Protection against anthrax toxemia by hexa-D-arginine in vitro and in vivo

Experimental VLP vaccine displaying a furin antigen elicits production of autoantibodies and is well tolerated in mice

Proprotein convertase (PCSK) enzymes serve a wide range of regulatory roles in mammals, for example in metabolism and immunity, and altered activity of PCSKs is associated with disorders, such as cardiovascular disease and cancer. Inhibition of PCSK9 activity with therapeutic antibodies or small interfering RNAs is used in the clinic to lower blood cholesterol, and RNA interference -based silencing of FURIN (PCSK3) is being evaluated in clinical trials as a cancer treatment. Inhibiting these proteins through vaccine-induced autoantibodies could be a patient-friendly way to reduce the frequency of intervention and the overall price of treatment. Here, we show that a self-directed immune response against PCSK9 and furin can be generated in mice by presenting fragments of the proteins on norovirus-like particles (noro-VLPs). We genetically fused three PCSK peptides and the P domain of furin to the SpyCatcher linker protein and covalently conjugated them on noro-VLPs via SpyCatcher/SpyTag linkage. Both PCSK9 peptides and the furin P domain generated antigen specific IgGs even without conventional adjuvants. Importantly, vaccinating against furin did not cause adverse events or immune-mediated inflammatory disease. This study adds further support for the feasibility of VLP-based anti-PCSK9 vaccines and shows that the same principles can be applied to make novel vaccine candidates against other endogenous proteins such as furin. We also demonstrate that the noro-VLP can be used as a vaccine platform for presenting self-antigens.

Development and Prospects of Furin Inhibitors for Therapeutic Applications

Furin, a serine protease enzyme located in the Golgi apparatus of animal cells, plays a crucial role in cleaving precursor proteins into their mature, active forms. It is ubiquitously expressed across various tissues, including the brain, lungs, gastrointestinal tract, liver, pancreas, and reproductive organs. Since its discovery in 1990, furin has been recognized as a significant therapeutic target, leading to the active development of furin inhibitors for potential use in antiviral, antibacterial, anticancer, and other therapeutic applications. This review provides a comprehensive overview of the progress in the development and characterization of furin inhibitors, encompassing peptides, linear and macrocyclic peptidomimetics, and non-peptide compounds, highlighting their potential in the treatment of both infectious and non-infectious diseases.

Targeting furin, a cellular proprotein convertase, for COVID-19 prevention and therapeutics

SARS-CoV-2 has triggered an international outbreak of the highly contagious acute respiratory disease known as COVID-19. Identifying key targets in the virus infection lifecycle is crucial for developing effective prevention and therapeutic strategies against it. Furin is a serine endoprotease that belongs to the family of proprotein convertases and plays a critical role in the entry of host cells by SARS-CoV-2. Furin can cleave a specific S1/S2 site, PRRAR, on the spike protein of SARS-CoV-2, which promotes viral transmission by facilitating membrane fusion. Hence, targeting furin could hold clinical implications for the prevention and treatment of COVID-19. This review offers an overview of furin's structure, substrates, function, and inhibitors, with a focus on its potential role in SARS-CoV-2 infection.

Proprotein convertases regulate trafficking and maturation of key proteins within the secretory pathway

Proprotein Convertases (PCs) are serine endoproteases that regulate the homeostasis of protein substrates in the cell. The PCs family counts 9 members-PC1/3, PC2, PC4, PACE4, PC5/6, PC7, Furin, SKI-1/S1P, and PCSK9. The first seven PCs are known as Basic Proprotein Convertases due to their propensity to cleave after polybasic clusters. SKI-1/S1P requires the additional presence of hydrophobic residues for processing, whereas PCSK9 is catalytically dead after autoactivation and exerts its functions using mechanisms alternative to direct cleavage. All PCs traffic through the canonical secretory pathway, reaching different compartments where the various substrates reside. Despite PCs members do not share the same subcellular localization, most of the cellular organelles count one or more Proprotein Convertases, including ER, Golgi stack, endosomes, secretory granules, and plasma membranes. The widespread expression of these enzymes at the systemic level speaks for their importance in the homeostasis of a large number of biological functions. Among others, PCs cleave precursors of hormones and growth factors and activate receptors and transcription factors. Notably, dysregulation of the enzymatic activity of Proprotein Convertases is associated to major human pathologies, such as cardiovascular diseases, cancer, diabetes, infections, inflammation, autoimmunity diseases, and Parkinson. In the current COVID-19 pandemic, Furin has further attracted the attention as a key player for conferring high pathogenicity to SARS-CoV-2. Here, we review the Proprotein Convertases family and their most important substrates along the secretory pathway. Knowledge about the complex functions of PCs is important to identify potential drug strategies targeting this class of enzymes.

The Path to Therapeutic Furin Inhibitors: From Yeast Pheromones to SARS-CoV-2

The spurious acquisition and optimization of a furin cleavage site in the SARS-CoV-2 spike protein is associated with increased viral transmission and disease, and has generated intense interest in the development and application of therapeutic furin inhibitors to thwart the COVID-19 pandemic. This review summarizes the seminal studies that informed current efforts to inhibit furin. These include the convergent efforts of endocrinologists, virologists, and yeast geneticists that, together, culminated in the discovery of furin. We describe the pioneering biochemical studies which led to the first furin inhibitors that were able to block the disease pathways which are broadly critical for pathogen virulence, tumor invasiveness, and atherosclerosis. We then summarize how these studies subsequently informed current strategies leading to the development of small-molecule furin inhibitors as potential therapies to combat SARS-CoV-2 and other diseases that rely on furin for their pathogenicity and progression.

COVID-19 pandemic: Insights into genetic susceptibility to SARS-CoV-2 and host genes implications on virus spread, disease severity and outcomes

The outbreak of the newly emerging severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) all over the world has caused global public health emergencies, international concern and economic crises. The systemic SARS-CoV-2 disease (COVID-19) can lead to death through causing unrestrained cytokines-storm and subsequent pulmonary shutdown among the elderly and patients with pre-existing comorbidities. Additionally, in comparison with poor nations without primary health care services, in developed countries with advanced healthcare system we can witness higher number of infections per one million people. In this review, we summarize the latest studies on genes associated with SARS-CoV-2 pathogenesis and propose possible mechanisms of the virus replication cycle and its triggered signaling pathways to encourage researchers to investigate genetic and immune profiles of the disease and try strategies for its treatment. Our review shows that immune response in people with different genetic background might vary as African and then Asian populations have lowest number of affected cases compared with European and American nations. Considering SARS-CoV-2 pathogenesis, we put forward some potentially important genetic gateways to COVID-19 infection including genes involved in the entry and replication of SARS-CoV-2 and the regulation of host immune response which might represent explanation for its spread, severity, and morality. Finally, we suggest that genetic alterations within these gateways could be critical factors in influencing geographical discrepancies of the virus, so it is essential to fully study them and design appropriated and reliable therapeutic agents against COVID-19.

