Antiviral Therapy 2015; 20:121–130 (doi: 10.3851/IMP2939)

Review Antivirals against : a critical review from a public-health perspective

Kimberley SM Benschop1*, Harrie GAM van der Avoort1, Erwin Duizer1, Marion PG Koopmans1,2

1Centre for Infectious Diseases Research, Diagnostics and Screening, National Institute for Public Health and the Environment, Bilthoven, the Netherlands 2Department of Viroscience, Erasmus Medical Centre, Rotterdam, the Netherlands

*Corresponding author e-mail: [email protected]

The enteroviruses (EVs) of the Picornaviridae family are potential emergence of drug-resistant strains and their the most common viral pathogens known. Most EV infec- impact on EV transmission and endemic circulation. We tions are mild and self-limiting but manifestations can include non- antivirals that inhibit EV rep- be severe in children and immunodeficient individuals. lication, for example, , a treatment for infection Antiviral development is actively pursued to benefit these with HCV, and , a treatment for A. high-risk patients and, given the alarming problem of They may have spurred resistance emergence in HCV or antimicrobial drug resistance, resistance influenza A patients who are unknowingly coinfected is a public-health concern. Picornavirus antivirals can be with EV. The public-health challenge is always to find a used off-label or as part of outbreak control measures. balance between individual benefit and the long-term They may be used in the final stages of eradi- health of the larger population. cation and to mitigate EV-A71 outbreaks. We review the

Introduction

Enteroviruses (EVs) are among the most common of the PV eradication process and as part of outbreak circulating known. They are non-enveloped control measures when PV is eradicated [6]. Further- RNA viruses belonging to the large genus more, severe disease outbreaks with EV-A71 [7–9], may within the Picornaviridae family, which also includes be mitigated with EV antivirals, which are thus being human (HRVs). In addition to poliovirus explored in Asia along with EV-A71 vaccine develop- (PV1–3, of the EV-C species), there are over 200 types ment [10,11]. of human non-polio enterovirus (NPEV) classified However, their effect on EV transmission and into four genetic species: EV-A to D (Table 1). Most endemic circulation are unknown, as is the potential EV infections remain asymptomatic or cause only mild for emergence and circulation of EV-resistant strains in gastrointestinal or respiratory disease in immunologi- the treatment-naive high-risk population. That poten- cally competent individuals. Severe disease manifes- tial is a major public-health concern in light of the anti- tations such as paralysis/poliomyelitis, meningitis, microbial resistance (AMR) caused by uncontrolled hand-foot-and-mouth disease (HFMD) and cardiac large-scale use of antimicrobial compounds in animal disease have been reported in immunocompetent per- production and the dissemination of these compounds sons but occur predominantly among infants, young into the environment [12–14]. The sheer magnitude of children and individuals with a humoral immune defi- these AMR strains circulating in the world population ciency. The development of antivirals to treat these has crippled antibiotic treatment options on a global high-risk patients has proceeded at a very rapid pace level. AMR has become a top priority for public-health over the past three decades [1–5]. institutes worldwide, with a strong focus on surveil- In addition to reducing pathology or disease duration lance of strains and monitoring drug usage. Critical for infected patients, antivirals can reduce excre- public-health scrutiny is required for EV antivirals as tion and shedding in the environment. These drugs are well, given their potential to be used on a large-scale being actively explored for use during the final stages basis, as an off-label drug, as part of outbreak control

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Table 1. Classification of human enteroviruses into 4 genetic species EV-A EV-B EV-C EV-D

Polioviruses – – 1–3 – Coxsackie A viruses 2–8, 10, 12, 14, 16 9 1, 11, 13, 15, 17–22, 24 – Coxsackie B viruses – 1–6 – – Echo viruses – 1–9, 11–21, 24–27, 29–34 – – EVs 71, 76, 89–92, 114, 119 69, 73–75, 77–88, 93, 97–98, 95–96, 99, 102, 104–105, 68, 70, 94, 111 101, 106–107 109, 113, 116–118

Adapted from van der Sanden et al. [98] with kind permission of Springer Science+Business Media. Numbers represent types. EV, enterovirus.

