Synthetic α-Hydroxytropolones Inhibit Replication of Wild-Type and Acyclovir-Resistant Herpes Simplex Viruses

Harmol used for the treatment of herpes simplex virus induced keratitis

Herpes simplex virus type 1 (HSV-1) infection of the eyes results in herpes simplex keratitis (HSK), which has led to vision loss and even blindness in patients. However, the rate of drug resistance in HSV is on the rise; therefore, new antiviral agents with sufficient safety profiles must be developed. At present, we assessed the anti-HSV-1 activity of 502 natural compounds and their ability to reduce the HSV-1-induced cytopathic effect. We chose harmol for further studies because it exhibited the highest antiviral activity. We found that harmol inhibited both HSV-1 F and HSV-1/153 (a clinical drug-resistant strain) replication, with an EC50 of 9.34 µM and 5.84 µM, respectively. Moreover, harmol reduced HSV-1 replication in corneal tissues and viral progeny production in tears, and also alleviated early corneal surface lesions related to HSK. For example, harmol treatment preserved corneal thickness and nerve density in HSK mice. Interestingly, harmol also showed a promising antiviral effect on HSV-1/153 induced HSK in mouse model. Furthermore, harmol combined with acyclovir (ACV) treatment showed a greater antiviral effect than either one alone in vitro. Therefore, harmol may be a promising therapeutic agent for managing HSK.

In vitro evaluation of tropolone absorption, metabolism, and clearance

Tropolone compounds can inhibit hepatitis B virus (HBV) replication at sub-micromolar levels and are synergistic upon co-treatment with nucleos(t)ide analog drugs. However, only a few compounds within this chemotype have been screened for their pharmacological properties. Here, we chose 36 structurally diverse tropolones from six subclasses to characterize their in vitro pharmacological parameters. All compounds were more soluble in pHs that reflect the gastrointestinal tract (pH 5 and 6.5) than plasma (pH 7.4). Those compounds that had solubility limits >100 μM were tested in a passive permeability assay, and there was no general trend in the compounds' passive permeability at any pH. Twenty-nine compounds with the best absorption parameters were tested in HEK293 cells to assess potential cytotoxicity; measured toxicities were similar to those in the hepatic HepDES19 cells used for screening (R2 = 0.55). Sixteen representative compounds were tested against five major CYP450 isoforms and there was no substantial inhibition by any compound against any of the enzymes tested (<50%). The t1/2 values of 15 compounds were determined in the microsome stability assay and 12 compounds were evaluated in plasma protein binding assays to assess factors affecting their rate of clearance. All compounds with detectable analyte peaks had t1/2 > 30 min, and while 4 of 12 had statistically significant decreased potency in conditions with increased albumin concentrations, only one compound's potency was biologically significant. These data indicate that the tropolones have pharmacological characteristics that reflect approved drugs and inform future structure activity relationships during drug design.

Antiviral activity of amide-appended α-hydroxytropolones against herpes simplex virus-1 and -2

α-Hydroxytropolones (αHTs) have potent antiviral activity against herpes simplex virus-1 and -2 (HSV-1 and HSV-2) in cell culture, including against acyclovir-resistant mutants, and as a result have the potential to be developed as antiviral drugs targeting these viruses. We recently described a convenient final-step amidation strategy to their synthesis, and this was used to generate 57 amide-substituted αHTs that were tested against hepatitis B virus. The following manuscript describes the evaluation of this library against HSV-1, as well as a subset against HSV-2. The structure-function analysis obtained from these studies demonstrates the importance of lipophilicity and rigidity to αHT-based anti-HSV potency, consistent with our prior work on smaller libraries. We used this information to synthesize and test a targeted library of 4 additional amide-appended αHTs. The most potent of this new series had a 50% effective concentration (EC₅₀) for viral inhibition of 72 nM, on par with the most potent αHT antivirals we have found to date. Given the ease of synthesis of amide-appended αHTs, this new class of antiviral compounds and the chemistry to make them should be highly valuable in future anti-HSV drug development.

A Systematic Review of Second-Line Treatments in Antiviral Resistant Strains of HSV-1, HSV-2, and VZV