Design and Structure-Activity Relationship of a Potent Furin Inhibitor Derived from Influenza Hemagglutinin

Furin plays an important role in various pathological states, especially in bacterial and viral infections. A detailed understanding of the structural requirements for inhibitors targeting this enzyme is crucial to develop new therapeutic strategies in infectious diseases, including an urgent unmet need for SARS-CoV-2 infection. Previously, we have identified a potent furin inhibitor, peptide Ac-RARRRKKRT-NH ₂ (CF1), based on the highly pathogenic avian influenza hemagglutinin. The goal of this study was to determine how its N-terminal part (the P8-P5 positions) affects its activity profile. To do so, the positional-scanning libraries of individual peptides modified at the selected positions with natural amino acids were generated. Subsequently, the best substitutions were combined together and/or replaced by unnatural residues to expand our investigations. The results reveal that the affinity of CF1 can be improved (2-2.5-fold) by substituting its P5 position with the small hydrophobic residues (Ile or Val) or a basic Lys.

The biogenesis of SARS-CoV-2 spike glycoprotein: multiple targets for host-directed antiviral therapy

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of COVID-19 (coronavirus disease-19), represents a far more serious threat to public health than SARS and MERS coronaviruses, due to its ability to spread more efficiently than its predecessors. Currently, there is no worldwide-approved effective treatment for COVID-19, urging the scientific community to intense efforts to accelerate the discovery and development of prophylactic and therapeutic solutions against SARS-CoV-2 infection. In particular, effective antiviral drugs are urgently needed. With few exceptions, therapeutic approaches to combat viral infections have traditionally focused on targeting unique viral components or enzymes; however, it has now become evident that this strategy often fails due to the rapid emergence of drug-resistant viruses. Targeting host factors that are essential for the virus life cycle, but are dispensable for the host, has recently received increasing attention. The spike glycoprotein, a component of the viral envelope that decorates the virion surface as a distinctive crown ("corona") and is essential for SARS-CoV-2 entry into host cells, represents a key target for developing therapeutics capable of blocking virus invasion. This review highlights aspects of the SARS-CoV-2 spike biogenesis that may be amenable to host-directed antiviral targeting.

TMPRSS2 and furin are both essential for proteolytic activation of SARS-CoV-2 in human airway cells

The novel emerged SARS-CoV-2 has rapidly spread around the world causing acute infection of the respiratory tract (COVID-19) that can result in severe disease and lethality. For SARS-CoV-2 to enter cells, its surface glycoprotein spike (S) must be cleaved at two different sites by host cell proteases, which therefore represent potential drug targets. In the present study, we show that S can be cleaved by the proprotein convertase furin at the S1/S2 site and the transmembrane serine protease 2 (TMPRSS2) at the S2' site. We demonstrate that TMPRSS2 is essential for activation of SARS-CoV-2 S in Calu-3 human airway epithelial cells through antisense-mediated knockdown of TMPRSS2 expression. Furthermore, SARS-CoV-2 replication was also strongly inhibited by the synthetic furin inhibitor MI-1851 in human airway cells. In contrast, inhibition of endosomal cathepsins by E64d did not affect virus replication. Combining various TMPRSS2 inhibitors with furin inhibitor MI-1851 produced more potent antiviral activity against SARS-CoV-2 than an equimolar amount of any single serine protease inhibitor. Therefore, this approach has considerable therapeutic potential for treatment of COVID-19.

A Multibasic Cleavage Site in the Spike Protein of SARS-CoV-2 Is Essential for Infection of Human Lung Cells

The pandemic coronavirus SARS-CoV-2 threatens public health worldwide. The viral spike protein mediates SARS-CoV-2 entry into host cells and harbors a S1/S2 cleavage site containing multiple arginine residues (multibasic) not found in closely related animal coronaviruses. However, the role of this multibasic cleavage site in SARS-CoV-2 infection is unknown. Here, we report that the cellular protease furin cleaves the spike protein at the S1/S2 site and that cleavage is essential for S-protein-mediated cell-cell fusion and entry into human lung cells. Moreover, optimizing the S1/S2 site increased cell-cell, but not virus-cell, fusion, suggesting that the corresponding viral variants might exhibit increased cell-cell spread and potentially altered virulence. Our results suggest that acquisition of a S1/S2 multibasic cleavage site was essential for SARS-CoV-2 infection of humans and identify furin as a potential target for therapeutic intervention.

Inhibition of Pore-Forming Proteins

Perforation of cellular membranes by pore-forming proteins can affect cell physiology, tissue integrity, or immune response. Since many pore-forming proteins are toxins or highly potent virulence factors, they represent an attractive target for the development of molecules that neutralize their actions with high efficacy. There has been an assortment of inhibitors developed to specifically obstruct the activity of pore-forming proteins, in addition to vaccination and antibiotics that serve as a plausible treatment for the majority of diseases caused by bacterial infections. Here we review a wide range of potential inhibitors that can specifically and effectively block the activity of pore-forming proteins, from small molecules to more specific macromolecular systems, such as synthetic nanoparticles, antibodies, antibody mimetics, polyvalent inhibitors, and dominant negative mutants. We discuss their mechanism of inhibition, as well as advantages and disadvantages.

Anthrax protective antigen is a calcium-dependent serine protease

Bacillus anthracis secretes a three component exotoxin-complex, which contributes to anthrax pathogenesis. Formation of this complex starts with the binding of protective antigen (PA) to its cellular receptor. In this study, we report that PA is a calcium-dependent serine protease and that the protein potentially uses this proteolytic activity for receptor binding. Additionally our findings shed new light on previous research describing the inhibition of anthrax toxins and exotoxin formation. Importantly, inhibition of the proteolytic activity of protective antigen could be a novel therapeutic strategy in fighting B. anthracis-related infections.

