A standardized approach to the evaluation of antivirals against DNA viruses: Orthopox-, adeno-, and herpesviruses

Indirubin-3'-monoxime exhibits potent antiviral and anti-inflammatory effects against human adenoviruses in vitro and in vivo

Human adenovirus (HAdV) infection is a major cause of respiratory disease, yet no antiviral drugs have been approved for its treatment. Herein, we evaluated the antiviral and anti-inflammatory effects of cyclin-dependent protein kinase (CDK) inhibitor indirubin-3'-monoxime (IM) against HAdV infection in cells and a transgenic mouse model. After evaluating its cytotoxicity, cytopathic effect reduction, antiviral replication kinetics, and viral yield reduction assays were performed to assess the anti-HAdV activity of IM. Quantitative real-time polymerase chain reaction (qPCR), quantitative reverse transcription PCR (qRT-PCR), and western blotting were used to assess the effects of IM on HAdV DNA replication, transcription, and protein expression, respectively. IM significantly inhibited HAdV DNA replication as well as E1A and Hexon transcription, in addition to significantly suppressing the phosphorylation of the RNA polymerase II C-terminal domain (CTD). IM mitigated body weight loss, reduced viral burden, and lung injury, decreasing cytokine and chemokine secretion to a greater extent than cidofovir. Altogether, IM inhibits HAdV replication by downregulating CTD phosphorylation to suppress viral infection and corresponding innate immune reactions as a promising therapeutic agent.

A ProTide of AZT Shows Activity Against Human Papillomaviruses

Infection by human papillomaviruses (HPV) can cause warts and tumors. So far, no small molecule antiviral has been approved for the treatment of infections with this DNA virus, although preclinical studies show activity for nucleosidic compounds, such as 9-(2-phosphonylmethoxy)ethylguanine (PMEG) or cidofovir. This prompted us to test new prodrug versions of the nucleoside analog 3'-azido-2',3'-dideoxythymidine (AZT), known to be active against reverse transcriptases and approved for the treatment of HIV. Here we report the synthesis of an ethylbutyl alaninyl ester phosphosphoramidate prodrug of AZT, dubbed AZAEB, and its activity against HPV, a target not known to be sensitive to AZT. A methyl ester derivative was found to be inactive against this and three other DNA viruses, while the phosphoramidate prodrug AZAEB showed a modest inhibitory effect against HPV types 6, 11, 18 and 31. Our results open up new avenues of study for the treatment of diseases caused by members of the papillomaviridae family.

In Silico Design and Experimental Validation of Novel Oxazole Derivatives Against Varicella zoster virus

Varicella zoster virus (VZV) infection causes severe disease such as chickenpox, shingles, and postherpetic neuralgia, often leading to disability. Reactivation of latent VZV is associated with a decrease in specific cellular immunity in the elderly and in patients with immunodeficiency. However, due to the limited efficacy of existing therapy and the emergence of antiviral resistance, it has become necessary to develop new and effective antiviral drugs for the treatment of diseases caused by VZV, particularly in the setting of opportunistic infections. The goal of this work is to identify potent oxazole derivatives as anti-VZV agents by machine learning, followed by their synthesis and experimental validation. Predictive QSAR models were developed using the Online Chemical Modeling Environment (OCHEM). Data on compounds exhibiting antiviral activity were collected from the ChEMBL and uploaded in the OCHEM database. The predictive ability of the models was tested by cross-validation, giving coefficient of determination q2 = 0.87-0.9. The validation of the models using an external test set proves that the models can be used to predict the antiviral activity of newly designed and known compounds with reasonable accuracy within the applicability domain (q2 = 0.83-0.84). The models were applied to screen a virtual chemical library with expected activity of compounds against VZV. The 7 most promising oxazole derivatives were identified, synthesized, and tested. Two of them showed activity against the VZV Ellen strain upon primary in vitro antiviral screening. The synthesized compounds may represent an interesting starting point for further development of the oxazole derivatives against VZV. The developed models are available online at OCHEM http://ochem.eu/article/145978 and can be used to virtually screen for potential compounds with anti-VZV activity.