Drug-resistant variants of herpes simplex viruses (HSV) have been reported that are not effectively treated with first-line antiviral agents. The objective of this study was to evaluate available literature on the possible efficacy of second-line treatments in HSV and the use of second-line treatments in HSV strains that are resistant to first-line treatments. Following Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, a final search was conducted in six databases on November 5, 2021 for all relevant literature using terms related to antiviral resistance, herpes, and HSV. Eligible manuscripts were required to report the presence of an existing or proposed second-line treatment for HSV-1, HSV-2, or varicella zoster virus (VZV); have full-text English-language access; and potentially reduce the rate of antiviral resistance. Following screening, 137 articles were included in qualitative synthesis. Of the included studies, articles that examined the relationship between viral resistance to first-line treatments and potential second-line treatments in HSV were included. The Cochrane risk-of-bias tool for randomized trials was used to assess risk of bias. Due to the heterogeneity of study designs, a meta-analysis of the studies was not performed. The dates in which accepted studies were published spanned from 2015-2021. In terms of sample characteristics, the majority (72.26%) of studies used Vero cells. When looking at the viruses on which the interventions were tested, the majority (84.67%) used HSV-1, with (34.31%) of these studies reporting testing on resistant HSV strains. Regarding the effectiveness of the proposed interventions, 91.97% were effective as potential managements for resistant strains of HSV. Of the papers reviewed, nectin in 2.19% of the reviews had efficacy as a second-line treatments in HSV, amenamevir in 2.19%, methanol extract in 2.19%, monoclonal antibodies in 1.46%, arbidol in 1.46%, siRNA swarms in 1.46%, Cucumis melo sulfated pectin in 1.46%, and components from Olea europeae in 1.46%. In addition to this griffithsin in 1.46% was effective, Morus alba L. in 1.46%, using nucleosides in 1.46%, botryosphaeran in 1.46%, monoterpenes in 1.46%, almond skin extracts in 1.46%, bortezomib in 1.46%, flavonoid compounds in 1.46%, andessential oils were effective in 1.46%, but not effective in 0.73%. The available literature reviewed consistently supports the existence and potentiality of second-line treatments for HSV strains that are resistant to first-line treatments. Immunocompromised patients have been noted to be the population most often affected by drug-resistant variants of HSV. Subsequently, we found that HSV infections in this patient population are challenging to manage clinically effectively. The goal of this systematic review is to provide additional information to patients on the potentiality of second-line treatment in HSV strains resistant to first-line treatments, especially those who are immunocompromised. All patients, whether they are immunocompromised or not, deserve to have their infections clinically managed in a manner supported by comprehensive research. This review provides necessary information about treatment options for patients with resistant HSV infections and their providers.

Synthetic derivatives of the antifungal drug ciclopirox are active against herpes simplex virus 2

We previously showed that the anti-fungal drug ciclopirox olamine effectively inhibits replication of herpes simplex virus (HSV)-1 and HSV-2. Given the rise of HSV strains that are resistant to nucleos(t)ide analog treatment, as well as the incomplete efficacy of nucleos(t)ide analogs, new inhibitory compounds must be explored for potential use in the treatment of HSV infection. In the present study, we analyzed 44 compounds derived from the core structure of ciclopirox olamine for inhibitory activity against HSV. Thirteen of these derivative compounds inhibited HSV-2 replication by > 1000- to ∼100,000-fold at 1 μM and displayed EC50 values lower than that of acyclovir, as well as low cytotoxicity, indicating their strong therapeutic potential. Through structural comparison, we also provide evidence for the importance of various structural motifs to the efficacy of ciclopirox and its derivatives, namely hydrophobic groups at R4 and R6 of the ciclopirox core structure. Like ciclopirox, representative analogs exhibit some oral bioavailability but are rapidly cleared in vivo. Together, these results will guide further development of N-hydroxypyridones as HSV therapeutics.

Viral Nucleases from Herpesviruses and Coronavirus in Recombination and Proofreading: Potential Targets for Antiviral Drug Discovery

In this review, we explore recombination in two very different virus families that have become major threats to human health. The Herpesviridae are a large family of pathogenic double-stranded DNA viruses involved in a range of diseases affecting both people and animals. Coronaviridae are positive-strand RNA viruses (CoVs) that have also become major threats to global health and economic stability, especially in the last two decades. Despite many differences, such as the make-up of their genetic material (DNA vs. RNA) and overall mechanisms of genome replication, both human herpes viruses (HHVs) and CoVs have evolved to rely heavily on recombination for viral genome replication, adaptation to new hosts and evasion of host immune regulation. In this review, we will focus on the roles of three viral exonucleases: two HHV exonucleases (alkaline nuclease and PolExo) and one CoV exonuclease (ExoN). We will review the roles of these three nucleases in their respective life cycles and discuss the state of drug discovery efforts against these targets.

Two-Metal Ion-Dependent Enzymes as Potential Antiviral Targets in Human Herpesviruses

The majority of drug discovery efforts against herpesviruses have focused on nucleoside analogs that target viral DNA polymerases, agents that are associated with dose-limiting toxicity and/or a narrow spectrum of activity. We are pursuing a strategy based on targeting two-metal ion-dependent (TMID) viral enzymes. This family of enzymes consists of structurally related proteins that share common active sites containing conserved carboxylates predicted to coordinate divalent cations essential for catalysis. Compounds that target TMID enzymes, such as HIV integrase and influenza endoribonuclease, have been successfully developed for clinical use. HIV integrase inhibitors have been reported to inhibit replication of herpes simplex virus (HSV) and other herpesviruses; however, the molecular targets of their antiviral activities have not been identified. We employed a candidate-based approach utilizing several two-metal-directed chemotypes and the potential viral TMID enzymatic targets in an effort to correlate target-based activity with antiviral potency. The panel of compounds tested included integrase inhibitors, the anti-influenza agent baloxavir, three natural products previously shown to exhibit anti-HSV activity, and two 8-hydroxyquinolines (8-HQs), AK-157 and AK-166, from our in-house program. The integrase inhibitors exhibited weak overall anti-HSV-1 activity, while the 8-HQs were shown to inhibit both HSV-1 and cytomegalovirus (CMV). Target-based analysis demonstrated that none of the antiviral compounds acted by inhibiting ICP8, contradicting previous reports. On the other hand, baloxavir inhibited the proofreading exonuclease of HSV polymerase, while AK-157 and AK-166 inhibited the alkaline exonuclease UL12. In addition, AK-157 also inhibited the catalytic activity of the HSV polymerase, which provides an opportunity to potentially develop dual-targeting agents against herpesviruses. IMPORTANCE Human herpesviruses (HHVs) establish lifelong latent infections, which undergo periodic reactivation and remain a major cause of morbidity and mortality, especially in immunocompromised individuals. Currently, HHV infections are treated primarily with agents that target viral DNA polymerase, including nucleoside analogs; however, long-term treatment can be complicated by the development of drug resistance. New therapies with novel modes of action would be important not only for the treatment of resistant viruses but also for use in combination therapy to reduce dose-limiting toxicities and potentially eliminate infection. Since many essential HHV proteins are well conserved, inhibitors of novel targets would ideally exhibit broad-spectrum activity against multiple HHVs.

