treatment of recalcitrant molluscum contagiosum virus and orf virus in immunocompromised patients. CDV remains a reference compound against poxviruses and holds potential for the therapy and short-term prophylaxis of not only orthopox- but also parapox- and molluscipoxvirus infections.