Antiviral prophylaxis with twice daily topical cidofovir protects against challenge in the adenovirus type 5/New Zealand rabbit ocular model

Cyclopentenylcytosine (CPE-C): In Vitro and In Vivo Evaluation as an Antiviral against Adenoviral Ocular Infections

Adenoviruses are the major cause of ocular viral infections worldwide. Currently, there is no approved antiviral treatment for these eye infections. Cyclopentenylcytosine (CPE-C) is an antiviral that has demonstrated activity against more than 20 viruses. The goals of the current study were to determine the in vitro and in vivo antiviral activity of CPE-C as well as its ocular toxicity. Antiviral activity was evaluated in vitro using standard plaque reduction assays to determine the 50% effective concentrations (EC₅₀s) and in vivo in the Ad5/NZW rabbit ocular replication model. Ocular toxicity was determined in uninfected rabbit eyes following topical ocular application. The in vitro EC50s for CPE-C ranged from 0.03 to 0.059 μg/mL for nine adenovirus types that commonly infect the eye. Ocular toxicity testing determined CPE-C to be non-irritating or practically non-irritating by Draize scoring. In vivo, 3% CPE-C topically administered 4X or 2X daily for 7 days to adenovirus-infected eyes demonstrated effective antiviral activity compared with the negative control and comparable antiviral activity to the positive control, 0.5% cidofovir, topically administered twice daily for 7 days. We conclude CPE-C was relatively non-toxic to rabbit eyes and demonstrated potent anti-adenoviral activity in vitro and in vivo.

Antiviral Drugs in Adenovirus-Induced Keratoconjunctivitis

Human adenovirus (HAdV) is one of the most common causes of conjunctivitis worldwide. Depending on specific serotypes and other factors, it can lead to several ocular manifestations, ranging from isolated, self-limited disease to epidemic and potentially sight-threatening keratoconjunctivitis. To date, no antiviral agent against ocular adenovirus has been licensed, and its management is still based on hygienic and supportive measures alone. In this review, a literature search up to August 2021 was performed to find peer-reviewed articles, with the primary aim to investigate drugs or other compounds with any antiviral activity against adenovirus. Finally, we included 70 articles, consisting of both in vitro, and in vivo studies on animal models and clinical trials of any phase, as well as a case-report, and analyzed each compound separately. Many antiviral agents proved to be effective on in vivo and in vitro studies on animal models, and in pre-clinical trials, but lacked reliability in large, controlled clinical investigations. The design of such studies, though, presented several hurdles, due to the nature and the specific characteristics of adenovirus-induced ocular diseases. Nevertheless, some promising compounds are currently under study, and further investigations are needed to prove their efficacy in the management of adenovirus conjunctivitis.

Animal Models in Human Adenovirus Research

Simple Summary

Animal models are widely used to study various aspects of human diseases and disorders. Likewise, they are indispensable for preclinical testing of medicals and vaccines. Human adenovirus infections are usually self-limiting, and can cause mild respiratory symptoms with fever, eye infection or gastrointestinal symptoms, but occasional local outbreaks with severe disease courses have been reported. In addition, adenovirus infections pose a serious risk for children and patients with a weakened immune system. Human adenovirus research in animal models to study adenovirus-induced disease and tumor development started in the 1950s. Various animal species have been tested for their susceptibility to human adenovirus infection since then, and some have been shown to mimic key characteristics of the infection in humans, including persistent infection. Furthermore, some rodent species have been found to develop tumors upon human adenovirus infection. Our review summarizes the current knowledge on animal models in human adenovirus research, describing the pros and cons along with important findings and future perspectives.

Abstract

Human adenovirus (HAdV) infections cause a wide variety of clinical symptoms, ranging from mild upper respiratory tract disease to lethal outcomes, particularly in immunocompromised individuals. To date, neither widely available vaccines nor approved antiadenoviral compounds are available to efficiently deal with HAdV infections. Thus, there is a need to thoroughly understand HAdV-induced disease, and for the development and preclinical evaluation of HAdV therapeutics and/or vaccines, and consequently for suitable standardizable in vitro systems and animal models. Current animal models to study HAdV pathogenesis, persistence, and tumorigenesis include rodents such as Syrian hamsters, mice, and cotton rats, as well as rabbits. In addition, a few recent studies on other species, such as pigs and tree shrews, reported promising data. These models mimic (aspects of) HAdV-induced pathological changes in humans and, although they are relevant, an ideal HAdV animal model has yet to be developed. This review summarizes the available animal models of HAdV infection with comprehensive descriptions of virus-induced pathogenesis in different animal species. We also elaborate on rodent HAdV animal models and how they contributed to insights into adenovirus-induced cell transformation and cancer.