Synthesis of Polyoxygenated Tropolones and their Antiviral Activity against Hepatitis B Virus and Herpes Simplex Virus-1

Polyoxygenated tropolones possess a broad range of biological activity, and as a result are promising lead structures or fragments for drug development. However, structure-function studies and subsequent optimization have been challenging, in part due to the limited number of readily available tropolones and the obstacles to their synthesis. Oxidopyrylium [5+2] cycloaddition can effectively generate a diverse array of seven-membered ring carbocycles, and as a result can provide a highly general strategy for tropolone synthesis. Here, we describe the use of 3-hydroxy-4-pyrone-based oxidopyrylium cycloaddition chemistry in the synthesis of functionalized 3,7-dimethoxytropolones, 3,7-dihydroxytropolones, and isomeric 3-hydroxy-7-methoxytropolones through complementary benzyl alcohol-incorporating procedures. The antiviral activity of these molecules against herpes simplex virus-1 and hepatitis B virus is also described, highlighting the value of this approach and providing new structure-function insights relevant to their antiviral activity.

Effects of Troponoids on Mitochondrial Function and Cytotoxicity

The α-hydroxytropolones (αHTs) are troponoid inhibitors of hepatitis B virus (HBV) replication that can target HBV RNase H with submicromolar efficacies. αHTs and related troponoids (tropones and tropolones) can be cytotoxic in cell lines as measured by 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) assays that assess mitochondrial function. Previous studies suggest that tropolones induce cytotoxicity through inhibition of mitochondrial respiration. Therefore, we screened 35 diverse troponoids for effects on mitochondrial function, mitochondrial/nuclear genome ratios, cytotoxicity, and reactive oxygen species (ROS) production. Troponoids as a class did not inhibit respiration or glycolysis, although the α-ketotropolone subclass interfered with these processes. The troponoids had no impact on the mitochondrial DNA/nuclear DNA ratio after 3 days of compound exposure. The patterns of troponoid-induced cytotoxicity among three hepatic cell lines were similar for all compounds, but three potent HBV RNase H inhibitors were not cytotoxic in primary human hepatocytes. Tropolones and αHTs increased ROS production in cells at cytotoxic concentrations but had no effect at lower concentrations that efficiently inhibit HBV replication. Troponoid-mediated cytotoxicity was significantly decreased upon the addition of the ROS scavenger N-acetylcysteine. These studies show that troponoids can increase ROS production at high concentrations within cell lines, leading to cytotoxicity, but are not cytotoxic in primary hepatocytes. Future development of αHTs as potential therapeutics against HBV may need to mitigate ROS production by altering compound design and/or by coadministering ROS antagonists to ameliorate increased ROS levels.

Characterization of the NiRAN domain from RNA-dependent RNA polymerase provides insights into a potential therapeutic target against SARS-CoV-2

Apart from the canonical fingers, palm and thumb domains, the RNA dependent RNA polymerases (RdRp) from the viral order Nidovirales possess two additional domains. Of these, the function of the Nidovirus RdRp associated nucleotidyl transferase domain (NiRAN) remains unanswered. The elucidation of the 3D structure of RdRp from the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), provided the first ever insights into the domain organisation and possible functional characteristics of the NiRAN domain. Using in silico tools, we predict that the NiRAN domain assumes a kinase or phosphotransferase like fold and binds nucleoside triphosphates at its proposed active site. Additionally, using molecular docking we have predicted the binding of three widely used kinase inhibitors and five well characterized anti-microbial compounds at the NiRAN domain active site along with their drug-likeliness. For the first time ever, using basic biochemical tools, this study shows the presence of a kinase like activity exhibited by the SARS-CoV-2 RdRp. Interestingly, a well-known kinase inhibitor- Sorafenib showed a significant inhibition and dampened viral load in SARS-CoV-2 infected cells. In line with the current global COVID-19 pandemic urgency and the emergence of newer strains with significantly higher infectivity, this study provides a new anti-SARS-CoV-2 drug target and potential lead compounds for drug repurposing against SARS-CoV-2.

The on-going coronavirus disease 2019 (COVID-19) pandemic caused by the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is significantly affecting the world health. Unfortunately, over 180 million cases of COVID-19 resulting in nearly 4 million deaths have been reported till June, 2021. In this study, using a combination of bioinformatics, biochemical and mass spectrometry methods, we show that the Nidovirus RdRp associated Nucleotidyl transferase (NiRAN) domain of the RNA-dependent RNA polymerase (RdRp) of SARS-CoV-2 exhibits a kinase like activity. Additionally, we also show that few broad spectrum anti-cancer and anti-microbial drugs dampen this kinase like activity. Of note, Sorafenib, an FDA approved anti-cancer kinase inhibiting drug significantly reduces the SARS-CoV-2 load in cell lines. Our study suggests that NiRAN domain of the SARS-CoV-2 RdRp is indispensible for the successful viral life cycle and shows that abolishing this enzymatic function of RdRp by small molecule inhibitors may open novel avenues for COVID-19 therapeutics.

