Orally available nucleoside analog UMM-766 provides protection in a murine model of orthopox disease

Although smallpox has been eradicated, other orthopoxviruses continue to be a public health concern as exemplified by the ongoing Mpox (formerly monkeypox) global outbreak. While medical countermeasures (MCMs) previously approved by the Food and Drug Administration for the treatment of smallpox have been adopted for Mpox, previously described vulnerabilities coupled with the questionable benefit of at least one of the therapeutics during the 2022 Mpox outbreak reinforce the need for identifying and developing other MCMs against orthopoxviruses. Here, we screened a panel of Merck proprietary small molecules and identified a novel nucleoside inhibitor with potent broad-spectrum antiviral activity against multiple orthopoxviruses. Efficacy testing of a 7-day dosing regimen of the orally administered nucleoside in a murine model of severe orthopoxvirus infection yielded a dose-dependent increase in survival. Treated animals had greatly reduced lesions in the lung and nasal cavity, particularly in the 10 µg/mL dosing group. Viral levels were also markedly lower in the UMM-766-treated animals. This work demonstrates that this nucleoside analog has anti-orthopoxvirus efficacy and can protect against severe disease in a murine orthopox model.IMPORTANCEThe recent monkeypox virus pandemic demonstrates that members of the orthopoxvirus, which also includes variola virus, which causes smallpox, remain a public health issue. While currently FDA-approved treatment options exist, risks that resistant strains of orthopoxviruses may arise are a great concern. Thus, continued exploration of anti-poxvirus treatments is warranted. Here, we developed a template for a high-throughput screening assay to identify anti-poxvirus small-molecule drugs. By screening available drug libraries, we identified a compound that inhibited orthopoxvirus replication in cell culture. We then showed that this drug can protect animals against severe disease. Our findings here support the use of existing drug libraries to identify orthopoxvirus-targeting drugs that may serve as human-safe products to thwart future outbreaks.

Reassessment of Evidence about Coinfection of Chickenpox and Monkeypox (Mpox) in African Children

In west and central Africa, monkeypox occurs mainly in older children, adolescents and young adults. In two large epidemiology studies of monkeypox outbreaks, the investigators observed a sizable number of coinfections of chickenpox (varicella) and monkeypox. Based on a review of the literature, we propose that chickenpox (human herpesvirus-3 infection) is a risk factor for acquisition of monkeypox infection. Our hypothesis states that the chickenpox skin lesion provides an entry site for the monkeypox virus, which is harbored on a fomite in the environment of the patient. The fact that monkeypox can enter via a scratch or abrasion is a known mechanism of spread for three other poxviruses, including mousepox (ectromelia), orf and molluscum contagiosum. There are many similarities in pathogenesis between certain poxviruses and chickenpox, including a viremia with a cellular stress response leading to high levels of the IL-6 cytokine. One very revealing observation in the two epidemiology studies was that the number of pox as well as the severity of disease in children with chickenpox and monkeypox coinfection was not greater than found in children with monkeypox alone. Based on the above observations, we conclude that, when chickenpox precedes monkeypox, priming of the immune system by the earlier chickenpox infection moderates the severity of the secondary infection with monkeypox. This conclusion also has important public health implications about chickenpox surveillance.

The Disease Ecology, Epidemiology, Clinical Manifestations, Management, Prevention, and Control of Increasing Human Infections with Animal Orthopoxviruses

Zoonotic orthopoxvirus outbreaks have occurred repeatedly worldwide, including monkeypox in Africa and the United States, cowpox in Europe, camelpox in the Middle East and India, buffalopox in India, vaccinia in South America, and novel emerging orthopoxvirus infections in the United States, Europe, Asia, and South America. Waning smallpox immunity may increase the potential for animal-to-human transmission followed by further community transmission person-to-person (as demonstrated by monkeypox and buffalopox outbreaks) and by contact with fomites (as demonstrated by camelpox, cowpox, and, possibly, Alaskapox). The objectives of this review are to describe the disease ecology, epidemiology, clinical manifestations, prevention, and control of human infections with animal orthopoxviruses and to discuss the association with diminished population herd immunity formerly induced by vaccinia vaccination against smallpox. Internet search engines were queried with key words, and case reports, case series, seroprevalence studies, and epidemiologic investigations were found for review.