Anti-CMV therapy, what next? A systematic review

Human cytomegalovirus (HCMV) is one of the main causes of serious complications in immunocompromised patients and after congenital infection. There are currently drugs available to treat HCMV infection, targeting viral polymerase, whose use is complicated by toxicity and the emergence of resistance. Maribavir and letermovir are the latest antivirals to have been developed with other targets. The approval of letermovir represents an important innovation for CMV prevention in hematopoietic stem cell transplant recipients, whereas maribavir allowed improving the management of refractory or resistant infections in transplant recipients. However, in case of multidrug resistance or for the prevention and treatment of congenital CMV infection, finding new antivirals or molecules able to inhibit CMV replication with the lowest toxicity remains a critical need. This review presents a range of molecules known to be effective against HCMV. Molecules with a direct action against HCMV include brincidofovir, cyclopropavir and anti-terminase benzimidazole analogs. Artemisinin derivatives, quercetin and baicalein, and anti-cyclooxygenase-2 are derived from natural molecules and are generally used for different indications. Although they have demonstrated indirect anti-CMV activity, few clinical studies were performed with these compounds. Immunomodulating molecules such as leflunomide and everolimus have also demonstrated indirect antiviral activity against HCMV and could be an interesting complement to antiviral therapy. The efficacy of anti-CMV immunoglobulins are discussed in CMV congenital infection and in association with direct antiviral therapy in heart transplanted patients. All molecules are described, with their mode of action against HCMV, preclinical tests, clinical studies and possible resistance. All these molecules have shown anti-HCMV potential as monotherapy or in combination with others. These new approaches could be interesting to validate in clinical trials.

Design and experimental validation of the oxazole and thiazole derivatives as potential antivirals against of human cytomegalovirus

QSAR studies of a set of previously synthesized azole derivatives tested against human cytomegalovirus (HCMV) were performed using the OCHEM web platform. The predictive ability of the classification models has a balanced accuracy (BA) of 73-79%. The validation of the models using an external test set proved that the models can be used to predict the activity of newly designed compounds with a reasonable accuracy within the applicability domain (BA = 76-83%). The models were applied to screen a virtual chemical library with expected activity of compounds against HCMV. The five most promising new compounds were identified, synthesized and their antiviral activities against HCMV were evaluated in vitro. Two of them showed some activity against the HCMV strain AD169. According to the results of docking analysis, the most promising biotarget associated with HCMV is DNA polymerase. The docking of the most active compounds 1 and 5 in the DNA polymerase active site shows calculated binding energies of -8.6 and -7.8 kcal/mol, respectively. The ligand's complexation was stabilized by the formation of hydrogen bonds and hydrophobic interactions with amino acids Lys60, Leu43, Ile49, Pro77, Asp134, Ile135, Val136, Thr62 and Arg137.

NPP-669, a Novel Broad-Spectrum Antiviral Therapeutic with Excellent Cellular Uptake, Antiviral Potency, Oral Bioavailability, Preclinical Efficacy, and a Promising Safety Margin

DNA viruses are responsible for many diseases in humans. Current treatments are often limited by toxicity, as in the case of cidofovir (CDV, Vistide), a compound used against cytomegalovirus (CMV) and adenovirus (AdV) infections. CDV is a polar molecule with poor bioavailability, and its overall clinical utility is limited by the high occurrence of acute nephrotoxicity. To circumvent these disadvantages, we designed nine CDV prodrug analogues. The prodrugs modulate the polarity of CDV with a long sulfonyl alkyl chain attached to one of the phosphono oxygens. We added capping groups to the end of the alkyl chain to minimize β-oxidation and focus the metabolism on the phosphoester hydrolysis, thereby tuning the rate of this reaction by altering the alkyl chain length. With these modifications, the prodrugs have excellent aqueous solubility, optimized metabolic stability, increased cellular permeability, and rapid intracellular conversion to the pharmacologically active diphosphate form (CDV-PP). The prodrugs exhibited significantly enhanced antiviral potency against a wide range of DNA viruses in infected human foreskin fibroblasts. Single-dose intravenous and oral pharmacokinetic experiments showed that the compounds maintained plasma and target tissue levels of CDV well above the EC50 for 24 h. These experiments identified a novel lead candidate, NPP-669. NPP-669 demonstrated efficacy against CMV infections in mice and AdV infections in hamsters following oral (p.o.) dosing at a dose of 1 mg/kg BID and 0.1 mg/kg QD, respectively. We further showed that NPP-669 at 30 mg/kg QD did not exhibit histological signs of toxicity in mice or hamsters. These data suggest that NPP-669 is a promising lead candidate for a broad-spectrum antiviral compound.

