Antimicrobial peptides: has their time arrived?

Abstract

Future MicrobiologyVol. 10, No. 7 EditorialFree AccessAntimicrobial peptides: has their time arrived?Enea Sancho-Vaello & Kornelius ZethEnea Sancho-Vaello*Author for correspondence: E-mail Address: enea.sancho@gmail.com Unidad de Biofísica, Centro Mixto Consejo Superior de Investigaciones Científicas-Universidad del País Vasco/Euskal Herriko Unibertsitatea (CSIC, UPV/EHU), Barrio Sarriena s/n, Leioa, Bizkaia, SpainSearch for more papers by this author & Kornelius Zeth Department of Biochemistry & Molecular Biology, University of the Basque Country, Barrio Sarriena s/n, Leioa, Bizkaia, Spain NanoGUNE, San Sebastian, Spain IKERBASQUE, Basque Foundation for Science, Bilbao, SpainSearch for more papers by this authorPublished Online:29 Jun 2015https://doi.org/10.2217/fmb.15.45AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsPermissionsReprints ShareShare onFacebookTwitterLinkedInRedditEmail Keywords: antimicrobial peptidesantibacterial resistancemembranepeptidoglycanThe survival of human beings on planet earth is becoming severely difficult owing to the threat of deathly microbes including viruses, bacteria and fungi. During the last decades, humans have been effectively fighting against these threats using antibiotics, many of which are based on Fleming's groundbreaking discovery of penicillin as an effective natural antibiotic. Subsequent work by Ehrlich and Domagk led to the large-scale development of the first synthetic antibiotics and their introduction in modern medicine [1]. Due to the extensive misuse of follow-up antibiotics for the prevention of farm animal diseases and human medicine, the world currently faces an important and eminent problem: the large increase of antimicrobial resistance. Although sulfonamide-resistant Streptococcus pyogenes first appeared in military hospitals in the 1930s [1], during the recent two decades the increase of resistances threatens the effective prevention and treatment of ‘simple’ infections caused by ‘superbugs’ [2]. Some clear examples of this evolution are the appearance of 450,000 new cases of multidrug-resistant tuberculosis in 2012 and the emergence of superbugs in hospitals around the world, such as methicillin-resistant Staphylococcus aureus (MRSA) or multidrug-resistant (MDR) Gram-negative bacteria such as Klebsiella pneumoniae [3]. In fact, the last report from the US Centers for Disease Control and Prevention estimated that over two million illnesses and 23,000 deaths were caused by drug-resistant microbes in the USA in 2013 [4]. This trend will become even more drastic in the future due to a reduced engagement of the pharmaceutical industry in the development of new antibiotics for the last two decades [5,6].Since the magnitude of this problem that threatens the global public health from an economical and sociological perspective, the response has to be fast and accurate. For this reason, both academia and the industry should become more involved in the important task of searching for new therapeutic agents. These potential therapeutic agents are already available and produced, for example, by the variety of soil bacteria and fungi that had previously delivered penicillin and vancomycin antibiotics and, more recently, the highly effective teixobactin antibiotic [7]. Additional putative antibiotics from organisms as different as sponges or oaks were shown to be useful weapons to fight against MDR strains [8]. However, many of these antibiotics follow traditional targeting schemes by inhibiting enzymatic functions, a strategy that can be easily bypassed via genetic adaptation or horizontal gene transfer between bacteria. In contrast, some of the currently most promising candidates target precursors of lipids or LPS as well as integrate into bacterial membranes [7]. Antimicrobial peptides (AMPs) belong to these promising therapeutic agents targeting membranes with a retained activity against MDR strains in spite of their co-evolution for millions of years [9].AMPs are produced by essentially all multicellular and many unicellular organisms as defense molecules against pathogenic microbes. In spite of their large number in nature, their existence was not reported until the middle of the last century when gramicidin from Bacillus brevis, cecropin from moths and magainin from frogs were reported [10]. AMPs can be classified into various groups (defensins, cecropins, magainins, cathelicidins, etc.) exhibiting different secondary structures and often stabilized by disulfides [11,12]. There are essentially no hallmarks that could help to clearly define AMPs other than that the peptides are usually small (10–50 residues), contain about 40% hydrophobic amino acids and a surplus of positive charges [12]. The overall positive charge is important for their interaction with the negatively charged LPS of the outer membrane and negatively charged membrane lipids to exert their mechanism of action, mainly based on the integration and disruption of membranes [13]. AMPs are also known to suppress biofilm formation, a common problem in many bacterial infections. Due to these interesting properties, AMPs are getting into the focus of research aiming to develop new therapeutic antibiotics. Currently, more than 2000 different AMPs have been classified in databases [14] and the scientific community has determined a variety of properties from diverse natural AMPs. One important number characterizing AMPs is their minimal inhibitor concentration (MIC) closely related with their efficiency in killing microbes. MICs can be as low as in the 500 nM range for horseshoe crab polyphemusin I or pig protegrin, similar in range with traditional antibiotics [15].What are the current trends in antibiotic research? The loss of activity of traditional