Discovery of novel dual inhibitors of the wild-type and the most prevalent drug-resistant mutant, S31N, of the M2 proton channel from influenza A virus

Virtual screening and molecular growth guide the design of inhibitors for the influenza virus drug-resistant mutant M2-V27A/S31N

The influenza A virus matrix protein 2 (AM2) protein is a proton-gated, proton-selective ion channel essential for influenza replication that has been identified as an antiviral target. The drug-resistance of the M2-V27A/S31N strain, which has been growing more prevalent in recent years and has the potential to spread globally, prevents current amantadine inhibitors from having the desired impact. In this study, we compiled the most common influenza A virus strains from 2001-2020 from the U.S. National Center for Biotechnology Information database and hypothesized that M2-V27A/S31N would become a common strain. The lead compound ZINC299830590 was screened for M2-V27A/S31N in the ZINC15 database using a pharmacophore model and molecular descriptors. This lead compound was then optimized by molecular growth, which allowed us to identify important amino acid residues and create interactions with them to produce compound 4. Molecular dynamics simulation showed that the complex of compound 4 and M2-V27A/S31N had certain degrees of stability and flexibility. The binding free energy of compound 4 was calculated using the MM/PB(GB)SA method and totaled -106.525 kcal/mol. Finally, physicochemical and pharmacokinetic profiles were predicted using the Absorption, Distribution, Metabolism, Excretion, and Toxicity model, which indicated the good bioavailability of compound 4. These results provide the basis for further in vivo and in vitro studies to demonstrate that compound 4 is a promising drug candidate against M2-V27A/S31N.Communicated by Ramaswamy H. Sarma.

Antiviral Compounds to Address Influenza Pandemics: An Update from 2016-2022

In recent decades, the world has gained experience of the dangerous effects of pandemic events caused by emerging respiratory viruses. In particular, annual epidemics of influenza are responsible for severe illness and deaths. Even if conventional influenza vaccines represent the most effective tool for preventing virus infections, they are not completely effective in patients with severe chronic disease and immunocompromised and new small molecules have emerged to prevent and control the influenza viruses. Thus, the attention of chemists is continuously focused on the synthesis of new antiviral drugs able to interact with the different molecular targets involved in the virus replication cycle. To date, different classes of influenza viruses inhibitors able to target neuraminidase enzyme, hemagglutinin protein, Matrix-2 (M2) protein ion channel, nucleoprotein or RNAdependent RNA polymerase have been synthesized using several synthetic strategies comprising the chemical modification of currently used drugs. The best results, in terms of inhibitory activity, are in the nanomolar range and have been obtained from the chemical modification of clinically used drugs such as Peramivir, Zanamivir, Oseltamir, Rimantadine, as well as sialylated molecules, and hydroxypyridinone derivatives. The aim of this review is to report, covering the period 2016-2022, the most recent routes related to the synthesis of effective influenza virus inhibitors.

Aprotinin-Drug against Respiratory Diseases

Aprotinin (APR) was discovered in 1930. APR is an effective pan-protease inhibitor, a typical "magic shotgun". Until 2007, APR was widely used as an antithrombotic and anti-inflammatory drug in cardiac and noncardiac surgeries for reduction of bleeding and thus limiting the need for blood transfusion. The ability of APR to inhibit proteolytic activation of some viruses leads to its use as an antiviral drug for the prevention and treatment of acute respiratory virus infections. However, due to incompetent interpretation of several clinical trials followed by incredible controversy in the literature, the usage of APR was nearly stopped for a decade worldwide. In 2015-2020, after re-analysis of these clinical trials' data the restrictions in APR usage were lifted worldwide. This review discusses antiviral mechanisms of APR action and summarizes current knowledge and prospective regarding the use of APR treatment for diseases caused by RNA-containing viruses, including influenza and SARS-CoV-2 viruses, or as a part of combination antiviral treatment.

Discovery of oseltamivir-based novel PROTACs as degraders targeting neuraminidase to combat H1N1 influenza virus

Annual and sporadic influenza outbreaks pose a great threat to human health and the economy worldwide. Moreover, the frequent mutation of influenza viruses caused by antigen drift complicates the application of antiviral therapeutics. As such, there is an urgent need for novel antiviral agents to tackle the problem of insufficient efficacy of licensed drugs. Inspired by the success of the newly emerged PROTACs (PROteolysis TArgeting Chimeras) strategy, we report herein the design and synthesis of novel PROTAC molecules based on an oseltamivir scaffold to combat severe annual influenza outbreaks. Among these, several compounds showed good anti-H1N1 activity and efficient influenza neuraminidase (NA) degradation activity. The best compound, 8e, effectively induced influenza NA degradation in a dose-dependent manner and relied on the ubiquitin-proteasome pathway. Moreover, Compound 8e exhibited potent antiviral activity toward both wild-type H1N1 virus and an oseltamivir-resistant strain (H1N1, H274Y). A molecular docking study demonstrated that Compound 8e had good hydrogen-bonding and hydrophobic interactions with both the active sites of NA and Von Hippel-Lindau (VHL) proteins, which could effectively drive the favorable interaction of these two proteins. Thus, as the first report of a successful anti-influenza PROTAC, this proof of concept will greatly widen the application range of the PROTAC technique to antiviral drug discovery.

Inhibition of Influenza Virus Replication by Oseltamivir Derivatives

Characterized by the high morbidity and mortality and seasonal surge, the influenza virus (IV) remains a major public health challenge. Oseltamivir is commonly used as a first-line antiviral. As a neuraminidase inhibitor, it attenuates the penetration of viruses through the mucus on the respiratory tract and inhibits the release of virus progeny from infected cells. However, over the years, oseltamivir-resistant strains have been detected in the IV surveillance programs. Therefore, new antivirals that circumvent the resistant strains would be of great importance. In this study, two novel secondary amine derivatives of oseltamivir CUHK326 (6f) and CUHK392 (10i), which bear heteroaryl groups of M2-S31 proton channel inhibitors, were designed, synthesized and subjected to biological evaluation using plaque assay. Influenza A virus (A/Oklahoma/447/2008, H1N1), influenza B viruses (B/HongKong/CUHK33261/2012), an oseltamivir-resistant influenza A virus (A/HongKong/CUHK71923/2009, H1N1) and an oseltamivir-resistant influenza B virus (B/HongKong/CUHK33280/2012) were included in the antiviral effect assessment compared to oseltamivir carboxylate (OC). Both novel compounds significantly reduced the plaque size of seasonal IV A and B, and performed similarly to OC at their corresponding half-maximal inhibitory concentration (IC₅₀). CUHK392 (10i) functioned more effectively than CUHK326 (6f). More importantly, these compounds showed an inhibitory effect on the oseltamivir-resistant strain under 10 nM with selective index (SI) of >200.