Investigating N-arylpyrimidinamine (NAPA) compounds as early-stage inhibitors against human cytomegalovirus

Human cytomegalovirus (CMV) is a ubiquitous β-herpesvirus that establishes latent asymptomatic infections in healthy individuals but can cause serious infections in immunocompromised people, resulting in increased risk of morbidity and mortality. The current FDA-approved CMV drugs target late stages of the CMV life-cycle. While these drugs are effective in most cases, they have serious drawbacks, including poor oral bioavailability, dose-limiting toxicity, and a low barrier to resistance. Given the clinical relevance of CMV-associated diseases, novel therapies are needed. Thus, a novel class of compounds that inhibits the early stages of the CMV life-cycle was identified and found to block infection of different strains in physiologically relevant cell types. This class of compounds, N-arylpyrimidinamine (NAPA), demonstrated potent anti-CMV activity against ganciclovir-sensitive and -resistant strains in in vitro replication assays, a selectivity index >30, and favorable in vitro ADME properties. Mechanism of action studies demonstrated that NAPA compounds inhibit an early step of virus infection. NAPA compounds are specific inhibitors of cytomegaloviruses and exhibited limited anti-viral activity against other herpesviruses. Collectively, we have identified a novel class of CMV inhibitor that effectively limits viral infection and proliferation.

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.

Design of new imidazole derivatives with anti-HCMV activity: QSAR modeling, synthesis and biological testing

The problem of designing new antiviral drugs against Human Cytomegalovirus (HCMV) was addressed using the Online Chemical Modeling Environment (OCHEM). Data on compound antiviral activity to human organisms were collected from the literature and uploaded in the OCHEM database. The predictive ability of the regression models was tested through cross-validation, giving coefficient of determination q² = 0.71-0.76. The validation of the models using an external test set proved that the models can be used to predict the activity of newly designed compounds with reasonable accuracy within the applicability domain (q² = 0.70-0.74). The models were applied to screen a virtual chemical library of imidazole derivatives, which was designed to have antiviral activity. The six most promising compounds were identified, synthesized and their antiviral activities against HCMV were evaluated in vitro. However, only two of them showed some activity against the HCMV AD169 strain.

Inhibiting cytomegalovirus replication through targeting the host electron transport chain

Human cytomegalovirus (HCMV) is a near ubiquitous herpesvirus that relies on host cell metabolism for efficient replication. Although it has been shown that HCMV requires functional host cell mitochondria for efficient replication, it is unknown whether mitochondrial targeted pharmacological agents can be repurposed as antivirals. Here we report that treatment with drugs targeting the electron transport chain (ETC) complexes inhibit HCMV replication. Addition of rotenone, oligomycin, antimycin and metformin resulted in decreased HCMV titers in vitro, independent of HCMV strain. This further illustrates the dependence of HCMV replication on functional mitochondria. Metformin, an FDA approved drug, delays HCMV replication kinetics resulting in a reduction of viral titers. Repurposing metformin as an antiviral is advantageous as its safety profile and epidemiological data are well accepted. Our findings provide new insight into the potential for targeting HCMV infection through host cell metabolism and how these pharmacological interventions function.