measures or as an over-the-counter (OTC) medication. compound acts by preventing viral attachment through There is also the risk of dissemination of antiviral drugs deforming the receptor binding region [26]. Among the in the environment [15], with consequences unknown. capsid inhibitors developed for EVs, pleconaril, disoxaril, Of particular concern is that non-picornavirus- pirodavir, V-073 and BTA-798 have advanced into clini- directed drugs on the market, such as ribavirin (treatment cal trials (Table 2). of HCV infections [16]) and amantadine (treatment of Pleconaril was the first compound to exhibit a Parkinson’s disease [17] and previously broad-spectrum activity against EV and HRV. The infections [18]), have been shown to inhibit replication drug was clinically evaluated for treatment of the com- of PV and several NPEV types such as EV-A71, echovi- mon cold caused by HRV [27] and for compassion- rus (E)-5 and E-18 [19–24]. Thus the current and past ate use in patients with a potentially life-threatening use of these drugs in their target populations could be EV infection [28–30]. Clinical development was halted leading to circulation of drug-resistant EVs if targeted in 2002 due to inconsistent results in treating EV individuals are unknowingly coinfected with EV. infection and minimal clinical efficacy in treating the This review addresses the potential public-health HRV-associated . Furthermore, plecon- implications of the use of antivirals against PV, NPEVs aril showed adverse effects [31]. The drug inhibits the and the related HRVs that have advanced into clinical cytochrome P-450 3A (CYP3A) enzyme activity [32], trials (Table 2). Furthermore, it explores the implica- leading to menstrual irregularities, and reduces the tions of the past and present use of licensed non-picor- efficacy of some hormonal contraceptives and anti- navirus-directed drugs that are also active against EVs. HIV drugs [31]. However, pleconaril was relicensed to Schering–Plough in 2003 and is being re-evaluated for Search strategies and selection criteria EV neonatal disease (clinicaltrial.gov NTC0031512) We searched Pubmed, with no date restriction, with the and for HRV-associated colds in high-risk patients terms: ‘antiviral(s)’; ‘antiviral(s)’ AND (‘enterovirus’ OR (clinicaltrial.gov NTC00394914; Table 2). ‘poliovirus’ OR ‘picornavirus’ OR ‘’ OR ‘influ- The capsid inhibitor V-073 (Viro-Defense, Atlanta, enza virus’ OR ‘HIV’); ‘antimicrobial/antibiotic’ AND GA, USA) is a potent in vitro inhibitor of PV replica- ‘resistance’; ‘antiviral(s)’ AND ‘resistance’; (‘antiviral(s)’ tion [33] has a good oral bioavailability and was well AND ‘resistance’) AND (‘transmission’ OR ‘circulation’ tolerated in mice [34]. OR ‘reatment-naïve’ or ‘public health’); ‘antiviral(s)’ It has therefore advanced into clinical trials aimed AND ‘recombination’; ‘recombination’ AND (‘enterovi- at PV eradication [6] (Table 2). The three other capsid rus’ OR ‘poliovirus’ OR ‘picornavirus’ OR ‘rhinovirus’). inhibitors, disoxaril, pirodavir and BTA-798 (vapen- For topics on drug resistance in the non-picornavirus davir; Biota Holdings, Notting Hill, Australia; Table 2) fields, we focused on the most recent comprehensive have until now been evaluated only for the treatment of reviews, but cite older reviews when appropriate. common cold caused by HRV. Disoxaril and pirodavir have failed to advance in clinical trials due to limited Antiviral compounds against enteroviruses clinical efficacy [35,36]. BTA-798 has completed the Phase II proof of concept in humans and was described Capsid inhibitors by Biota as achieving a significant reduction in severity of Capsid inhibitors are small compounds that can integrate community-acquired common cold in high-risk patients within the hydrophobic pocket of the capsid composed with . by the outer capsid proteins, in particular VP3 and VP1 (Figure 1) [1]. Their integration leads to immobilization Protease inhibitors and compression of the capsid, whereby the virus is unable The protease inhibitors designed for are to release its RNA within the cell [25]. In some cases, the directed against the viral proteases 2A and 3C (Figure 1).