Characterization of the NiRAN domain from RNA-dependent RNA polymerase provides insights into a potential therapeutic target against SARS-CoV-2

Apart from the canonical fingers, palm and thumb domains, the RNA dependent RNA polymerases (RdRp) from the viral order Nidovirales possess two additional domains. Of these, the function of the Nidovirus RdRp associated nucleotidyl transferase domain (NiRAN) remains unanswered. The elucidation of the 3D structure of RdRp from the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), provided the first ever insights into the domain organisation and possible functional characteristics of the NiRAN domain. Using in silico tools, we predict that the NiRAN domain assumes a kinase or phosphotransferase like fold and binds nucleoside triphosphates at its proposed active site. Additionally, using molecular docking we have predicted the binding of three widely used kinase inhibitors and five well characterized anti-microbial compounds at the NiRAN domain active site along with their drug-likeliness. For the first time ever, using basic biochemical tools, this study shows the presence of a kinase like activity exhibited by the SARS-CoV-2 RdRp. Interestingly, a well-known kinase inhibitor- Sorafenib showed a significant inhibition and dampened viral load in SARS-CoV-2 infected cells. In line with the current global COVID-19 pandemic urgency and the emergence of newer strains with significantly higher infectivity, this study provides a new anti-SARS-CoV-2 drug target and potential lead compounds for drug repurposing against SARS-CoV-2.

Anti-herpetic Activity of Macrocystis pyrifera and Durvillaea antarctica Algae Extracts Against HSV-1 and HSV-2

Herpes simplex viruses (HSVs) type 1 (HSV-1) and type 2 (HSV-2) are highly prevalent in the human population, and the infections they produce are lifelong with frequent reactivations throughout life. Both viruses produce uncomfortable and sometimes painful lesions in the orofacial and genital areas, as well as herpetic gingivostomatitis, among other clinical manifestations. At present, the most common treatments against HSVs consist of nucleoside analogs that target the viral polymerases. However, such drugs are poorly effective for treating skin lesions, as they only reduce in 1-2 days the duration of the herpetic lesions. Additionally, viral isolates resistant to these drugs can emerge in immunosuppressed individuals, and second-line drugs for such variants are frequently accompanied by adverse effects requiring medical supervision. Thus, novel or improved therapeutic drugs for treating HSV lesions are needed. Here, we assessed the potential antiviral activity of aqueous extracts obtained from two brown macroalgae, namely Macrocystis pyrifera and Durvillaea antarctica against HSVs. Both extracts showed antiviral activity against acyclovir-sensitive and acyclovir-resistant HSV-1 and HSV-2. Our analyses show that there is a significant antiviral activity associated with proteins in the extract, although other compounds also seem to contribute to inhibiting the replication cycle of these viruses. Evaluation of the algae extracts as topical formulations in an animal model of HSV-1 skin infection significantly reduced the severity of the disease more than acyclovir, as well as the duration of the herpetic lesions, when compared to mock-treated animals, with the D. antarctica extract performing best. Taken together, these findings suggest that these algae extracts may be potential phytotherapeutics against HSVs and may be useful for the treatment and reduction of common herpetic manifestations in humans.

Cetylpyridinium chloride blocks herpes simplex virus replication in gingival fibroblasts

Infections with herpes simplex viruses are lifelong and highly prevalent worldwide. Individuals with clinical symptoms elicited by HSVs may suffer from occasional or recurrent herpetic lesions in the orofacial and genital areas. Despite the existence of nucleoside analogues that interfere with HSV replication, such as acyclovir, these drugs are somewhat ineffective in treating skin lesions as topical formulations only reduce in one or few days the duration of the herpetic ulcers. Cetylpyridinium chloride (CPC) is a quaternary ammonium compound present in numerous hygiene products, such as mouthwashes, deodorants, aphtae-treating formulations and oral tablets as an anti-septic to limit bacterial growth. Some reports indicate that CPC can also modulate host signaling pathways, namely NF-κB signaling. Because HSV infection is modulated by NF-κB, we sought to assess whether CPC has antiviral effects against HSVs. Using wild-type HSV-1 and HSV-2, as well as viruses that are acyclovir-resistant or encode GFP reporter genes, we assessed the antiviral capacity of CPC in epithelial cells and human gingival fibroblasts expanded from the oral cavity and its mechanism of action. We found that a short, 10-min exposure to CPC added after HSV entry into the cells, significantly limited viral replication in both cell types by impairing viral gene expression. Interestingly, our results suggest that CPC blocks HSV replication by interfering with the translocation of NF-κB into the nucleus of HSV-infected cells. Taken together, these findings suggest that formulations containing CPC may help limit HSV replication in infected tissues and consequently reduce viral shedding.