Elongated and Shortened Peptidomimetic Inhibitors of the Proprotein Convertase Furin

Novel elongated and shortened derivatives of the peptidomimetic furin inhibitor phenylacetyl-Arg-Val-Arg-4-amidinobenzylamide were synthesized. The most potent compounds, such as Nα (carbamidoyl)Arg-Arg-Val-Arg-4-amidinobenzylamide (Ki =6.2 pm), contain additional basic residues at the N terminus and inhibit furin in the low-picomolar range. Furthermore, to decrease the molecular weight of this inhibitor type, compounds that lack the P5 moiety were prepared. The best inhibitors of this series, 5-(guanidino)valeroyl-Val-Arg-4-amidinobenzylamide and its P3 tert-leucine analogue displayed Ki values of 2.50 and 1.26 nm, respectively. Selected inhibitors, together with our previously described 4-amidinobenzylamide derivatives as references, were tested in cell culture for their activity against furin-dependent infectious pathogens. The propagation of the alphaviruses Semliki Forest virus and chikungunya virus was strongly inhibited in the presence of selected derivatives. Moreover, a significant protective effect of the inhibitors against diphtheria toxin was observed. These results confirm that the inhibition of furin should be a promising approach for the short-term treatment of acute infectious diseases.

Protection against Shiga Toxins

Shiga toxins consist of an A-moiety and five B-moieties able to bind the neutral glycosphingolipid globotriaosylceramide (Gb3) on the cell surface. To intoxicate cells efficiently, the toxin A-moiety has to be cleaved by furin and transported retrogradely to the Golgi apparatus and to the endoplasmic reticulum. The enzymatically active part of the A-moiety is then translocated to the cytosol, where it inhibits protein synthesis and in some cell types induces apoptosis. Protection of cells can be provided either by inhibiting binding of the toxin to cells or by interfering with any of the subsequent steps required for its toxic effect. In this article we provide a brief overview of the interaction of Shiga toxins with cells, describe some compounds and conditions found to protect cells against Shiga toxins, and discuss whether they might also provide protection in animals and humans.

Endogenous secreted phospholipase A2 group X regulates cysteinyl leukotrienes synthesis by human eosinophils

BACKGROUND

Phospholipase A2s mediate the rate-limiting step in the formation of eicosanoids such as cysteinyl leukotrienes (CysLTs). Group IVA cytosolic PLA2α (cPLA2α) is thought to be the dominant PLA2 in eosinophils; however, eosinophils also have secreted PLA2 (sPLA2) activity that has not been fully defined.

OBJECTIVES

To examine the expression of sPLA2 group X (sPLA2-X) in eosinophils, the participation of sPLA2-X in the formation of CysLTs, and the mechanism by which sPLA2-X initiates the synthesis of CysLTs in eosinophils.

METHODS

Peripheral blood eosinophils were obtained from volunteers with asthma and/or allergy. A rabbit polyclonal anti-sPLA2-X antibody identified sPLA2-X by Western blot. We used confocal microscopy to colocalize the sPLA2-X to intracellular structures. An inhibitor of sPLA2-X (ROC-0929) that does not inhibit other mammalian sPLA2s, as well as inhibitors of the mitogen-activated kinase cascade (MAPK) and cPLA2α, was used to examine the mechanism of N-formyl-methionyl-leucyl-phenylalanine (fMLP)-mediated formation of CysLT.

RESULTS

Eosinophils express the mammalian sPLA2-X gene (PLA2G10). The sPLA2-X protein is located in the endoplasmic reticulum, golgi, and granules of eosinophils and moves to the granules and lipid bodies during fMLP-mediated activation. Selective sPLA2-X inhibition attenuated the fMLP-mediated release of arachidonic acid and CysLT formation by eosinophils. Inhibitors of p38, extracellular-signal-regulated kinases 1/2 (p44/42 MAPK), c-Jun N-terminal kinase, and cPLA2α also attenuated the fMLP-mediated formation of CysLT. The sPLA2-X inhibitor reduced the phosphorylation of p38 and extracellular-signal-regulated kinases 1/2 (p44/42 MAPK) as well as cPLA2α during cellular activation, indicating that sPLA2-X is involved in activating the MAPK cascade leading to the formation of CysLT via cPLA2α. We further demonstrate that sPLA2-X is activated before secretion from the cell during activation. Short-term priming with IL-13 and TNF/IL-1β increased the expression of PLA2G10 by eosinophils.

CONCLUSIONS

These results demonstrate that sPLA2-X plays a significant role in the formation of CysLTs by human eosinophils. The predominant role of the enzyme is the regulation of MAPK activation that leads to the phosphorylation of cPLA2α. The sPLA2-X protein is regulated by proteolytic cleavage, suggesting that an inflammatory environment may promote the formation of CysLTs through this mechanism. These results have important implications for the treatment of eosinophilic disorders such as asthma.

Cationic Cell-Penetrating Peptides Are Potent Furin Inhibitors

Cationic cell-penetrating peptides have been widely used to enhance the intracellular delivery of various types of cargoes, such as drugs and proteins. These reagents are chemically similar to the multi-basic peptides that are known to be potent proprotein convertase inhibitors. Here, we report that both HIV-1 TAT47-57 peptide and the Chariot reagent are micromolar inhibitors of furin activity in vitro. In agreement, HIV-1 TAT47-57 reduced HT1080 cell migration, thought to be mediated by proprotein convertases, by 25%. In addition, cyclic polyarginine peptides containing hydrophobic moieties which have been previously used as transfection reagents also exhibited potent furin inhibition in vitro and also inhibited intracellular convertases. Our finding that cationic cell-penetrating peptides exert potent effects on cellular convertase activity should be taken into account when biological effects are assessed.

Peptidomimetic furin inhibitor MI-701 in combination with oseltamivir and ribavirin efficiently blocks propagation of highly pathogenic avian influenza viruses and delays high level oseltamivir resistance in MDCK cells

Antiviral medication is used for the treatment of severe influenza infections, of which the neuraminidase inhibitors (NAIs) are the most effective drugs, approved so far. Here, we investigated the antiviral efficacy of the peptidomimetic furin inhibitor MI-701 in combination with oseltamivir carboxylate and ribavirin against the infection of highly pathogenic avian influenza viruses (HPAIV) that are activated by the host protease furin. Cell cultures infected with the strains A/Thailand/1(KAN-1)/2004 (H5N1) and A/FPV/Rostock/1934 (H7N1) were treated with each agent alone, or in double and triple combinations. MI-701 alone achieved a concentration-dependent reduction of virus propagation. Double treatment of MI-701 with oseltamivir carboxylate and triple combination with ribavirin showed synergistic inhibition and a pronounced delay of virus propagation. MI-701 resistant mutants were not observed. Emergence of NA mutation H275Y conferring high oseltamivir resistance was significantly delayed in the presence of MI-701. Our data indicate that combination with a potent furin inhibitor significantly enhances the therapeutic efficacy of conventional antivirals drugs against HPAIV infection.