Filociclovir Is an Active Antiviral Agent against Ocular Adenovirus Isolates In Vitro and in the Ad5/NZW Rabbit Ocular Model

Presently, there is no FDA- or EMA-approved antiviral for the treatment of human adenovirus (HAdV) ocular infections. This study determined the antiviral activity of filociclovir (FCV) against ocular HAdV isolates in vitro and in the Ad5/NZW rabbit ocular model. The 50% effective concentrations (EC₅₀) of FCV and cidofovir (CDV) were determined for several ocular HAdV types using standard plaque reduction assays. Rabbits were topically inoculated in both eyes with HAdV5. On day 1, the rabbits were divided into four topical treatment groups: (1) 0.5% FCV 4x/day × 10 d; (2) 0.1% FCV 4x/day × 10 d; (3) 0.5% CDV 2x/day × 7 d; (4) vehicle 4x/day × 10 d. Eyes were cultured for virus on days 0, 1, 3, 4, 5, 7, 9, 11, and 14. The resulting viral eye titers were determined using standard plaque assays. The mean in vitro EC₅₀ for FCV against tested HAdV types ranged from 0.50 to 4.68 µM, whereas those treated with CDV ranged from 0.49 to 30.3 µM. In vivo, compared to vehicle, 0.5% FCV, 0.1% FCV, and 0.5% CDV produced lower eye titers, fewer numbers of positive eye cultures, and shorter durations of eye infection. FCV demonstrated anti-adenovirus activity in vitro and in vivo.

Filociclovir Is a Potent In Vitro and In Vivo Inhibitor of Human Adenoviruses

Human adenovirus (HAdV) infection is common in the general population and can cause a range of clinical manifestations, among which pneumonia and keratoconjunctivitis are the most common. Although HAdV infections are mostly self-limiting, infections in immunocompromised individuals can be severe. No antiviral drug has been approved for treating adenoviruses. Filociclovir (FCV) is a nucleoside analogue which has successfully completed phase I human clinical safety studies and is now being developed for treatment of human cytomegalovirus (HCMV)-related disease in immunocompromised patients. In this report, we show that FCV is a potent broad-spectrum inhibitor of HAdV types 4 to 8, with 50% effective concentrations (EC50s) ranging between 1.24 and 3.6 μM and a 50% cytotoxic concentration (CC50) of 100 to 150 μM in human foreskin fibroblasts (HFFs). We also show that the prophylactic oral administration of FCV (10 mg/kg of body weight) 1 day prior to virus challenge and then daily for 14 days to immunosuppressed Syrian hamsters infected intravenously with HAdV6 was sufficient to prevent morbidity and mortality. FCV also mitigated tissue damage and inhibited virus replication in the liver. The 10-mg/kg dose had similar effects even when the treatment was started on day 4 after virus challenge. Furthermore, FCV administered at the same dose after intranasal challenge with HAdV6 partially mitigated body weight loss but significantly reduced pathology and virus replication in the lung. These findings suggest that FCV could potentially be developed as a pan-adenoviral inhibitor.

Antiviral Effects of Curcumin on Adenovirus Replication

Human adenovirus (HAdV) is a common pathogen that can cause severe morbidity and mortality in certain populations, including pediatric, geriatric, and immunocompromised patients. Unfortunately, there are no approved therapeutics to combat HAdV infections. Curcumin, the primary curcuminoid compound found in turmeric spice, has shown broad activity as an antimicrobial agent, limiting the replication of many different bacteria and viruses. In this study, we evaluated curcumin as an anti-HAdV agent. Treatment of cells in culture with curcumin reduced HAdV replication, gene expression, and virus yield, at concentrations of curcumin that had little effect on cell viability. Thus, curcumin represents a promising class of compounds for further study as potential therapeutics to combat HAdV infection.

Recent Advances in Novel Antiviral Therapies against Human Adenovirus

Human adenovirus (HAdV) is a very common pathogen that typically causes minor disease in most patients. However, the virus can cause significant morbidity and mortality in certain populations, including young children, the elderly, and those with compromised immune systems. Currently, there are no approved therapeutics to treat HAdV infections, and the standard treatment relies on drugs approved to combat other viral infections. Such treatments often show inconsistent efficacy, and therefore, more effective antiviral therapies are necessary. In this review, we discuss recent developments in the search for new chemical and biological anti-HAdV therapeutics, including drugs that are currently undergoing preclinical/clinical testing, and small molecule screens for the identification of novel compounds that abrogate HAdV replication and disease.

