Structure-activity relationship studies of a series of antiviral and antibacterial aglycon derivatives of the glycopeptide antibiotics vancomycin, eremomycin, and dechloroeremomycin

The Use of Antibiotics as Chiral Selectors in Capillary Electrophoresis: A Review

Chirality is becoming an essential issue in modern pharmaceutical research as regulatory agencies emphasize the safety and efficiency of enantiomers in drug development. The development of efficient and reliable chiral separation methods became a necessity in the last 30 years, and capillary electrophoresis (CE), due to its relatively low costs and “green” features, is attracting increased attention. Cyclodextrin (CD) and their derivatives are the most frequently used chiral selectors (CSs) in CE, however, the use of antibiotics as CSs represents an interesting alternative. Various classes of antibiotics (aminoglycosides, ansamycins, glycopeptides, lincosamides, macrolides, tetracyclines) have been used more or less successfully for the enantio-separation of pharmaceuticals. Antibiotics offer the possibility of a multitude of potential interactions (electrostatic, inclusion, hydrogen bonding, etc.) due to their chemical diversity, allowing the enantio-separation of analytes with a wide range of structural characteristics. This article aims to review the application of various classes of antibiotics in the CE enantio-separation of pharmaceuticals. Antibiotic physiochemical characteristics, variables impacting enantio-separation, advantages, and disadvantages when certain antibiotics are used as CSs in CE are also explored.

Emerging Roles of Glycopeptide Antibiotics: Moving beyond Gram-Positive Bacteria

Glycopeptides, a class of cell wall biosynthesis inhibitors, have been the antibiotics of choice against drug-resistant Gram-positive bacterial infections. Their unique mechanism of action involving binding to the substrate of cell wall biosynthesis and substantial longevity in clinics makes this class of antibiotics an attractive choice for drug repurposing and reprofiling. However, resistance to glycopeptides has been observed due to alterations in the substrate, cell wall thickening, or both. The emergence of glycopeptide resistance has resulted in the development of synthetic and semisynthetic glycopeptide analogues to target acquired resistance. Recent findings demonstrate that these derivatives, along with some of the FDA approved glycopeptides have been shown to have antimicrobial activity against Gram-negative bacteria, Mycobacteria, and viruses thus expanding their spectrum of activity across the microbial kingdom. Additional mechanisms of action and identification of novel targets have proven to be critical in broadening the spectrum of activity of glycopeptides. This review focuses on the applications of glycopeptides beyond their traditional target group of Gram-positive bacteria. This will aid in making the scientific community aware about the nontraditional activity profiles of glycopeptides, identify the existing loopholes, and further explore this antibiotic class as a potential broad-spectrum antimicrobial agent.

The multifaceted nature of antimicrobial peptides: current synthetic chemistry approaches and future directions

Bacterial infections caused by 'superbugs' are increasing globally, and conventional antibiotics are becoming less effective against these bacteria, such that we risk entering a post-antibiotic era. In recent years, antimicrobial peptides (AMPs) have gained significant attention for their clinical potential as a new class of antibiotics to combat antimicrobial resistance. In this review, we discuss several facets of AMPs including their diversity, physicochemical properties, mechanisms of action, and effects of environmental factors on these features. This review outlines various chemical synthetic strategies that have been applied to develop novel AMPs, including chemical modifications of existing peptides, semi-synthesis, and computer-aided design. We will also highlight novel AMP structures, including hybrids, antimicrobial dendrimers and polypeptides, peptidomimetics, and AMP-drug conjugates and consider recent developments in their chemical synthesis.

Glycopeptides as Potential Interventions for COVID-19

Coronavirus disease 2019 (COVID-19), an infectious disease that primarily attacks the human pulmonary system, is caused by a viral strain called severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The outbreak emerged from Wuhan, China, and later spread throughout the world. Until the first week of May 2020, over 3.7 million cases had been reported worldwide and more than 258,000 had died due to the disease. So far, off label use of various drugs has been tried in many clinical settings, however, at present, there is no vaccine or antiviral treatment for human and animal coronaviruses. Therefore, repurposing of the available drugs may be promising to control emerging infections of SARS-COV2; however, new interventions are likely to require months to years to develop. Glycopeptides, which are active against gram-positive bacteria, have demonstrated significant activity against viral infections including SARS-COV and MERS-COV and have a high resemblance of sequence homology with SARS-COV2. Recent in vitro studies have also shown promising activities of aglycon derivative of glycopeptides and teicoplanin against SARS-COV2. Hydrophobic aglycon derivatives and teicoplanin, with minimal toxicity to human cell lines, inhibit entry and replication of SARS-COV2. These drugs block proteolysis of polyprotein a/b with replicase and transcription domains. Teicoplanin use was associated with complete viral clearance in a cohort of patients with severe COVID-19 symptoms. This review attempts to describe the activity, elucidate the possible mechanisms and potential clinical applications of existing glycopeptides against corona viruses, specifically SARS-COV2.

