Protective effect of protease inhibitors in influenza virus infected animals

Aprotinin (II): Inhalational Administration for the Treatment of COVID-19 and Other Viral Conditions

Aprotinin is a broad-spectrum inhibitor of human proteases that has been approved for the treatment of bleeding in single coronary artery bypass surgery because of its potent antifibrinolytic actions. Following the outbreak of the COVID-19 pandemic, there was an urgent need to find new antiviral drugs. Aprotinin is a good candidate for therapeutic repositioning as a broad-spectrum antiviral drug and for treating the symptomatic processes that characterise viral respiratory diseases, including COVID-19. This is due to its strong pharmacological ability to inhibit a plethora of host proteases used by respiratory viruses in their infective mechanisms. The proteases allow the cleavage and conformational change of proteins that make up their viral capsid, and thus enable them to anchor themselves by recognition of their target in the epithelial cell. In addition, the activation of these proteases initiates the inflammatory process that triggers the infection. The attraction of the drug is not only its pharmacodynamic characteristics but also the possibility of administration by the inhalation route, avoiding unwanted systemic effects. This, together with the low cost of treatment (≈2 Euro/dose), makes it a good candidate to reach countries with lower economic means. In this article, we will discuss the pharmacodynamic, pharmacokinetic, and toxicological characteristics of aprotinin administered by the inhalation route; analyse the main advances in our knowledge of this medication; and the future directions that should be taken in research in order to reposition this medication in therapeutics.

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.

The Antiviral Potential of Host Protease Inhibitors

The replication of numerous pathogenic viruses depends on host proteases, which therefore emerged as potential antiviral drug targets. In some cases, e.g., for influenza viruses, their function during the viral propagation cycle is relatively well understood, where they cleave and activate viral surface glycoproteins. For other viruses, e.g., Ebola virus, the function of host proteases during replication is still not clear. Host proteases may also contribute to the pathogenicity of virus infection by activating proinflammatory cytokines. For some coronaviruses, human proteases can also serve in a nonproteolytical fashion simply as receptors for virus entry. However, blocking of such protein-protein contacts is challenging, because receptor surfaces are often flat and difficult to address with small molecules. In contrast, many proteases possess well-defined binding pockets. Therefore, they can be considered as well-druggable targets, especially, if they are extracellularly active. The number of their experimental crystal structures is steadily increasing, which is an important prerequisite for a rational structure-based inhibitor design using computational chemistry tools in combination with classical medicinal chemistry approaches. Moreover, host proteases can be considered as stable targets, and their inhibition should prevent rapid resistance developments, which is often observed when addressing viral proteins. Otherwise, the inhibition of host proteases can also affect normal physiological processes leading to a higher probability of side effects and a narrow therapeutic window. Therefore, they should be preferably used in combination therapies with additional antiviral drugs. This strategy should provide a stronger antiviral efficacy, allow to use lower drug doses, and minimize side effects. Despite numerous experimental findings on their antiviral activity, no small-molecule inhibitors of host proteases have been approved for the treatment of virus infections, so far.

Influenza virus activating host proteases: Identification, localization and inhibitors as potential therapeutics

Cellular proteases are reponsible for activation of influenza virus hemagglutinin (HA) in epithelial tissues of the respiratory tract. The trans-Golgi network (TGN) is the main subcellular compartment where HA cleavage occurs during its biosynthesis. The proteolytic HA cleavage is an indispensable prerequisite for the fusion of viral with endosomal membrane and the delivery of the virus genome into the cell. Both, the structure and accessibility of the HA cleavage site determine the responsible host protease(s) for cutting. Most influenza virus strains contain a HA sequence with a single arginine at the cleavage site suitable for processing by the trypsin-like serine proteases human airway trypsin-like protease (HAT) and transmembrane protease serine 2 (TMPRSS2), albeit a minority of viruses possesses HA cleavage site motifs that are processed by other proteases. TMPRSS2-deficient mice demonstrated the relevance of TMPRSS2 for pneumotropism and pathogenicity of H1N1 and H7N9 virus infections. In contrast, H3N2 virus infections are promoted by an additional not yet identified protease. Highly pathogenic avian H5 and H7 viruses are characterized by an enlarged cleavage site loop containing a multibasic amino acid motif, where the eukaryotic subtilases furin or PC5/6 cleave. Their ubiquitous presence in the organism allows a systemic virus infection. Peptidomimetic inhibitors derived from the HA cleavage site inhibit the HA-activating proteases and thus virus propagation.

