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Antimicrobial peptides: Coming to the end of antibiotic era, the most promising agents.

INTRODUCTION

The misuse of antibiotics which are the main forces in antimicrobial therapy has led to the development of widespread resistance in microorganisms. Especially the nosocomial infections are caused by the Gram positive or Gram negative pathogens which have increased antibiotic resistance in intensive care units (Akova 2016; Rosenrhal et al. 2016). While some microorganisms are resistant to only one antimicrobial agent, many of them developed the resistance to multiple antimicrobials, so they called multidrug-resistant (MDR) strains. Infections caused by the MDR bacteria are generally not respond to antimicrobial therapy and they comprises a major risk for the mortality. Worse still, sometimes MDR microorganisms become resistant to all available antibiotics, named "pan-resistant organisms", and they could not treated with any single agent (Giamarellon 2010; Naim et al. 2016).

World Health Organization (WHO), had taken an extensive interest for that problem and published a report named "Antimicrobial resistance: Global report on surveillance 2014" to capture the attractions for antimicrobial resistance (WHO 2014). Then the "global action plan on antimicrobial resistance" was composed in 2015, by WHO as a World Health Assembly documents. That document outlines five objectives including improving the awareness and understandings, strengthening the knowledges and evidences, reducing the incidence of infections, optimizing the antimicrobial usage, and increasing the investment in new medicines (WHO 2015).

While combating the infectious diseases, there is conspicuous decrease in existing antibiotics, and as indicated by the WHO, finding of the alternative antimicrobial agents which have a new mechanism of action is very crucial. Antimicrobial activities of natural substances are always known, and there is a reinterest for their potential usages due to the rise of multidrug resistance in a variety of bacteria. Among them antimicrobial cationic peptides (AMPs) seems very promising antibacterial agents to controlling the resistant bacterial infections Donadio et al. 2010).

This substances which, have AMPs are very prevalent in nature as important component of the innate immune response to the surrounding microorganisms, and they can be isolated from most of the living organisms such as insects, plants, microorganisms, mammals or non-mammalian vertebrates. AMPs have rapid action and various activity like broad spectrum antibacterial, antifungal, antiviral, and antiprotozoal (Hancock and Chapple 1999; Otvos 2005).

Structure of AMPs

AMPs are generally relatively short molecules, including 10-100 amino-acid residues display an positive net charge ranging from +2 to +11, and containing an amphiphilic and hydrophobic residues (in general 50%) (Hancock and Sahl 2006). Due to the increasing number of natural, semi-synthetic or synthetic AMPs, there are several databases are exist for today, which manage information and conduct peptide analysis (Wang 2015). As shown in Figure 1, despite their similar general physical properties, AMPs are classified based on the composition of their amino-acid, size and conformational secondary structures into major groups including peptides with [beta]-strands, amphipathic a-helices, loop structure, and extended structures. Most of the AMPs are belongs to the first two categories (Hancock 2001; Jenssen et al. 2006).

Among AMPs, the most studied ones are colistin, melittin, indolicidin, nisin, CAMA, defensins, protegrins, magainins, etc. Colistin is a non-ribosomally synthesized AMP which is used as a prodrug as the methanesulfonic acid derivative of polymyxin E from Bacillus polymyxa var colistinus, and it's bactericidal to Gram negative bacteria especially Pseudomonas aeruginosa due to a detergent-like mechanism (Bechinger and Lohner 2006). Indolicidin is a haemolytic and antimicrobial peptide isolated from bovine neutrophils. It has a 13 tridecapeptide amide and an extremely high tryptophan content corresponding to its wide range of antimicrobial activities (Selsted et al. 1992). Nisin is an important bacteriocin AMP and one of the most studied lantibiotic which has a 34 amino acid including lanthionine and methyl-lanthionine. It is isolated from the bacteria Lactococcus lactis, and shows rapid bactericidal effects against Gram-positive bacteria with no discrimination between multidrug resistant or sensitive pathogens due to its' dual mode of action against bacterial cell membranes (Willey and Van der Donk 2007). Melittin is a main toxic element of the honey bee venom, and an active AMP against Gram positive and Gram negative bacteria. It is known as a hemolytic peptide which have 26 amino acid and amphipathic a-helices (Raghuraman and Chattopadyay 2007). Because of the toxic characters of some effective AMPs, hybride peptides such as CAMA (Cecropin (1-7)-melittin A (2-9) amide) are designed for increasing the antimicrobial activity while decreasing toxicity. CAMA included the some amino acid regions of cecropin-A and melittin and is very active against Gram-positive bacteria, forming the ion-permeable channels in lipid membranes (Cao et al. 2010).