Topical Astodrimer Sodium, a Non-Toxic Polyanionic Dendrimer, Demonstrates Antiviral Activity in an Experimental Ocular Adenovirus Infection Model

There is no approved antiviral therapy for adenovirus (HAdV) ocular infections. Astodrimer sodium (SPL7013) is a polyanionic dendrimer with antiviral activity. The current study evaluated the ocular tolerability and anti-adenoviral efficacy of topical SPL7013 in rabbit ocular models. In a tolerability study, rabbits were treated with 3% SPL7013, vehicle, or 0.5% cidofovir. Their eyes were graded using the Draize scale. In antiviral efficacy studies, HAdV5 inoculated eyes were treated with 3% SPL7013, vehicle, or 0.5% cidofovir. Eyes were cultured for the virus on days 0, 1, 3, 4, 5, 7, 9, 11, and 14. Viral titers were determined. There were no differences in Draize scores between 3% SPL7013 and vehicle on any day. Cidofovir produced significantly higher Draize scores on day 12 than SPL7013 and vehicle. The 3% SPL7013 and 0.5% cidofovir significantly reduced daily viral titers and positive cultures per total compared with vehicle on several different days. The 3% SPL7013 and 0.5% cidofovir significantly reduced the duration of HAdV5 shedding compared to vehicle. The 3% SPL7013 demonstrated significantly more antiviral activity compared with vehicle in the Ad5/NZW rabbit ocular model. The 3% SPL7013 induced “minimal” to “practically non-irritating” Draize scores in the ocular tolerability study. Further development of astodrimer sodium as a topical antiviral therapy for adenoviral ocular infections is indicated.

Management of Adenoviral Keratoconjunctivitis: Challenges and Solutions

Human adenovirus (HAdV) is the most common cause of infectious conjunctivitis, accounting for up to 75% of all conjunctivitis cases and affecting people of all ages and demographics. In addition to ocular complications, it can cause systemic infections in the form of gastroenteritis, respiratory disease, and dissemination in immunocompromised individuals. HAdV causes lytic infection of the mucoepithelial cells of the conjunctiva and cornea, as well as latent infection of lymphoid and adenoid cells. Epidemic keratoconjunctivitis (EKC) is the most severe ocular manifestation of HAdV infection, in which the presence of subepithelial infiltrates (SEIs) in the cornea is a hallmark feature of corneal involvement. SEIs have the tendency to recur and may lead to long-term visual disability. HAdV persistence and dissemination are linked to sporadic outbreaks of adenoviral keratoconjunctivitis. There is no FDA-approved antiviral for treating adenoviral keratoconjunctivitis, and as such, solutions should be proffered to handle the challenges associated with viral persistence and dissemination. Several treatment modalities have been investigated, both systemically and locally, to not only mitigate symptoms but reduce the course of the infection and prevent the risk of long-term complications. These options include systemic and topical antivirals, in-office povidone-iodine irrigation (PVI), immunoglobulin-based therapy, anti-inflammatory therapy, and immunotherapy. More recently, combination PVI/dexamethasone ophthalmic formulations have shown favorable outcomes and were well tolerated in clinical trials for the treatment of EKC. Possible, future treatment considerations include sialic acid analogs, cold atmospheric plasma, N-chlorotaurine, and benzalkonium chloride. Continued investigation and evaluation of treatment are warranted to reduce the economic burden and potential long-term visual debilitation in affected patients. This review will focus on how persistence and dissemination of HAdV pose a significant challenge to the management of adenoviral keratoconjunctivitis. Furthermore, current and future trends in prophylactic and therapeutic modalities for adenoviral keratoconjunctivitis will be discussed.