Activation of influenza A viruses by host proteases from swine airway epithelium

Pigs are important natural hosts of influenza A viruses, and due to their susceptibility to swine, avian, and human viruses, they may serve as intermediate hosts supporting adaptation and genetic reassortment. Cleavage of the influenza virus surface glycoprotein hemagglutinin (HA) by host cell proteases is essential for viral infectivity. Most influenza viruses, including human and swine viruses, are activated at a monobasic HA cleavage site, and we previously identified TMPRSS2 and HAT to be relevant proteases present in human airways. We investigated the proteolytic activation of influenza viruses in primary porcine tracheal and bronchial epithelial cells (PTEC and PBEC, respectively). Human H1N1 and H3N2 viruses replicated efficiently in PTECs and PBECs, and viruses containing cleaved HA were released from infected cells. Moreover, the cells supported the proteolytic activation of HA at the stage of entry. We found that swine proteases homologous to TMPRSS2 and HAT, designated swTMPRSS2 and swAT, respectively, were expressed in several parts of the porcine respiratory tract. Both proteases cloned from primary PBECs were shown to activate HA with a monobasic cleavage site upon coexpression and support multicycle replication of influenza viruses. swAT was predominantly localized at the plasma membrane, where it was present as an active protease that mediated activation of incoming virus. In contrast, swTMPRSS2 accumulated in the trans-Golgi network, suggesting that it cleaves HA in this compartment. In conclusion, our data show that HA activation in porcine airways may occur by similar proteases and at similar stages of the viral life cycle as in human airways.

Activation of influenza viruses by proteases from host cells and bacteria in the human airway epithelium

Influenza is an acute infection of the respiratory tract, which affects each year millions of people. Influenza virus infection is initiated by the surface glycoprotein hemagglutinin (HA) through receptor binding and fusion of viral and endosomal membranes. HA is synthesized as a precursor protein and requires cleavage by host cell proteases to gain its fusion capacity. Although cleavage of HA is crucial for virus infectivity, little was known about relevant proteases in the human airways for a long time. Recent progress in the identification and characterization of HA‐activating host cell proteases has been considerable however and supports the idea of targeting HA cleavage as a novel approach for influenza treatment. Interestingly, certain bacteria have been demonstrated to support HA activation either by secreting proteases that cleave HA or due to activation of cellular proteases and thereby may contribute to virus spread and enhanced pathogenicity. In this review, we give an overview on activation of influenza viruses by proteases from host cells and bacteria with the main focus on recent progress on HA cleavage by proteases HAT and TMPRSS2 in the human airway epithelium. In addition, we outline investigations of HA‐activating proteases as potential drug targets for influenza treatment.

The authors, who are leading experts in this field, present a timely, authoritative review on the proteolytic cleavage of the influenza hemagglutinin (HA), an activation mechanism that is essential for the infectivity of influenza viruses, including the recently emerged H7N9. They also address the potential of host proteases as targets for developing new influenza drugs. This review will be of considerable interest to virologists, microbiologists and pharmaceutical companies alike.

Plasminogen controls inflammation and pathogenesis of influenza virus infections via fibrinolysis

Detrimental inflammation of the lungs is a hallmark of severe influenza virus infections. Endothelial cells are the source of cytokine amplification, although mechanisms underlying this process are unknown. Here, using combined pharmacological and gene-deletion approaches, we show that plasminogen controls lung inflammation and pathogenesis of infections with influenza A/PR/8/34, highly pathogenic H5N1 and 2009 pandemic H1N1 viruses. Reduction of virus replication was not responsible for the observed effect. However, pharmacological depletion of fibrinogen, the main target of plasminogen reversed disease resistance of plasminogen-deficient mice or mice treated with an inhibitor of plasminogen-mediated fibrinolysis. Therefore, plasminogen contributes to the deleterious inflammation of the lungs and local fibrin clot formation may be implicated in host defense against influenza virus infections. Our studies suggest that the hemostatic system might be explored for novel treatments against influenza.

Influenza viruses, including H5N1 bird influenza viruses continue to form a major threat for public health. Available antiviral drugs for the treatment of influenza are effective to a limited extent and the emergence of resistant viruses may further undermine their use. The symptoms associated with influenza are caused by replication of the virus in the respiratory tract and the host immune response. Here, we report that a molecule of the fibrinolytic system, plasminogen, contributes to inflammation caused by influenza. Inhibiting the action of plasminogen protected mice from severe influenza infections, including those caused by H5N1 and H1N1 pandemic 2009 viruses and may be a promising novel strategy to treat influenza.