Identification of human adenovirus replication inhibitors from a library of small molecules targeting cellular epigenetic regulators

Human adenovirus (HAdV) can cause severe disease in certain at-risk populations such as newborns, young children, the elderly and individuals with a compromised immune system. Unfortunately, no FDA-approved antiviraldrug is currently available for the treatment of HAdV infections. Within the nucleus of infected cells, the HAdV genome associates with histones and forms a chromatin-like structure during early infection, and viral gene expression appears to be regulated by cellular epigenetic processes. Thus, one potential therapeutic strategy to combat HAdV disease may be to target the cellular proteins involved in modifying the viral nucleoprotein structure and facilitating HAdV gene expression and replication. We have screened a panel of small molecules that modulate the activity of epigenetic regulatory proteins for compounds affecting HAdV gene expression. Several of the compounds, specifically chaetocin, gemcitabine and lestaurtinib, reduced HAdV recovery by 100- to 1000-fold, while showing limited effects on cell health, suggesting that these compounds may indeed be promising as anti-HAdV therapeutics.

The discovery and development of filociclovir for the prevention and treatment of human cytomegalovirus-related disease

Human cytomegalovirus (HCMV) infections are widespread among the human population. Infection is persistent and mostly asymptomatic, except in immunocompromised individuals, particularly transplant patients, where significant morbidity and mortality can occur. Currently approved drugs for treating HCMV-related disease [including ganciclovir (GCV), valganciclovir (VGCV), cidofovir (CDV) and foscarnet (FOS)] all target the viral DNA polymerase and suffer from dose-limiting toxicity and resistance issues. The most recently approved drug, letermovir (LMV), was approved only for prophylaxis in adult HCMV-seropositive stem cell transplant recipients. Although LMV is highly potent, high-grade resistance mutations in the terminase gene were shown to readily emerge in vitro and in treated patients. Therefore, there is a need for new drugs that can be used for combinatorial therapeutic and/or prophylactic regimens to counteract the emergence of resistant mutants. Filociclovir (FCV), also known as cyclopropavir or MBX-400, is a methylenecyclopropane nucleoside analog, which has successfully completed Phase I safety studies, and is now entering Phase II clinical efficacy studies for the treatment of HCMV-related disease in transplant patients. FCV is 10-fold more active than GCV against HCMV in vitro, and has activity against all human herpesviruses except HSV-1 and HSV-2. Recently, FCV was also shown to be highly potent against human adenoviruses. This activity spectrum suggests that FCV could be used to treat/prevent infection with several viruses that pose significant risk to transplant patients. The active triphosphate form of FCV (FCV-TP) reaches higher peak levels than GCV-TP in HCMV-infected cells, and exhibits about 10-fold higher affinity to HCMV DNA polymerase UL54. Furthermore, FCV was shown to retain activity against a panel of GCV-resistant HCMV isolates, suggesting that it could be a useful alternative therapy for treating patients infected with some GCV-resistant HCMV strains. This review summarizes the early discovery work of FCV and highlights the recent advances in the continued development of this clinical candidate.

Development of a novel screening platform for the identification of small molecule inhibitors of human adenovirus

Human adenovirus (HAdV) can cause severe disease and death in both immunocompromised and immunocompetent patients. The current standards of treatment are often ineffective, and no approved antiviral therapy against HAdV exists. We report here the design and validation of a fluorescence-based high-content screening platform for the identification of novel anti-HAdV compounds. The screen was conducted using a wildtype-like virus containing the red fluorescent protein (RFP) gene under the regulation of the HAdV major late promoter. Thus, RFP expression allows monitoring of viral late gene expression (a surrogate marker for virus replication), and compounds affecting virus growth can be easily discovered by quantifying RFP intensity. We used our platform to screen ~1200 FDA-approved small molecules, and identified several cardiotonic steroids, corticosteroids and chemotherapeutic agents as anti-HAdV compounds. Our screening platform provides the stringency necessary to detect compounds with varying degrees of antiviral activity, and facilitates drug discovery/repurposing to combat HAdV infections.