Physiologically Active Compounds Based on Membranotropic Cage Carriers–Derivatives of Adamantane and Polyhedral Boron Clusters (Review)

Data on compounds based on cage structures―boron clusters (polyhedral boron hydrides, carboranes, metallacarboranes) and compounds of the adamantane series, which possess physiological activity, have been generalized. The main emphasis is placed on the antiviral activity of the compounds. The mechanism of the possible action of the replication inhibitors of influenza A virus strains is considered, the molecular model of viroporin inhibitors is discussed. The proposed model consists of a cage hydrophobic core that performs the function of a membranotropic carrier (a boron cluster or adamantane fragment), into which physiologically active functional groups are introduced. The relationship between the structure of the cage compound with the introduced substitute and the biologically active properties of this molecular structure has been analyzed.

Ultrasensitive chemiluminescent neuraminidase probe for rapid screening and identification of small-molecules with antiviral activity against influenza A virus in mammalian cells

Influenza A virus is the most virulent influenza subtype and is associated with large-scale global pandemics characterized by high levels of morbidity and mortality. Developing simple and sensitive molecular methods for detecting influenza viruses is critical. Neuraminidase, an exo-glycosidase displayed on the surface of influenza virions, is responsible for the release of the virions and their spread in the infected host. Here, we present a new phenoxy-dioxetane chemiluminescent probe (CLNA) that can directly detect neuraminidase activity. The probe exhibits an effective turn-on response upon reaction with neuraminidase and produces a strong emission signal at 515 nm with an extremely high signal-to-noise ratio. Comparison measurements of our new probe with previously reported analogous neuraminidase optical probes showed superior detection capability in terms of response time and sensitivity. Thus, as far as we know, our probe is the most sensitive neuraminidase probe known to date. The chemiluminescence turn-on response produced by our neuraminidase probe enables rapid screening for small molecules that inhibit viral replication through different mechanisms as validated directly in influenza A-infected mammalian cells using the known inhibitors oseltamivir and amantadine. We expect that our new chemiluminescent neuraminidase probe will prove useful for various applications requiring neuraminidase detection including drug discovery assays against various influenza virus strains in mammalian cells.

A new chemiluminescence neuraminidase probe enables rapid screening of small molecules that inhibit viral replication, directly in influenza A-infected mammalian cells.

Rational design of a deuterium-containing M2-S31N channel blocker UAWJ280 with in vivo antiviral efficacy against both oseltamivir sensitive and -resistant influenza A viruses

Seasonal influenza A virus (IAV) infections are among the most important global health problems. FDA-approved antiviral therapies against IAV include neuraminidase inhibitors, M2 inhibitors, and polymerase inhibitor baloxavir. Resistance against adamantanes (amantadine and rimantadine) is widespread as virtually all IAV strains currently circulating in the human population are resistant to adamantanes through the acquisition of the S31N mutation. The neuraminidase inhibitor-resistant strains also contain the M2-S31N mutant, suggesting M2-S31N is a high-profile antiviral drug target. Here we report the development of a novel deuterium-containing M2-S31N inhibitor UAWJ280. UAWJ280 had broad-spectrum antiviral activity against both oseltamivir sensitive and -resistant influenza A strains and had a synergistic antiviral effect in combination with oseltamivir in cell culture. In vivo pharmacokinetic (PK) studies demonstrated that UAWJ280 had favourable PK properties. The in vivo mouse model study showed that UAWJ280 was effective alone or in combination with oseltamivir in improving clinical signs and survival after lethal challenge with an oseltamivir sensitive IAV H1N1 strain. Furthermore, UAWJ280 was also able to ameliorate clinical signs and increase survival when mice were challenged with an oseltamivir-resistant IAV H1N1 strain. In conclusion, we show for the first time that the M2-S31N channel blocker UAWJ280 has in vivo antiviral efficacy in mice that are infected with either oseltamivir sensitive or -resistant IAVs, and it has a synergistic antiviral effect with oseltamivir.

Understanding the Impact of Resistance to Influenza Antivirals

Influenza poses a significant burden on society and health care systems. Although antivirals are an integral tool in effective influenza management, the potential for the emergence of antiviral-resistant viruses can lead to uncertainty and hesitation among front-line prescribers and policy makers. Here, we provide an overview of influenza antiviral resistance in context, exploring the key concepts underlying its development and clinical impact. Due to the acute nature of influenza in immunocompetent patients, resistant viruses that develop during antiviral treatment of a single patient ("treatment-emergent resistance") are usually cleared in a relatively short time, with no impact on future antiviral efficacy. In addition, although available data are limited by small numbers of patients, they show that antiviral treatment still provides clinical benefit to the patient within whom resistance emerges. In contrast, the sustained community transmission of resistant variants in the absence of treatment ("acquired resistance") is of greater concern and can potentially render front-line antivirals ineffective. Importantly, however, resistant viruses are usually associated with reduced fitness such that their widespread transmission is relatively rare. Influenza antivirals are an essential part of effective influenza management due to their ability to reduce the risk of complications and death in infected patients. Although antiviral resistance should be taken seriously and requires continuous careful monitoring, it is not comparable to antibiotic resistance in bacteria, which can become permanent and widespread, with far-reaching medical consequences. The benefits of antiviral treatment far outweigh concerns of potential resistance, which in the vast majority of cases does not have a significant clinical impact.

Highlighting membrane protein structure and function: A celebration of the Protein Data Bank

Biological membranes define the boundaries of cells and compartmentalize the chemical and physical processes required for life. Many biological processes are carried out by proteins embedded in or associated with such membranes. Determination of membrane protein (MP) structures at atomic or near-atomic resolution plays a vital role in elucidating their structural and functional impact in biology. This endeavor has determined 1198 unique MP structures as of early 2021. The value of these structures is expanded greatly by deposition of their three-dimensional (3D) coordinates into the Protein Data Bank (PDB) after the first atomic MP structure was elucidated in 1985. Since then, free access to MP structures facilitates broader and deeper understanding of MPs, which provides crucial new insights into their biological functions. Here we highlight the structural and functional biology of representative MPs and landmarks in the evolution of new technologies, with insights into key developments influenced by the PDB in magnifying their impact.