antibiotics is likely caused by two mechanisms: genetic adaptation through mutation or the horizontal gene transfer which may cause the degradation of antibiotics in cells that were previously not resistant. Because of that, new and successful antibiotics need to deviate from these old schemes targeting alternative chemical moieties such as cellular biochemical intermediates of peptidoglycan or LPS or biological membranes. In this context, the strength of AMPs is their killing mechanism through their interaction with bacterial membranes, the ‘Achilles heel’ of bacteria [15,16]. Membranes, LPS and peptidoglycan are essential components of bacterial cells and can be changed only moderately if at all. This is one reason why vancomycin, which binds a precursor of peptidoglycan, remains active when penicillin, cephalosporin or macrolide fail. More recently another promising antibiotic called teixobactin, which inhibits the synthesis of peptidoglycan, has been discovered [7].The potential of AMPs is probably wider since they have been also found as potential drugs against cancer. In this case, their anticancer activity is based on the differences between normal and cancer cells, since malignant cells have membrane net negative charge, higher fluidity and expanded surface area. AMPs have shown their ability in disrupting and permeating the cancer cell membranes with higher preference [17].What are the current limitations in the development of antimicrobial peptides toward new antibiotics? In spite of a breadth of data collected on AMPs, their rational design or the prediction of properties is lacking behind. AMPs are required to have high stability under physiological conditions, low cytotoxicity against host cells and small MIC against bacteria. For that, ‘brute force’ approaches based on random changes of amino acid sequences in a given peptide are being employed [15]. This approach was applied to the well-studied LL-37 peptide [18], downsizing it to 22-mer resulting in a compound named OP-145, a promising drug against middle ear infection, currently in clinical trials in Phase II/III [19]. While ‘brute force’ approaches are based on random trials, there are modern computer-assisted design strategies that can more easily relate peptide sequence and structure, although the success of these programs is rather limited [15]. One reason of this failure is the complex environment of biological membranes (AMPs act on), which is difficult if not impossible to model. Another problem is the number of reliable peptide structures deposited in the PDB database as a basis for rational design. Although many AMPs are known to form oligomers in solution and even more likely in the presence of lipids there are only a few structures available showing these states [12]. The only currently known true oligomeric peptide structure is the hexameric channel of human dermcidin, which might be an interesting model for optimizing physiological activity [20,21]. However, dermicidin demonstrates another problem in the development of AMPs: while small peptides up to a length of approximately 30 residues can be produced at large quantities and low prices, longer peptides are difficult to synthesize and are expensive. As a consequence, their large-scale production involves excessive costs, with prices between $100 and $600 per gram but may be lowered when produced in microbial expression systems [16]. While small peptides have entered clinical trials for long, many of which became commercial drugs, only about 10–20 AMPs are currently in clinical Phase I, II or III [15].In spite of these obstacles, one may wonder why the time for more clinical trials with AMPs has not come. Although these molecules come into the focus at a time which provides sophisticated technical equipment for various studies, their potential may be explored without knowing every mechanistic detail. Retrospectively seen when the first antibiotics appeared in the early 19th century, little was known about the molecular mechanisms (e.g., inhibiting ribosomal β-lactamase activity) and the techniques in biochemistry or structural biology we use today were unknown. Still antibiotics were successfully used to treat various diseases, possibly also because the standards for their approval were lower. Since humans express some AMPs at high concentrations to protect themselves against the uncontrolled growth of microbes, one additional strategy not available for conventional antibiotics is to use their immunomodulatory potential to self-stimulate the human immune system [16].In any case, our society has to be careful with the management of these new drugs after approval, trying to limit their use by means of strict medical guidelines and the health education of the citizens in order to prevent the apparition of new resistances. Our society should not step into the same trap as the American epidemiologist William Stewart who, in 1967, facing the impressive increase in life expectancy and not suspecting the appearance of resistances, said: ‘The time has come to close the book on infectious diseases.’ Unfortunately, he was wrong.Financial & competing interests disclosureK Zeth is currently supported by two research grants 522/2-4 and 522/5-1 of the German Science Foundation (DFG) and the BFU2013-48581-P number from MINECO 2013 Spain. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.No writing assistance was utilized in the production of this manuscript.References1 Sköld O. Sulfonamide resistance: mechanisms and trends. Drug Resist. Updat. 3, 155–160 (2000).Crossref, Medline, CAS, Google Scholar2 Levy S, Marshall B. 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The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.No writing assistance was utilized in the production of this manuscript.PDF download