Identification of potent and compartment-selective small molecule furin inhibitors using cell-based assays

The proprotein convertase furin is implicated in a variety of pathogenic processes such as bacterial toxin activation, viral propagation, and cancer. Several groups have identified non-peptide compounds with high inhibitory potency against furin in vitro, although their efficacy in various cell-based assays is largely unknown. In this study we show that certain guanidinylated 2,5-dideoxystreptamine derivatives exhibit interesting ex vivo properties. Compound 1b (1,1'-(4-((2,4-diguanidino-5-(4-guanidinophenoxy)cyclohexyl)oxy)-1,3-phenylene)diguanidine) is a potent and cell-permeable inhibitor of cellular furin, since it was able to retard tumor cell migration, block release of a Golgi reporter, and protect cells against Bacillus anthracis (anthrax) and Pseudomonas aeruginosa intoxication, with no evident cell toxicity. Other compounds based on the 2,5-dideoxystreptamine scaffold, such as compound 1g (1,1'-(4,6-bis(4-guanidinophenoxy)cyclohexane-1,3-diyl)diguanidine) also efficiently protected cells against anthrax, but displayed only moderate protection against Pseudomonas exotoxin A and did not inhibit cell migration, suggesting poor cell permeability. Certain bis-guanidinophenyl ether derivatives such as 2f (1,3-bis(2,4-diguanidinophenoxy) benzene) exhibited micromolar potency against furin in vitro, low cell toxicity, and highly efficient protection against anthrax toxin; this compound only slightly inhibited intracellular furin. Thus, compounds 1g and 2f both represent potent furin inhibitors at the cell surface with low intracellular inhibitory action, and these particular compounds might therefore be of preferred therapeutic interest in the treatment of certain bacterial and viral infections.

Novel Furin Inhibitors with Potent Anti-infectious Activity

New peptidomimetic furin inhibitors with unnatural amino acid residues in the P3 position were synthesized. The most potent compound 4-guanidinomethyl-phenylacteyl-Arg-Tle-Arg-4-amidinobenzylamide (MI-1148) inhibits furin with a Ki value of 5.5 pM. The derivatives also strongly inhibit PC1/3, whereas PC2 is less affected. Selected inhibitors were tested in cell culture for antibacterial and antiviral activity against infectious agents known to be dependent on furin activity. A significant protective effect against anthrax and diphtheria toxin was observed in the presence of the furin inhibitors. Furthermore, the spread of the highly pathogenic H5N1 and H7N1 avian influenza viruses and propagation of canine distemper virus was strongly inhibited. Inhibitor MI-1148 was crystallized in complex with human furin. Its N-terminal guanidinomethyl group in the para position of the P5 phenyl ring occupies the same position as that found previously for a structurally related inhibitor containing this substitution in the meta position, thereby maintaining all of the important P5 interactions. Our results confirm that the inhibition of furin is a promising strategy for a short-term treatment of acute infectious diseases.

An overview of investigational toxin-directed therapies for the adjunctive management of Bacillus anthracis infection and sepsis

INTRODUCTION

Sepsis with Bacillus anthracis infection has a very high mortality rate despite appropriate antibiotic and supportive therapies. Over the past 15 years, recent outbreaks in the US and in Europe, coupled with anthrax's bioterrorism weapon potential, have stimulated efforts to develop adjunctive therapies to improve clinical outcomes. Since lethal toxin and edema toxin (LT and ET) make central contributions to the pathogenesis of B. anthracis, these have been major targets in this effort.

AREAS COVERED

Here, the authors review different investigative biopharmaceuticals that have been recently identified for their therapeutic potential as inhibitors of LT or ET. Among these inhibitors are two antibody preparations that have been included in the Strategic National Stockpile (SNS) and several more that have reached Phase I testing. Presently, however, many of these candidate agents have only been studied in vitro and very few tested in bacteria-challenged models.

EXPERT OPINION

Although a large number of drugs have been identified as potential therapeutic inhibitors of LT and ET, in most cases their testing has been limited. The use of the two SNS antibody therapies during a large-scale exposure to B. anthracis will be difficult. Further testing and development of agents with oral bioavailability and relatively long shelf lives should be a focus for future research.

Therapeutic uses of furin and its inhibitors: a patent review

INTRODUCTION

Since the discovery of furin, numerous reports have studied its role in health and diseases, including cancer, inflammatory and infectious diseases. This interest has led to the development of both large protein- and peptide-based inhibitors aiming to control furin activity to treat these disorders. The most recent advances include the development of potent peptidomimetic furin inhibitors, considerably expanding the field of therapeutic applications.

AREA COVERED

In this review, the use of furin or its inhibitors for therapeutic conditions is described through the patent literature since 1994. Only compounds with biological efficacy or augmented properties demonstrated within the patent literature or the associated publications concerning their claimed uses are discussed.

EXPERT OPINION

Considering the diseases that may benefit from furin inhibition, several patents detail the use of the restricted number of furin inhibitors. However, there have been recent reports of new scaffolds, and even the use of furin itself, as a therapeutic agent. Despite considerable evidence of in vivo efficacy, limited confirmation from clinical trials supports or refutes the further use of these compounds in a therapeutic context. The most advanced application is the use of furin knockdown in the generation of an autologous cancer vaccine, which has initiated clinical trials.

Entecavir combined with furin inhibitor simultaneously reduces hepatitis B virus replication and e antigen secretion

BACKGROUND

The antiviral therapy of chronic hepatitis B virus (HBV) infection pursues the dual goals, virological response (undetectable serum HBV DNA) and hepatitis B e antigen (HBeAg) serological response (serum HBeAg loss/seroconversion). It is relatively difficult, however, to realize the serological response, especially for nucleotide/nucleoside analogs. Furin, a proprotein convertase, is involved in HBeAg maturation. The suppression of furin using inhibitors accordingly reduces HBeAg secretion, but possibly enhances HBV replication. For these reasons, the strategy based on the combination of nucleoside analog entecavir (ETV) and furin inhibitors to inhibit HBV replication and HBeAg secretion simultaneously were studied here.