Amide-containing α-hydroxytropolones as inhibitors of hepatitis B virus replication

The Hepatitis B Virus (HBV) ribonuclease H (RNaseH) is a promising but unexploited drug target. Here, we synthesized and analyzed a library of 57 amide-containing α-hydroxytropolones (αHTs) as potential leads for HBV drug development. Fifty percent effective concentrations ranged from 0.31 to 54 μM, with selectivity indexes in cell culture of up to 80. Activity against the HBV RNaseH was confirmed in semi-quantitative enzymatic assays with recombinant HBV RNaseH. The compounds were overall poorly active against human ribonuclease H1, with 50% inhibitory concentrations of 5.1 to >1,000 μM. The αHTs had modest activity against growth of the fungal pathogen Cryptococcus neoformans, but had very limited activity against growth of the Gram - bacterium Escherichia coli and the Gram + bacterium Staphylococcus aureus, indicating substantial selectivity for HBV. A molecular model of the HBV RNaseH templated against the Ty3 RNaseH was generated. Docking the compounds to the RNaseH revealed the anticipated binding pose with the divalent cation coordinating motif on the compounds chelating the two Mn++ ions modeled into the active site. These studies reveal that that amide αHTs can be strong, specific HBV inhibitors that merit further assessment toward becoming anti-HBV drugs.

Antiviral activity of Α-hydroxytropolones on caprine alphaherpesvirus 1 in vitro

The emergence of human alphaherpesvirus strains (i.e. HHV-1 and -2) resistant to commonly used antiviral drugs has prompted the research for alternative, biologically active anti-herpetic agents. Natural-product and synthetic α-hydroxytropolones (αHTs) have been identified as lead therapeutic agents for a number of infections, including HHV-1 and -2, and several veterinary herpesviruses, i.e. bovine alphaherpesvirus 1 (BoHV-1), equine alphaherpesvirus 1 (EHV-1) and feline alphaherpesvirus 1 (FHV-1). In the present study we evaluated the activity in vitro of two natural and two synthetic α-hydroxytropolones (αHTs) against Caprine alphaherpesvirus 1 (CpHV-1) which is regarded as a useful homologous animal model for the study of HSV-2 infection, chiefly for the assessment of antiviral drugs in in vivo studies. AlphaHTs were able to decrease significantly CpHV-1 viral titres up to 4.25 log10 TCID₅₀/50 μl and suppressed extensively CpHV-1 nucleic acids up to 8.71 log10 viral DNA copy number/10 μl. This study demonstrated the efficacy of αHTs against CpHV-1 in vitro, adding to their activity observed against the human and animal alphaherpesviruses in vitro. The activity of αHTs against CpHV-1 appeared similar but not identical to the patterns of activity observed against other alphaherpesviruses, suggesting virus-related variability in terms of response to specific αHT molecules. These findings open several perspectives in terms of future studies using the CpHV-1 homologous animal model, for the development of therapeutic tools against herpesviruses.

Divergent synthesis of a thiolate-based α-hydroxytropolone library with a dynamic bioactivity profile

Here we describe a rapid and divergent synthetic route toward structurally novel αHTs functionalized with either one or two thioether or sulfonyl appendages. Evaluation of this library against hepatitis B and herpes simplex virus, as well as the pathogenic fungus Cryptococcus neoformans, and a human hepatoblastoma (HepDES19) revealed complementary biological profiles and new lead compounds with sub-micromolar activity against each pathogen.

Importance of lipophilicity for potent anti-herpes simplex virus-1 activity of α-hydroxytropolones

We previously reported that troponoid compounds profoundly inhibit replication of herpes simplex virus (HSV)-1 and HSV-2 in cell culture, including acyclovir-resistant mutants. Synthesis of 26 alpha-hydroxylated tropolones (αHTs) led to a preliminary structure-activity relationship highlighting the potency of bi-phenyl side chains. Here, we explore the structure-activity relationship in more detail, with a focus on various biaryl and other lipophilic molecules. Along with our prior structure-function analysis, we present a refined structure-activity relationship that reveals the importance of the lipophilicity and nature of the side chain for potent anti-HSV-1 activity in cells. We expect this new information will help guide future optimization of αHTs as HSV antivirals.

Overview of Recent Strategic Advances in Medicinal Chemistry

Introducing novel strategies, concepts, and technologies that speed up drug discovery and the drug development cycle is of great importance both in the highly competitive pharmaceutical industry as well as in academia. This Perspective aims to present a "big-picture" overview of recent strategic innovations in medicinal chemistry and drug discovery.

Synthesis and Evaluation of Troponoids as a New Class of Antibiotics

Novel antibiotics are urgently needed. The troponoids [tropones, tropolones, and α-hydroxytropolones (α-HT)] can have anti-bacterial activity. We synthesized or purchased 92 troponoids and evaluated their antibacterial activities against Staphylococcus aureus, Escherichia coli, Acinetobacter baumannii, and Pseudomonas aeruginosa. Preliminary hits were assessed for minimum inhibitory concentrations (MIC₈₀) and cytotoxicity (CC₅₀) against human hepatoma cells. Sixteen troponoids inhibited S. aureus/E. coli/A. baumannii growth by ≥80% growth at <30 μM with CC₅₀ values >50 μM. Two selected tropolones (63 and 285) inhibited 18 methicillin-resistant S. aureus (MRSA) strains with similar MIC₈₀ values as against a reference strain. Two selected thiotropolones (284 and 363) inhibited multidrug-resistant (MDR) E. coli with MIC₈₀ ≤30 μM. One α-HT (261) inhibited MDR-A. baumannii with MIC₈₀ ≤30 μM. This study opens new avenues for development of novel troponoid antibiotics to address the critical need to combat MDR bacterial infections.