Mechanism and Kinetics of Copper Complexes Binding to the Influenza A M2 S31N and S31N/G34E Channels

Copper(II) is known to bind in the influenza virus His37 cluster in the homotetrameric M2 proton channel and block the proton current needed for uncoating. Copper complexes based on iminodiacetate also block the M2 proton channel and show reduced cytotoxicity and zebrafish-embryo toxicity. In voltage-clamp oocyte studies using the ubiquitous amantadine-insensitive M2 S31N variant, the current block showed fast and slow phases, in contrast to the single phase found for amantadine block of wild-type M2. Here, we evaluate the mechanism of block by copper adamantyl iminodiacitate and copper cyclooctyl iminodiacitate complexes and address whether the complexes can coordinate with one or more of the His37 imidazoles. The current traces were fitted to parametrized master equations. The energetics of binding and the rate constants suggest that the first step is copper complex binding within the channel, and the slow step in the current block is the formation of a Cu-histidine coordination complex. Solution-phase isothermal titration calorimetry and density functional theory (DFT) calculations indicate that imidazole binds to the copper complexes. Structural optimization using DFT reveals that the complexes fit inside the channel and project the Cu(II) toward the His37 cluster, allowing one imidazole to form a coordination complex with Cu(II). Electrophysiology and DFT studies also show that the complexes block the G34E amantadine-resistant mutant despite some crowding in the binding site by the glutamates.

Chemical Probes for Blocking of Influenza A M2 Wild-type and S31N Channels

We report on using the synthetic aminoadamantane-CH₂-aryl derivatives 1-6 as sensitive probes for blocking M2 S31N and influenza A virus (IAV) M2 wild-type (WT) channels as well as virus replication in cell culture. The binding kinetics measured using electrophysiology (EP) for M2 S31N channel are very dependent on the length between the adamantane moiety and the first ring of the aryl headgroup realized in 2 and 3 and the girth and length of the adamantane adduct realized in 4 and 5. Study of 1-6 shows that, according to molecular dynamics (MD) simulations and molecular mechanics Poisson-Boltzmann surface area (MM/PBSA) calculations, all bind in the M2 S31N channel with the adamantyl group positioned between V27 and G34 and the aryl group projecting out of the channel with the phenyl (or isoxazole in 6) embedded in the V27 cluster. In this outward binding configuration, an elongation of the ligand by only one methylene in rimantadine 2 or using diamantane or triamantane instead of adamantane in 4 and 5, respectively, causes incomplete entry and facilitates exit, abolishing effective block compared to the amantadine derivatives 1 and 6. In the active M2 S31N blockers 1 and 6, the phenyl and isoxazolyl head groups achieve a deeper binding position and high k_on/low k_off and high k_on/high k_off rate constants, compared to inactive 2-5, which have much lower k_on and higher k_off. Compounds 1-5 block the M2 WT channel by binding in the longer area from V27-H37, in the inward orientation, with high k_on and low k_off rate constants. Infection of cell cultures by influenza virus containing M2 WT or M2 S31N is inhibited by 1-5 or 1-4 and 6, respectively. While 1 and 6 block infection through the M2 block mechanism in the S31N variant, 2-4 may block M2 S31N virus replication in cell culture through the lysosomotropic effect, just as chloroquine is thought to inhibit SARS-CoV-2 infection.

Synthetic Analogues of Aminoadamantane as Influenza Viral Inhibitors-In Vitro, In Silico and QSAR Studies

A series of nineteen amino acid analogues of amantadine (Amt) and rimantadine (Rim) were synthesized and their antiviral activity was evaluated against influenza virus A (H3N2). Among these analogues, the conjugation of rimantadine with glycine illustrated high antiviral activity combined with low cytotoxicity. Moreover, this compound presented a profoundly high stability after in vitro incubation in human plasma for 24 h. Its thermal stability was established using differential and gravimetric thermal analysis. The crystal structure of glycyl-rimantadine revealed that it crystallizes in the orthorhombic Pbca space group. The structure–activity relationship for this class of compounds was established, with CoMFA (Comparative Molecular Field Analysis) 3D-Quantitative Structure Activity Relationships (3D-QSAR) studies predicting the activities of synthetic molecules. In addition, molecular docking studies were conducted, revealing the structural requirements for the activity of the synthetic molecules.

Site-directed M2 proton channel inhibitors enable synergistic combination therapy for rimantadine-resistant pandemic influenza

Pandemic influenza A virus (IAV) remains a significant threat to global health. Preparedness relies primarily upon a single class of neuraminidase (NA) targeted antivirals, against which resistance is steadily growing. The M2 proton channel is an alternative clinically proven antiviral target, yet a near-ubiquitous S31N polymorphism in M2 evokes resistance to licensed adamantane drugs. Hence, inhibitors capable of targeting N31 containing M2 (M2-N31) are highly desirable. Rational in silico design and in vitro screens delineated compounds favouring either lumenal or peripheral M2 binding, yielding effective M2-N31 inhibitors in both cases. Hits included adamantanes as well as novel compounds, with some showing low micromolar potency versus pandemic “swine” H1N1 influenza (Eng195) in culture. Interestingly, a published adamantane-based M2-N31 inhibitor rapidly selected a resistant V27A polymorphism (M2-A27/N31), whereas this was not the case for non-adamantane compounds. Nevertheless, combinations of adamantanes and novel compounds achieved synergistic antiviral effects, and the latter synergised with the neuraminidase inhibitor (NAi), Zanamivir. Thus, site-directed drug combinations show potential to rejuvenate M2 as an antiviral target whilst reducing the risk of drug resistance.

"Swine flu" illustrated that the spread of influenza pandemics in the modern era is rapid, making antiviral drugs the best way of limiting disease. One proven influenza drug target is the M2 proton channel, which plays an essential role during virus entry. However, resistance against licensed drugs targeting this protein is now ubiquitous, largely due to an S31N change in the M2 sequence. Understandably, considerable effort has focused on developing M2-N31 inhibitors, yet this has been hampered by controversy surrounding two potential drug binding sites. Here, we show that both sites can in fact be targeted by new M2-N31 inhibitors, generating synergistic antiviral effects. Developing such drug combinations should improve patient outcomes and minimise the emergence of future drug resistance.

Design and synthesis of pinane oxime derivatives as novel anti-influenza agents

Parasitic characteristics, mutations and resistance of influenza A virus make it difficult for current influenza antiviral drugs to maintain long-term effectiveness. Currently, to design non-adamantane compounds targeting the S31N mutant of M2 proton channel is a promising direction for the development of novel anti-influenza drugs. In our previous research, a pinanamine-based antiviral M090 was discovered to target hemagglutinin instead of M2, with its structure being highly similar to reported M2-S31N inhibitors. Herein, a series of pinane oxime derivatives were designed from scratch and evaluated for anti-influenza activity and their cytotoxicity in vitro. Utilizing a combination of structure-activity relationship analysis, electrophysiological assay and molecular docking, the most potent compound 11h, as a M2-S31N blocker, exhibited excellent activity with EC₅₀ value at the low micromolar level against both H3N2 and H1N1. No significant toxicity of 11h was observed. In addition, compound 11h was located tightly in the pore of the drug-binding site with the thiophene moiety facing down toward the C-terminus, and did not adopt a similar position and orientation as the reference inhibitor.