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Table 2. Clinically evaluated compounds active against enteroviruses

Drug Target Mode of action Treatment status

Pleconaril Capsid Rigidification of pocket preventing RNA release Compassionate use for Compassionate use [30] life-threatening infections HRV common cold Phase III halted [31] HRV-induced exacerbation Phase II completed of asthma/COPD in high-risk NCT00394914 patients Neonatal sepsis Phase II ongoing NCT00031512 BTA-798 Capsid Rigidification of pocket preventing RNA release HRV-induced exacerbation Phase IIb completed of asthma/COPD in high-risk patients V-073 Capsid Rigidification of pocket preventing RNA release Polio eradication Phase IIb POC ongoing Disoxaril Capsid Rigidification of pocket preventing RNA release E9-induced paralysis Mice studies [99] Pirodavir Capsid Rigidification of pocket preventing RNA release HRV common cold Phase II halted [35] Enviroxime PI4KIIIb Disruption of PI4P conversion required for 3Dpol HRV common cold Phase II halted [39,43] recruitment to replication complex Rupintrivir 3C-protease Induction of conformational change involving HRV common cold Phase II halted [37] key amino acids required for activity AG7404 3C-protease Induction of conformational change involving HRV common cold Phase I halted [38] key amino acids required for activity Ribavirin 3Dpol Incorporation into RNA chain leads to erroneous HCV in combination with FDA approved incorporation of paired nucleosides pegylated Amantadine IRES Inhibition of IRES-dependent translation (HAV) Influenza A (no longer FDA approved recommended) and Parkinson’s disease Fluoxetine 2C Inhibition of viral replication, mechanism not Depression FDA approved yet known

COPD, chronic obstructive pulmonary disease; FDA, Food and Drug Administration (USA); HRV, human rhinovirus; IRES, internal ribosome entry site; PI4KIIIb, phosphatidylinositol-4-kinase IIIb; POC, proof of concept.

Of these, only AG7088/rupintrivir and AG7404, both Antivirals to aid in PV eradication directed against 3C, have been clinically evaluated (Table 2). However, trials evaluating their ability to In 2006, the Global Polio Eradication Initiative pro- reduce the severity of HRV infection were halted on posed that antivirals be used to aid PV eradication finding insufficient therapeutic effect [37–39]. Rupin- by supplementing the anti-PV infection armament trivir, also active against EV-A71 [40], has a good of inactivated and live (oral) PV vaccines (IPV and safety profile and is a candidate for future treatment OPV) [6]. While OPV is generally safe and very effec- of EV-A71-associated HFMD [41]. Compounds tar- tive in areas with wild-type PV transmission, its use geting the 2A protease are still in development and is not without risks. As the virus can still replicate in have not yet been clinically evaluated (reviewed by the gut, it is likely to mutate towards a variant called Chen and Shih [3]). vaccine-derived PV (VDPV) that has a neurovirulent phenotype [44]. These variants can be transmitted Replication inhibitors from person to person as effectively as wild PVs and Replication inhibitors that have been developed for can be shed in the environment. Furthermore, immu- EVs target either the three viral non-structural proteins nocompromised patients shed these variants for a 3A, 2C and 3Dpol (the RNA-dependent polymerase prolonged period [44,45]. In a population with low [RdRp]), or host proteins of the replication machin- vaccine coverage [46] these VDPVs can cause out- ery (Figure 1). Enviroxime, which targets host protein breaks. In a majority of these outbreaks, the circulat- phosphatidylinositol-4-kinase II b (PI4KIIIb) [42], is ing VDPVs (cVDPVs) were identified as recombinants the only enteroviral replication inhibitor to be clini- containing the non-structural region of NPEVs of the cally evaluated. The trial to evaluate treatment of HRV- EV-C species (reviewed by Combelas et al. [47]). The associated common cold was halted at Phase II due to ability of EVs to recombine is of concern when con- findings of toxicity (Table 2) [43]. sidering combination drug therapy (discussed below).