The HIV Integrase Inhibitor Raltegravir Inhibits Felid Alphaherpesvirus 1 Replication by Targeting both DNA Replication and Late Gene Expression

Alphaherpesvirus-associated ocular infections in humans caused by human alphaherpesvirus 1 (HHV-1) remain challenging to treat due to the frequency of drug application required and the potential for the selection of drug-resistant viruses. Repurposing on-the-market drugs is a viable strategy to accelerate the pace of drug development. It has been reported that the human immunodeficiency virus (HIV) integrase inhibitor raltegravir inhibits HHV-1 replication by targeting the DNA polymerase accessory factor and limits terminase-mediated genome cleavage of human betaherpesvirus 5 (HHV-5). We have previously shown, both in vitro and in vivo, that raltegravir can also inhibit the replication of felid alphaherpesvirus 1 (FeHV-1), a common ocular pathogen of cats with a pathogenesis similar to that of HHV-1 ocular disease. In contrast to what was reported for HHV-1, we were unable to select for a raltegravir-resistant FeHV-1 strain in order to define any basis for drug action. A candidate-based approach to explore the mode of action of raltegravir against FeHV-1 showed that raltegravir did not impact FeHV-1 terminase function, as described for HHV-5. Instead, raltegravir inhibited DNA replication, similarly to HHV-1, but by targeting the initiation of viral DNA replication rather than elongation. In addition, we found that raltegravir specifically repressed late gene expression independently of DNA replication, and both activities are consistent with inhibition of ICP8. Taken together, these results suggest that raltegravir could be a valuable therapeutic agent against herpesviruses.IMPORTANCE The rise of drug-resistant herpesviruses is a longstanding concern, particularly among immunocompromised patients. Therefore, therapies targeting viral proteins other than the DNA polymerase that may be less likely to lead to drug-resistant viruses are urgently needed. Using FeHV-1, an alphaherpesvirus closely related to HHV-1 that similarly causes ocular herpes in its natural host, we found that the HIV integrase inhibitor raltegravir targets different stages of the virus life cycle beyond DNA replication and that it does so without developing drug resistance under the conditions tested. This shows that the drug could provide a viable strategy for the treatment of herpesvirus infections.

Sensitivity of the C-Terminal Nuclease Domain of Kaposi's Sarcoma-Associated Herpesvirus ORF29 to Two Classes of Active-Site Ligands

Kaposi's sarcoma-associated herpesvirus (KSHV), the etiological agent of Kaposi's sarcoma, belongs to the Herpesviridae family, whose members employ a multicomponent terminase to resolve nonparametric viral DNA into genome-length units prior to their packaging. Homology modeling of the ORF29 C-terminal nuclease domain (pORF29C) and bacteriophage Sf6 gp2 have suggested an active site clustered with four acidic residues, D476, E550, D661, and D662, that collectively sequester the catalytic divalent metal (Mn2+) and also provided important insight into a potential inhibitor binding mode. Using this model, we have expressed, purified, and characterized the wild-type pORF29C and variants with substitutions at the proposed active-site residues. Differential scanning calorimetry demonstrated divalent metal-induced stabilization of wild-type (WT) and D661A pORF29C, consistent with which these two enzymes exhibited Mn2+-dependent nuclease activity, although the latter mutant was significantly impaired. Thermal stability of WT and D661A pORF29C was also enhanced by binding of an α-hydroxytropolone (α-HT) inhibitor shown to replace divalent metal at the active site. For the remaining mutants, thermal stability was unaffected by divalent metal or α-HT binding, supporting their role in catalysis. pORF29C nuclease activity was also inhibited by two classes of small molecules reported to inhibit HIV RNase H and integrase, both of which belong to the superfamily of nucleotidyltransferases. Finally, α-HT inhibition of KSHV replication suggests ORF29 nuclease function as an antiviral target that could be combined with latency-activating compounds as a shock-and-kill antiviral strategy.

MODERN ETHIOTROPIC CHEMOTHERAPY OF HERPESVIRUS INFECTIONS: ADVANCES, NEW TRENDS AND PERSPECTIVES. ALPHAHERPESVIRUSES (PART II)

A key role in the treatment of herpesviral infections is played by modified nucleosides and their predecessors - acyclovir, its L-valine ester (valaciclovir) and famciclovir (prodrug of penciclovir). The biological activity of compounds of this class is determined by their similarity to natural nucleosides. After phosphorylation by viral thymidine kinase and then cell enzymes to the triphosphate forms, acyclovir and penciclovir inhibit the activity of viral DNA polymerase and synthesis of viral DNA. The increasing role of herpesvirus infections in human infectious pathology, as well as the development of drug resistance in viruses, mainly in patients with immunodeficiencies of various origins, necessitate the search for new compounds possessing anti-herpesvirus activity, using as a biological target not DNA polymerase, but other viral proteins and enzymes, unique or different from cellular proteins, performing similar functions.

Amidation Strategy for Final-Step α-Hydroxytropolone Diversification

α-Hydroxytropolones (αHTs) are excellent metalloenzyme-inhibiting fragments that have been the basis for the development of potent inhibitors of various therapeutically important enzymes. The following manuscript describes a final-step amidation approach for αHT diversification. The method takes advantage of a scalable, chromatography-free synthesis of a carboxylic acid-appended αHT, and in the present manuscript we describe the synthesis of eight amide-containing αHTs, three of which we envision using as chemical probes. We expect that the general strategy will find widespread usage in both chemical biology and medicinal chemistry studies on αHTs.