[The effect of combination of glycyrrhizic acid with alpha-glutamyl-tryptophan on the experimental adenoviral infection]

In this work, the activity of glycyrrhizic acid (GL) and dipeptide alpha-glutamyl-tryptophane (EW) as single preparations or in combination (GL+EW) against experimental adenoviral infection in the syrian hamsters was studied. Application of gl and GL+EW was shown to decrease the level of the adenovirus replication in liver tissue by 0.6 - 1.2 lgTCID50 depending on the composition and time point of the post infection. It was also demonstrated that normalization of the structure of the liver tissue was required, which was shown on the level of both optical and electron microscopy. The results obtained in this work suggest that gl and GL+EW may be considered as potential component of the complex therapy of adenoviral infection.

Human adenovirus: Viral pathogen with increasing importance

The aim of this review is to describe the biology of human adenovirus (HAdV), the clinical and epidemiological characteristics of adenoviral epidemic keratoconjunctivitis and to present a practical update on its diagnosis, treatment, and prophylaxis. There are two well-defined adenoviral keratoconjunctivitis clinical syndromes: epidemic keratoconjunctivitis (EKC) and pharyngoconjunctival fever (PCF), which are caused by different HAdV serotypes. The exact incidence of adenoviral conjunctivitis is still poorly known. However, cases are more frequent during warmer months. The virus is endemic in the general population, and frequently causes severe disease in immunocompromised patients, especially the pediatric patients. Contagion is possible through direct contact or fomites, and the virus is extremely resistant to different physical and chemical agents. The clinical signs or symptoms of conjunctival infection are similar to any other conjunctivitis, with a higher incidence of pseudomembranes. In the cornea, adenoviral infection may lead to keratitis nummularis. Diagnosis is mainly clinical, but its etiology can be confirmed using cell cultures, antigen detection, polymerase chain reaction or immunochromatography. Multiple treatments have been tried for this disease, but none of them seem to be completely effective. Prevention is the most reliable and recommended strategy to control this contagious infection.

Virucidal mechanism of action of NVC-422, a novel antimicrobial drug for the treatment of adenoviral conjunctivitis

Human adenoviral conjunctivitis is a highly contagious eye infection affecting millions of people world-wide. If untreated, it can further develop into keratitis, corneal ulceration, scarring and possible blindness. Despite the significant patient morbidity and socio-economic costs, it is an unmet medical need with no FDA approved treatment. Here, we demonstrate the virucidal activity of NVC-422 (N,N-dichloro-2,2-dimethyltaurine) against adenovirus type 5 (Ad5) and investigated its mechanism of action of Ad5 inactivation. NVC-422 inhibits Ad5-induced loss of cell viability in vitro with 50% inhibitory concentration (IC(50)) ranging from 9 to 23 μM. NVC-422 does not cause any cytotoxicity at concentrations as high as 250 μM. Invitro, NVC-422 inactivates Ad5 but does not interfere with viral replication, indicating that NVC-422 acts on the extracellular adenovirus as a virucidal agent. NVC-422 inactivates Ad5 by oxidative inactivation of key viral proteins such as fiber and hexon as evidenced by SDS-PAGE, Western blotting and reversed-phase HPLC. These data, combined with measurements of the kinetics of the NVC-422 reactivity with selected amino acids, indicate that the changes in the viral proteins are caused by the selective oxidation of sulfur-containing amino acids. The conformational changes of the viral proteins result in the destruction of the viral morphology as shown by transmission electron microscopy. In summary, NVC-422 exhibits virucidal activity against Ad5 by the oxidative inactivation of key viral proteins, leading to the loss of viral integrity and infectivity.

Treatment of viral conjunctivitis with antiviral drugs

Viral conjunctivitis is one of the most common disorders observed in ophthalmic emergency departments, yet no established treatment exists. Lately, antiviral medications have been introduced into clinical practice; however, a systematic review focusing on their use and effectiveness in the treatment of viral conjunctivitis has not been previously reported. We systemically reviewed the literature to identify studies where antiviral drugs were used to treat viral conjunctivitis. Currently, aciclovir, trifluridine and valaciclovir are commonly used as antiviral agents to treat herpesvirus infections. Cidofovir has been used successfully to treat some cases of adenoviral conjunctivitis, although toxicity has also been reported. The use of other medications, such as idoxuridine, has been minimized in clinical practice due to their high toxicity. Interestingly, most of the antiviral drugs developed are used to treat herpesvirus infections, while less progress has been made in the field of adenoviral infections. For other viral causes of conjunctivitis, no effective remedy is currently available, and treatment focuses on the relief of symptoms. Caution should be exercised when coadministering other pharmacological agents, such as corticosteroids, because of emerging adverse effects.