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Figure 1. Antiviral targets within the enteroviral life cycle

Capsid inhibitors

Translation inhibitor (for example, amantadine)

VPg protein Protease inhibitors Genomic RNA IRES (+) sense P1 Replication P2 inhibitors P3 viral/host Autocleavage

Non-picornavirus target replication VP4 VP2 VP3 VP1 2A 2B 2C 3A 3C 3D inhibitors VPg (for example, ribavirin, prozac) Replication

Assembly

Infectious virion Provirion

Adapted with permission of Frontiers in Bioscience from Lim et al. [100]. Antiviral compounds for and non-polio enteroviruses are classified as capsid inhibitors, protease inhibitors and replication inhibitors either targeting viral or host proteins.

To eliminate OPV/VDPV circulation, steps will be antiviral that impairs a good immune response is out of taken to stop OPV vaccination once wild-type PV the question. transmission has been interrupted. However, since A second important concern is the emergence of resist- immunocompromised persons will continue to serve as ance. Emergence is inevitable when using antivirals for a reservoir of OPV/VDPVs in the community [44,45], single stranded RNA (sRNA) viruses because the RdRp the interruption of OPV/VDPV transmission from lacks proofreading function. For the capsid inhibitor immunocompromised persons is set as a specific goal in V-073, now in trial for PV eradication (Table 2), two the eradication strategy [6]. Antivirals will be of further mutations have been found in vitro to confer resistance use to contain outbreaks in the final stages of the eradi- against this drug: -194- (I194F) cation campaign. These outbreaks are often caused by in VP1 (position 192 in PV-3) and alanine-24- in a re-introduction from an endemic country like Paki- VP3 (A24V) [50,51]. Combination therapy is often rec- stan, Nigeria or Afghanistan, as has occurred in Syria. ommended to minimize the reduced efficacy of a drug due In addition, antivirals will be stockpiled to be readily to the emergence of resistance [52]. As AG7404 is active available to contain outbreaks caused by accidental or in inhibiting PV, including V-073-resistant strains [53], deliberate release of PV (reviewed by Collet et al. [6]) the drug has potential for combination therapy regard- after PV has been eradicated. ing polio eradication. When evaluating and licensing a PV antiviral, an However, given the recombination capacity of important public-health concern is that it does not EVs [54,55] and particularly the PV and Coxsackie A disrupt vaccine response [48,49]. As the eradication viruses, combined use of V-073 and AG7404 or any other campaign relies mainly on effective immunity of the drug combination should be cautiously approached. population through vaccination, any in vivo use of an When licensing and using V-073 and AG7404 for