Fluorous-Phase Approach to α-Hydroxytropolone Synthesis

α-Hydroxytropolones (αHTs) are troponoids that demonstrate inhibition against an array of therapeutically significant targets, making them potential drug leads for several human diseases. We have utilized a recently discovered one-pot three-component oxidopyrylium cycloaddition in a solid-supported synthesis of αHTs. Though the procedure is time efficient and generates assay-ready molecules, the system suffers from low yields and an inability to perform reaction modifications on resin-bound intermediates. In order to combat these issues with the solid-phase platform, we incorporated fluorous tags into our synthetic route. Through the implementation of fluorous phase chemistry, we demonstrate a substantial increase in the overall yield of αHTs, as well as an ability to execute metal-catalyzed cross coupling and amide coupling on fluorous tagged intermediates. We also show that tagged molecules can be separated from nonfluorous impurities, and vice versa, by utilizing fluorous liquid-liquid and solid-phase extractions. Hence, these proof-of-principle investigations describe the viability of a fluorous phase approach to αHT synthesis and its potential to serve as a combinatorial technique to produce structurally diverse substrates.

Broad anti-herpesviral activity of α-hydroxytropolones

Herpesviruses are ubiquitous in animals and cause economic losses concomitant with many diseases. Most of the domestic animal herpesviruses are within the subfamily Alphaherpesvirinae, which includes human herpes simplex virus 1 (HSV-1). Suppression of HSV-1 replication has been reported with α-hydroxytropolones (αHTs), aromatic ring compounds that have broad bioactivity due to potent chelating activity. It is postulated that αHTs inhibit enzymes within the nucleotidyltransferase superfamily (NTS). These enzymes require divalent cations for nucleic acid cleavage activity. Potential targets include the nuclease component of the herpesvirus terminase (pU_L15C), a highly conserved NTS-like enzyme that cleaves viral DNA into genomic lengths prior to packaging into capsids. Inhibition of pU_L15C activity in biochemical assays by various αHTs previously revealed a spectrum of potencies. Interestingly, the most potent anti-pU_L15C αHT inhibited HSV-1 replication to a limited extent in cell culture. The aim of this study was to evaluate three different αHT molecules with varying biochemical anti-pU_L15C activity for a capacity to inhibit replication of veterinary herpesviruses (BoHV-1, EHV-1, and FHV-1) and HSV-1. Given the known discordant potencies between anti-pU_L15C and HSV-1 replication inhibition, a second objective was to elucidate the mechanism of action of these compounds. The results show that αHTs broadly inhibit herpesviruses, with similar inhibitory effect against HSV-1, BoHV-1, EHV-1, and FHV-1. Based on immunoblotting, Southern blotting, and real-time qPCR, the compounds were found to specifically inhibit viral DNA replication. Thus, αHTs represent a new class of broadly active anti-herpesviral compounds with potential veterinary applications.

Synthesis and biological assessment of 3,7-dihydroxytropolones

3,7-Dihydroxytropolones (3,7-dHTs) are highly oxygenated troponoids that have been identified as lead compounds for several human diseases. To date, structure-function studies on these molecules have been limited due to a scarcity of synthetic methods for their preparation. New synthetic strategies towards structurally novel 3,7-dHTs would be valuable in further studying their therapeutic potential. Here we describe the successful adaptation of a [5 + 2] oxidopyrilium cycloaddition/ring-opening for 3,7-dHT synthesis, which we apply in the synthesis of a plausible biosynthetic intermediate to the natural products puberulic and puberulonic acid. We have also tested these new compounds in several biological assays related to human immunodeficiency virus (HIV), hepatitis B virus (HBV) and herpes simplex virus (HSV) in order to gain insight into structure-functional analysis related to antiviral troponoid development.

The Exonuclease Activity of Herpes Simplex Virus 1 UL12 Is Required for Production of Viral DNA That Can Be Packaged To Produce Infectious Virus

The herpes simplex virus (HSV) type I alkaline nuclease, UL12, has 5'-to-3' exonuclease activity and shares homology with nucleases from other members of the Herpesviridae family. We previously reported that a UL12-null virus exhibits a severe defect in viral growth. To determine whether the growth defect was a result of loss of nuclease activity or another function of UL12, we introduced an exonuclease-inactivating mutation into the viral genome. The recombinant virus, UL12 D340E (the D340E mutant), behaved identically to the null virus (AN-1) in virus yield experiments, exhibiting a 4-log decrease in the production of infectious virus. Furthermore, both viruses were severely defective in cell-to-cell spread and produced fewer DNA-containing capsids and more empty capsids than wild-type virus. In addition, DNA packaged by the viral mutants was aberrant, as determined by infectivity assays and pulsed-field gel electrophoresis. We conclude that UL12 exonuclease activity is essential for the production of viral DNA that can be packaged to produce infectious virus. This conclusion was bolstered by experiments showing that a series of natural and synthetic α-hydroxytropolones recently reported to inhibit HSV replication also inhibit the nuclease activity of UL12. Taken together, our results demonstrate that the exonuclease activity of UL12 is essential for the production of infectious virus and may be considered a target for development of antiviral agents.IMPORTANCE Herpes simplex virus is a major pathogen, and although nucleoside analogs such as acyclovir are highly effective in controlling HSV-1 or -2 infections in immunocompetent individuals, their use in immunocompromised patients is complicated by the development of resistance. Identification of additional proteins essential for viral replication is necessary to develop improved therapies. In this communication, we confirm that the exonuclease activity of UL12 is essential for viral replication through the analysis of a nuclease-deficient viral mutant. We demonstrate that the exonuclease activity of UL12 is essential for the production of viral progeny and thus provides an attractive, druggable enzymatic target.