Clinical and antiviral efficacy of an ophthalmic formulation of dexamethasone povidone-iodine in a rabbit model of adenoviral keratoconjunctivitis

PURPOSE

To determine the efficacy of a new formulation of topical dexamethasone 0.1%/povidone-iodine 0.4% (FST-100) in reducing clinical symptoms and infectious viral titers in a rabbit model of adenoviral keratoconjunctivitis.

METHODS

Rabbit corneas were inoculated bilaterally with 2×10(6) plaque-forming-units (PFU) of adenovirus type 5 (Ad5) after corneal scarification. Animals were randomized 1:1:1:1 (five rabbits per group) to FST-100, 0.5% cidofovir, tobramycin/dexamethasone (Tobradex; Alcon Laboratories, Fort Worth, TX) ophthalmic suspension, and balanced salt solution (BSS; Alcon Laboratories). Treatment began 12 hours after viral inoculation and continued for 7 consecutive days. The eyes were clinically scored daily for scleral inflammation (injection), ocular neovascularization, eyelid inflammation (redness), friability of vasculature, inflammatory discharge (pus), and epiphora (excessive tearing). Eye swabs were collected daily before treatment for the duration of the study. Virus was eluted from the swabs and PFU determined by titration on human A549 cells, according to standard procedures.

RESULTS

The FST-100 treatment resulted in significantly lower clinical scores (P<0.05) than did the other treatments. The 0.5% cidofovir exhibited the most ocular toxicity compared with FST-100, tobramycin/dexamethasone, and balanced salt solution treatments. FST-100 and 0.5% cidofovir significantly (P<0.05) reduced viral titers compared with tobramycin/dexamethasone or balanced salt solution.

CONCLUSIONS

FST-100 was the most efficacious in minimizing the clinical symptoms of adenovirus infection in rabbit eyes. FST-100 and 0.5% cidofovir were both equally effective in reducing viral titers and decreasing the duration of viral shedding. By providing symptomatic relief in addition to reducing infectious virus titers, FST-100 should be a valuable addition to treatment of epidemic adenoviral keratoconjunctivitis.

Clinical features and treatment of adenovirus infections

Adenoviruses (Ads) are common opportunistic pathogens that are rarely associated with severe clinical symptoms in healthy individuals. In contrast, in patients with compromised immunity, Ad infections often result in disseminated and potentially life-threatening disease. Among these are AIDS patients, individuals with hereditary immunodeficiencies and recipients of solid organ or haematopoietic stem cell transplants (HSCT) who receive immunosuppressive therapy. The latter account for the largest number of severe Ad infections. There is currently no formally approved antiviral therapy for the treatment of severe Ad keratoconjunctivitis and life-threatening Ad infections in immunocompromised patients. Here we update current knowledge on Ad biology, the clinical features observed in different patient groups and specific immune responses towards Ad infections. In addition, we review current and future treatment options, including: (i) the antiviral drugs cidofovir, ribavirin and new investigational compounds, as evaluated in the clinic or in relevant animal models, as well as (ii) novel immunotherapeutic strategies.

Hexadecyloxypropyl-cidofovir, CMX001, prevents adenovirus-induced mortality in a permissive, immunosuppressed animal model

Adenoviruses (Ads) cause a wide array of end-organ and disseminated diseases in severely immunosuppressed patients. For example, approximately 20% of pediatric allogeneic hematopoietic stem cell transplant recipients develop disseminated Ad infection, and the disease proves fatal in as many as 50-80% of these patients. Ad infections are a serious problem for solid-organ transplant recipients and AIDS patients as well. Unfortunately, there are no antiviral drugs approved specifically to treat these infections. A suitable animal model that is permissive for Ad replication would help in the discovery process. Here we identify an animal model to study Ad pathogenesis and the efficacy of antiviral compounds. We show that human serotype 5 Ad (Ad5) causes severe systemic disease in immunosuppressed Syrian hamsters that is similar to that seen in immunocompromised patients. We also demonstrate that hexadecyloxypropyl-cidofovir (CMX001) rescues the hamsters from a lethal challenge with Ad5. The antiviral drug provided protection both prophylactically and when given up to 2 days after i.v. exposure to Ad5. CMX001 acts by reducing Ad replication in key target organs. Thus, the immunosuppressed Syrian hamster is a powerful model to evaluate anti-Ad drugs, and its use can facilitate the entry of drugs such as CMX001 into clinical trials.