Virtual Screening Identifies Chebulagic Acid as an Inhibitor of the M2(S31N) Viral Ion Channel and Influenza A Virus

The increasing prevalence of drug-resistant influenza viruses emphasizes the need for new antiviral countermeasures. The M2 protein of influenza A is a proton-gated, proton-selective ion channel, which is essential for influenza replication and an established antiviral target. However, all currently circulating influenza A virus strains are now resistant to licensed M2-targeting adamantane drugs, primarily due to the widespread prevalence of an M2 variant encoding a serine to asparagine 31 mutation (S31N). To identify new chemical leads that may target M2(S31N), we performed a virtual screen of molecules from two natural product libraries and identified chebulagic acid as a candidate M2(S31N) inhibitor and influenza antiviral. Chebulagic acid selectively restores growth of M2(S31N)-expressing yeast. Molecular modeling also suggests that chebulagic acid hydrolysis fragments preferentially interact with the highly-conserved histidine residue within the pore of M2(S31N) but not adamantane-sensitive M2(S31). In contrast, chebulagic acid inhibits in vitro influenza A replication regardless of M2 sequence, suggesting that it also acts on other influenza targets. Taken together, results implicate chebulagic acid and/or its hydrolysis fragments as new chemical leads for M2(S31N) and influenza-directed antiviral development.

Put a cork in it: Plugging the M2 viral ion channel to sink influenza

The ongoing threat of seasonal and pandemic influenza to human health requires antivirals that can effectively supplement existing vaccination strategies. The M2 protein of influenza A virus (IAV) is a proton-gated, proton-selective ion channel that is required for virus replication and is an established antiviral target. While licensed adamantane-based M2 antivirals have been historically used, M2 mutations that confer major adamantane resistance are now so prevalent in circulating virus strains that these drugs are no longer recommended. Here we review the current understanding of IAV M2 structure and function, mechanisms of inhibition, the rise of drug resistance mutations, and ongoing efforts to develop new antivirals that target resistant forms of M2.

Investigation of the Drug Resistance Mechanism of M2-S31N Channel Blockers through Biomolecular Simulations and Viral Passage Experiments

Recent efforts in drug development against influenza A virus (IAV) M2 proton channel S31N mutant resulted in conjugates of amantadine linked with aryl head heterocycles. To understand the mechanism of drug resistance, we chose a representative M2-S31N inhibitor, compound 3, as a chemical probe to identify resistant mutants. To increase the possibility of identifying novel resistant mutants, serial viral passage experiments were performed with multiple strains of H1N1 and H3N2 viruses in different cell lines. This approach not only identified M2 mutations around the drug-binding site, including the pore-lining residues (V27A, V27F, N31S, and G34E) and an interhelical residue (I32N), but also a new allosteric mutation (R45H), in addition to L46P previously identified, located at the C-terminus of M2 that is more than 10 Å away from the drug-binding site. The effects of each mutation were next investigated using electrophysiology, recombinant viruses, and molecular dynamics (MD) simulations. The reduced sensitivity in channel blockage correlated with increased drug resistance in antiviral assays using recombinant viruses. The MD simulations show that the V27A, V27F, G34E, and R45H mutations increase the diameter and hydration state of the pore in complex with compound 3. The Molecular Mechanics Generalized Born (MM-GBSA) calculations result in more positive binding free energies for the complexes of resistant M2 (V27A, V27F, G34E, R45H) with compound 3 compared to the stable complexes (S31N and I32N). Overall, this is the first systematic study of the drug resistance mechanism of M2-S31N channel blockers using multiple viruses in different cell lines.

Discovery of M2 channel blockers targeting the drug-resistant double mutants M2-S31N/L26I and M2-S31N/V27A from the influenza A viruses

Influenza virus infections are a persistent threat to human health due to seasonal outbreaks and sporadic pandemics. Amantadine and rimantadine are FDA-approved influenza antiviral drugs and work by inhibiting the viral M2 proton channel. However, the therapeutic potential for the antiviral amantadine/rimantadine was curtailed by the emergence of drug-resistant mutations in its target protein M2. In this study, we identified four amantadine-resistant M2 mutants among avian and human influenza A H5N1 strains circulating between 2002 and 2019: the single S31N and V27A mutants, and the S31N/L26I and S31N/V27A double mutants. Herein, utilizing two-electrode voltage clamp (TEVC) assays, we screened a panel of structurally diverse M2 inhibitors against these single and double mutant channels. Three compounds 6, 7, and 15 were found to significantly block all three M2 mutants: M2-S31N, M2-S31N/L26I, and M2-S31N/V27A. Using recombinant viruses generated from reverse genetics, we further showed that these compounds also inhibited the replication of recombinant viruses harboring either the single S31N or double S31N/L26I and S31N/V27A mutants. This work represents the first example in developing antivirals by targeting the drug-resistant double mutants of M2 proton channels.

Discovery of novel oxoindolin derivatives as atypical dual inhibitors for DNA Gyrase and FabH

The antibacterial agents and therapies today are facing serious problems such as drug resistance. Introducing dual inhibiting effect is a valid approach to solve this trouble and bring advantages including wide adaptability, favorable safety and superiority of combination. We started from potential DNA Gyrase inhibitory backbone isatin to develop oxoindolin derivatives as atypical dual Gyrase (major) and FabH (assistant) inhibitors via a two-round screening. Aiming at blocking both duplication (Gyrase) and survival (FabH), most of synthesized compounds indicated potency against Gyrase and some of them inferred favorable inhibitory effect on FabH. The top hit I18 suggested comparable Gyrase inhibitory activity (IC₅₀ = 0.025 μM) and antibacterial effect with the positive control Novobiocin (IC₅₀ = 0.040 μM). FabH inhibitory activity (IC₅₀ = 5.20 μM) was also successfully introduced. Docking simulation hinted possible important interacted residues and binding patterns for both target proteins. Adequate Structure-Activity Relation discussions provide the future orientations of modification. With high potency, low initial toxicity and dual inhibiting strategy, advanced compounds with therapeutic methods will be developed for clinical application.

Novel N-Substituted oseltamivir derivatives as potent influenza neuraminidase inhibitors: Design, synthesis, biological evaluation, ADME prediction and molecular docking studies

The discovery of novel potent neuraminidase (NA) inhibitors remains an attractive approach for treating infectious diseases caused by influenza. In this study, we describe the design and synthesis of novel N-substituted oseltamivir derivatives for probing the 150-cavity which is nascent to the activity site of NA. NA inhibitory studies showed that new derivatives demonstrated the inhibitory activity with IC₅₀ values at nM level against NA of a clinical influenza virus strain. Moreover, the in silico ADME predictions showed that the selected compounds had comparable properties with oseltamivir carboxylate, which demonstrated the druggablity of these derivatives. Furthermore, molecular docking studies showed that the most potent compound 6f and 10i could adopt different modes of binding interaction with NA, which may provide novel solutions for treating oseltamivir-resistant influenza. Based on the research results, we consider that compounds 6f and 10i have the potential for further studies as novel antiviral agents.