METHODS

The suppression of furin was performed using inhibitors decanoyl-RVKR-chloromethylketone (CMK) and hexa-D-arginine (D6R) or the expression of furin inhibitory prosegment. The influence of furin suppression on HBV replication and the effect of CMK combined with nucleoside analog entecavir (ETV) on HBV replication and HBeAg secretion was investigated in HepG2.2.15 cells. HBeAg level in media was detected using enzyme-linked immunosorbent assay. Intracellular viral antigens and HBV DNA were detected using Western and Southern blotting analyses, respectively.

RESULTS

CMK, D6R and the expression of inhibitory prosegment all significantly reduced HBeAg secretion, but only CMK enhance HBV replication. Concordantly, only CMK post-transcriptionally accumulated cytosolic HBV replication-essential hepatitis B core antigen (HBcAg). The HBcAg-accumulating effect of CMK was further found to be resulted from its redundant inhibitory effect on the trypsin-like activity of cellular proteasomes that are responsible for HBcAg degradation. Moreover, the viral replication-enhancing effect of CMK was abrogated by ETV and ETV combined with CMK reduced HBV replication and HBeAg secretion simultaneously.

CONCLUSION

The suppression of furin itself does not enhance HBV replication. Nucleotide/nucleoside analogs combined with furin inhibitors may be a potential easy way to realize the dual goals of the antiviral therapy for chronic hepatitis B in the future.

X-ray structures of human furin in complex with competitive inhibitors

Furin inhibitors are promising therapeutics for the treatment of cancer and numerous infections caused by bacteria and viruses, including the highly lethal Bacillus anthracis or the pandemic influenza virus. Development and improvement of inhibitors for pharmacological use require a detailed knowledge of the protease’s substrate and inhibitor binding properties. Here we present a novel preparation of human furin and the first crystal structures of this enzyme in complex with noncovalent inhibitors. We show the inhibitor exchange by soaking, allowing the investigation of additional inhibitors and substrate analogues. Thus, our work provides a basis for the rational design of furin inhibitors.

Hemojuvelin modulates iron stress during acute kidney injury: improved by furin inhibitor

AIMS

Free iron plays an important role in the pathogenesis of acute kidney injury (AKI) via the formation of hydroxyl radicals. Systemic iron homeostasis is controlled by the hemojuvelin-hepcidin-ferroportin axis in the liver, but less is known about this role in AKI.

RESULTS

By proteomics, we identified a 42 kDa soluble hemojuvelin (sHJV), processed by furin protease from membrane-bound hemojuvelin (mHJV), in the urine during AKI after cardiac surgery. Biopsies from human and mouse specimens with AKI confirm that HJV is extensively increased in renal tubules. Iron overload enhanced the expression of hemojuvelin-hepcidin signaling pathway. The furin inhibitor (FI) decreases furin-mediated proteolytic cleavage of mHJV into sHJV and augments the mHJV/sHJV ratio after iron overload with hypoxia condition. The FI could reduce renal tubule apoptosis, stabilize hypoxic induced factor-1, prevent the accumulation of iron in the kidney, and further ameliorate ischemic-reperfusion injury. mHJV is associated with decreasing total kidney iron, secreting hepcidin, and promoting the degradation of ferroportin at AKI, whereas sHJV does the opposite.

INNOVATION

This study suggests the ratio of mHJV/sHJV affects the iron deposition during acute kidney injury and sHJV could be an early biomarker of AKI.

CONCLUSION

Our findings link endogenous HJV inextricably with renal iron homeostasis for the first time, add new significance to early predict AKI, and identify novel therapeutic targets to reduce the severity of AKI using the FI.

Designing inhibitors of anthrax toxin

INTRODUCTION

Present-day rational drug design approaches are based on exploiting unique features of the target biomolecules, small- or macromolecule drug candidates and physical forces that govern their interactions. The 2013 Nobel Prize in chemistry awarded 'for the development of multiscale models for complex chemical systems' once again demonstrated the importance of the tailored drug discovery that reduces the role of the trial-and-error approach to a minimum. The intentional dissemination of Bacillus anthracis spores in 2001 via the so-called anthrax letters has led to increased efforts, politically and scientifically, to develop medical countermeasures that will protect people from the threat of anthrax bioterrorism.

AREAS COVERED

This article provides an overview of the recent rational drug design approaches for discovering inhibitors of anthrax toxin. The review also directs the readers to the vast literature on the recognized advances and future possibilities in the field.

EXPERT OPINION

Existing options to combat anthrax toxin lethality are limited. With the only anthrax toxin inhibiting therapy (protective antigen-targeting with a monoclonal antibody, raxibacumab) approved to treat inhalational anthrax, the situation, in our view, is still insecure. Further, the FDA's animal rule for drug approval, which clears compounds without validated efficacy studies on humans, creates a high level of uncertainty, especially when a well-characterized animal model does not exist. Better identification and validation of anthrax toxin therapeutic targets at the molecular level as well as elucidation of the parameters determining the corresponding therapeutic windows are still necessary for more effective therapeutic options.

Potential biological targets ofBacillus anthracisin anti-infective approaches against the threat of bioterrorism

The terrorist attacks of 2001 involving anthrax underscore the imperative that safe and effective medical countermeasures should be readily available. Vaccination appears to be the most effective form of mass protection against a biological attack, but the current vaccines have drawbacks that justify the enormous amount of effort currently being put into developing more effective vaccines and other treatment modalities. After providing a comprehensive overview of the organism Bacillus anthracis as a biological weapon and its pathogenicity, this review briefly summarizes the current knowledge vital to the management of anthrax disease. This knowledge has been acquired since 2001 as a result of the progress on anthrax research and focuses on the possible development of improved human anti-infective strategies targeting B. anthracis spore components, as well as strategies based on host-pathogen interactions.

Small molecule inhibitors of anthrax lethal factor toxin

This manuscript describes the preparation of new small molecule inhibitors of Bacillus anthracis lethal factor. Our starting point was the symmetrical, bis-quinolinyl compound 1 (NSC 12155). Optimization of one half of this molecule led to new LF inhibitors that were desymmetrized to afford more drug-like compounds.

Therapeutic potential of furin inhibitors for the chronic infection of hepatitis B virus

BACKGROUND & AIMS

Hepatitis B e antigen (HBeAg) is essential for the development of chronic hepatitis B virus (HBV) infection. Furin, a proprotein convertase, plays a key role in processing of HBeAg precursor into maturated HBeAg. For these reasons, the therapeutic potential of furin inhibition for chronic HBV infection was studied.