AMPs' mechanisms of actions

Most of the active AMPs interact with bacterial membranes, especially the Gram negative's lipopolysaccharide (LPS) layer. They create an ion-permeable channels, and increase membrane permeability to developing the cleavages. While crossing the small molecules into the bacterial cells from that cracks, some antimicrobial agents including the AMPs' are passing through the membrane. To explain how AMPs damages the membranes, as shown in Figure 2, a variety of possible membrane-weakening mechanisms such as toroidal, barrel-stave or carpet models have been maintained. As a result of this interactions, AMPs are taken up by "self-promoted uptake" pathway and they also affect many intracellular mechanisms to toward the bacterial cell death. The explanation both how AMPs bind to and inhibit endotoxins and how they shown the synergistic interactions with conventional antibiotics are existing by this pathway (Steinberg et al. 1997; Yeaman and Yount 2003; Wang et al. 2015).

Besides of the antimicrobial activities, AMPs also has an "enhancer" activity for classical antibiotics. That enhancement in activities of the antibiotics with appropriate AMPs, especially for MDR strains, caused by not only a permeability-increasing, but also the result of an increased access to the intracellular targets. Thus AMPs could be serve as anti-resistance compounds against planktonic cells (Sawyer et al. 1998; McCafferty et al. 1999).

On the other hand, when the planktonic forms of bacteria generally causes an acute infections, biofilm-associated forms cause persistent and chronic infections. A biofilm is a clusters of microorganisms in their extracellular matrix (EPS), on the biotic or abiotic surfaces. The cells growing in a biofilms are physiologically different from their planktonic forms. The bacteria in biofilms become more resistant to antimicrobial agents up to 1000-fold and the host's immune responses. That increased resistance in biofilm forming bacteria, could be explained by the decreased diffusion of antimicrobials, increased activity of multidrug efflux pumps, quorum-sensing systems, antimicrobial tolerance, and slow-growing cells,... etc (Donlan 2001; Hoiby et al. 2010).

To prevent or delay the emergence of resistance caused by the biofilm, AMPs might be use in the antimicrobial combinations to providing the synergistic interactions. There are many kind of AMPs were found active against bacterial biofilms in that way (Dosler and Mataraci 2013; Dosler and Karaaslan 2014). However, the anti-biofilm activities of AMPs are not completely understood, there are some possible explanations provided by some limited studies. That explanations includes matrix disruption; inhibition of some biofilm-related genes in DNA, blocking the quorum sensing systems, and AMPs dual mode of actioss on both the cytoplasmic membrane and intracellular targets (Hancock and Sahl 2006; Jorge et al. 2012).

AMPs' resistance mechanisms

AMPs are generally not affected by many resistance mechanisms that influenced the antibiotics. Also, there are a few number of resistance mechanisms which can effect antimicrobial cationic peptides and these mechanisms perform independently from others that effect antibiotics. A general mechanism causing resistance bacteria is the association of positively charged molecules onto the cell surface which reduces the interaction and binding of AMPs. The most important resistance mechanisms against AMPs are decreased permeability towards the cells, secretion of proteases, release of AMP degrading enzymes, down-regulation of host responses, active efflux, and alteration of the membrane physiology. In addition, evolutionary changes like mutations that allow innate expression of intrinsic resistance genes, also help for the bacteria to protecting from AMPs,. However there were some mechanisms that affect the AMPs, development of resistance to cationic peptides is quite rare because of the substantial theories such as "the death of the organisms occurs rapidly, hence, it does not leave much time for the bacteria to mutate and divide" and "AMPs possess multiple targets, hence, even if one fails, others remain to take the task forward" (Guilhelmelli et al. 2013; Nawrocki et al. 2014; Yeung et al. 2011).