Efficacy and cytotoxicity in cell culture of novel α-hydroxytropolone inhibitors of hepatitis B virus ribonuclease H

Chronic Hepatitis B virus (HBV) infection is a major worldwide public health problem. Current direct-acting anti-HBV drugs target the HBV DNA polymerase activity, but the equally essential viral ribonuclease H (RNaseH) activity is unexploited as a drug target. Previously, we reported that α-hydroxytropolone compounds can inhibit the HBV RNaseH and block viral replication. Subsequently, we found that our biochemical RNaseH assay underreports efficacy of the α-hydroxytropolones against HBV replication. Therefore, we conducted a structure-activity analysis of 59 troponoids against HBV replication in cell culture. These studies revealed that antiviral efficacy is diminished by larger substitutions on the tropolone ring, identified key components in the substitutions needed for high efficacy, and revealed that cytotoxicity correlates with increased lipophilicity of the α-hydroxytropolones. These data provide key guidance for further optimization of the α-hydroxytropolone scaffold as novel HBV RNaseH inhibitors.

Troponoids Can Inhibit Growth of the Human Fungal Pathogen Cryptococcus neoformans

Cryptococcus neoformans is a pathogen that is common in immunosuppressed patients. It can be treated with amphotericin B and fluconazole, but the mortality rate remains 15 to 30%. Thus, novel and more effective anticryptococcal therapies are needed. The troponoids are based on natural products isolated from western red cedar, and have a broad range of antimicrobial activities. Extracts of western red cedar inhibit the growth of several fungal species, but neither western red cedar extracts nor troponoid derivatives have been tested against C. neoformans We screened 56 troponoids for their ability to inhibit C. neoformans growth and to assess whether they may be attractive candidates for development into anticryptococcal drugs. We determined MICs at which the compounds inhibited 80% of cryptococcal growth relative to vehicle-treated controls and identified 12 compounds with MICs ranging from 0.2 to 15 μM. We screened compounds with MICs of ≤20 μM for cytotoxicity in liver hepatoma cells. Fifty percent cytotoxicity values (CC₅₀s) ranged from 4 to >100 μM. The therapeutic indexes (TI, CC₅₀/MIC) for most of the troponoids were fairly low, with most being <8. However, two compounds had TI values that were >8, including a tropone with a TI of >300. These tropones are fungicidal and are not antagonistic when used in combination with fluconazole or amphotericin B. Inhibition by these two tropones remains unchanged under conditions favoring cryptococcal capsule formation. These data support the hypothesis that troponoids may be a productive scaffold for the development of novel anticryptococcal therapies.

Traceless Solid-Phase α-Hydroxytropolone Synthesis

α-Hydroxytropolones are established inhibitors of several therapeutically relevant binuclear metalloenzymes, and thus lead drug targets for various human diseases. We have leveraged a recently-disclosed three-component oxidopyrylium cycloaddition in the first solid-phase synthesis of α-hydroxytropolones. We also showed that, while minor impurities exist after cleavage and aqueous wash, the semi-crude products display activity in HIV RT-associated RNaseH enzymatic and cell-based assays consistent with pure molecules made in solution phase. These proof-of-principle studies demonstrate the feasibility of solid-phase α-hydroxytropolone synthesis and its potential to serve as a powerful platform for α-hydroxytropolone-based drug discovery and development.

Synthetic α-Hydroxytropolones as Inhibitors of HIV Reverse Transcriptase Ribonuclease H Activity

HIV Reverse Transcriptase-associated ribonuclease H activity is a promising enzymatic target for drug development that has not been successfully targeted in the clinic. While the α-hydroxytropolone-containing natural products β-thujaplicinol and manicol have emerged as some of the most potent leads described to date, structure-function studies have been limited to the natural products and semi-synthetic derivatives of manicol. Thus, a library of α-hydroxytropolones synthesized through a convenient oxidopyrylium cycloaddition/ring-opening sequence have been tested in in vitro and cell-based assays, and have been analyzed using computational support. These studies reveal new synthetic α-hydroxytropolones that, unlike the natural product leads they are derived from, demonstrate protective antiviral activity in cellular assays.

Discovery and Development of a Three-Component Oxidopyrylium [5 + 2] Cycloaddition

α-Hydroxy-γ-pyrone-based oxidopyrylium cycloaddition reactions are useful methods for accessing a highly diverse range of oxabicyclo[3.2.1]octane products. Intermolecular variants of the reaction require the formation of a methyl triflate-based pre-ylide salt that upon treatment with base in the presence of alkenes or alkynes leads to α-methoxyenone-containing bicyclic products. Herein, we describe our discovery that the use of ethanol-stabilized chloroform as solvent leads to the generation of α-ethoxyenone-containing bicyclic byproducts. This three-component process was further optimized by gently heating a mixture of a purified version of the oxidopyrylium dimer in the presence of an alcohol prior to addition of a dipolarophile. Using this convenient procedure, several new oxidopyrylium cycloaddition products can be generated in moderate yields. We also highlight the method in a tandem ring-opening/debenzylation method for the generation of α-hydroxytropolones.