Nanoparticles containing constrained phospholipids deliver mRNA to liver immune cells in vivo without targeting ligands

Once inside the cytoplasm of a cell, mRNA can be used to treat disease by upregulating the expression of any gene. Lipid nanoparticles (LNPs) can deliver mRNA to hepatocytes in humans, yet systemic non-hepatocyte delivery at clinical doses remains difficult. We noted that LNPs have historically been formulated with phospholipids containing unconstrained alkyl tails. Based on evidence that constrained adamantyl groups have unique properties that can improve small molecule drug delivery, we hypothesized that a phospholipid containing an adamantyl group would facilitate mRNA delivery in vivo. We quantified how 109 LNPs containing "constrained phospholipids" delivered mRNA to 16 cell types in mice, then using a DNA barcoding-based analytical pipeline, related phospholipid structure to in vivo delivery. By analyzing delivery mediated by constrained phospholipids, we identified a novel LNP that delivers mRNA to immune cells at 0.5 mg/kg. Unlike many previous LNPs, these (a) did not preferentially target hepatocytes and (b) delivered mRNA to immune cells without targeting ligands. These data suggest constrained phospholipids may be useful LNP components.

Antivirals Against Coronaviruses: Candidate Drugs for SARS-CoV-2 Treatment?

Coronaviruses (CoVs) are a group of viruses from the family Coronaviridae that can infect humans and animals, causing mild to severe diseases. The ongoing pandemic of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) represents a global threat, urging the development of new therapeutic strategies. Here we present a selection of relevant compounds that have been described from 2005 until now as having in vitro and/or in vivo antiviral activities against human and/or animal CoVs. We also present compounds that have reached clinical trials as well as further discussing the potentiality of other molecules for application in (re)emergent CoVs outbreaks. Finally, through rationalization of the data presented herein, we wish to encourage further research encompassing these compounds as potential SARS-CoV-2 drug candidates.

Reprogramming of the Antibacterial Drug Vancomycin Results in Potent Antiviral Agents Devoid of Antibacterial Activity

Influenza A and B viruses are a global threat to human health and increasing resistance to the existing antiviral drugs necessitates new concepts to expand the therapeutic options. Glycopeptide derivatives have emerged as a promising new class of antiviral agents. To avoid potential antibiotic resistance, these antiviral glycopeptides are preferably devoid of antibiotic activity. We prepared six vancomycin aglycone hexapeptide derivatives with the aim of obtaining compounds having anti-influenza virus but no antibacterial activity. Two of them exerted strong and selective inhibition of influenza A and B virus replication, while antibacterial activity was successfully eliminated by removing the critical N-terminal moiety. In addition, these two molecules offered protection against several other viruses, such as herpes simplex virus, yellow fever virus, Zika virus, and human coronavirus, classifying these glycopeptides as broad antiviral molecules with a favorable therapeutic index.

N-Terminal guanidine derivatives of teicoplanin antibiotics strongly active against glycopeptide resistant Enterococcus faecium

Antibiotic resistance is one of the major challenges in healthcare of our time. To meet this challenge, we designed and prepared guanidine and lipophilic guanidine derivatives of the glycopeptide antibiotic teicoplanin to armed them with activity against the most threatening nosocomial bacteria, multiresistant enterococci. From teicoplanin and its pseudoaglycone, a series of N-terminal guanidine derivatives have been prepared with free and amide C-terminal parts. Six aliphatic and aromatic lipophilic carbodiimides were prepared and used for the synthesis of lipophilic guanidine teicoplanin conjugates. All new N-terminal guanidine antibiotics showed high activity against a standard panel of Gram-positive bacteria. Four selected derivatives displayed excellent antibacterial activity against a series of nosocomial VanA Enterococcus faecium strains.