METHODS

The effects of furin inhibitor I (decanoyl-RVKR-chloromethylketone, CMK) and furin inhibitor II (hexa-D-arginine, D6R) on HBeAg secretion, the destination of unprocessed precursor and cellular secretory functions were comparatively investigated.

RESULTS

CMK and D6R significantly decreased the supernatant level of HBeAg and increased the intracellular level of HBeAg precursor in HepG2.2.15 cells in vitro. The accumulated HBeAg precursor was not found to be retro-transported into the cytosol to inhibit HBV replication as expected, but was found to be expressed on the cell surface, where it may be more convenient to mediate host immune responses. Furthermore, these inhibitors at effective concentrations were not found to interfere with the maturations of albumin and prothrombin. Compared with CMK, D6R was suboptimal in effectiveness; however, D6R neither enhanced HBV replication through the accumulation of cytosolic HBcAg nor did it cause severe cell damage in an elongated safety analyses.

CONCLUSION

Furin inhibitors CMK and D6R reduce HBeAg secretion and increase cell surface expression of the HBeAg precursor in HepG2.2.15 cells. Novel furin inhibitors or modified forms of D6R may promote the reduction of immune tolerance and the elimination of infected hepatocytes in patients with chronic HBV infection.

Furin mRNA expression in peripheral blood correlates with chronic hepatitis B virus infection

AIM

  The mechanisms underlying development of chronic hepatitis B virus (HBV) infection are related to immune tolerance, but are as yet incompletely understood. Furin has been found to be essential for maintenance of peripheral immune tolerance mediated by regulatory T cells (Treg). Such effect of furin on chronic HBV infection was investigated in this study.

METHODS

  Peripheral blood from 40 individuals with self-limited HBV infection, 40 patients with asymptomatic persistent HBV infection and 40 patients with chronic hepatitis B (CHB) was collected and mRNA expression levels of furin, transforming growth factor (TGF)-β1 and the Treg-function-related forkhead transcription factor FoxP3 were detected using quantitative real-time polymerase chain reaction. CD4(+) CD25(+) FoxP3(+) Treg were detected using flow cytometry.

RESULTS

  Furin mRNA expression in peripheral blood was significantly higher in patients with persistent HBV infection than in individuals with self-limited infection (P < 0.01), and was much higher in CHB patients than in those with asymptomatic persistent infection (P < 0.01). Furthermore, furin mRNA was relatively higher in patients with positive hepatitis B e antigen and higher levels of serum HBV DNA (>10 000 copies/mL). In patients with CHB, furin mRNA expression was found to correlate with TGF-β1 mRNA and FoxP3 mRNA expression using Spearman's rank correlation coefficient test. It was 5.7-times higher in CD4(+) CD25(+) T cells than in CD4(+) CD25(-) T cells and correlated with the frequency of Treg (P < 0.05).

CONCLUSION

  Furin mRNA expression in peripheral blood correlates with chronic HBV infection and liver damage, and seems to participate in immune inhibitory and anti-inflammatory mechanisms in HBV infection, mediated by TGF-β1 and/or Treg.

Generation and characterization of non-competitive furin-inhibiting nanobodies

The PC (proprotein convertase) furin cleaves a large variety of proproteins and hence plays a major role in many pathologies. Therefore furin inhibition might be a good strategy for therapeutic intervention, and several furin inhibitors have been generated, although none are entirely furin-specific. To reduce potential side effects caused by cross-reactivity with other proteases, dromedary heavy-chain-derived nanobodies against catalytically active furin were developed as specific furin inhibitors. The nanobodies bound only to furin but not to other PCs. Upon overexpression in cell lines, they inhibited the cleavage of two different furin substrates, TGFβ (transforming growth factor β) and GPC3 (glypican 3). Purified nanobodies could inhibit the cleavage of diphtheria toxin into its enzymatically active A fragment, but did not inhibit cleavage of a small synthetic peptide-based substrate, suggesting a mode-of-action based on steric hindrance. The dissociation constant of purified nanobody 14 is in the nanomolar range. The nanobodies were non-competitive inhibitors with an inhibitory constant in the micromolar range as demonstrated by Dixon plot. Furthermore, anti-furin nanobodies could protect HEK (human embryonic kidney)-293T cells from diphtheria-toxin-induced cytotoxicity as efficiently as the PC inhibitor nona-D-arginine. In conclusion, these antibody-based single-domain nanobodies represent the first generation of highly specific non-competitive furin inhibitors.

Small molecule inhibitors of Bacillus anthracis protective antigen proteolytic activation and oligomerization

Protective antigen (PA), lethal factor, and edema factor, the protein toxins of Bacillus anthracis , are among its most important virulence factors and play a key role in infection. We performed a virtual ligand screen of a library of 10000 members to identify compounds predicted to bind to PA and prevent its oligomerization. Four of these compounds slowed PA association in a FRET-based oligomerization assay, and two of those protected cells from intoxication at concentrations of 1-10 μM. Exploration of the protective mechanism by Western blot showed decreased SDS-resistant PA oligomer on cells and, surprisingly, decreased amounts of activated PA. In vitro assays showed that one of the inhibitors blocked furin-mediated cleavage of PA, apparently through its binding to the PA substrate. Thus, we have identified inhibitors that can independently block both PA's cleavage by furin and its subsequent oligomerization. Lead optimization on these two backbones may yield compounds with high activity and specificity for the anthrax toxins.

Highly potent inhibitors of proprotein convertase furin as potential drugs for treatment of infectious diseases

Optimization of our previously described peptidomimetic furin inhibitors was performed and yielded several analogs with a significantly improved activity. The most potent compounds containing an N-terminal 4- or 3-(guanidinomethyl)phenylacetyl residue inhibit furin with K(i) values of 16 and 8 pM, respectively. These analogs inhibit other proprotein convertases, such as PC1/3, PC4, PACE4, and PC5/6, with similar potency, whereas PC2, PC7, and trypsin-like serine proteases are poorly affected. Incubation of selected compounds with Madin-Darby canine kidney cells over a period of 96 h revealed that they exhibit great stability, making them suitable candidates for further studies in cell culture. Two of the most potent derivatives were used to inhibit the hemagglutinin cleavage and viral propagation of a highly pathogenic avian H7N1 influenza virus strain. The treatment with inhibitor 24 (4-(guanidinomethyl)phenylacetyl-Arg-Val-Arg-4-amidinobenzylamide) resulted in significantly delayed virus propagation compared with an inhibitor-free control. The same analog was also effective in inhibiting Shiga toxin activation in HEp-2 cells. This antiviral effect, as well as the protective effect against a bacterial toxin, suggests that inhibitors of furin or furin-like proprotein convertases could represent promising lead structures for future drug development, in particular for the treatment of infectious diseases.