Acute Late-Stage Myocarditis in the Crab-Eating Macaque Model of Hemorrhagic Smallpox

Hemorrhagic smallpox, caused by variola virus (VARV), was a rare but nearly 100% lethal human disease manifestation. Hemorrhagic smallpox is frequently characterized by secondary bacterial infection, coagulopathy, and myocardial and subendocardial hemorrhages. Previous experiments have demonstrated that intravenous (IV) cowpox virus (CPXV) exposure of macaques mimics human hemorrhagic smallpox. The goal of this experiment was to further understand the onset, nature, and severity of cardiac pathology and how it may contribute to disease. The findings support an acute late-stage myocarditis with lymphohistiocytic infiltrates in the CPXV model of hemorrhagic smallpox.

Modelling the impact of a smallpox attack in India and influence of disease control measures

OBJECTIVES

To estimate the impact of a smallpox attack in Mumbai, India, examine the impact of case isolation and ring vaccination for epidemic containment and test the health system capacity under different scenarios with available interventions.

SETTING

The research is based on Mumbai, India population.

INTERVENTIONS

We tested 50%, 70%, 90% of case isolation and contacts traced and vaccinated (ring vaccination) in the susceptible, exposed, infected, recovered model and varied the start of intervention between 20, 30 and 40 days after the initial attack.

PRIMARY AND SECONDARY OUTCOME MEASURES

We estimated and incorporated in the model the effect of past vaccination protection, age-specific immunosuppression and contact rates and Mumbai population age structure in modelling disease morbidity and transmission.

RESULTS

The estimated duration of an outbreak ranged from 127 days to 8 years under different scenarios, and the number of vaccine doses needed for ring vaccination ranged from 16 813 to 8 722 400 in the best-case and worst-case scenarios, respectively. In the worst-case scenario, the available hospital beds in Mumbai would be exceeded. The impact of a smallpox epidemic may be severe in Mumbai, especially compared with high-income settings, but can be reduced with early diagnosis and rapid response, high rates of case finding and isolation and ring vaccination.

CONCLUSIONS

This study tells us that if smallpox re-emergence occurs, it may have significant health and economic impact, the extent of which will depend on the availability and delivery of interventions such as a vaccine or antiviral agent, and the capacity of case isolation and treatment. Further research on health systems requirements and capacity across the diverse states and territories of India could improve the preparedness and management strategies in the event of re-emergent smallpox or other serious emerging infections.

An overview of tecovirimat for smallpox treatment and expanded anti-orthopoxvirus applications

Introduction

Tecovirimat (TPOXX®; ST-246) was approved for the treatment of symptomatic smallpox by the USFDA in July of 2018 and has been stockpiled by the US government for use in a smallpox outbreak. While there has not been a reported case of smallpox since 1978 it is still considered a serious bioterrorism threat.

Areas covered

A brief history of smallpox from its proposed origins as a human disease through its eradication in the late 20th century is presented. The current smallpox threat and the current public health response plans are described. The discovery, and development of tecovirimat through NDA submission and subsequent approval for treatment of smallpox are discussed. Google Scholar and PubMed were searched over all available dates for relevant publications.

Expert opinion

Approval of tecovirimat to treat smallpox represents an important milestone in biosecurity preparedness. Incorporating tecovirimat into the CDC smallpox response plan, development of pediatric liquid and intravenous formulations, and approval for post-exposure prophylaxis would provide additional health security benefit.

Tecovirimat shows broad efficacy against orthopoxviruses in vitro and in vivo and could be developed for use against emerging orthopoxvirus diseases such as monkeypox, vaccination-associated adverse events, and side effects of vaccinia oncolytic virus therapy.

Biological agents of bioterrorism - preparedness is vital

Bioterrorist threats and attacks are still an issue of concern in the world. Biological agents are divided into three categories. The highest priority agents classified in category A pose a massive risk to public health and national security. The society should be prepared for this risk. Health professionals in the Czech Republic should be aware of the diseases caused by category A agents, which are not common in the country. In this context, the project of the Ministry of the Interior of the Czech Republic “Decontamination of the injured persons” has been implemented at the Faculty of Health Sciences, Palacký University, Olomouc. The article provides an overview of selected serious infectious risks and information on the project the aim of which is to create certified methodical procedures and guidelines on situations related to bioterrorism.

The Virology of Taterapox Virus In Vitro

Taterapox virus (TATV) is phylogenetically the closest related virus to variola—the etiological agent of smallpox. Despite the similarity, few studies have evaluated the virus. In vivo, TATV can infect several animals but produces an inapparent infection in wild-type mice; however, TATV does cause morbidity and mortality in some immunocompromised strains. We employed in vitro techniques to compare TATV to ectromelia (ECTV) and vaccinia (VACV) viruses. Both ECTV and TATV replicate efficiently in primate cell lines but TATV replicates poorly in murine cells lines. Furthermore, TATV induces cytopathic effects, but to a lesser extent than ECTV, and changes cytoskeletal networks differently than both ECTV and VACV. Bioinformatic studies revealed differences in several immunomodulator open reading frames that could contribute to the reduced virulence of TATV, which were supported by in vitro cytokine assays.