Effect of 6-azacytidine on the course of experimental adenoviral infection in newborn Syrian hamsters

Adenoviral infection is a serious human pathology leading to respiratory, gastrointestinal and ocular disorders and epidemic outbreaks, especially in children's groups. Here we present the results from an investigation of anti- adenoviral effect of 6-azacytidine (6-AC) both in vitro and in vivo. The selectivity index of 6-AC for adenovirus type 5 in HEp-2 cells was 374, the 50% effective concentration was 0.5 mg/ml. For in vivo investigations we developed a model of disseminated adenoviral infection in newborn Syrian hamsters. The infectious virus was recovered from the liver, kidney, lungs and heart. Application of 6-AC led to a reduced period of the virus presence (7 days in the liver and 4 days in the kidney and heart) and lowered virus titers on day 3 post-inoculation (p.i.) (liver - 2.7 and 4.1, heart - 0 and 3.2, kidney - 0 and 2.4 log(10 )CPD(50)/mg tissue weight, in the presence and absence of 6-AC, respectively). Application of 6-AC to newborn Syrian hamsters led to partial destruction of their splenocytes. The results obtained suggest that 6-AC or 6-ACbased drugs with lower toxicity or applied topically may be suitable for therapy and prevention of adenoviral infection in humans.

Adenovirus infections in transplant recipients

Adenoviruses are increasingly recognized as contributors to morbidity and mortality among stem cell and solid-organ transplant recipients. Clinical presentations range from asymptomatic viremia to respiratory and gastrointestinal disease, hemorrhagic cystitis, and severe disseminated illness. The limited clinical data available support the use of cidofovir for many of these illnesses. Prospective studies are needed to better understand the pathogenesis of and therapeutic options for adenoviral infections in this patient population.

Antiviral therapy for adenovirus infections

The treatment of severe adenovirus keratoconjunctivitis and life-threatening adenovirus infections in immunocompromised patients is still unsatisfactory. We here review the mode of action and antiviral data for cidofovir and ribavirin, obtained in cell culture, animal models or patients. Several nucleoside or nucleotide analogues have been described that target the adenovirus polymerase, whereas other antiviral targets have been poorly investigated. Furthermore, optimal therapeutic response may be achieved by combining antiviral therapy with immunotherapeutic approaches, as currently being explored.

Prospects for adenovirus antivirals

Adenoviruses cause a number of self-limiting but often highly infectious diseases that affect multiple organs, most commonly those associated with respiratory, genitourinary and gastrointestinal tracts and the ocular surface. Many factors have driven a search for effective topical and systemic antivirals to adenoviruses. These include patient morbidity, economic losses and chronic visual disturbances associated with epidemic keratoconjunctivitis; and the startling recent trend of high morbidity and rising mortality associated with systemic adenoviral infections in the immunosuppressed, particularly paediatric bone marrow transplant recipients. The development of effective antivirals has proven to be a complex task, owing to the fact that multiple and often genetically divergent adenovirus serotypes can cause similar diseases. Currently, there remains no licensed systemic or topical treatment in the USA or Europe. However, many compounds have been explored for activity against adenoviruses, and some have been evaluated clinically in either a topical setting for ocular disease or in the setting of systemic treatment in the face of life-threatening adenovirus infections. This article outlines such compounds, discusses the potential for their clinical development, and highlights some problems that may be faced in evaluating their efficacy clinically.

Amphibian virology

Amphibians are a diverse group of species; much work remains to be done to elucidate the viruses of amphibians. Viral diseases may play an important role in wild and captive amphibian populations. Iridoviruses are a leading cause of wild amphibian mortality events in the United States and are a common cause of amphibian mortality events worldwide. In addition to frank viral diseases, viral infections may play a role in the establishment of bacterial, fungal, and parasitic diseases and are an underlying cause of neoplasia. It is important for the amphibian clinician to recognize disease syndromes and pathology that are consistent with viral etiology.