Main trends in the design of semi-synthetic antibiotics of a new generation

This review summarizes main advances achieved by Russian researchers in the synthesis and characterization of semi-synthetic antibiotics of a new generation in the period from 2004 to 2019. The following classes of compounds are considered as the basis for modification: polycyclic antibacterial glycopeptides of the vancomycin group, classical macrolides, antifungal polyene macrolides, the antitumour antibiotic olivomycin A, antitumour anthracyclines and broad-spectrum antibiotics, in particular, oligomycin A, heliomycin and some other. Main trends in the design of modern anti-infective and antitumour agents over this period are considered in relation to original natural antibiotics, which have been independently discovered by Russian researchers. It is shown that a new type of hybrid structures can, in principle, be synthesized based on glycopeptides, macrolides and other antibiotics, including heterodimers containing a new benzoxaborole pharmacophore. The review addresses the influence of the length of the spacer between two antibiotic molecules on the biological activity of hybrid structures. A combination of genetic engineering techniques and methods of organic synthesis is shown to be useful for the design of new potent antifungal antibiotics based on polyenes of the amphotericin B group. Many new semi-synthetic analogues exhibit important biological properties, such as a broad spectrum of activity and low toxicity. Emphasis is given to certain aspects related to investigation of a broad range of biological activity and mechanisms of action of new derivatives.

The bibliography includes 101 references.

Hydrophobic Derivatives of Glycopeptide Antibiotics as Inhibitors of Protein Kinases

As key regulators of cell signaling, protein kinases (PKs) are attractive targets for therapeutic intervention in a variety of diseases. Herein, we report for the first time the inhibitory activity of polycyclic peptides, particularly, derivatives of glycopeptide antibiotics teicoplanin and eremomycin, against a panel of 12 recombinant human protein kinases and two protein kinases (CK1 and CK2) isolated from rat liver. Several of the investigated compounds inhibited various PKs with IC50 values below 10 μM and caused >90% suppression of the enzyme activity at 10 µM concentration. Kinetic analysis of the protein kinase CK2α inhibition by the teicoplanin aglycon analogue (7) demonstrated the non-competitive mechanism of inhibition (with regard to ATP). Interestingly, the inhibitory activity of some investigated compounds correlated with the earlier described antiviral activity against HIV, HCV, and other corona- and flaviviruses.

Design, synthesis, biological evaluation and molecular docking of amide and sulfamide derivatives as Escherichia coli pyruvate dehydrogenase complex E1 inhibitors

Optimal binding mode of novel E. coli PDHc E1 inhibitor 9d.

The next ten stories on antiviral drug discovery (part E): advents, advances, and adventures

This review article presents the fifth part (part E) in the series of stories on antiviral drug discovery. The ten stories belonging to this fifth part are dealing with (i) aurintricarboxylic acid; (ii) alkenyldiarylmethanes; (iii) human immunodeficiency virus (HIV) integrase inhibitors; (iv) lens epithelium‐derived growth factor as a potential target for HIV proviral DNA integration; (v) the status presens of neuraminidase inhibitors NAIs in the control of influenza virus infections; (vi) the status presens on respiratory syncytial virus inhibitors; (vii) tricyclic (1,N‐2‐ethenoguanine)‐based acyclovir and ganciclovir derivatives; (viii) glycopeptide antibiotics as antivirals targeted at viral entry; (ix) the potential (off‐label) use of cidofovir in the treatment of polyoma (JC and BK) virus infections; and (x) finally, thymidine phosphorylase as a target for both antiviral and anticancer agents. © 2009 Wiley Periodicals, Inc. Med Res Rev, 31, No. 1, 118–160, 2010

Inhibition of hepatitis C virus replication by semi-synthetic derivatives of glycopeptide antibiotics

Objectives

Some semi-synthetic derivatives of glycopeptide antibiotics have been shown to exert in vitro antiviral activity against HIV and coronaviruses. Here we report and characterize the in vitro anti-hepatitis C virus (HCV) activity of several semi-synthetic derivatives of teicoplanin aglycone.

Methods

Anti-HCV activity was analysed in: (i) three different subgenomic HCV replicon systems using a luciferase or quantitative RT–PCR (qRT–PCR) assay; and (ii) an infectious HCV cell culture system by means of qRT–PCR and immunofluorescence assays.