A dual-functional molecular strategy for in situ suppressing and visualizing of neuraminidase in aqueous solution using iridium(iii) complexes

We have designed for the first time a dual-functional luminescent probe and inhibitor of neuraminidase (NA), a key influenza target. The lead iridium(iii) complex exhibited enhanced inhibition against NA compared to the FDA-approved antiviral drug, oseltamivir, and could detect NA even in the presence of an autofluorescent background.

The L46P mutant confers a novel allosteric mechanism of resistance toward the influenza A virus M2 S31N proton channel blockers

The Food and Drug Administration-approved influenza A antiviral amantadine inhibits the wild-type (WT) AM2 channel but not the S31N mutant predominantly found in circulating strains. In this study, serial viral passages were applied to select resistance against a newly developed isoxazole-conjugated adamantane inhibitor that targets the AM2 S31N channel. This led to the identification of the novel drug-resistant mutation L46P located outside the drug-binding site, which suggests an allosteric resistance mechanism. Intriguingly, when the L46P mutant was introduced to AM2 WT, the channel remained sensitive toward amantadine inhibition. To elucidate the molecular mechanism, molecular dynamics simulations and binding free energy molecular mechanics-generalized born surface area (MM-GBSA) calculations were performed on WT and mutant channels. It was found that the L46P mutation caused a conformational change in the N terminus of transmembrane residues 22-31 that ultimately broadened the drug-binding site of AM2 S31N inhibitor 4, which spans residues 26-34, but not of AM2 WT inhibitor amantadine, which spans residues 31-34. The MM-GBSA calculations showed stronger binding stability for 4 in complex with AM2 S31N compared with 4 in complex with AM2 S31N/L46P, and equal binding free energies of amantadine in complex with AM2 WT and AM2 L46P. Overall, these results demonstrate a unique allosteric resistance mechanism toward AM2 S31N channel blockers, and the L46P mutant represents the first experimentally confirmed drug-resistant AM2 mutant that is located outside of the pore where drug binds. SIGNIFICANCE STATEMENT: AM2 S31N is a high-profile antiviral drug target, as more than 95% of currently circulating influenza A viruses carry this mutation. Understanding the mechanism of drug resistance is critical in designing the next generation of AM2 S31N channel blockers. Using a previously developed AM2 S31N channel blocker as a chemical probe, this study was the first to identify a novel resistant mutant, L46P. The L46P mutant is located outside of the drug-binding site. Molecular dynamics simulations showed that L46P causes a dilation of drug-binding site between residues 22 and 31, which affects the binding of AM2 S31N channel blockers, but not the AM2 WT inhibitor amantadine.

Structure-Property Relationship Studies of Influenza A Virus AM2-S31N Proton Channel Blockers

Majority of current circulating influenza A viruses carry the S31N mutation in their M2 genes, rendering AM2-S31N as a high profile antiviral drug target. With our continuous interest in developing AM2-S31N channel blockers as novel antivirals targeting both oseltamivir-sensitive and -resistant influenza A viruses, we report herein the structure-property relationship studies of AM2-S31N inhibitors. The goal was to identify lead compounds with improved microsomal stability and membrane permeability. Two lead compounds, 10d and 10e, were found to have high mouse and human liver microsomal stability (T _1/2 > 145 min) and membrane permeability (>200 nm/s). Both compounds also inhibit both currently circulating oseltamivir-sensitive and -resistant human influenza A viruses (H1N1 and H3N2) with EC₅₀ values ranging from 0.4 to 2.8 μM and a selectivity index of >100. We also showed for the first time that AM2-S31N channel blockers such as 10e inhibited influenza virus replication at both low and high multiply of infection (10²-10⁶ pfu/mL) and the inhibition was not cell-type dependent. Overall, these studies have identified two promising lead candidates for further development as antiviral drugs against drug-resistant influenza A viruses.

Random Mutagenesis Analysis of the Influenza A M2 Proton Channel Reveals Novel Resistance Mutants

The influenza M2 proton channel is a major drug target, but unfortunately, the acquisition of resistance mutations greatly reduces the functional life span of a drug in influenza treatment. New M2 inhibitors that inhibit mutant M2 channels otherwise resistant to the early adamantine-based drugs have been reported, but it remains unclear whether and how easy resistance could arise to such inhibitors. We have combined a newly developed proton conduction assay with an established method for selection and screening, both Escherichia coli-based, to enable the study of M2 function and inhibition. Combining this platform with two groups of structurally different M2 inhibitors allowed us to isolate drug resistant M2 channels from a mutant library. Two groups of M2 variants emerged from this analysis. A first group appeared almost unaffected by the inhibitor, M_089 (N13I, I35L, and F47L) and M_272 (G16C and D44H), and the single-substitution variants derived from these (I35L, L43P, D44H, and L46P). Functionally, these resemble the known drug resistant M2 channels V27A, S31N, and swine flu. In addition, a second group of tested M2 variants were all still inhibited by drugs but to a lesser extent than wild type M2. Molecular dynamics simulations aided in distinguishing the two groups where drug binding to the wild type and the less resistant M2 group showed a stable positioning of the ligand in the canonical binding pose, as opposed to the drug resistant group in which the ligand rapidly dissociated from the complex during the simulations.

A Robust Proton Flux (pHlux) Assay for Studying the Function and Inhibition of the Influenza A M2 Proton Channel

The M2 protein is an important target for drugs in the fight against the influenza virus. Because of the emergence of resistance against antivirals directed toward the M2 proton channel, the search for new drugs against resistant M2 variants is of high importance. Robust and sensitive assays for testing potential drug compounds on different M2 variants are valuable tools in this search for new inhibitors. In this work, we describe a fluorescence sensor-based assay, which we termed "pHlux", that measures proton conduction through M2 when synthesized from an expression vector in Escherichia coli. The assay was compared to a previously established bacterial potassium ion transport complementation assay, and the results were compared to simulations obtained from analysis of a computational model of M2 and its interaction with inhibitor molecules. The inhibition of M2 was measured for five different inhibitors, including Rimantadine, Amantadine, and spiro type compounds, and the drug resistance of the M2 mutant variants (swine flu, V27A, and S31N) was confirmed. We demonstrate that the pHlux assay is robust and highly sensitive and shows potential for high-throughput screening.