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the sole purpose of PV eradication, the likelihood of Treating NPEV infections a V-073-resistant OPV/VDPV recombining with an AG7404-resistant OPV/VDPV or EV-C NPEV is low. Although PV is the most well-known, it is not the However, the likelihood increases when AG7404 will only enterovirus of public-health concern. EV-A71 be used as an off-label drug on a large-scale basis or has been associated with outbreaks worldwide and when it has been re-evaluated and licensed for NPEV/ most recently in the Asia–Pacific region, with fatal HRV treatment. In the latter case, the combined use of outcomes among children [8,58]. EV-A71 typically V-073 and AG7404 should be investigated. To under- causes HFMD or herpangina, for which severe and stand the potential effect of the combination V-073 and even fatal complications include brainstem encephali- AG7404, data are required on the susceptibility and the tis, aseptic meningitis and pulmonary oedema [8,58]. emergence of resistance of PV and the EV-C species to Moreover, EV-A71 infections have been related to AG7404, in particular those that have been identified long-term neurological sequellae including acute as successful recombination partners for VDPVs (for flaccid paralysis [59], hence the nickname ‘poliovi- example, CV-A13 and -17) [47,56]. rus 4’. Given its clinical impact, both vaccines and antivirals specifically targeting EV-A71 are widely Cross-resistance of PV and NPEV antivirals researched [10,11]. Because of the large number of EV types with an overlapping disease profile, antivi- As resistance is inevitable, so is cross-resistance. The rals with broad-spectrum activity are a major focus emergence of cross-resistance can not only jeopard- for the industry and clinicians [4]. However, the use ize the use of the drug in question but also the use of of broad-spectrum antivirals can have a major effect another drug. The capsid inhibitor BTA-798, currently on the transmission and endemic circulation of these being evaluated for treatment of community-acquired viruses. In addition, the emergence of resistance must common cold in asthma patients, has also proven be monitored, in particular the cross-resistance of EV effective in inhibiting PV [53]. Of concern is that the drugs and non-picornavirus drugs. V-073-resistant strains with the I194F marker were also resistant to BTA-798 [53]. Whether BTA-798 can NPEV transmissibility and circulation select for PV-I194F resistance is not yet known, but should be investigated. If or when BTA-798 is licensed The faecal–oral transmission route of EV leads to res- for HRV treatment, the drug could potentially be sold ervoir-to-human transmission as individuals become as an OTC medication or used off-label for NPEVs in infected by contact with contaminated fomites or the future. The use of BTA-798 during a PV outbreak water or by ingestion of contaminated water [60,61] during the final stages of the eradication campaign or or food [62,63]. Although EV infections are there- thereafter can thus lead to the emergence of PV I194F fore preventable by better hygiene, we know that the mutants. Similarly, the use of BTA-798 by immune- downside of good hygiene is that the resulting limited deficient individuals still shedding OPV can lead to contact to these viruses will yield a lower seropreva- transmission of these V-073 cross-resistant strains. lence rate in the community. Reduced early exposure This could jeopardize the effectiveness of V-073 to to EVs may shift the balance of EVs as a common treat immune-deficient individuals and devalue the and mild infection towards a more severe infection, as stockpiling of V-073 for post-eradication use. Further- clinical impact increases with age [64,65]. Reducing more, if BTA-798 can be excreted in stools and shed the exposure to EV infection at a young age also cre- in the environment, it could aggravate the risk. A simi- ates a setting in which the number of susceptible indi- lar scenario can be envisioned for other capsid inhibi- viduals in the community increases [64], heightening tors that will be clinically re-evaluated and licensed. the risk of an outbreak following an introduction. For example, the I192F V-073-mutation in PV3 was This has been notably suggested for PVs, where the already described as a mutation resulting from selec- increase in hygiene measures was linked to the succes- tion by the capsid inhibitor disoxaril (Table 2) [57]. sive appearance of poliomyelitis cases and outbreaks Conversely, V-073 could select for the cross-resistance of a disease rarely seen before the 1800s [64,66,67]. in NPEVs and HRVs. As such, uncontrolled use of Similarly for HAV, another member of the picorna- V-073 could disarm the use of any drug developed virus family, improved hygiene led to an increased and licensed for the treatment of these infections. average age of first exposure and, as a consequence, Therefore, it is recommended that resistance muta- a rise in outbreaks among non-vaccinated individu- tions induced by the anti-PV drug of choice should be als [68,69]. Effective antivirals may have a similar evaluated for their potential to confer cross-resistance effect, in particular when used on a large-scale basis to other HRV and NPEV antivirals that are or will be for treatment of severe EV disease, as an off-label PV licensed, and vice versa. drug or as an OTC medication.