Results

Several teicoplanin aglycone derivatives elicited selective anti-HCV activity in replicons as well as infectious cell culture systems, with LCTA-949 being the most potent derivative. LCTA-949 proved, in contrast to several directly acting antivirals for HCV, efficient in clearing cells of their replicons. When LCTA-949 was combined with HCV protease or polymerase inhibitors an overall additive effect was observed. Likewise, LCTA-949 was equipotent against wild-type replicons as well as against replicons resistant to polymerase and protease inhibitors. Following up to 4 months of selective pressure, no drug-resistant replicons were selected. When combined with the HCV NS3 protease inhibitor VX-950, LCTA-949 prevented the development of VX-950-resistant variants.

Conclusions

Semi-synthetic derivatives of teicoplanin aglycone constitute a novel class of HCV replication inhibitors that are not cross-resistant with various HCV protease and polymerase inhibitors and in particular are potent in clearing hepatoma cells of their replicons. This class of molecules also provides a good tool to obtain novel insights into the replication cycle of HCV and into cellular factors/processes that are crucial for viral replication.

High-throughput screening for inhibitors of Mycobacterium tuberculosis H37Rv

There is an urgent need for the discovery and development of new antitubercular agents that target new biochemical pathways and treat drug resistant forms of the disease. One approach to addressing this need is through high-throughput screening of medicinally relevant libraries against the whole bacterium in order to discover a variety of new, active scaffolds that will stimulate new biological research and drug discovery. Through the Tuberculosis Antimicrobial Acquisition and Coordinating Facility (www.taacf.org), a large, medicinally relevant chemical library was screened against M. tuberculosis strain H37Rv. The screening methods and a medicinal chemistry analysis of the results are reported herein.

Novel Semisynthetic Derivative of Antibiotic Eremomycin Active against Drug-Resistant Gram-Positive Pathogens Including Bacillus anthracis

Five adamantyl-containing carboxamides of eremomycin or vancomycin were synthesized and their antibacterial activities against some Gram-positive clinical isolates were investigated in vitro and in vivo. The adamantyl-2 amide of glycopeptide antibiotic eremomycin (1a in Chart 1, AN0900) was the most active compound and showed high activity against several Gram-positive pathogens: vancomycin-susceptible staphylococci and enterococci, glycopeptide-intermediate-resistant Staphylococcus aureus, and glycopeptide-resistant enterococci. Compound 1a was equally active in vitro against both Ciprofloxacin-susceptible and -resistant Bacillus anthracis strains (MICs 0.25-0.5 microg/mL). It was distinguished by having a 2.8 h half-life (t1/2) in mice and a volume of distribution of 2.18 L/kg. Compound 1a was active against Staphylococcus aureus in mice (iv) and provided complete protection against a lethal intravenous challenge with vegetative B. anthracis bacilli and also in a murine pulmonary anthrax model in which mice were challenged with Bacillus anthracis spores.

Efficient Peptide Coupling Involving Sterically Hindered Amino Acids

Hindered amino acids have been introduced into peptide chains by coupling N-(Cbz- and Fmoc-alpha-aminoacyl)benzotriazoles with amino acids, wherein at least one of the components was sterically hindered, to provide compounds 3a-e, (3c +3 c'), 5a-d, (5a + 5a'), 6a-c, (6b + 6b'), 8a-c, 9a-e, 10a-d, and (10a + 10a') in isolated yields of 41-95% with complete retention of chirality as evidenced by NMR and HPLC analysis. The benzotriazole activation methodology is a new route for the synthesis of sterically hindered peptides. (Note: compound numbers written within brackets represent diastereomeric mixtures or racemates; compound numbers without brackets represent enantiomers.).

A new series of glycopeptide antibiotics incorporating a squaric acid moiety. Synthesis, structural and antibacterial studies

The aglycones of the antibiotics eremomycin, vancomycin and ristocetin (3, 4 and 6, respectively) were prepared by deglycosidation of the parent antibiotics with hydrogen fluoride, and complete assignation of their 1H, 13C and 15N spectra was performed. The squaric acid amide esters (11-14), were prepared from dimethyl squarate. The corresponding asymmetric diamides (16-19, 22, 23) were also synthesized using 4-phenylbenzylamine and triglycine. The advantage of the method is the high regioselectivity and that no protecting group strategy is required. Electrospray mass spectroscopic method was elaborated for the determination of the site of substitution of the modified antibiotics. The antibacterial activity of the prepared compounds is discussed in detail.