Drug development against human adenoviruses and its advancement by Syrian hamster models

The symptoms of human adenovirus infections are generally mild and self-limiting. However, these infections have been gaining importance in recent years because of a growing number of immunocompromised patients. Solid organ and hematopoietic stem cell transplant patients are subjected to severe immunosuppressive regimes and cannot efficaciously eliminate virus infections. In these patients, adenovirus infections can develop into deadly multi-organ disseminated disease. Presently, in the absence of approved therapies, physicians rely on drugs developed for other purposes to treat adenovirus infections. As there is a need for anti-adenoviral therapies, researchers have been developing new agents and repurposing existing ones to treat adenovirus infections. There are several small molecule drugs that are being tested for their efficacy against human adenoviruses; some of these have reached clinical trials, while others are still in the preclinical phase. Besides these compounds, research on immunotherapy against adenoviral infection has made significant progress, promising another modality for treatment. The availability of an animal model confirmed the activity of some drugs already in clinical use while proving that others are inactive. This led to the identification of several lead compounds that await further development. In the present article, we review the current status of anti-adenoviral therapies and their advancement by in vivo studies in the Syrian hamster model.

African swine fever virus replication events and cell nucleus: New insights and perspectives

African swine fever (ASF) is currently matter for major concerns in global swine industry as it is highly contagious and causes acute fatal haemorrhagic fever in domestic pigs and wild boar. The absence of effective vaccines and treatments pushes ASF control to relay on strict sanitary and stamping out measures with costly socio-economic impacts. The current epidemic scenario of fast spreading throughout Asiatic countries impels further studies on prevention and combat strategies against ASF. Herein we review knowledge on African Swine Fever Virus (ASFV) interactions with the host cell nucleus and on the functional properties of different viral DNA-replication related proteins. This entails, the confirmation of an intranuclear viral DNA replication phase, the characterization of cellular DNA damage responses (DDR), the subnuclear compartments disruption due to viral modulation, and the unravelling of the biological role of several viral proteins (A104R, I215 L, P1192R, QP509 L and Q706 L), so to contribute to underpin rational strategies for vaccine candidates development.

Expanding the activity spectrum of antiviral agents

Broad-spectrum antivirals (BSAs) are agents that inhibit replication of several human viruses. Here, we review 108 approved, investigational, and experimental BSAs, for which safety profiles in humans are available. The most effective and tolerable BSAs could reinforce the arsenal of available antiviral therapeutics pending the results of further pre-clinical and clinical studies.

In vitro comparison of currently available and investigational antiviral agents against pathogenic human double-stranded DNA viruses: A systematic literature review

BACKGROUND

Double-stranded (ds) DNA virus infections often occur concomitantly in immunocompromised patients. We performed a systematic search of published in vitro activity for nine approved and investigational antivirals to understand the spectrum of in vitro activity against dsDNA viruses.

METHODS

A literature search was performed (PubMed and the WoS Core Collection) using keywords related to: 1) targeted approved/developmental antivirals (acyclovir, artesunate, brincidofovir, cidofovir, cyclopropavir (filociclovir), foscarnet, ganciclovir, letermovir, and maribavir); 2) pathogenic dsDNA viruses; 3) in vitro activity. We summarized data from 210 publications.

RESULTS

Activity against ≤3 viruses was documented for maribavir (cytomegalovirus, Epstein-Barr virus), and letermovir, while activity against > 3 viruses was shown for ganciclovir, cidofovir, acyclovir, foscarnet, cyclopropavir, artesunate, and brincidofovir. The EC₅₀ values of brincidofovir were the lowest, ranging from 0.001 to 0.27 μM, for all viruses except papillomaviruses. The next most potent agents included cidofovir, ganciclovir, foscarnet, and acyclovir with EC₅₀ values between 0.1 μM and >10 μM for cytomegalovirus, herpes simplex virus, and adenovirus.

CONCLUSION

Most of the identified antivirals had in vitro activity against more than one dsDNA virus. Brincidofovir and cidofovir have broad-spectrum activity, and brincidofovir has the lowest EC₅₀ values. These findings could assist clinical practice and developmental research.