AMPs' therapeutic potentials

Nowadays, there is a rising interest in potential therapeutic uses of AMPs. Especially linear and circular AMPs are preferable classes, because of the simple molecular structure and easy synthesis, and inherent stability to degrading by proteases, respectively. However the AMPs have desirable facilities for their therapeutic uses, there is some limitations especially lability depends on the environment including the presence of protease, pH change, and etc. for drug development. There are many ongoing studies to overcome the disadvantages that limits the potential clinical applications of AMPs. They generally focused on using the unusual amino acids such as D-forms, acetylation or amidation of the terminal regions, increasing the peptide stability against proteases, immobilization of AMPs on solid materials, using the modern drug delivery systems, such as liposome encapsulation or nanoparticles (Seo et al. 2012; Wang et al. 2015; Mendez-Samperio 2013). Some AMPs in clinical trials for therapeutic usage, are summarized in Table 1.

In conclusion, AMPs have attracted attention as alternative antibiotics due to their desirable properties such as potency, rapid and multiple mechanisms of action, broad spectrum activities, and low potential to induce novel resistance mechanisms, make them excellent prospects. There are a lot of clinical study which about local or systemic infections, showed that the AMPs can contribute to the rapid clearance of microorganisms through direct killing, inhibition of pro-inflammatory mediators such as lipopolysaccharide, and by modulating the inflammatory response to infection. However, in addition to ongoing phase II and phase III clinical trials, major concerns surrounding cationic peptides such as stability, toxicity, immunogenity, pharmacokinetic and pharmacodynamic parameters and antimicrobial activity will prove resolvability and they have to be formulated properly to develop into a drug form. According to those clinical trials, AMPs seems to be the new and active group of antimicrobial agent, not only as a single agent but also an adjuvant for the treatment of serious infections without a significant resistance problem in the near future (Hancock and Sahl 2006; Yeung et al. 2011; Findlay et al. 2016; Felicio et al. 2017; Griffith et al. 2017).

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Sibel Dosler

Department of Pharmaceutical Microbiology, Faculty of Pharmacy, Istanbul University, 34116, Istanbul-Turkey

Address for Correspondence: Sibel Dosler, e-mail: sibel.dosler@Istanbul.edu.tr

Received: 07.08.2017

Accepted: 22.08.2017

Cite this article as: Dosler S (2017). Antimicrobial peptides: Coming to the end of antibiotic era, the most promising agents. Istanbul J Pharm 47 (2): 72-76.

DOI: 10.5152/IstanbulJPharm.2017.0012
Table 1. AMPs in commercial development

Peptide                    Medical usage          Clinical trials

Pexiganan (MSI-78)      Diabetic foot ulcers         Phase III
Iseganan (IB-367)    Ulcerative oral mucositis       Phase III
Omiganan (MBI-226)     Bloodstream infections        Phase III
RDP58               Inflammatory bowel diseases    Post phase II
MBI-594 AN                Acne infections            Phase II
P113                      Oral candidiasis           Phase II
P113D                     Lung infections            Phase II
D2A21               Skin infections, burn wounds     Phase II
Neuprex (rBPI21)          Meningococcemia            Phase II
XMP 629                Topical dermal therapy        Phase II
hLF-1-11             Transplantation infections      Phase II
CZEN-002             Vulvo-vaginal candidiasis       Phase II
MX-226               Dermatological infections       Phase II
Glutoxim                    Tuberculosis             Phase II
IMX-942               Immunomodulation, fevers       Phase IA
                          in chemotherapy
HB-1345                         Acne                Pre Phase I
Plectasin           Systemic anti Gram positive     Preclinical
HB-107                     Wound healing            Preclinical
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Author:Dosler, Sibel
Publication:Journal of the Faculty of Pharmacy of Istanbul University
Date:Aug 1, 2017
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