Evaluation of Taterapox Virus in Small Animals

Taterapox virus (TATV), which was isolated from an African gerbil (Tatera kempi) in 1975, is the most closely related virus to variola; however, only the original report has examined its virology. We have evaluated the tropism of TATV in vivo in small animals. We found that TATV does not infect Graphiurus kelleni, a species of African dormouse, but does induce seroconversion in the Mongolian gerbil (Meriones unguiculatus) and in mice; however, in wild-type mice and gerbils, the virus produces an unapparent infection. Following intranasal and footpad inoculations with 1 × 10⁶ plaque forming units (PFU) of TATV, immunocompromised stat1^−/− mice showed signs of disease but did not die; however, SCID mice were susceptible to intranasal and footpad infections with 100% mortality observed by Day 35 and Day 54, respectively. We show that death is unlikely to be a result of the virus mutating to have increased virulence and that SCID mice are capable of transmitting TATV to C57BL/6 and C57BL/6 stat1^−/− animals; however, transmission did not occur from TATV inoculated wild-type or stat1^−/− mice. Comparisons with ectromelia (the etiological agent of mousepox) suggest that TATV behaves differently both at the site of inoculation and in the immune response that it triggers.

Smallpox Vaccination of Laboratory Workers at US Variola Testing Sites

To evaluate the need to revaccinate laboratory workers against smallpox, we assessed regular revaccination at the US Laboratory Response Network’s variola testing sites by examining barriers to revaccination and the potential for persistence of immunity. Our data do not provide evidence to suggest prolonging the recommended interval for revaccination.

Poxvirus viability and signatures in historical relics

Although it has been >30 years since the eradication of smallpox, the unearthing of well-preserved tissue material in which the virus may reside has called into question the viability of variola virus decades or centuries after its original occurrence. Experimental data to address the long-term stability and viability of the virus are limited. There are several instances of well-preserved corpses and tissues that have been examined for poxvirus viability and viral DNA. These historical specimens cause concern for potential exposures, and each situation should be approached cautiously and independently with the available information. Nevertheless, these specimens provide information on the history of a major disease and vaccination against it.

Poxvirus viability and signatures in historical relics

Although it has been >30 years since the eradication of smallpox, the unearthing of well-preserved tissue material in which the virus may reside has called into question the viability of variola virus decades or centuries after its original occurrence. Experimental data to address the long-term stability and viability of the virus are limited. There are several instances of well-preserved corpses and tissues that have been examined for poxvirus viability and viral DNA. These historical specimens cause concern for potential exposures, and each situation should be approached cautiously and independently with the available information. Nevertheless, these specimens provide information on the history of a major disease and vaccination against it.

Comparative pathology of smallpox and monkeypox in man and macaques

In the three decades since the eradication of smallpox and cessation of routine vaccination, the collective memory of the devastating epidemics caused by this orthopoxvirus has waned, and the human population has become increasingly susceptible to a disease that remains high on the list of possible bioterrorism agents. Research using surrogate orthopoxviruses in their natural hosts, as well as limited variola virus research in animal models, continues worldwide; however, interpretation of findings is often limited by our relative lack of knowledge about the naturally occurring disease. For modern comparative pathologists, many of whom have no first-hand knowledge of naturally occurring smallpox, this work provides a contemporary review of this historical disease, as well as discussion of how it compares with human monkeypox and the corresponding diseases in macaques.

Whither monkeypox vaccination

Monkeypox (MPX) is a virulent orthopoxvirus that is endemic in some regions of Central Africa. MPX incidence has been rising since the cessation of routine smallpox immunization. While it causes significant disease, there is limited person-to-person spread, the incidence is still relatively low, and cases are generally restricted to remote areas that are difficult to access. Therefore, initiating vaccine trials or implementing vaccination programs would be challenging. This paper considers the factors that may influence future decisions on whether MPX vaccination should be pursued.