Aprotinin and similar protease inhibitors as drugs against influenza

Efforts to develop new antiviral chemotherapeutic approaches are focusing on compounds that target either influenza virus replication itself or host factor(s) that are critical to influenza replication. Host protease mediated influenza hemagglutinin (HA) cleavage is critical for activation of virus infectivity and as such is a chemotherapeutic target. Influenza pathogenesis involves a "vicious cycle" in which host proteases activate progeny virus which in turn amplifies replication and stimulates further protease activities which may be detrimental to the infected host. Aprotinin, a 58 amino acid polypeptide purified from bovine lung that is one of a family of host-targeted antivirals that inhibit serine proteases responsible for influenza virus activation. This drug and similar agents, such as leupeptin and camostat, suppress virus HA cleavage and limit reproduction of human and avian influenza viruses with a single arginine in the HA cleavage site. Site-directed structural modifications of aprotinin are possible to increase its intracellular targeting of cleavage of highly virulent H5 and H7 hemagglutinins possessing multi-arginine/lysine cleavage site. An additional mechanism of action for serine protease inhibitors is to target a number of host mediators of inflammation and down regulate their levels in virus-infected hosts. Aprotinin is a generic drug approved for intravenous use in humans to treat pancreatitis and limit post-operative bleeding. As an antiinfluenzal compound, aprotinin might be delivered by two routes: (i) a small-particle aerosol has been approved in Russia for local respiratory application in mild-to-moderate influenza and (ii) a proposed intravenous administration for severe influenza to provide both an antiviral effect and a decrease in systemic pathology and inflammation.

TMPRSS2 and TMPRSS4 facilitate trypsin-independent spread of influenza virus in Caco-2 cells

Proteolysis of influenza virus hemagglutinin by host cell proteases is essential for viral infectivity, but the proteases responsible are not well defined. Recently, we showed that engineered expression of the type II transmembrane serine proteases (TTSPs) TMPRSS2 and TMPRSS4 allows hemagglutinin (HA) cleavage. Here we analyzed whether TMPRSS2 and TMPRSS4 are expressed in influenza virus target cells and support viral spread in the absence of exogenously added protease (trypsin). We found that transient expression of TMPRSS2 and TMPRSS4 resulted in HA cleavage and trypsin-independent viral spread. Endogenous expression of TMPRSS2 and TMPRSS4 in cell lines correlated with the ability to support the spread of influenza virus in the absence of trypsin, indicating that these proteases might activate influenza virus in naturally permissive cells. Indeed, RNA interference (RNAi)-mediated knockdown of both TMPRSS2 and TMPRSS4 in Caco-2 cells, which released fully infectious virus without trypsin treatment, markedly reduced the spread of influenza virus, demonstrating that these proteases were responsible for efficient proteolytic activation of HA in this cell line. Finally, TMPRSS2 was found to be coexpressed with the major receptor determinant of human influenza viruses, 2,6-linked sialic acids, in human alveolar epithelium, indicating that viral target cells in the human respiratory tract express TMPRSS2. Collectively, our results point toward an important role for TMPRSS2 and possibly TMPRSS4 in influenza virus replication and highlight the former protease as a potential therapeutic target.

Chapter 3 Antiviral drugs: general considerations

The development of an antiviral drug as well as of other drugs is a long process. In most programmes the screening and evaluation start using inhibition of virus multiplication in cell cultures, but in some instances the screening starts in animal models of different viral diseases. In these cases, the mechanism of action has to be analyzed after the in vivo effect has been found. It is not possible to specify precisely the time and resources required in a newly started project to find a compound active against a virus infection but 5-10 years is a reasonable estimation. For some viruses such as herpesviruses, where a number of active inhibitors are already known, the task is simpler than it is to find inhibitors of a virus such as influenza against which only a few active inhibitors have been reported. Evaluation of clinical efficacy in humans is a large and difficult part of the development of an antiviral drug. The number of uncontrolled clinical studies claiming efficacy of different drugs against viral diseases is depressingly large. It is essential to perform double-blind, placebo-controlled and statistically well evaluated studies to be able to judge the clinical efficacy of an antiviral drug. As the knowledge of the detailed natural history and molecular biology of viral diseases and viruses themselves increases, one will obviously have better opportunities to find new drugs. Methods such as X-ray diffraction measurement and NMR determinations will probably lead to a detailed understanding of the structures and interactions taking place at the active site of viral enzymes and their cellular counterparts.

