Role of the viral hemagglutinin in the anti-influenza virus activity of newly synthesized polycyclic amine compounds

The adamantane scaffold: Beyond a lipophilic moiety

Adamantyl-based compounds have been successful clinically for the treatment of neurological disorders and viral infections. Whilst the effects of incorporating adamantane into a drug scaffold is contextual by nature, its unique structural and physicochemical properties have attracted considerable attention. Previous reviews have highlighted its ability to alter physicochemical properties such as lipophilicity. However, with the movement to incorporate structural complexity and escape the 'flat land' of modern drug discovery, adamantane provides value beyond a hydrophobic substituent. The non-planar three-dimensional rigid scaffold allows for the precise positioning of substituents to probe drug targets more effectively. This review evaluates the synthetic accessibility and subsequent integration of multi-substituted and hetero-adamantane containing ligands in drug discovery programs. The vast benefits of adamantyl-based motifs beyond increasing the lipophilicity of a target compound are discussed thus emphasising its multi-dimensional value in drug discovery.

Antiviral mechanisms and preclinical evaluation of amantadine analogs that continue to inhibit influenza A viruses with M2S31N-based drug resistance

To better manage seasonal and pandemic influenza infections, new drugs are needed with enhanced activity against amantadine- and rimantadine-resistant influenza A virus (IAV) strains containing the S31N variant of the viral M2 ion channel (M2S31N). Here we tested 36 amantadine analogs against a panel of viruses containing either M2S31N or the parental, M2 S31 wild-type variant (M2WT). We found that several analogs, primarily those with sizeable lipophilic adducts, inhibited up to three M2S31N-containing viruses with activities at least 5-fold lower than rimantadine, without inhibiting M2S31N proton currents or modulating endosomal pH. While M2WT viruses in passaging studies rapidly gained resistance to these analogs through the established M2 mutations V27A and/or A30T, resistance development was markedly slower for M2S31N viruses and did not associate with additional M2 mutations. Instead, a subset of analogs, exemplified by 2-propyl-2-adamantanamine (38), but not 2-(1-adamantyl)piperidine (26), spiro[adamantane-2,2'-pyrrolidine] (49), or spiro[adamantane-2,2'-piperidine] (60), inhibited cellular entry of infectious IAV following pre-treatment and/or H1N1 pseudovirus entry. Conversely, an overlapping subset of the most lipophilic analogs including compounds 26, 49, 60, and others, disrupted viral M2-M1 protein colocalization required for intracellular viral assembly and budding. Finally, a pilot toxicity study in mice demonstrated that 38 and 49 were tolerated at 30 mg/kg. Together, these results indicate that amantadine analogs act on multiple, complementary mechanisms to inhibit replication of M2S31N viruses.

A Study of the Activity of Adamantyl Amines against Mutant Influenza A M2 Channels Identified a Polycyclic Cage Amine Triple Blocker, Explored by Molecular Dynamics Simulations and Solid-State NMR

We compared the anti-influenza potencies of 57 adamantyl amines and analogs against influenza A virus with serine-31 M2 proton channel, usually termed as WT M2 channel, which is amantadine sensitive. We also tested a subset of these compounds against viruses with the amantadine-resistant L26F, V27A, A30T, G34E M2 mutant channels. Four compounds inhibited WT M2 virus in vitro with mid-nanomolar potency, with 27 compounds showing sub-micromolar to low micromolar potency. Several compounds inhibited L26F M2 virus in vitro with sub-micromolar to low micromolar potency, but only three compounds blocked L26F M2-mediated proton current as determined by electrophysiology (EP). One compound was found to be a triple blocker of WT, L26F, V27A M2 channels by EP assays, but did not inhibit V27A M2 virus in vitro, and one compound inhibited WT, L26F, V27A M2 in vitro without blocking V27A M2 channel. One compound blocked only L26F M2 channel by EP, but did not inhibit virus replication. The triple blocker compound is as long as rimantadine, but could bind and block V27A M2 channel due to its larger girth as revealed by molecular dynamics simulations, while MAS NMR informed on the interaction of the compound with M2(18-60) WT or L26F or V27A.