Preliminary microRNA analysis in lung tissue to identify potential therapeutic targets against H5N1 infection

Within the past decade, human infections with the highly pathogenic avian influenza H5N1 have resulted in approximately 60% mortality and increased the need for vaccines and therapeutics. Understanding the molecular events associated with pathology can aid this effort; therefore, this study was conducted to assess microRNA (miRNA) expression in mouse lungs infected with H5N1 A/Vietnam/1203/04. Intranasal administration of 1500 median tissue culture infectious dose of H5N1 promoted differences in the number and expression pattern of miRNA from lung tissue collected at 2, 4, 6, 24, and 96 h post-exposure that mapped to common biological functions. Informatics analysis identified miRNA-specific predicted genes known to be therapeutic drug targets in which Furin was common to all time periods. This study provides insight into the differential miRNA expression with respect to the host-pathogen relationship and identification of potential therapeutic drug targets.

Domain flexibility modulates the heterogeneous assembly mechanism of anthrax toxin protective antigen

The three protein components of anthrax toxin are nontoxic individually, but they form active holotoxin complexes upon assembly. The role of the protective antigen (PA) component of the toxin is to deliver two other enzyme components, lethal factor and edema factor, across the plasma membrane and into the cytoplasm of target cells. PA is produced as a proprotein, which must be proteolytically activated; generally, cell surface activation is mediated by a furin family protease. Activated PA can then assemble into one of two noninterconverting oligomers, a homoheptamer and a homooctamer, which have unique properties. Herein we describe molecular determinants that influence the stoichiometry of PA in toxin complexes. By tethering PA domain 4 (D4) to domain 2 with two different-length cross-links, we can control the relative proportions of PA heptamers and octamers. The longer cross-link favors octamer formation, whereas the shorter one favors formation of the heptamer. X-ray crystal structures of PA (up to 1.45 Å resolution), including these cross-linked PA constructs, reveal that a hinge-like movement of D4 correlates with the relative preference for each oligomeric architecture. Furthermore, we report the conformation of the flexible loop containing the furin cleavage site and show that, for efficient processing, the furin site cannot be moved ~5 or 6 residues within the loop. We propose that there are different orientations of D4 relative to the main body of PA that favor the formation of either the heptamer or the octamer.

Select human anthrax protective antigen epitope-specific antibodies provide protection from lethal toxin challenge

Bacillus anthracis remains a serious bioterrorism concern, and the currently licensed vaccine remains an incomplete solution for population protection from inhalation anthrax and has been associated with concerns regarding efficacy and safety. Thus, understanding how to generate long-lasting protective immunity with reduced immunizations or provide protection through postexposure immunotherapeutics are long-sought goals. Through evaluation of a large military cohort, we characterized the levels of antibodies against protective antigen and found that over half of anthrax vaccinees had low serum levels of in vitro toxin neutralization capacity. Using solid-phase epitope mapping and confirmatory assays, we identified several neutralization-associated humoral epitopes and demonstrated that select antipeptide responses mediated protection in vitro. Finally, passively transferred antibodies specific for select epitopes provided protection in an in vivo lethal toxin mouse model. Identification of these antigenic regions has important implications for vaccine design and the development of directed immunotherapeutics.

Inhibition of furin/proprotein convertase-catalyzed surface and intracellular processing by small molecules

Furin is a ubiquitously expressed proprotein convertase (PC) that plays a vital role in numerous disease processes including cancer metastasis, bacterial toxin activation (e.g. anthrax and Pseudomonas), and viral propagation (e.g. avian influenza and human immunodeficiency virus). To identify small molecule inhibitors of furin and related processing enzymes, we performed high-throughput screens of chemical diversity libraries utilizing both enzyme-based and cell-based assays. The screens identified partially overlapping sets of compounds that were further characterized for affinity, mechanism, and efficacy in additional cellular processing assays. Dicoumarols were identified as a class of compounds that inhibited furin non-competitively and reversibly with Ki values in the micromolar range. These compounds inhibited furin/furin-like activity both at the cell surface (protecting against anthrax toxin) and in the secretory pathway (blocking processing of the metastasis factor membrane-type 1 matrix metalloproteinase/MT1-MMP) at concentrations close to Ki values. Compounds tested exhibited distinct patterns of inhibition of other furin-family PCs (rat PACE4, human PC5/6 and human PC7), showing that dicoumarol derivatives might be developed as either generic or selective inhibitors of the PCs. The extensive clinical use, high bioavailability and relatively low toxicity of dicoumarols suggests that the dicoumarol structure will be a good starting point for development of drug-like inhibitors of furin and other PCs that can act both intracellularly and at the cell surface.

Targeting virulence: a new paradigm for antimicrobial therapy

Clinically significant antibiotic resistance has evolved against virtually every antibiotic deployed. Yet the development of new classes of antibiotics has lagged far behind our growing need for such drugs. Rather than focusing on therapeutics that target in vitro viability, much like conventional antibiotics, an alternative approach is to target functions essential for infection, such as virulence factors required to cause host damage and disease. This approach has several potential advantages including expanding the repertoire of bacterial targets, preserving the host endogenous microbiome, and exerting less selective pressure, which may result in decreased resistance. We review new approaches to targeting virulence, discuss their advantages and disadvantages, and propose that in addition to targeting virulence, new antimicrobial development strategies should be expanded to include targeting bacterial gene functions that are essential for in vivo viability. We highlight both new advances in identifying these functions and prospects for antimicrobial discovery targeting this unexploited area.

Defensive strategies of Bacillus anthracis that promote a fatal disease

Bacillus anthracis is a Gram-positive bacterium that causes anthrax. Bacterial spores that enter the host germinate into metabolically active bacilli that disseminate throughout the body and replicate to high numbers. Two virulence factors are essential for this unrestrained growth. The first is a weakly immunogenic poly gamma-D-glutamic acid capsule that surrounds the bacilli and confers resistance to phagocytosis. The second virulence factor, anthrax toxin, disrupts multiple host functions to diminish the immune response.