Synthesis of Some N4-Substituted Derivatives of 1-[(S)-3-Hydroxy-2-(phosphonomethoxy)propyl]cytosine (HPMPC, Cidofovir)

N4-Substituted derivatives of HPMPC were synthesized in four-step synthesis which included treatment of 4-methoxypyrimidin-2(1H)-one (1) with (S)-[(trityloxy)methyl]oxirane in DMF. Condensation of intermediary 1-[2-hydroxy-3-(trityloxy)propyl]-4-methoxypyrimidin-2(1H)-one (2) with (diisopropoxyphosphoryl)methyl tosylate in the presence of sodium hydride resulted in fully protected 4-methoxypyrimidin-2(1H)-one derivative 3 which gave on reaction with an appropriate primary amine in dioxane N4-substituted products 4a-4i. The reaction with bromotrimethylsilane simultaneously cleaved the trityl group and deprotected the phosphonate residue and gave the title HPMP analogues substituted at the cytosine amino group in position N4 5a-5i. Compound 4j was prepared from 4-methoxypyrimidin-2(1H)-one (1) by reaction with cyclopropylamine in dioxane. The intermediary 4-(cyclopropylamino)pyrimidin-2(1H)-one (6) then reacts with (S)-[(trityloxy)methyl]oxirane in DMF. Fully protected phosphonate 4j and its deprotected counterpart 5j was obtained by the same sequence of reactions as in the case of compounds 5a-5i.

Reptile virology

Reptiles are hosts to diverse viral infections. This article reviews the viruses that are known to infect reptiles and discusses associated pathology, available diagnostic methods, and management techniques for the reptile clinician.

Clinical potential of the acyclic nucleoside phosphonates cidofovir, adefovir, and tenofovir in treatment of DNA virus and retrovirus infections

The acyclic nucleoside phosphonates HPMPC (cidofovir), PMEA (adefovir), and PMPA (tenofovir) have proved to be effective in vitro (cell culture systems) and in vivo (animal models and clinical studies) against a wide variety of DNA virus and retrovirus infections: cidofovir against herpesvirus (herpes simplex virus types 1 and 2 varicella-zoster virus, cytomegalovirus [CMV], Epstein-Barr virus, and human herpesviruses 6, 7, and 8), polyomavirus, papillomavirus, adenovirus, and poxvirus (variola virus, cowpox virus, vaccinia virus, molluscum contagiosum virus, and orf virus) infections; adefovir against herpesvirus, hepadnavirus (human hepatitis B virus), and retrovirus (human immunodeficiency virus types 1 [HIV-1] and 2 [HIV-2], simian immunodeficiency virus, and feline immunodeficiency virus) infections; and tenofovir against both hepadnavirus and retrovirus infections. Cidofovir (Vistide) has been officially approved for the treatment of CMV retinitis in AIDS patients, tenofovir disoproxil fumarate (Viread) has been approved for the treatment of HIV infections (i.e., AIDS), and adefovir dipivoxil (Hepsera) has been approved for the treatment of chronic hepatitis B. Nephrotoxicity is the dose-limiting side effect for cidofovir (Vistide) when used intravenously (5 mg/kg); no toxic side effects have been described for adefovir dipivoxil and tenofovir disoproxil fumarate, at the approved doses (Hepsera at 10 mg orally daily and Viread at 300 mg orally daily).

Sequence changes in the human adenovirus type 5 DNA polymerase associated with resistance to the broad spectrum antiviral cidofovir

Although there is currently no FDA approved antiviral treatment for adenovirus (Ad) infections, the broad spectrum antiviral cidofovir (CDV) has demonstrated potent inhibitory activity against many Ad serotypes in vitro and in an in vivo ocular replication model. The clinical potential of CDV prompted the assessment for the emergence of CDV resistance in Ad5. Serial passage of Ad5 in increasing concentrations of CDV resulted in derivation of four different Ad5 variants with increased resistance to CDV. CDV resistance was demonstrated by ability to replicate viral DNA in infected cells at CDV concentrations that inhibit the parental virus, by ability to form plaques in CDV concentrations of >20 microg/ml and by increased progeny release following infection and growth in media containing CDV. Using marker rescue, the loci for CDV resistance in variant R1 was shown to be mediated by one residue change L741S, one of two mutations within the R1 encoded DNA polymerase. The CDV-resistant variants R4, R5 and R6 also contained mutations in their respective DNA polymerase sequences, but these were different from R1; variant R4 contained two changes (F740I and V180I), whereas both R5 and R6 variants contained the non-conserved mutation A359E. R6 contained additional alterations L554F and V817L. The location of the R1 change is close to a region of the DNA polymerase which is conserved with other polymerases that is predicted to involve nucleotide binding.