Designing a Propellane-based Nonlinear Optically Active System Absorbing in Three Different Wavelength Regions

The aim of this work is to demonstrate the possibility of using propellane in designing a molecule that can absorb in three different wavelength regions and their nonlinear optical (NLO) activity can be fine-tuned by varying the three wings. We considered 22 tailor-made propellane derivatives consisting of phenyl, naphthyl, and biphenyl wings for this purpose. Using the state-of-the-art linear and quadratic response methods within TD-DFT and RI-CC2 theories and a suitable generalized few-state model that quantifies the effect of orientation of different transition moments on NLO properties, we discussed how and why the linear and nonlinear optical activity of propellane vary when the three wings are assembled successively to construct a full-propellane.

Emerging and state of the art hemagglutinin-targeted influenza virus inhibitors

Introduction: Seasonal influenza vaccination, together with FDA-approved neuraminidase (NA) and polymerase acidic (PA) inhibitors, is the most effective way for prophylaxis and treatment of influenza infections. However, the low efficacy of prevailing vaccines to newly emerging influenza strains and increasing resistance to available drugs drives intense research to explore more effective inhibitors. Hemagglutinin (HA), one of the major surface proteins of influenza strains, represents an attractive therapeutic target to develop such new inhibitors.Areas covered: This review summarizes the current progress of HA-based influenza virus inhibitors and their mechanisms of action, which may facilitate further research in developing novel antiviral inhibitors for controlling influenza infections.Expert opinion: HA-mediated entry of influenza virus is an essential step for successful infection of the host, which makes HA a promising target for the development of antiviral drugs. Recent progress in delineating the crystal structures of HA, especially HA-inhibitors complexes, has revealed a number of key residues and conserved binding pockets within HA. This has opened up important insights for developing HA-based antiviral inhibitors that have a high resistance barrier and broad-spectrum activities.

Heterocyclic Inhibitors of Viroporins in the Design of Antiviral Compounds

Ion channels of viruses (viroporins) represent a common type of protein targets for drugs. The relative simplicity of channel architecture allows convenient computational modeling and enables virtual search for new inhibitors. In this review, we analyze the data published over the last 10 years on known ion channels of viruses that cause socially significant diseases. The effectiveness of inhibition by various types of heterocyclic compounds of the viroporins of influenza virus, hepatitis С virus, human immunodeficiency virus, human papillomaviruses, coronaviruses, and respiratory syncytial virus is discussed. The presented material highlights the promise held by the search for heterocyclic antiviral compounds that act by inhibition of viroporins.

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.

Aniline-Based Inhibitors of Influenza H1N1 Virus Acting on Hemagglutinin-Mediated Fusion

Two series of easily accessible anilines were identified as inhibitors of influenza A virus subtype H1N1, and extensive chemical synthesis and analysis of the structure-activity relationship were performed. The compounds were shown to interfere with low pH-induced membrane fusion mediated by the H1 and H5 (group 1) hemagglutinin (HA) subtypes. A combination of virus resistance, HA interaction, and molecular dynamics simulation studies elucidated the binding site of these aniline-based influenza fusion inhibitors, which significantly overlaps with the pocket occupied by some H3 HA-specific inhibitors, indicating the high relevance of this cavity for drug design.

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.

Propellanes-From a Chemical Curiosity to "Explosive" Materials and Natural Products

Propellanes are a unique class of compounds currently consisting of well over 10 000 representatives, all featuring two more or less inverted tetrahedral carbon atoms that are common to three bridging rings. The central single bond between the two bridgeheads is significantly weakened in the smaller entities, which leads to unusual reactivities of these structurally interesting propeller-like molecules. This Review highlights the synthesis of such propellanes and their occurrence in material sciences, natural products, and medicinal chemistry. The conversion of [1.1.1]propellane into bridgehead derivatives of bicyclo[1.1.1]pentane, including oligomers and polymers with bicyclo[1.1.1]penta-1,3-diyl repeat units, is also featured. A selection of natural products with larger propellane subunits are discussed in detail. Heteropropellanes and inorganic propellanes are also addressed. The historical background is touched in brief to show the pioneering work of David Ginsburg, Günther Snatzke, Kenneth B. Wiberg, Günter Szeimies, and others.