Development of CMX001 for the Treatment of Poxvirus Infections

CMX001 (phosphonic acid, [[(S)-2-(4-amino-2-oxo-1(2H)-pyrimidinyl)-1-(hydroxymethyl)ethoxy]methyl]mono[3-(hexadecyloxy)propyl] ester) is a lipid conjugate of the acyclic nucleotide phosphonate, cidofovir (CDV). CMX001 is currently in Phase II clinical trials for the prophylaxis of human cytomegalovirus infection and under development using the Animal Rule for smallpox infection. It has proven effective in reduction of morbidity and mortality in animal models of human smallpox, even after the onset of lesions and other clinical signs of disease. CMX001 and CDV are active against all five families of double-stranded DNA (dsDNA) viruses that cause human morbidity and mortality, including orthopoxviruses such as variola virus, the cause of human smallpox. However, the clinical utility of CDV is limited by the requirement for intravenous dosing and a high incidence of acute kidney toxicity. The risk of nephrotoxicity necessitates pre-hydration and probenecid administration in a health care facility, further complicating high volume CDV use in an emergency situation. Compared with CDV, CMX001 has a number of advantages for treatment of smallpox in an emergency including greater potency in vitro against all dsDNA viruses that cause human disease, a high genetic barrier to resistance, convenient oral administration as a tablet or liquid, and no evidence to date of nephrotoxicity in either animals or humans. The apparent lack of nephrotoxicity observed with CMX001 in vivo is because it is not a substrate for the human organic anion transporters that actively secrete CDV into kidney cells. The ability to test the safety and efficacy of CMX001 in patients with life-threatening dsDNA virus infections which share many basic traits with variola is a major advantage in the development of this antiviral for a smallpox indication.

Ectromelia virus infections of mice as a model to support the licensure of anti-orthopoxvirus therapeutics

The absence of herd immunity to orthopoxviruses and the concern that variola or monkeypox viruses could be used for bioterroristic activities has stimulated the development of therapeutics and safer prophylactics. One major limitation in this process is the lack of accessible human orthopoxvirus infections for clinical efficacy trials; however, drug licensure can be based on orthopoxvirus animal challenge models as described in the "Animal Efficacy Rule". One such challenge model uses ectromelia virus, an orthopoxvirus, whose natural host is the mouse and is the etiological agent of mousepox. The genetic similarity of ectromelia virus to variola and monkeypox viruses, the common features of the resulting disease, and the convenience of the mouse as a laboratory animal underscores its utility in the study of orthopoxvirus pathogenesis and in the development of therapeutics and prophylactics. In this review we outline how mousepox has been used as a model for smallpox. We also discuss mousepox in the context of mouse strain, route of infection, infectious dose, disease progression, and recovery from infection.

ACAM2000: the new smallpox vaccine for United States Strategic National Stockpile

Smallpox was eradicated more than 30 years ago, but heightened concerns over bioterrorism have brought smallpox and smallpox vaccination back to the forefront. The previously licensed smallpox vaccine in the United States, Dryvax^® (Wyeth Laboratories, Inc.), was highly effective, but the supply was insufficient to vaccinate the entire current US population. Additionally, Dryvax^® had a questionable safety profile since it consisted of a pool of vaccinia virus strains with varying degrees of virulence, and was grown on the skin of calves, an outdated technique that poses an unnecessary risk of contamination. The US government has therefore recently supported development of an improved live vaccinia virus smallpox vaccine. This initiative has resulted in the development of ACAM2000™ (Acambis, Inc.™), a single plaque-purified vaccinia virus derivative of Dryvax^®, aseptically propagated in cell culture. Preclinical and clinical trials reported in 2008 demonstrated that ACAM2000™ has comparable immunogenicity to that of Dryvax^®, and causes a similar frequency of adverse events. Furthermore, like Dryvax^®, ACAM2000™ vaccination has been shown by careful cardiac screening to result in an unexpectedly high rate of myocarditis and pericarditis. ACAM2000™ received US Food and Drug Administration (FDA) approval in August 2007, and replaced Dryvax^® for all smallpox vaccinations in February 2008. Currently, over 200 million doses of ACAM2000™ have been produced for the US Strategic National Stockpile. This review of ACAM2000™ addresses the production, characterization, clinical trials, and adverse events associated with this new smallpox vaccine.

Experimental infection of an African dormouse (Graphiurus kelleni) with monkeypox virus

Suitable animal models are needed to study monkeypox virus (MPXV) as human monkeypox clinically resembles smallpox and MPXV is a zoonotic and potential bioterroristic agent. We have demonstrated that a species of African dormice, Graphiurus kelleni, is susceptible to a lethal infection of MPXV and that MPXV replicated in multiple organs of this species. Following intranasal administration, MPXV replicated locally in the nasal mucosa causing necrosis and hemorrhage with subsequent systemic spread to lymph nodes, spleen, liver, and other tissues where it caused severe necrosis and/or hemorrhage leading to death. The dormouse model was validated for testing prophylactic (Dryvax vaccine) and therapeutic (cidofovir) test articles against intranasal challenges with MPXV.