Synthesis and structure-activity relationship study of arylsulfonamides as novel potent H5N1 inhibitors

H5N1 virus, one subtype of highly pathogenic influenza A virus in human infection, has recently received attention due to its unpredictable and high mortality. In this study, a series of arylsulfonamide derivatives were identified as improved H5N1 inhibitors for the influenza treatment by systematic structure-activity relationship investigation. Among them, the most potent H5N1 inhibitor 3h exhibited excellent antiviral activity against H5N1 virus with EC₅₀ value of 0.006 μM and selectivity index 33543.3. Moreover, the molecular docking of 3h with M2 proton channel protein provides practical way for understanding the inhibition of H5N1 with this kind of compounds.

Discovery of Highly Potent Pinanamine-Based Inhibitors against Amantadine- and Oseltamivir-Resistant Influenza A Viruses

Influenza pandemic is a constant major threat to public health caused by influenza A viruses (IAVs). IAVs are subcategorized by the surface proteins hemagglutinin (HA) and neuraminidase (NA), in which they are both essential targets for drug discovery. While it is of great concern that NA inhibitor oseltamivir resistant strains are frequently identified from human or avian influenza virus, structural and functional characterization of influenza HA has raised hopes for new antiviral therapies. In this study, we explored a structure-activity relationship (SAR) of pinanamine-based antivirals and discovered a potent inhibitor M090 against amantadine-resistant viruses, including the 2009 H1N1 pandemic strains, and oseltamivir-resistant viruses. Mechanism of action studies, particularly hemolysis inhibition, indicated that M090 targets influenza HA and it occupied a highly conserved pocket of the HA₂ domain and inhibited virus-mediated membrane fusion by "locking" the bending state of HA₂ during the conformational rearrangement process. This work provides new binding sites within the HA protein and indicates that this pocket may be a promising target for broad-spectrum anti-influenza A drug design and development.

In Vitro Pharmacokinetic Optimizations of AM2-S31N Channel Blockers Led to the Discovery of Slow-Binding Inhibitors with Potent Antiviral Activity against Drug-Resistant Influenza A Viruses

Influenza viruses are respiratory pathogens that are responsible for both seasonal influenza epidemics and occasional influenza pandemics. The narrow therapeutic window of oseltamivir, coupled with the emergence of drug resistance, calls for the next-generation of antivirals. With our continuous interest in developing AM2-S31N inhibitors as oral influenza antivirals, we report here the progress of optimizing the in vitro pharmacokinetic (PK) properties of AM2-S31N inhibitors. Several AM2-S31N inhibitors, including compound 10b, were discovered to have potent channel blockage, single to submicromolar antiviral activity, and favorable in vitro PK properties. The antiviral efficacy of compound 10b was also synergistic with oseltamivir carboxylate. Interestingly, binding kinetic studies (K_d, K_on, and K_off) revealed several AM2-S31N inhibitors that have similar K_d values but significantly different K_on and K_off values. Overall, this study identified a potent lead compound (10b) with improved in vitro PK properties that is suitable for the in vivo mouse model studies.

Divalent copper complexes as influenza A M2 inhibitors

New M2 blockers effective against the ubiquitous amantadine-resistant S31N M2 mutation in influenza A are needed. Six copper complexes, 2, 4, 6, 8, 9, and 10, were synthesized and found to block both wild type and S31N M2. Free Cu²⁺ also blocks M2 S31N but not S31N/H37A. The copper complexes do not block M2 H37A (either S31 or S31N). The complexes were effective against three influenza A strains in cell-culture assays, but less toxic to cells than CuCl₂. For example 4, Cu(cyclooctylamineiminodiacetate), which was stable at pH > 4 in the buffers used, had an EC₅₀ against A/Calif/07/2009 H1N1 of 0.7 ± 0.1 μM with a CC₅₀ of 147 μM (therapeutic index, averaged over three strains, 67.8). In contrast, CuCl₂ had an EC₅₀ of 3.8 ± 0.9 μM and CC₅₀ of 19 μM. Because M2 H37 is highly conserved, these complexes show promise for further testing as drugs against all strains of influenza A.

Anti-influenza activity of diazaadamantanes combined with monoterpene moieties

The antiviral activity of several diaza-adamantanes containing monoterpenoid moieties against a rimantadine-resistant strain of the influenza A/Puerto Rico/8/34 (H1N1) virus was studied. Hetero-adamantanes containing monoterpene moieties at the aminal position of the heterocycle were found to exhibit lower activity compared to compounds with a diaza-adamantane fragment and a monoterpene moiety linked via an amino group at the 6-position of the hetero-adamantane ring. The highest selectivity index (a ratio of the 50% cytotoxic concentration to the 50% inhibitory concentration) out of 30 was observed for compound 8d, which contains a citronellal monoterpenoid moiety. Diaza-adamantane 8d was superior to its adamantane-containing analog 5 both in its anti-influenza activity and selectivity. Furthermore, 8d has more balanced physicochemical properties than 5, making the former a more promising drug candidate. Modelling these compounds against an influenza virus M2 ion channel predicted plausible binding modes to both the wild-type and the mutant (S31N).

Influenza A Virus Nucleoprotein: A Highly Conserved Multi-Functional Viral Protein as a Hot Antiviral Drug Target

Prevention and treatment of influenza virus infection is an ongoing unmet medical need. Each year, thousands of deaths and millions of hospitalizations are attributed to influenza virus infection, which poses a tremendous health and economic burden to the society. Aside from the annual influenza season, influenza viruses also lead to occasional influenza pandemics as a result of emerging or re-emerging influenza strains. Influenza viruses are RNA viruses that exist in quasispecies, meaning that they have a very diverse genetic background. Such a feature creates a grand challenge in devising therapeutic intervention strategies to inhibit influenza virus replication, as a single agent might not be able to inhibit all influenza virus strains. Both classes of currently approved anti-influenza drugs have limitations: the M2 channel blockers amantadine and rimantadine are no longer recommended for use in the U.S. due to predominant drug resistance, and resistance to the neuraminidase inhibitor oseltamivir is continuously on the rise. In pursuing the next generation of antiviral drugs with broad-spectrum activity and higher genetic barrier of drug resistance, the influenza virus nucleoprotein (NP) stands out as a high-profile drug target. This review summarizes recent developments in designing inhibitors targeting influenza NP and their mechanisms of action.

