The presence of various natural antimicrobial substances that contribute to a horde of body defense mechanisms was discovered during the late 19th century, and the research world has never rested since then. It was in the year 1963 when in vitro antimicrobial activities of leukocyte extracts was discovered to be an activity of basic proteins. Since then, there have been discoveries of various cationic peptides with antimicrobial properties. Most of these peptides were discovered in host cells and tissues and virtually in every other living species.
The respective properties of these substances that have also been termed as Nature’s Antibiotics, as well as their numerous functions in host defenses of multicellular organisms have been of immense help in supporting a variety of peptide-based therapeutics that primarily uses the effector mechanisms of the innate immunity. Antimicrobial peptides refer to a wide range of natural macromolecules as well as their synthetic analogs. In this piece, the focus is going to be on targeted antimicrobial peptides, and there will be no differentiating between their antimicrobial and immunomodulatory activities.
The main component of innate immunity peptides comes from their ability to interact with non-protein targets and the bacterial membranes that are supported by a polypharmacology that features various anti-inflammatory and immunomodulatory activities. The other unique and exceedingly attractive property of these peptides to the research world in a clinical context is their low susceptibility to classical mechanisms of drug resistance.
This is usually a factor of low propensity when it comes to the selection of resistant mutants as well as their ability to reduce both planktonic and biofilm bacterial counts as well as their ability to interact with both non-dividing and dividing cells by interacting with a conserved structure that is usually free from the cells’ proliferative status. Indeed, some of the mechanisms that antimicrobial peptides used for bacterial resistance have been discovered and thoroughly studied, it is worth noting that their emergence happens at very low frequency compared to most of the traditional antibiotics. Also, the host defense peptides have the properties to act synergistically with the classical antibiotic agents.
The peptides being considered for innate immunity are, therefore, being seen as a potential source of antibiotic candidates, with the added benefit of having extended clinical lifetimes. Most of them may have not been approved for clinical use to date, however, good progress has been made towards their commercial development with the help of recent advancements in technology. It is estimated that there are nearly 1700 antimicrobial peptide sequences that have already been identified or that have already been predicted.
Currently, it is estimated that there are approximately fifteen peptide-based therapeutic agents in clinical trials for use as anti-inflammatory and anti-infective indications, with a bulk of them being limited to topical applications. The challenges that have been perennially associated with most of the clinical development of host defense peptides need solutions that can address toxicity which is always a major concern and one of the top derailments to the process. Due to their fast metabolic degradation or secretion, it is common for these peptides to be used in high doses to preserve their therapeutic levels in vivo. This usually comes with a major concern when it comes to the acceptable margins of safety, even though they have very good selectivity for bacteria the cells of mammals. Also, there are always concerns about potential immunogenicity which must also be considered. Some of the other concerns that are also likely to be raised may be inherent to all peptide-based drug candidates like bioavailability when taken orally, and high costs of production of the peptide-based drugs.
It is worth noting that peptide therapeutics are currently occupying a larger focus on the global pharmaceutical pie. There have been incredible advances in peptide modification, formulation, and peptide delivery technologies that have been used to overcome or go around some of the concerns that impeded the development of peptide therapeutics in the past. Some of these concerns include but are not limited to peptide pharmacokinetics, toxicity, and bioavailability among others. The same approaches have also been applied to the development of innate immunity peptides.
Innate immunity peptides, for example, have been modified by optimizing the content as well the length of their sequences as means of increasing their selective antimicrobial activities. These peptides have also been modified by converting them into peptidomimetics as a means of improving their pharmacokinetic properties. When it comes to optimizing the content and the length of their sequences, this usually involves the systematic substitution of each residue using other coded amino acids to give a peptide candidate that has enhanced antimicrobial activities or increased activity differentials that exists between eukaryotic and prokaryotic cells. For the second case, the focus is always on the peptidomimetics. Peptidomimetics refers to compounds with structures that are slightly different from the traditional peptide stereochemistry but still exhibit biological activities of the parent sequence.
These have always been generated as candidates for dealing with proteolysis resistance. They include sequences that have been generated from non-natural amino acids C- or N- terminally modified lipophilic groups or chains and also from peptoid as well as non-peptide mimetics such as amphiphilic polymers and aminosteroids. The other methods that can be used to improve the pharmacokinetic properties of these compounds that prevent the immunogenicity as well as the serum protein inactivation of biopharmaceuticals such as the use of pegylation technology have also been implemented at different stages of the trials. The same approaches have also been used in addressing the challenges of unknown toxicology for systematically administered innate immunity peptides.
Conversely, studies show that there is a good number of peptidomimetics and modified peptides that have the ability to retain both the activity and the toxicity of the parent peptide. In order to reliably and effectively control the toxicity associated with a therapeutic candidate, the use of selective drug delivery technology may be applied as an alternative approach or alongside other well-established approaches. Some of these methods are designed to restrict the activity of the innate immunity peptides to the respective sites where the infection occurred to generate targeted microbial peptides. They may feature peptide sequences that have been conjugated to target moieties that have been modified as inactive precursors.