New insights into the pathogenesis and treatment of anthrax toxin-induced shock

Inhalational Bacillus anthracis infection is a leading bioterrorist health threat in the US today. Lethal (LeTx) and edema toxin production are key to the virulent effects of this lethal bacteria. Recent insights into the structure and function of these toxins have increased the understanding of both the pathogenesis and treatment of anthrax. These are binary type toxins comprised of protective antigen necessary for their cellular uptake and either lethal or edema factors, the toxigenic moieties. Primary cellular receptors for protective antigen have been identified and the processing of the completed toxins clarified. Consistent with the ability of lethal factor to cleave mitogen activated protein kinase kinases, the evidence indicates that an excessive inflammatory response does not contribute to shock with LeTx. Rather, the immunosuppressive effects of LeTx could promote infection; however, direct endothelial dysfunction may have an important role in shock due to LeTx. Recent studies show that edema factor, a potent adenyl cyclase, may have a major role in shock during anthrax and that it may also be immunosuppresive. Therapies under development which target several steps in the cellular uptake and function of these two toxins have been effective in both in vitro and in vivo systems. Understanding how best to apply these agents in combination with conventional treatments should be a goal of future research.

Targeting host cell furin proprotein convertases as a therapeutic strategy against bacterial toxins and viral pathogens

Pathogens or their toxins, including influenza virus, Pseudomonas, and anthrax toxins, require processing by host proprotein convertases (PCs) to enter host cells and to cause disease. Conversely, inhibiting PCs is likely to protect host cells from multiple furin-dependent, but otherwise unrelated, pathogens. To determine if this concept is correct, we designed specific nanomolar inhibitors of PCs modeled from the extended cleavage motif TPQRERRRKKR downward arrowGL of the avian influenza H5N1 hemagglutinin. We then confirmed the efficacy of the inhibitory peptides in vitro against the fluorescent peptide, anthrax protective antigen (PA83), and influenza hemagglutinin substrates and also in mice in vivo against two unrelated toxins, anthrax and Pseudomonas exotoxin. Peptides with Phe/Tyr at P1' were more selective for furin. Peptides with P1' Thr were potent against multiple PCs. Our strategy of basing the peptide sequence on a furin cleavage motif known for an avian flu virus shows the power of starting inhibitor design with a known substrate. Our results confirm that inhibiting furin-like PCs protects the host from the distinct furin-dependent infections and lay a foundation for novel, host cell-focused therapies against acute diseases.

Synthetic small molecule furin inhibitors derived from 2,5-dideoxystreptamine

Furin plays a crucial role in embryogenesis and homeostasis and in diseases such as Alzheimer's disease, cancer, and viral and bacterial infections. Thus, inhibition of furin may provide a feasible and promising approach for therapeutic intervention of furin-mediated disease mechanisms. Here, we report on a class of small molecule furin inhibitors based on 2,5-dideoxystreptamine. Derivatization of 2,5-dideoxystreptamine by the addition of guanidinylated aryl groups yielded a set of furin inhibitors with nanomolar range potency against furin when assayed in a biochemical cleavage assay. Moreover, a subset of these furin inhibitors protected RAW 264.7 macrophage cells from toxicity caused by furin-dependent processing of anthrax protective antigen. These inhibitors were found to behave as competitive inhibitors of furin and to be relatively specific for furin. Molecular modeling revealed that these inhibitors may target the active site of furin as they showed site occupancy similar to the alkylating inhibitor decanoyl-Arg-Val-Lys-Arg-CH(2)Cl. The compounds presented here are bona fide synthetic small molecule furin inhibitors that exhibit potency in the nanomolar range, suggesting that they may serve as valuable tools for studying furin action and potential therapeutics agents for furin-dependent diseases.

Lethal and edema toxins in the pathogenesis of Bacillus anthracis septic shock: implications for therapy

Recent research regarding the structure and function of Bacillus anthracis lethal (LeTx) and edema (ETx) toxins provides growing insights into the pathophysiology and treatment of shock with this lethal bacteria. These are both binary-type toxins composed of protective antigen necessary for their cellular uptake and either lethal or edema factors, the toxigenic moieties. The primary cellular receptors for protective antigen have been identified and constructed and key steps in the extracellular processing and internalization of the toxins clarified. Consistent with the lethal factor's primary action as an intracellular endopeptidase targeting mitogen-activated protein kinase kinases, growing evidence indicates that shock with this toxin does not result from an excessive inflammatory response. In fact, the potent immunosuppressive effects of LeTx may actually contribute to the establishment and persistence of infection. Instead, shock with LeTx may be related to the direct injurious effects of lethal factor on endothelial cell function. Despite the importance of LeTx, very recent studies show that edema factor, a potent adenyl cyclase, has the ability to make a substantial contribution to shock caused by B. anthracis and works additively with LeTx. Furthermore, ETx may contribute to the immunosuppressive effects of LeTx. Therapies under development that target several different steps in the cellular uptake and function of these two toxins have been effective in in vitro and in vivo systems. Understanding how best to apply these agents clinically and how they interact with conventional treatments should be goals for future research.

Cationic polyamines inhibit anthrax lethal factor protease

Background

Anthrax is a human disease that results from infection by the bacteria, Bacillus anthracis and has recently been used as a bioterrorist agent. Historically, this disease was associated with Bacillus spore exposure from wool or animal carcasses. While current vaccine approaches (targeted against the protective antigen) are effective for prophylaxis, multiple doses must be injected. Common antibiotics that block the germination process are effective but must be administered early in the infection cycle. In addition, new therapeutics are needed to specifically target the proteolytic activity of lethal factor (LF) associated with this bacterial infection.

Results

Using a fluorescence-based assay to identify and characterize inhibitors of anthrax lethal factor protease activity, we identified several chemically-distinct classes of inhibitory molecules including polyamines, aminoglycosides and cationic peptides. In these studies, spermine was demonstrated for the first time to inhibit anthrax LF with a K_i value of 0.9 ± 0.09 μM (mean ± SEM; n = 3). Additional linear polyamines were also active as LF inhibitors with lower potencies.

Conclusion

Based upon the studies reported herein, we chose linear polyamines related to spermine as potential lead optimization candidates and additional testing in cell-based models where cell penetration could be studied. During our screening process, we reproducibly demonstrated that the potencies of certain compounds, including neomycin but not neamine or spermine, were different depending upon the presence or absence of nucleic acids. Differential sensitivity to the presence/absence of nucleic acids may be an additional point to consider when comparing various classes of active compounds for lead optimization.