Effect of membranotropic and mucoadhesive formulations of protein proteinase inhibitors on bovine herpes virus-1 reproduction

The lipidized derivatives of Bowman-Birk soybean protease inhibitor (BBI) containing one to three oleoyl groups were synthesized and characterized. The (ole)(1)- and (ole)(2)BBI were demonstrated to have 200- and 100-fold higher uptake into Caco-2 cell monolayers compared to native BBI. The acylated BBI had increased affinity to elastase-like proteases. Aprotinin-loaded starch/bovine serum albumin microcapsules were prepared using interfacial cross-linking with terephthaloyl chloride and characterized for their morphology, size and release of the inhibitor. Various formulations of protein proteinase inhibitors were tested for their influence on BHV-1 reproduction in cell cultures. Native aprotinin possessed palpable dose-dependent effect inhibiting the virus reproduction up to 4.0 lg (10,000-fold). The bioadhesive, biodegradable aprotinin-loaded microcapsules were the most effective decreasing virus infectious titer up to 4.0 lg and delaying the cytopathic effect up to 144 h in lesser doses of aprotinin. The lipophilic derivative (ole)(1)BBI was shown to exhibit effective inhibition (>100-fold) of BHV-1 reproduction unlike native BBI.

Design of synthetic bactericidal peptides derived from the bactericidal domain P(18-39) of aprotinin

A bactericidal domain, P(18-39), of the proteinase inhibitor aprotinin, possesses the structural feature of two antiparallel beta-sheets connected by a short turn. In order to understand the structural requirements for antibacterial activity, two peptides, each having the sequence corresponding to a single beta-sheet structure of P(18-39), were synthesized and their antibacterial properties investigated. One peptide, P(18-28), with the sequence IIRYFYNAKAG, was active against almost all the bacterial strains investigated. However, the bactericidal activity of P(18-28) was reduced compared to the parent molecule, P(18-39). The other peptide, P(29-39), with the sequence LCQTFVYGGCR, was only weakly bactericidal against Pseudomonas aeruginosa. A peptide, P(18-26), devoid of the C-terminus dipeptide Ala-Gly of P(18-28), retained the bactericidal activity of P(18-28) against most of the bacterial strains investigated. Only Klebsiella pneumoniae, P. aeruginosa and Staphylococcus aureus were resistant to P(18-26). Replacement of lysine 26 by arginine in P(18-26) (IIRYFYNAR) improved the bactericidal activity. The retropeptide, RANYFYRII, retained the antibacterial activity of IIRYFYNAR toward Gram-negative bacteria, but it was less active against Gram-positive bacteria. The random peptide, IANRIYRYF, was as bactericidal as IIRYFYNAR. Moreover, the random peptide possessed, in contrast to IIRYFYNAR, a strong antifungal activity against Candida albicans. Elimination of the N-hydrophobic terminal Ile-Ile from P(18-26) (RYFYNAK) strongly reduced the bactericidal potency of the peptide. Attaching the hydrophobic peptide, FFVAP, to the C-terminal of P(18-26) (IIRYFYNAKFFVAP) increased the bactericidal potency of the peptides considerably. We concluded that the order of the amino acids in the sequence of the peptides is not, per se, a critical feature for bactericidal activity. Hydrophobic interaction between peptide and bacterial membrane is probably the most important feature involved in the bactericidal mechanism of the antibiotic peptides.

Evaluation of anti-influenza effects of camostat in mice infected with non-adapted human influenza viruses

The anti-influenza effects of camostat, a serine protease inhibitor, on in vivo influenza infections were evaluated. Mice which received non-adapted human influenza viruses intranasally, developed a reproducible infection with very low mortality. The infection was readily detected by the recovery of the virus from an oropharyngeal swab. Five-week-old ICR mice received intraperitoneal injections of saline (control), amantadine (known positive drug), or camostat, after infection with influenza A/Taiwan/1/86 virus. Virus detection was performed on day 1, 2, 3, 5, and 7 of postinfection. Both camostat and amantadine were effective in ameliorating mouse influenza. On day 5, mice injected with camostat (45%) or amantadine (50%) showed a lower virus secreting rate than those receiving saline (90%). Additionally, camostat showed strong anti-influenza effects on an amantadine-resistant type A virus and a type B virus infection in vitro. The results show that blocking the hemagglutinin cleavage is an effective target for development of an anti-influenza agent. They also demonstrate that virus detection from the oropharynx of mice, infected with non-adapted virus, is a useful in vivo influenza model.