These inactive precursors can be activated selectively at respective body sites or they can be loaded in drug delivery systems which can then be used to target their desired site of actions. With the first approach, the targeting moiety can be as simple as an antibody that has been directed against a specific antigen pathogen. In such a case, a host defense peptide sequence can be conjugated to the sequence of the antibody fragment by producing fusion protein with or without a cleavable linker connecting the antimicrobial domains and the targeting domains.
When this is done successfully, the final immunoconjugate may display specific resistance to a fungus obtained from a given plant species or discriminate against a specific periodontal pathogen from other bacteria originating from commensal flora. It is also possible for the targeting moiety to be another peptide sequence that has been selectively bound to a given cell surface receptor of a specific bacterial pathogen. It will then be possible to generate a targeted antimicrobial candidate as a synthetic peptide or a fusion that features pheromone and antimicrobial sequences. These peptides, can, for instance, tell the difference between MSSA and MRSA strains.
The generation of targeted antimicrobial peptides has also been extensively studied with the application of conjugation of classical antibiotics in host defense peptide sequence. This is done in a bid to increase the activity as well as the selectivity against certain bacteria that express the target of the traditional antibiotics. A good example of this conjugation of a classical antibiotic is magainin 2. Magainin 2 is a vancomycin-peptide conjugate whose construction featured copper (1) catalyzed azide-alkyne cyclo-addition.
Such a method has resulted in hybrid antibiotics that have two different pharmacophores that can use their dual activity to enhance their efficacy while at the same time delaying resistance development. Additionally, antibiotics developed from such a method also has the capability of restoring the activities the traditional agents against resistant bacteria.
Another typical application of these hybrid antibiotics is in the formation and production of antimicrobial peptide prodrug candidates. With this, the classical antibiotics are used as a promoiety instead of being used as an active agent. For instance, when cephalosporin is conjugated to a host defense peptide sequence, it exhibits the properties to reversibly modulate an activity of the determinant – the net charge of the primary peptide.
The primary peptide can then be selectively released from the conjugate with the help of lactamase-mediated hydrolysis of the classical bacteria’s lactam rings. Such a reaction normally features the main mechanism displayed in antibiotic resistance in Gram-positive pathogens. There has been a proposal for prodrug modifications as a way of potentiating the systematic application of the host defense peptides. The suggested modifications have also been used as a means of overcoming the toxicity usually associated with the low molecular weight drug candidates.
Nature also has its way of selecting a prodrug approach for regulating and controlling the activity of some of the innate immunity peptides. The natural processes for doing this can be easily mimicked synthetically to produce peptide sequences that have three domains which include the parent sequence of a host defense peptide, a linker connected to a target disease-associated protease, and an oligo-glutamyl profragment. The differences between toxicity and activity can be attained between a parent peptide and neutrophil elastase-dependent propeptide, however, enzymes that have bacterial origins but lacks mammalian homologs may have to be targeted differently for the activation of a propeptide systematic administration.
It is also possible for a peptide prodrug containing a promoiety to give a pharmacologically active entity when it is activated. In other words, the core drugs may target an antimicrobial peptide on a site of bacterial infection while at the same time allowing for co-delivery of another agent designed to complement activity.
Antimicrobial peptides can also be targeted with the help of their own loading in a nanoparticulate system featuring a selective delivery capacity. Such antimicrobial peptides include liposomes, nanocapsules, nanospheres, dendritic polymers, hydrogels, and carbon nanotubes among others. The drug loading capacities of these peptides are mainly based on their nanoscale sizes, which also helps in increasing their circulation times. Due to their structure, their cargo is normally protected against metabolic degradation. Their size also helps in reducing their toxicity by simply preventing their interaction with host cell surfaces and plasma proteins.
Additionally, they make it possible for the cargo to be released in a controlled environment and it is also possible to modify the surfaces of the nano-carriers with the help of targeting moieties which then allows deliveries to cells or tissues, including deliveries through the blood-brain barrier. Already, there is a lot of work that has been done in investigating these drug delivery systems with the host defense peptides as the cargoes. Some of the delivery systems that have already been investigated include polymer-carriers, proteins, and liposomes among others.
On concerns relating to the high cost of production of these peptide drug candidates, a lot of reprieves have come from advancement in technology that has led to more efficient production methods such as the use of recombinant expressions using heterologous microbial systems. The production of peptides obtained natural amino acids has been complemented by the use of solid-phase synthesis. Solid-phase synthesis has proven to be an efficient and cost-effective method for producing tons of peptides ever year. It has also proven to be deadly effective in modifying peptides as well as peptidomimetics that conforms to the requirements of the regulating agencies.
The potential of peptide therapeutics can be extended past the anti-inflammatory and anti-infective applications, especially regarding cancer therapies. Innate immunity peptides can also be used as active neoplastic eukaryotic cells because of their high anionic lipid content of the malignant cells and the mitochondrial membranes. Some of the methods developed for anti-cancer and anti-infective areas to deal with the shortcomings of antimicrobial peptides can ultimately be used for achieving the full potential of therapeutic and anti-microbial peptides.
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