As their name implies, antimicrobial peptides (AMPs) are small molecules with antimicrobial activity, despite wide variations in their mass, amino acid residue composition, charge, three-dimensional structure and biological characteristics. They are now known to be a vast group of molecules widely distributed throughout nature and produced in species of the kingdoms Monera (e.g. Eubacteria), Protista (e.g. protozoans and algae), Fungi (yeasts), Plantae (plants) and Animalia (e.g. insects, fish, amphibians, reptiles, birds and mammals) [1
]. In some species these peptides serve as the primary antimicrobial defence mechanism, yet in other species they serve as an adjunct to existing innate and adaptive immune systems.
Interest in AMPs as potential antibiotic pharmaceuticals has always been high [2
]. Because of their rapid and broad-spectrum antimicrobial properties, these peptides were quickly proposed as antimicrobials to treat microbial infections, particularly those caused by antibiotic-resistant bacteria. Current research is divided into many areas. One area continues to focus on identifying the spectrum of AMPs in nature, determining their range of antimicrobial activities against bacteria, fungi and viruses, identifying their mechanisms of antimicrobial activity in model membrane systems, identifying their mechanisms of antimicrobial activity in intact microorganisms, and assessing their cytotoxicity to eukaryotic cells and erythrocytes. A second area of research focuses on the role of AMPs in innate immunity, their ability to attenuate the induction of pro-inflammatory cytokines and their role in adaptive immune mechanisms [3
]. This is particularly true for defensins, which have direct antimicrobial activity, chemoattract phagocytic and mast cells, induce inflammatory mediators, regulate the functions of phagocytes, and regulate the complement system [5
]. A third area of research, and the focus of this review, is the development of modified peptides with unique properties. This includes AMP mimetics, hybrid AMPs, AMP congeners, stabilised AMPs, AMP conjugates and immobilised AMPs.
It is generally accepted by many investigators that naturally occurring AMPs per se may not be suitable for pharmaceutical development. In fact, commercial development of these peptides for even the simplest of applications has been very limited. However, there are some AMP compounds that have reached clinical trials [7
]. The stages of development of additional trials for immunomodulatory anti-infectives with antimicrobial actions, immunomodulatory anti-infective peptides lacking antimicrobial action, immunomodulatory peptides, and anti-infective peptides with unknown immunomodulatory activity are listed in of the recent article by Steinstraesser et al. [8
]. These compounds have been examined as active treatments for a variety of medical uses, including diabetic foot ulcers, prevention of catheter-related bloodstream infections, therapy of acute acne, etc. The discovery, preclinical status, phase status and post status of many of these compounds in clinical trials can be easily searched and cross-checked (http://clinicaltrials.gov/
Diverse biological activities of hybrid peptides
Despite the clear potential of these compounds and the extensive efforts to move them into a clinical realm, the resulting success has been limited. One nearly successful peptide in clinical trials was MSI-78 (pexiganan acetate). This topically administered peptide reached phase III clinical trials and was found to be as effective as the oral antibiotic ofloxacin in the treatment of diabetic foot ulcers. Unfortunately, the new drug application was ultimately not approved by the US Food and Drug Administration (FDA) [7
]. Omiganan (MX-226 or CPI-226) represents another near success story, as this peptide missed its primary endpoint in phase 3a clinical trials for preventing catheter-associated infections. Omiganan did achieve significance in other endpoints (reducing catheter colonisation), allowing it to progress into confirmatory 3b trials.
The near success of these compounds still helps to maintain an enthusiastic attitude amongst pharmaceutical companies and pre-clinical researchers about their potential for commercial development. However, there are still a number of roadblocks that must be addressed before clinical implementation of AMPs will be attainable. Most notable are the disadvantages in the production, properties and efficacy of AMPs. Potentially the largest issue in the field is the projected high manufacturing cost of these peptides. If these agents will ever truly move forward into clinical practice, a less costly means of production must be developed. Recombinant DNA methods have been explored to generate the compounds, using production systems in a variety of organisms (bacteria, fungi, plant and animal systems have all been explored); in general, however, these methods have failed to prove commercial feasibility. The pharmacokinetic properties of the AMPs can be somewhat unfavourable, as these peptides are highly susceptible to degradation by proteases and exhibit short half-lives. Lastly, AMPs have had low bacteriological efficacy in animal models, and the loss of activity under physiological conditions is a valid concern with the general applicability of this entire class of compounds.
Regardless of the challenges noted above with developing AMPs for clinical use, the rise of resistant bacteria has prompted the continual search for new AMPs and this interest has never waned. The purpose of this review is to describe the newer functional classes of modified AMPs that have the potential to overcome these hurdles and become an important class of available antibiotics. Here we present some of the contemporary strategies proposed to design and engineer AMPs for unique and specific applications. These applications are not all designed solely for preventing or treating microbial infections in humans or animals, but have expanded applications as slow delivery systems, wound dressings, food preservation systems, and coatings for implants, catheters and toys. Even when immobilised or in complex environments, AMPs still retain their antimicrobial activity, which questions earlier work on their mechanisms of action, particularly for AMPs known to form well structured pores. The descriptions, potential and mechanisms of action of these new groups of modified AMPs have served as the topics of many excellent and recent reviews ().
Relevant and recent reviews of composition, structure and activities of modified antimicrobial peptides