Total synthesis approaches to natural product derivatives based on the combination of chemical synthesis and metabolic engineering

Secondary metabolites are an extremely diverse and important group of natural products with industrial and biomedical implications. Advances in metabolic engineering of both native and heterologous secondary metabolite producing organisms have allowed the directed synthesis of desired novel products by exploiting their biosynthetic potentials. Metabolic engineering utilises knowledge of cellular metabolism to alter biosynthetic pathways. An important technique that combines chemical synthesis with metabolic engineering is mutasynthesis (mutational biosynthesis; MBS), which advanced from precursor-directed biosynthesis (PDB). Both techniques are based on the cellular uptake of modified biosynthetic intermediates and their incorporation into complex secondary metabolites. Mutasynthesis utilises genetically engineered organisms in conjunction with feeding of chemically modified intermediates. From a synthetic chemist's point of view the concept of mutasynthesis is highly attractive, as the method combines chemical expertise with Nature's synthetic machinery and thus can be exploited to rapidly create small libraries of secondary metabolites. However, in each case, the method has to be critically compared with semi- and total synthesis in terms of practicability and efficiency. Recent developments in metabolic engineering promise to further broaden the scope of outsourcing chemically demanding steps to biological systems.

Status Presens of Antiviral Drugs And Strategies: Part I: DNA Viruses and Retroviruses

More than 40 compounds have been formally licensed for clinical use as antiviral drugs, and half of these are used for the treatment of HIV infections. The others have been approved for the therapy of herpesvirus (HSV, VZV, CMV), hepadnavirus (HBV), hepacivirus (HCV) and myxovirus (influenza, RSV) infections. New compounds are in clinical development or under preclinical evaluation, and, again, half of these are targeting HIV infections. Yet, quite a number of important viral pathogens (i.e. HPV, HCV, hemorrhagic fever viruses) remain in need of effective and/or improved antiviral therapies.

Viruses and Viral Diseases

More than 40 compounds have been formally licensed for clinical use as antiviral drugs, and half of these are used for the treatment of human immunodeficiency virus (HIV) infections. The others have been approved for the therapy of herpesvirus (herpes simplex virus (HSV), varicella-zoster virus (VZV), cytomegalovirus (CMV)), hepadnavirus (hepatitis B virus (HBV)), hepacivirus (hepatitis C virus (HCV)), and myxovirus (influenza, respiratory synctural virus (RSV)) infections. New compounds are in clinical development or under preclinical evaluation, and, again, half of these target HIV infections. Yet, quite a number of important viral pathogens (i.e., human papilloma virus (HPV), HCV, hemorrhagic fever viruses) remain in need of effective and/or improved antiviral therapies.

Structures of Staphylococcus aureus cell-wall complexes with vancomycin, eremomycin, and chloroeremomycin derivatives by 13C{19F} and 15N{19F} rotational-echo double resonance

Solid-state NMR has been used to examine isolated cell walls and intact whole cells of Staphylococcus aureus complexed to five different vancomycin, eremomycin, and chloroeremomycin derivatives. The cell walls and whole cells were specifically labeled with d-[1-(13)C]alanine, or a combination of [1-(13)C]glycine and [epsilon-(15)N]lysine. Each of the bound glycopeptides had a (19)F-labeled substituent at either its C-terminus or its disaccharide position. The (13)C{(19)F} rotational-echo double-resonance (REDOR) dephasing for the cell-wall (13)C-labeled bridging pentaglycyl segment connecting a glycopeptide-complexed peptidoglycan stem with its neighboring stem indicates that the fluorine labels for all bound glycopeptides are positioned at one or the other end of the bridge. An exception is N'-(p-trifluoromethoxybenzyl)chloroeremomycin, whose hydrophobic substituent differs in length by one phenyl group compared to that of oritavancin, N'-4-[(4-chlorophenyl)benzyl)]chloroeremomycin. For this drug, the fluorine label is near the middle of the pentaglycyl segment. (15)N{(19)F} REDOR dephasing shows the proximity of the fluorine to the bridge-link site of the pentaglycyl bridge for C-terminus-substituted moieties and the cross-link site for disaccharide-substituted moieties. Full-echo REDOR spectra of cell-wall complexes from cells labeled by d-[1-(13)C]alanine (in the presence of an alanine racemase inhibitor) reveal three different carbonyl carbon chemical-shift environments, arising from the d-Ala-d-Ala binding site and the d-Ala-Gly-1 cross-link site. The REDOR results indicate a single fluorine dephasing center in each peptidoglycan complex. Molecular models of the mature cell-wall complexes that are consistent with internuclear distances obtained from (13)C{(19)F} and (15)N{(19)F} REDOR dephasing allow a correlation of structure and antimicrobial activity of the glycopeptides.