Affinity of Rimantadine Enantiomers against Influenza A/M2 Protein Revisited

Recent findings from solid state NMR (ssNMR) studies suggested that the (R)-enantiomer of rimantadine binds to the full M2 protein with higher affinity than the (S)-enantiomer. Intrigued by these findings, we applied functional assays, such as antiviral assay and electrophysiology (EP), to evaluate the binding affinity of rimantadine enantiomers to the M2 protein channel. Unexpectedly, no significant difference was found between the two enantiomers. Our experimental data based on the full M2 protein function were further supported by alchemical free energy calculations and isothermal titration calorimetry (ITC) allowing an evaluation of the binding affinity of rimantadine enantiomers to the M2TM pore. Both enantiomers have similar channel blockage, affinity, and antiviral potency.

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.

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.

Flipping in the pore: discovery of dual inhibitors that bind in different orientations to the wild-type versus the amantadine-resistant S31N mutant of the influenza A virus M2 proton channel

Influenza virus infections lead to numerous deaths and millions of hospitalizations each year. One challenge facing anti-influenza drug development is the heterogeneity of the circulating influenza viruses, which comprise several strains with variable susceptibility to antiviral drugs. For example, the wild-type (WT) influenza A viruses, such as the seasonal H1N1, tend to be sensitive to antiviral drugs, amantadine and rimantadine, while the S31N mutant viruses, such as the pandemic 2009 H1N1 (H1N1pdm09) and seasonal H3N2, are resistant to this class of drugs. Thus, drugs targeting both WT and the S31N mutant are highly desired. We report our design of a novel class of dual inhibitors along with their ion channel blockage and antiviral activities. The potency of the most active compound 11 in inhibiting WT and the S31N mutant influenza viruses is comparable with that of amantadine in inhibiting WT influenza virus. Solution NMR studies and molecular dynamics (MD) simulations of drug-M2 interactions supported our design hypothesis: namely, the dual inhibitor binds in the WT M2 channel with an aromatic group facing down toward the C-terminus, while the same drug binds in the S31N M2 channel with its aromatic group facing up toward the N-terminus. The flip-flop mode of drug binding correlates with the structure-activity relationship (SAR) and has paved the way for the next round of rational design of broad-spectrum antiviral drugs.

Azapropellanes with anti-influenza a virus activity

The synthesis of several [4,4,3], [4,3,3], and [3,3,3]azapropellanes is reported. Several of the novel amines displayed low-micromolar activities against an amantadine-resistant H1N1 strain, but they did not show activity against an amantadine-sensitive H3N2 strain. None of the tested compounds inhibit the influenza A/M2 proton channel function. Most of the compounds did not show cytotoxicity for MDCK cells.

Easily accessible polycyclic amines that inhibit the wild-type and amantadine-resistant mutants of the M2 channel of influenza A virus

Amantadine inhibits the M2 proton channel of influenza A virus, yet most of the currently circulating strains of the virus carry mutations in the M2 protein that render the virus amantadine-resistant. While most of the research on novel amantadine analogues has revolved around the synthesis of novel adamantane derivatives, we have recently found that other polycyclic scaffolds effectively block the M2 proton channel, including amantadine-resistant mutant channels. In this work, we have synthesized and characterized a series of pyrrolidine derivatives designed as analogues of amantadine. Inhibition of the wild-type M2 channel and the A/M2-S31N, A/M2-V27A, and A/M2-L26F mutant forms of the channel were measured in Xenopus oocytes using two-electrode voltage clamp assays. Most of the novel compounds inhibited the wild-type ion channel in the low micromolar range. Of note, two of the compounds inhibited the amantadine-resistant A/M2-V27A and A/M2-L26F mutant ion channels with submicromolar and low micromolar IC50, respectively. None of the compounds was found to inhibit the S31N mutant ion channel.