Slow but Steady Wins the Race: Dissimilarities among New Dual Inhibitors of the Wild-Type and the V27A Mutant M2 Channels of Influenza A Virus

New insights on the amantadine resistance mechanism of the V27A mutant were obtained through the study of novel, easily accessible 4-(1- and 2-adamantyl)piperidines, identified as dual binders of the wild-type and V27A mutant M2 channels of influenza A virus. Their antiviral activity and channel blocking ability were determined using cell-based assays and two-electrode voltage clamp (TEVC) technique on M2 channels, respectively. In addition, electrophysiology experiments revealed two interesting findings: (i) these inhibitors display a different behavior against the wild-type versus V27A mutant A/M2 channels, and (ii) the compounds display antiviral activity when they have k_d equal or smaller than 10^-6 while they do not exhibit antiviral activity when k_d is 10^-5 or higher although they may show blocking activity in the TEV assay. Thus, caution must be taken when predicting antiviral activity based on percent channel blockage in electrophysiological assays. These findings provide experimental evidence of the resistance mechanism of the V27A mutation to wild-type inhibitors, previously predicted in silico, offer an explanation for the lack of antiviral activity of compounds active in the TEV assay, and may help design new and more effective drugs.

Expeditious Lead Optimization of Isoxazole-Containing Influenza A Virus M2-S31N Inhibitors Using the Suzuki-Miyaura Cross-Coupling Reaction

The existence of multidrug-resistant influenza viruses, coupled with the continuously antigenic shift and antigenic drift of influenza viruses, necessitates the development of the next-generation of influenza antivirals. As the AM2-S31N mutant persists in more than 95% of current circulating influenza A viruses, targeting the AM2-S31N proton channel appears to be a logical and valid approach to combating drug resistance. Starting from compound 1, an isoxazole compound with potent AM2-S31N channel blockage and antiviral activity, in this study we report an expeditious synthetic strategy that allows us to promptly explore the structure-activity relationships of isoxazole-containing AM2-S31N inhibitors. Propelled by the convenient synthesis, the lead optimization effort yielded a number of potent antivirals with submicromolar efficacy against several human clinical isolates of influenza A viruses, including both oseltamivir-sensitive and -resistant strains.

Progress of small molecular inhibitors in the development of anti-influenza virus agents

The influenza pandemic is a major threat to human health, and highly aggressive strains such as H1N1, H5N1 and H7N9 have emphasized the need for therapeutic strategies to combat these pathogens. Influenza anti-viral agents, especially active small molecular inhibitors play important roles in controlling pandemics while vaccines are developed. Currently, only a few drugs, which function as influenza neuraminidase (NA) inhibitors and M2 ion channel protein inhibitors, are approved in clinical. However, the acquired resistance against current anti-influenza drugs and the emerging mutations of influenza virus itself remain the major challenging unmet medical needs for influenza treatment. It is highly desirable to identify novel anti-influenza agents. This paper reviews the progress of small molecular inhibitors act as antiviral agents, which include hemagglutinin (HA) inhibitors, RNA-dependent RNA polymerase (RdRp) inhibitors, NA inhibitors and M2 ion channel protein inhibitors etc. Moreover, we also summarize new, recently reported potential targets and discuss strategies for the development of new anti-influenza virus drugs.

An M2-V27A channel blocker demonstrates potent in vitro and in vivo antiviral activities against amantadine-sensitive and -resistant influenza A viruses

Adamantanes such as amantadine (1) and rimantadine (2) are FDA-approved anti-influenza drugs that act by inhibiting the wild-type M2 proton channel from influenza A viruses, thereby inhibiting the uncoating of the virus. Although adamantanes have been successfully used for more than four decades, their efficacy was curtailed by emerging drug resistance. Among the limited number of M2 mutants that confer amantadine resistance, the M2-V27A mutant was found to be the predominant mutant under drug selection pressure, thereby representing a high profile antiviral drug target. Guided by molecular dynamics simulations, we previously designed first-in-class M2-V27A inhibitors. One of the potent lead compounds, spiroadamantane amine (3), inhibits both the M2-WT and M2-V27A mutant with IC₅₀ values of 18.7 and 0.3 μM, respectively, in in vitro electrophysiological assays. Encouraged by these findings, in this study we further examine the in vitro and in vivo antiviral activity of compound 3 in inhibiting both amantadine-sensitive and -resistant influenza A viruses. Compound 3 not only had single to sub-micromolar EC₅₀ values against M2-WT- and M2-V27A-containing influenza A viruses in antiviral assays, but also rescued mice from lethal viral infection by either M2-WT- or M2-V27A-containing influenza A viruses. In addition, we report the design of two analogs of compound 3, and one was found to have improved in vitro antiviral activity over compound 3. Collectively, this study represents the first report demonstrating the in vivo antiviral efficacy of inhibitors targeting M2 mutants. The results suggest that inhibitors targeting drug-resistant M2 mutants are promising antiviral drug candidates worthy of further development.

On the role of water density fluctuations in the inhibition of a proton channel

Hv1 is a transmembrane four-helix bundle that transports protons in a voltage-controlled manner. Its crucial role in many pathological conditions, including cancer and ischemic brain damage, makes Hv1 a promising drug target. Starting from the recently solved crystal structure of Hv1, we used structural modeling and molecular dynamics simulations to characterize the channel's most relevant conformations along the activation cycle. We then performed computational docking of known Hv1 inhibitors, 2-guanidinobenzimidazole (2GBI) and analogs. Although salt-bridge patterns and electrostatic potential profiles are well-defined and distinctive features of activated versus nonactivated states, the water distribution along the channel lumen is dynamic and reflects a conformational heterogeneity inherent to each state. In fact, pore waters assemble into intermittent hydrogen-bonded clusters that are replaced by the inhibitor moieties upon ligand binding. The entropic gain resulting from releasing these conformationally restrained waters to the bulk solvent is likely a major contributor to the binding free energy. Accordingly, we mapped the water density fluctuations inside the pore of the channel and identified the regions of maximum fluctuation within putative binding sites. Two sites appear as outstanding: One is the already known binding pocket of 2GBI, which is accessible to ligands from the intracellular side; the other is a site located at the exit of the proton permeation pathway. Our analysis of the waters confined in the hydrophobic cavities of Hv1 suggests a general strategy for drug discovery that can be applied to any ion channel.

Discovery of Potent Antivirals against Amantadine-Resistant Influenza A Viruses by Targeting the M2-S31N Proton Channel

Despite the existence of flu vaccines and small-molecule antiviral drugs, influenza virus infection remains a public health concern that warrants immediate attention. As resistance to the only orally bioavailable drug, oseltamivir, has been continuously reported, there is a clear need to develop the next-generation of anti-influenza drugs. We chose the influenza A virus M2-S31N mutant proton channel as the drug target to address this need as it is one of the most conserved viral proteins and persist in >95% of currently circulating influenza A viruses. In this study, we report the development of a late-stage diversification strategy for the expeditious synthesis of M2-S31N inhibitors. The channel blockage and antiviral activity of the synthesized compounds were tested in two-electrode voltage clamp assays and antiviral assays, respectively. Several M2-S31N inhibitors were identified to have potent M2-S31N channel blockage and micromolar antiviral efficacy against several M2-S31N-containing influenza A viruses.