Identification of an aprotinin antiviral domain

Digestion of the proteinase inhibitor aprotinin, by clostripain, a cysteine proteinase, yielded five oligopeptide fragments. Two fragments exhibited both antiviral and antibacterial activities, two fragments only antiviral activity, and one fragment showed no antimicrobial activity. One of the former oligopeptides showed antiviral activity against human herpes simplex virus type 1 and bovine parainfluenza virus type 3. It consisted of the hexapeptide Y-F-Y-N-A-K corresponding to amino acids 21-26 of intact aprotinin. An identical synthetic peptide had the same antiviral spectrum as the natural hexapeptide, exhibited no antibacterial activity, and was also devoid of trypsin inhibiting activity. Intact aprotinin, in contrast, is ineffective against human herpes simplex virus 1 and bovine parainfluenza virus 3 but possesses antibacterial properties against several bacterial species [(1992) J. Appl. Bact. 72, 180-187].

Replication of influenza B virus in chicken embryos is suppressed by exogenous aprotinin

Chicken embryo proteinases, one of which is a blood clotting factor Xa-like proteinase, are known to effectively cleave the haemagglutinin (HA) of Influenza B viruses to permit their replication in chicken embryonated eggs. Here we show that injection of the serine proteinase inhibitor, aprotinin, into the allantoic cavity of eggs infected with Influenza B/Hong Kong/73 and B/Lee/40 viruses suppresses the viral HA cleavage and reduces the virus proteolytic activation and replication. Effective inhibition dose was determined as approximately 10.0 micrograms of aprotinin per embryo that corresponds to 0.1 microM concentration. However, heparin, which is known to be a direct inhibitor of the Factor Xa, was not able to suppress Influenza B virus hemagglutinin cleavage and replication in chicken embryo system. These data shed light on the pattern of proteinases involved in the Influenza B virus proteolytic activation and indicate that aprotinin possesses antiviral potential against Influenza B viruses.

Aprotinin aerosol treatment of influenza and paramyxovirus bronchopneumonia of mice

The therapeutic efficacy of aerosolized aprotinin, a natural proteinase inhibitor, against influenza and paramyxovirus bronchopneumonia of mice is shown. Small-particle aerosol of aprotinin solution was generated by a Collison type nebulizer and infected mice were exposed to aerosol atmosphere by four 30-40 min incubations per day for 6 days. This regimen provided an inhalation aprotinin dosage of approx. 6 micrograms/mouse/day. With such treatment more than 50% of mice infected with lethal doses of either influenza virus or paramyxovirus were protected from death. A suppression of the development of fatal hemorrhagic bronchopneumonia and a normalization of the body weight gain were observed in infected mice treated with aerosolized aprotinin. These data suggest that low doses of aerosolized proteinase inhibitors could be successfully applied against respiratory influenza-like virus diseases.

Bactericidal activities of lysozyme and aprotinin against gram-negative and gram-positive bacteria related to their basic character

Bactericidal properties of aprotinin, a proteinase inhibitor and possibly a defence molecule in bovine species, and of chicken egg white lysozyme, known as muramidase, were investigated. Incubation of various bacteria in the presence of either aprotinin or lysozyme showed that both proteins killed Gram-positive as well as Gram-negative bacteria without addition of complement or EDTA. Denaturation of the two proteins by dithiothreitol did not lead to loss of their bactericidal potency. Electron microscopic examination of Escherichia coli incubated either with lysozyme or aprotinin revealed that the bacterial cytoplasms gradually disintegrated. Both aprotinin and lysozyme were demonstrated within the affected cytoplasm by immunogold labelling. The results suggest that the bactericidal potency of lysozyme is not only due to muramidase activity but also to its cationic and hydrophobic properties. The bactericidal activity of aprotinin is probably also related to both these properties rather than to its activity as proteinase inhibitor.