Polycyclic peptide and glycopeptide antibiotics and their derivatives as inhibitors of HIV entry

Antiviral activity and other biological properties of two groups of polycyclic peptides are discussed. Antibiotics of the complestatin–kistamycin group have a structural motif similar to that of the peptide core of antibacterial antibiotics of the vancomycin–teicoplanin group though no amino acid component in the chloropeptin–kistamicin antibiotics is identical to an amino acid incorporated in the peptide core of the antibiotics of the vancomycin–teicoplanin group. Chloropeptins and the hydrophobic several derivatives of antibacterial antibiotics are inhibitors of HIV and some other viruses. They interfere with the viral (i.e. HIV) entry process. Chemical modifications of natural glycopeptide antibiotics led to the compounds with antiviral properties whereas antibacterial properties were lost. These glycopeptide aglycons derivatives can be envisaged as potential lead compounds for application as microbicides against sexual HIV transmission.

Inhibition of feline (FIPV) and human (SARS) coronavirus by semisynthetic derivatives of glycopeptide antibiotics

Various semisynthetic derivatives of glycopeptide antibiotics including vancomycin, eremomycin, teicoplanin, ristocetin A and DA-40926 have been evaluated for their inhibitory activity against feline infectious peritonitis virus (FIPV) and human (SARS-CoV, Frankfurt-1 strain) coronavirus in cell culture in comparison with their activity against human immunodeficiency virus (HIV). Several glycopeptide derivatives modified with hydrophobic substituents showed selective antiviral activity. For the most active compounds, the 50% effective concentrations (EC₅₀) were in the lower micromolar range. In general, removal of the carbohydrate parts of the molecules did not affect the antiviral activity of the compounds. Some compounds showed inhibitory activity against both, whereas other compounds proved inhibitory to either, FIPV or SARS-CoV. There was no close correlation between the EC₅₀ values of the glycopeptide derivatives for FIPV or SARS-CoV.

Structure-activity relationships in a series of semisynthetic polycyclic glycopeptide antibiotics

The main achievements in the development of methods for the design of semisynthetic antibiotics of a new generation belonging to the group of polycyclic glycopeptides directed against infections caused by multidrug-resistant bacteria and dangerous human and animal viruses are reviewed. The review is focused on the results obtained at the Gauze Institute in the area of chemical modification of natural antibiotics (eremomycin, vancomycin, teicoplanin, etc.) directed toward modification of their antibacterial and/or antiviral activity. A special emphasis is placed on the study of the mechanisms of action of these antibiotics, which could be the basis of a rational approach to their chemical modification involving the transformation of the inner binding pocket and the peripheral regions of the molecules that participate in the formation of their complexes with targets. The recently discovered antiviral activity of modified glycopeptides antibiotics is also discussed. A possibility of obtaining new highly active anti-HIV-1 and anti-HIV-2 preparations on the basis of hydrophobic derivatives of the aglycones of glycopeptide antibiotics was demonstrated. New semisynthetic derivatives of antibiotics that exhibit a high antibacterial activity in vivo, have good pharmacological characteristics, and are promising for practical use are described.

A guided tour through the antiviral drug field

Approximately 40 compounds have been formally licensed for clinical use as antiviral drugs, with half of these in use for the treatment of HIV infections. The remaining have been approved for use in the therapy of herpes virus (herpes simplex virus, varicella zoster virus and cytomegalovirus), hepadnavirus, hepacivirus and myxovirus (influenza and respiratory syncytial virus) infections. New compounds are in clinical development or under preclinical evaluation, and again, half of these are intended to target HIV infections. However, quite a number of important viral pathogens (i.e., human papillomavirus, hepatitis C virus and hemorrhagic fever viruses) remain in need of effective and/or improved antiviral therapies.