Aminoadamantanes with persistent in vitro efficacy against H1N1 (2009) influenza A

A series of 2-adamantanamines with alkyl adducts of various lengths were examined for efficacy against strains of influenza A including those having an S31N mutation in M2 proton channel that confer resistance to amantadine and rimantadine. The addition of as little as one CH₂ group to the methyl adduct of the amantadine/rimantadine analogue, 2-methyl-2-aminoadamantane, led to activity in vitro against two M2 S31N viruses A/Calif/07/2009 (H1N1) and A/PR/8/34 (H1N1) but not to a third A/WS/33 (H1N1). Solid state NMR of the transmembrane domain (TMD) with a site mutation corresponding to S31N shows evidence of drug binding. But electrophysiology using the full length S31N M2 protein in HEK cells showed no blockade. A wild type strain, A/Hong Kong/1/68 (H3N2) developed resistance to representative drugs within one passage with mutations in M2 TMD, but A/Calif/07/2009 S31N was slow (>8 passages) to develop resistance in vitro, and the resistant virus had no mutations in M2 TMD. The results indicate that 2-alkyl-2-aminoadamantane derivatives with sufficient adducts can persistently block p2009 influenza A in vitro through an alternative mechanism. The observations of an HA1 mutation, N160D, near the sialic acid binding site in both 6-resistant A/Calif/07/2009(H1N1) and the broadly resistant A/WS/33(H1N1) and of an HA1 mutation, I325S, in the 6-resistant virus at a cell-culture stable site suggest that the drugs tested here may block infection by direct binding near these critical sites for virus entry to the host cell.

Mechanistic insights from functional characterization of an unnatural His37 mutant of the influenza A/M2 protein

The influenza A/M2 protein is a homotetrameric single-pass integral membrane protein encoded by the influenza A viral genome. Its transmembrane domain represents both a crucial drug target and a minimalistic model system for transmembrane proton transport and charge stabilization. Recent structural and functional studies of M2 have suggested that the proton transport mechanism involves sequential extraviral protonation and intraviral deprotonation of a highly conserved His37 side chain by the transported proton, consistent with a pH-activated proton shuttle mechanism. Multiple tautomeric forms of His can be formed, and it is not known whether they contribute to the mechanism of proton shuttling. Here we present the thermodynamic and functional characterization of an unnatural amino acid mutant at His37, where the imidazole side chain is substituted with a 4-thiazolyl group that is unable to undergo tautomerization and has a significantly lower solution pKa. The mutant construct has a similar stability to the wild-type protein at pH8 in bilayers and is virtually inactive at external pH7.4 in a semiquantitative liposome flux assay as expected from its lower sidechain pKa. However when the external buffer pH is lowered to 4.9 and 2.4, the mutant shows increasing amantadine sensitive flux of a similar magnitude to that of the wild type construct at pH7.4 and 4.9 respectively. These findings are in line with mechanistic hypotheses suggesting that proton flux through M2 is mediated by proton exchange from adjacent water molecules with the His37 sidechain, and that tautomerization is not required for proton translocation. This article is part of a Special Issue entitled: Viral Membrane Proteins - Channels for Cellular Networking.

3-Azatetracyclo[5.2.1.1(5,8).0(1,5)]undecane derivatives: from wild-type inhibitors of the M2 ion channel of influenza A virus to derivatives with potent activity against the V27A mutant

We have synthesized and characterized a series of compounds containing the 3-azatetracyclo[5.2.1.1(5,8).0(1,5)]undecane scaffold designed as analogues of amantadine, an inhibitor of the M2 proton channel of influenza A virus. Inhibition of the wild-type (WT) M2 channel and the amantadine-resistant A/M2-S31N and A/M2-V27A mutant ion channels were measured in Xenopus oocytes using two-electrode voltage clamp (TEV) assays. Most of the novel compounds inhibited the WT ion channel in the low micromolar range. Of note, several compounds inhibited the A/M2 V27A mutant ion channel, one of them with submicromolar IC50. None of the compounds was found to inhibit the S31N mutant ion channel. The antiviral activity of three novel dual WT and A/M2-V27A channels inhibitors was confirmed by influenza virus yield assays.