Intraluminal proteolytic activation plays an important role in replication of type 1 reovirus in the intestines of neonatal mice

Oral inoculation of suckling mice with reovirus serotype 1 (strain Lang) results in the conversion of intact virions to intermediate subviral particles (ISVPs) in the intestinal lumen. Digestion of virus in vitro with chymotrypsin or trypsin reveals two distinct forms of ISVPs, while the predominant species of ISVPs found in the small intestinal lumen appears to be identical to the chymotrypsin product. The in vivo conversion of virions to ISVPs was blocked by pretreatment of mice with protease inhibitors, resulting in inefficient replication of reovirus in intestinal tissue. The early inhibition of viral replication in suckling mice pretreated with protease inhibitors was not observed when suckling mice were inoculated with ISVPs generated by in vitro digestion with either chymotrypsin or trypsin. However, replication was decreased during secondary rounds of replication in mice receiving repeated doses of protease inhibitors, suggesting that luminal proteolytic digestion is important in rendering progeny virions infectious in the gut.

Inhibition of proteolytic activation of influenza virus hemagglutinin by specific peptidyl chloroalkyl ketones

Lysates of cultured cells have been analyzed for arginine-specific endoproteases using peptidyl-p-nitroanilides as chromogenic substrates. The enzymes present in MDBK, MDCK, VERO, BHK, and chick embryo cells required lysine-arginine or arginine-arginine pairs as cleavage sites, whereas chorioallantoic membrane cells contained, in addition, an activity that could cleave at a single arginine. The effect of peptidyl chloroalkyl ketones on the activation of the fowl plague virus hemagglutinin by the proteases specific for paired basic residues has been investigated. When virions containing uncleaved hemagglutinin were incubated with lysates of uninfected cells, cleavage was completely inhibited by peptidyl chloroalkyl ketones containing paired basic residues at a concentration of 1 mM. In contrast a compound containing a single arginine had no inhibitory activity. When dibasic peptidyl chloroalkyl ketones were added to infected cell cultures, cleavage of hemagglutinin and multiple cycles of virus replication were inhibited at 10 mM. However, a 100- to 200-fold increase of the inhibitory activity in intact cells could be achieved by N-terminal acylation. These studies suggest a potential role of peptidyl chloroalkyl ketones as antiviral agents.

The molecular biology of influenza virus pathogenicity

It is an accepted concept that the pathogenicity of a virus is of polygenic nature. Because of their segmented genome, influenza viruses provide a suitable system to prove this concept. The studies employing virus mutants and reassortants have indicated that the pathogenicity depends on the functional integrity of each gene and on a gene constellation optimal for the infection of a given host. As a consequence, virtually every gene product of influenza virus has been reported to contribute to pathogenicity, but evidence is steadily growing that a key role has to be assigned to hemagglutinin. As the initiator of infection, hemagglutinin has a double function: (1) promotion of adsorption of the virus to the cell surface, and (2) penetration of the viral genome through a fusion process among viral and cellular membranes. Adsorption is based on the binding to neuraminic acid-containing receptors, and different virus strains display a distinct preference for specific oligosaccharides. Fusion capacity depends on proteolytic cleavage by host proteases, and variations in amino acid sequence at the cleavage site determine whether hemagglutinin is activated in a given cell. Differences in cleavability and presumably also in receptor specificity are important determinants for host tropism, spread of infection, and pathogenicity. The concept that proteolytic activation is a determinant for pathogenicity was originally derived from studies on avian influenza viruses, but there is now evidence that it may also be relevant for the disease in humans because bacterial proteases have been found to promote the development of influenza pneumonia in mammals.

High protection of animals lethally infected with influenza virus by aprotinin-rimantadine combination

The successful therapeutic synergism of aprotinin and rimantadine, which are known to attack different viral targets, was demonstrated in influenza-virus-infected animals. Combined treatment with these drugs of mice infected with a highly lethal dose of mouse-adapted influenza virus prevented the development of fatal haemorrhagic pneumonia and protected about 75% of animals; whereas the separate administration of aprotinin and rimantadine induced 35% and 15% protection, respectively.

Alphavirus replication in cultured cells and infected animals is inhibited by antiproteinase agents

The influence of different antiproteinase agents on alphavirus replication was examined. Sindbis virus multicycle replication in cultured cells was suppressed by N-tosyl-phenylalanine chloromethyl ketone (TPCK), an inhibitor of chymotrypsin-like proteinases, and by aprotinin, an inhibitor of a wide spectrum of proteinases. Antiviral activity of TPCK was also demonstrated in Sindbis virus-infected animals. Parenteral injections of TPCK in infected mice reduced virus titers in brain and blood. The possible mechanism(s) of antiviral action of the antiproteinase agents are discussed.