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Transmission and circulation of resistant This example illustrates the need to cast a wider net NPEVs: lessons learned from the antimicrobial when studying potential for antiviral resistance devel- world and from other viruses opment to include possible veterinary applications, for instance for FMDV [79], a picornavirus which infects The other side of the transmission coin is the transmission cloven-hoofed animals. Broad-spectrum antiviral and circulation of resistant viruses. The transmission and drugs developed against FMDVs [4] could potentially circulation of AMR strains is painfully well known [12,70] be disseminated into the environment where human but provides us with the valuable lesson that large-scale uptake of the drug can act on PVs and NPEVs with use of a drug needs to be cautiously approached and the potential for resistance emergence in a treatment- resistance development vigilantly monitored and tackled. naive population. The evolution of resistance towards the currently limited antimicrobial armament is crippling for the antimicrobial Cross-resistance due to widespread use of field, where treatment options are likewise limited [12]. non-picornavirus antivirals Similarly for antiviral resistance, a resistant viral strain can be transmitted into the environment if The development and clinical evaluation of antivirals to the virus is fit enough, reaching the treatment-naive use against such infections as HIV, herpes simplex virus community and presenting an increased risk of trans- and HCV is booming and exceeds by far the develop- mission to high-risk patients, who thus cannot be ment of EV antivirals. The serendipitous exploration treated. This situation has become a reality for HIV by several scientists [80] led to the notion that these transmission and circulation, with approximately 27% non-picornavirus antivirals could also be active against of newly infected individuals harbouring a HAART- EV and HRVs, acting by the same or a different (pleo- resistant HIV-1 (reviewed by Frentz et al. [71]), which tropic) mode of action. The broad-spectrum activity is untreatable with the current anti-HIV armament. As of the non-picornavirus antivirals now available or in for influenza A, all strains are currently amantadine- development may increase our armament for treating resistant [72]. Their rapid emergence and spread has EV infections, but also pose a risk for (cross-) resist- led to the termination of the use of amantadine as ance emergence in the circulating EV population. While influenza treatment [73]. It is assumed that the high EV-associated illness is primarily seen in children, EV prevalence of amantadine-resistance was triggered circulation without clinical symptoms is quite common by large-scale use of the drug by poultry farmers in in older age groups undergoing treatment with these [74], coupled with the fact that the resistance non-picornavirus antivirals. This may have or already mutation does not crucially affect the fitness of influ- could have potentially led to selection and circulation enza A strains [73]. This contrasts with of resistance. resistance, which typically impairs viral fitness. Con- One of the non-picornavirus antivirals is ribavirin, sequently, the incidence of oseltamivir-resistant influ- which is currently being used to treat HCV (in combi- enza A strains among the treatment-naive population nation with pegylated interferon [IFN]-a) [16] respira- is currently low [73] but this can change. Before the tory syncytial virus infection [81] and Lassa [82]. 2009 pandemic, an osteltamivir-resistant strain spread Ribavirin is able to inhibit replication of both PV and rapidly and dominated as the seasonal influenza EV-A71 [19,21]. A Korean study on circulating E-5 and strain [75,76]. The spread of the oseltamivir-resistant E-18 strains showed ribavirin to be effective in inhibit- strains in the treatment-naive population was most ing replication of these viruses as well [22,23]. probably the result of a non-drug-induced compensa- Amantadine is another broad-spectrum non-picor- tion mutation incorporated into the viral genome [77]. navirus antiviral on the market that is being explored The seasonal resistant strain was replaced by the as to its effectiveness against EVs and HRVs. It can oseltamivir-sensitive pandemic H1N1 in 2009, but inhibit replication of both DNA and RNA viruses the previous dissemination of oseltamivir resistance and has been licensed for the treatment of influenza required vigilance in the use of oseltamivir during A [18] and Parkinson’s disease [83]. The compound and after the pandemic. Furthermore, a study in Swe- has already been shown to inhibit replication of E-5, den found that oseltamivir was not degraded in the E-18 [22,23] and EV-A71 [84] but not that of PV [85]. environment [78] and could therefore contribute to The mode of action of amantadine is pleotropic: for selection for oseltamivir-resistant viruses in the avian influenza A, the M2 channel is targeted [86] while for population, which could spread in the human popu- the picornavirus HAV, the internal ribosome entry site lation via reassortment or further evolution. Luckily, (IRES)-mediated translation is affected [84]. Amanta- human uptake of the drug was not a risk because the dine is no longer used to treat influenza A due to the active compound cannot be taken in by nose, where rapid emergence of resistant mutations [72] but is still the drug needs to act. used for Parkinson’s disease.