Potential antiviral therapeutics for smallpox, monkeypox and other orthopoxvirus infections

We assessed the activities of 24 different antiviral compounds against smallpox (two strains of variola major and one of variola minor), monkeypox, vaccinia and cowpox viruses by a neutral red uptake assay. To establish assay parameters, we examined viral replication and its inhibition at various times postinfection and at several multiplicities of infection. Drugs were selected to target a range of functions involved in viral replication. Eight compounds (cidofovir, cyclic HPMPC (cHPMPC), HPMPA, ribavirin, tiazofurin, carbocyclic 3-deazaadenosine, 3-deazaneplanocin A and DFBA (1-(2,4-difluorobenzyloxy)adenosine perchlorate)-a derivative of adenosine N1-oxide) inhibited the replication of all three variola strains and the other orthopoxviruses at drug concentrations within a pharmacologically achievable range. Two others (methisazone and bis-POM-PMEA) showed a lesser degree of antiviral effect, while the remainder were inactive. To examine possible naturally occurring drug resistance among a large number of variola isolates obtained from different geographical regions and at different times, we examined the sensitivity of 35 different strains of variola as well as other orthopoxviruses to a subset of three of the most active compounds: cidofovir, cHPMPC, and ribavirin. Preliminary data indicate that nearly all isolates appear to have similar drug sensitivities. These findings are currently being verified and expanded.

Passive antibody administration (immediate immunity) as a specific defense against biological weapons

The potential threat of biological warfare with a specific agent is proportional to the susceptibility of the population to that agent. Preventing disease after exposure to a biological agent is partially a function of the immunity of the exposed individual. The only available countermeasure that can provide immediate immunity against a biological agent is passive antibody. Unlike vaccines, which require time to induce protective immunity and depend on the host's ability to mount an immune response, passive antibody can theoretically confer protection regardless of the immune status of the host. Passive antibody therapy has substantial advantages over antimicrobial agents and other measures for postexposure prophylaxis, including low toxicity and high specific activity. Specific antibodies are active against the major agents of bioterrorism, including anthrax, smallpox, botulinum toxin, tularemia, and plague. This article proposes a biological defense initiative based on developing, producing, and stockpiling specific antibody reagents that can be used to protect the population against biological warfare threats.

Cidofovir in the treatment of poxvirus infections

Cidofovir [(S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine, HPMPC] has since 1996 been licensed for clinical use in the treatment of cytomegalovirus (CMV) retinitis in AIDS patients. Cidofovir has broad-spectrum activity against virtually all DNA viruses, including herpes-, adeno-, polyoma-, papilloma- and poxviruses. Among the poxviruses, vaccinia, variola (smallpox), cowpox, monkeypox, camelpox, molluscum contagiosum and orf have proven sensitive to the inhibitory effects of cidofovir. In vivo, cidofovir has shown high efficacy, even after administration of a single systemic (intraperitoneal) or intranasal (aerosolized) dose, in protecting mice from a lethal respiratory infection with either vaccinia or cowpox. Cidofovir has also demonstrated high effectiveness in the treatment of vaccinia virus infection in severe combined immune deficiency mice. In humans, cidofovir has been used successfully in the treatment, by both the topical and intravenous route, of recalcitrant molluscum contagiosum and orf in immunocompromised patients. Taken together, these data indicate that cidofovir should be effective in the therapy and short-term prophylaxis of smallpox and related poxvirus infections in humans, as well as the treatment of the complications of vaccinia that may arise in immunocompromised patients inadvertently inoculated with the smallpox vaccine (vaccinia).

The detection of subtle differences between different orthopoxvirus genomes by heteroduplex analysis

Corresponding DNA fragments from variola (Harvey) and monkeypox (Denmark) viruses which had been cloned into different plasmid vectors were subjected to heteroduplex analysis. Characteristic deletion loops corresponding to differences between the cloning vectors served as internal markers to identify and to orientate the heteroduplexed molecules. Partial denaturation of the resulting heteroduplexes was used as a primary screen to locate regions of heterogeneity between the poxvirus inserts. The denaturation threshold for homoduplexes was consistently higher than that for heteroduplexes. However, significant sequence divergence between corresponding fragments was indicated by larger than usual differences in thresholds between corresponding homo- and heteroduplexes. Denaturation bubbles of 0.1-0.5 kb were detected and hence small regions of heterogeneity between the genomes (180 kb) of variola and monkeypox viruses were localised. This procedure has a general application in comparative studies on large, complex but closely related DNA molecules.