Antiviral therapies on the horizon for influenza

Adequate response to severe influenza infections or pandemic outbreaks requires two complementary strategies: preventive vaccination and antiviral therapy. The existing influenza drugs, M2 blockers and neuraminidase inhibitors, show modest clinical efficacy and established or potential resistance. In the past three years, several new agents have entered the clinical pipeline and already yielded some promising data from Phase 2 trials. For two main categories, that is, the broadly neutralizing anti-hemagglutinin antibodies and small-molecule inhibitors of the viral polymerase complex, crystallography was instrumental to guide drug development. These structural insights also aid to expand the activity spectrum towards influenza A plus B viruses, or conceive nucleoprotein or polymerase assembly inhibitors. The practice of influenza therapy should radically change in the next decade.

Antivirals for Respiratory Viral Infections: Problems and Prospects

In the past two decades, several newly emerging and reemerging viral respiratory pathogens including several influenza viruses (avian influenza and pandemic influenza), severe acute respiratory syndrome coronavirus (SARS-CoV), and Middle East respiratory syndrome coronavirus (MERS-CoV), have continued to challenge medical and public health systems. Thereafter, the development of cost-effective, broad-spectrum antiviral agents is the urgent mission of both virologists and pharmacologists. Current antiviral developments have focused targets on viral entry, replication, release, and intercellular pathways essential for viral life cycle. Here, we review the current literature on challenges and prospects in the development of these antivirals.

Recent progress in designing inhibitors that target the drug-resistant M2 proton channels from the influenza A viruses

Influenza viruses are the causative agents for seasonal influenza, which results in thousands of deaths and millions of hospitalizations each year. Moreover, sporadic transmission of avian or swan influenza viruses to humans often leads to an influenza pandemic, as there is no preimmunity in the human body to fight against such novel strains. The metastable genome of the influenza viruses, coupled with the reassortment of different strains from a wide range of host origins, leads to the continuous evolution of the influenza virus diversity. Such characteristics of influenza viruses present a grand challenge in devising therapeutic strategies to combat influenza virus infection. This review summarizes recent progress in designing small molecule inhibitors that target the drug-resistant influenza A virus M2 proton channels and highlights the contribution of mechanistic studies of proton conductance to drug discovery. The lessons learned throughout the course of M2 drug discovery might provide insights for designing inhibitors that target other therapeutically important ion channels.

Pharmacological Characterization of the Spectrum of Antiviral Activity and Genetic Barrier to Drug Resistance of M2-S31N Channel Blockers

Adamantanes (amantadine and rimantadine) are one of the two classes of Food and Drug Administration-approved antiviral drugs used for the prevention and treatment of influenza A virus infections. They inhibit viral replication by blocking the wild-type (WT) M2 proton channel, thus preventing viral uncoating. However, their use was discontinued due to widespread drug resistance. Among a handful of drug-resistant mutants, M2-S31N is the predominant mutation and persists in more than 95% of currently circulating influenza A strains. We recently designed two classes of M2-S31N inhibitors, S31N-specific inhibitors and S31N/WT dual inhibitors, which are represented by N-[(5-cyclopropyl-1,2-oxazol-3-yl)methyl]adamantan-1-amine (WJ379) and N-[(5-bromothiophen-2-yl)methyl]adamantan-1-amine (BC035), respectively. However, their antiviral activities against currently circulating influenza A viruses and their genetic barrier to drug resistance are unknown. In this report, we evaluated the therapeutic potential of these two classes of M2-S31N inhibitors (WJ379 and BC035) by profiling their antiviral efficacy against multidrug-resistant influenza A viruses, in vitro drug resistance barrier, and synergistic effect with oseltamivir. We found that M2-S31N inhibitors were active against several influenza A viruses that are resistant to one or both classes of Food and Drug Administration-approved anti-influenza drugs. In addition, M2-S31N inhibitors display a higher in vitro genetic barrier to drug resistance than amantadine. The antiviral effect of WJ379 was also synergistic with oseltamivir carboxylate. Overall, these results reaffirm that M2-S31N inhibitors are promising antiviral drug candidates that warrant further development.

Mechanisms of Action of Novel Influenza A/M2 Viroporin Inhibitors Derived from Hexamethylene Amiloride

The increasing prevalence of influenza viruses with resistance to approved antivirals highlights the need for new anti-influenza therapeutics. Here we describe the functional properties of hexamethylene amiloride (HMA)-derived compounds that inhibit the wild-type and adamantane-resistant forms of the influenza A M2 ion channel. For example, 6-(azepan-1-yl)-N-carbamimidoylnicotinamide ( 9: ) inhibits amantadine-sensitive M2 currents with 3- to 6-fold greater potency than amantadine or HMA (IC50 = 0.2 vs. 0.6 and 1.3 µM, respectively). Compound 9: competes with amantadine for M2 inhibition, and molecular docking simulations suggest that 9: binds at site(s) that overlap with amantadine binding. In addition, tert-butyl 4'-(carbamimidoylcarbamoyl)-2',3-dinitro-[1,1'-biphenyl]-4-carboxylate ( 27: ) acts both on adamantane-sensitive and a resistant M2 variant encoding a serine to asparagine 31 mutation (S31N) with improved efficacy over amantadine and HMA (IC50 = 0.6 µM and 4.4 µM, respectively). Whereas 9: inhibited in vitro replication of influenza virus encoding wild-type M2 (EC50 = 2.3 µM), both 27: and tert-butyl 4'-(carbamimidoylcarbamoyl)-2',3-dinitro-[1,1'-biphenyl]-4-carboxylate ( 26: ) preferentially inhibited viruses encoding M2(S31N) (respective EC50 = 18.0 and 1.5 µM). This finding indicates that HMA derivatives can be designed to inhibit viruses with resistance to amantadine. Our study highlights the potential of HMA derivatives as inhibitors of drug-resistant influenza M2 ion channels.

Novel spirothiazamenthane inhibitors of the influenza A M2 proton channel

The development of treatments for influenza that inhibit the M2 proton channel without being susceptible to the widespread resistance mechanisms associated with the adamantanes is an ongoing challenge. Using a yeast high-throughput yeast growth restoration assay designed to identify M2 channel inhibitors, a single screening hit was uncovered. This compound (3), whose structure was incorrectly identified in the literature, is an inhibitor with similar potency to amantadine against WT M2. A library of derivatives of 3 was prepared and activity against WT M2 and the two principal mutant strains (V27A and S31N) was assessed in the yeast assay. The best compounds were further evaluated in an antiviral plaque reduction assay using engineered WT, V27A and S31N M2 influenza A strains with otherwise identical genetic background. Compound 63 was found to inhibit all three virus strains in this cell based antiviral assay at micromolar concentrations, possibly through a mechanism other than M2 inhibition.