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Commonly used antivirals against HIV include non- strategic monitoring of drug use and the effect of the nucleoside reverse transcriptase inhibitors (NNRTIs), drugs on NPEV transmission changes. of which several are on the market and several still in Surveillance should include analysis of both the development [87]. These compounds have been found phenotypic susceptibility and the genetic markers that to inhibit NPEV replication [88]. An NNRTI in devel- indicate reduced susceptibility or resistance, both for opment, TBZE-029, inhibits CV-B3 replication by tar- untreated and treated patients. In the Netherlands, a geting the 2C protein [88]. genetic surveillance system, VIRO-Typened, is organ- The search for compounds with potential activ- ized [95] to include patient characteristics such as age ity against EVs has also found its way into the field and sex, clinical data such as disease and mortality, of symptomatic treatment. Through the screening of epidemiological data such as when and where a strain compound libraries, fluoxetine, a selective serotonin was isolated, and the genetic data (currently partial reuptake inhibitor used to treat depression and anxiety VP1) of a virus of interest. This allows a more in-depth disorders, was found to be active against CV-B3 by tar- analysis of the circulation of EV types in the Nether- geting the 2C protein [89]. The drugs bosentan and val- lands. By including the different genes of the proteins sartan, commonly used to treat myocarditis and other targeted by drugs on the market, the system could heart conditions, were found to protect the human further be used to monitor the genetic markers that endothelial cell from CV-B3 infection [90]. confer resistance and also to trace back the origin of The emergence of mutations conferring resistance resistance emergence through phylogenetic ancestry. could result from the long-term use of these non- Moreover, inclusion of phenotypic susceptibility test- picornavirus antivirals in unknowingly EV-infected ing would allow analysis of unidentified or non-drug- Parkinson’s patients using amantadine, in chronically induced or naturally resistant markers not previously depressed individuals using fluoxetine or in patients described. with chronic myocarditis using bosentan and valsar- A possible extension of the Dutch system could be the tan. Similarly, chronically infected HCV patients using inclusion of samples from environmental surveillance, ribavirin and/or HIV patients using NNRTIs, who also an approach already implemented in the Netherlands have unknown HRV or EV infection [91–94], could for PV surveillance [60] and now being evaluated for have induced the emergence of resistance. Of concern EV-A71 surveillance in Asia [97]. This would further aid is that ribavirin is available as an OTC medication in in establishing the effect of antivirals on transmission Mexico for influenza treatment (ribavirina, Vilona, and circulation. Furthermore, in addition to surveillance, Mexico) and could have been used in an uncontrolled drug resistance should be monitored not only in PVs and manner in double-infected cases. For example, ribavi- NPEVs but also in other picornaviruses such as HAV and rina has occasionally been brought to the USA for HIV FMDV, given the potential of these drugs to be shed in treatment, although it shows no documented effect on the environment when used on a large-scale basis. HIV. For all of the above-mentioned non-picornavirus antivirals, the occurrence of cross-resistance needs to Conclusion be investigated. In addition, future research will be needed on whether the resistant mutations induced by The field of antiviral development against PVs and these drugs in the circulating EV population are cross- NPEVs is developing fast, and compounds licensed resistant to future-licensed EV antivirals. for other uses are being screened for their effectiveness against EVs. The armament of drugs to treat EV infec- Surveillance for antiviral resistance tions is growing, but with the use of any compound, the implications for public health should also be consid- Once antivirals are licensed as treatment for PVs and ered. As painfully shown in the antimicrobial field and NPEVs, surveillance is essential to detect strains in the now showing signs among antivirals, it is important to treatment-naive population that are naturally resistant investigate the possible effects of off-label treatment or susceptible to developing resistance [95]. Such over- or uncontrolled use or shedding of compounds in the sight allows public-health officials to tackle and curtail environment. While resistance and cross-resistance can the circulation of resistant strains as early as possible to arise for many antivirals in development or on the mar- avoid falling into the AMR predicament. ket, antivirals for EVs and their consequences for pub- Among the antivirals currently under clinical evalu- lic health and treatment are of extraordinary concern ation, efficacy can vary between types/subgenogroups given the number and ubiquity of EVs, and their often and even strains [33,96], stressing the need for con- asymptomatic infections. The dangers do not necessar- tinuous updates of the baseline susceptibility profiles. ily mean that we cannot use EV antivirals, but they urge In addition, the various temporal and geographical cir- us to find a balance between the benefit of an individual culation patterns for different NPEV types require the patient and the implications for public health. In this

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