Natural antimicrobial peptides and substances, and their mechanisms of operations in host defenses, are things that the research world has been exploring since the late nineteenth century. In the early 1960s, the discovery of in vitro antibacterial actions of leukocyte extracts came into light, and it was majorly associated with the actions of basic proteins. Since then, there has been a lot of discovery and identification of a variety of cationic peptides that possess antimicrobial properties in tissues and other host cells present in nearly all the living species.
Also known as “Nature antibiotics,” the properties of these cationic peptides and their modes of operations, as well as their multiple functions in the host defenses of cellular organisms has led the research world to believe the rationale of the possibility of developing a whole novel peptide-based therapeutic with the ability to exploit the effector mechanisms of the innate immunity.
Antimicrobial peptide is a phrase used to refer to a variety of natural macromolecules that are synthesized ribosomally, and with the ability to innately modify immunity peptides or their synthetic analogs. The main features of innate immunity peptides come from their primary activity that is mainly directed at a universal non-protein target.
In most cases, the target is always a bacterial membrane that has been adequately strengthened by a polypharmacology, which may feature both anti-inflammatory and immunomodulatory properties. Other unique and appealing properties that have already been identified during a variety of clinical trials features the peptide’s low susceptibility to the classical model of operations, upon which drug resistance develops, and this is mainly linked to the peptide’s low propensity when it comes to the selection of their resistant mutants, their interaction with dividing and non-dividing cells and their effects in reducing both planktonic and biofilm bacterial counts.
Most of these properties normally come into light when the peptides target a conserved structure that is free from the proliferative nature of the cells. It may be true that some of the mechanisms of bacterial resistance to antimicrobial peptides have been identified and mapped. It has also been noted that their instances usually occur at very low frequencies when compared to those of traditional antibiotics. Besides, it has been shown that host defense peptides have the ability to act synergistically with these types of antibiotic agents.
Innate immunity peptides are consequently being viewed as very strong potential candidates for the development and formulation of antibiotics, given the nature of their extended clinical lifetimes. Not a lot of them have been approved for clinical trials, but a lot of progress has been made towards their commercial development with the help of recent technological advancements. Currently, it is estimated that approximately 1500 antimicrobial peptide sequences have been either identified or predicted.
Also, about 20 various types of peptide-based therapeutic agents have been allowed for clinical trials for anti-inflammatory or anti-infective indications, which are mostly limited to topical applications. It should be noted that most of the challenges that are normally associated with the traditional clinical development of host defense peptide candidates that may be used for a variety of systematic therapies, demand ideal solutions that will address the issue of possible toxicity. Due to their rapid metabolic degradation, there is always the possibility that the peptides may be applied in high doses so that they maintain their therapeutic integrity in vivo. This, may, however, come with undesired margins for safety, even though they have very good selectivity when it comes to choosing the bacteria over the mammalian cells.
Additionally, potential immunogenicity is always a factor that ought to be considered under such circumstances. The other notable concerns that should never be brushed aside include the very high cost of production, their poor oral bioavailability, and their complex pharmacology, which is likely to lead to unnecessary and uncontrolled off-target toxicity.
Presently, the global pharmaceutical pipeline seems to be dominated by various peptide therapeutics. Due to the advancement in peptide modification, improved bioavailability, better formulation, and advanced delivery technology, the research world is optimistic that some of the inherent challenges that have been encountered in their development, may be helpful in the creation of more innate immunity peptides.
It is also worth pointing out that most innate immunity peptides can be modified through the optimization of the length, and their content sequences, which in turn help in increasing their selective antibacterial activity or by converting them into peptidomimetics that further help to improve their pharmacokinetic properties. In the case of increasing their selective antimicrobial activity, this can always be achieved by reducing the length of the sequence and by systematically substituting each residue with a uniquely coded amino acid.
These alterations usually lead to a peptide candidate, characterized by improved antimicrobial activities, and with enhanced activity differentials between both eukaryotic and prokaryotic cells. In the past, this modification has always been performed alongside the structure-activity relationship studies targeting the rational design of the therapeutic candidates. The main focus was to investigate the direct antibacterial activities of some of the candidate peptides, though certain immunomodulatory peptides lack in vitro antimicrobial activities that have also been modified using these methods.
Secondly, peptidomimetics, the structure that tends to move away from the traditional peptide structures but still has the ability to produce the biological activities associated with the parent sequence, have also been generated for consideration as candidates for proteolysis. Some of these peptides feature sequences obtained from non-natural amino acids - proteinogenic amino acid analogs, which have increased hydrophobicity, peptoids, non-peptide mimetics, and N- or C- terminally modified amino acids with lipophilic chains or groups.
Other methods that can be used to enhance the pharmacokinetic properties of the peptides, lower their immunogenicity, and reduce their serum protein activation, such as the use of methods including pegylation technology, have also been thoroughly explored and implemented in some cases.
The challenges of unknown toxicology for innate immunity peptides for systematic administration, have also been explored using a variety of approaches. Some modified peptidomimetics or peptides, are believed to possess the properties to retain not just the activity, but also the toxicity levels of the parent peptide. In order to efficiently and reliably control these levels of toxicity, present in the therapeutic candidate. There may be a need for the use of a selective delivery technology as an alternative method, or it could be used in conjunction with any or a combination of the approaches that have already been explored.
Such approaches may be used to restrict the activity of the innate immunity peptides, so that they are only active on the sites of the infection. Such peptides may feature peptide sequences that have been specially conjugated to target moieties that have been modified to act as inactive precursors. With such precursors, it may be possible to selectively activate them at a target body site, or load them in drug delivery mechanisms that can then be used in targeting certain desired sites of action. When they are used as part of the drug delivery mechanism, the targeting moiety can be an antibody that has been modified to work against the pathogen-specific antigen.
In this case, a host defense peptide sequence is conjugated to the sequence of an antibody fragment. This is a process that normally happens through the production of a fusion protein that has a cleavable linker between the antimicrobial and the targeting domains. With such an approach, the ensuing immunoconjugate may possess specific resistance to a fungus present in transgenic plants, or it may discriminate a certain periodontal pathogen obtained from other bacteria. It is also possible for the targeting moiety to be in the form of another peptide sequence that may have the ability to selectively bind to certain specific cell surface receptors of a bacterial pathogen like pheromone receptors.
Through such, it is possible to generate a targeted antimicrobial candidate through fusion or synthetic peptide that displays both the pheromone, and the antimicrobial sequence. Such peptides are usually referred to as chimeric peptides, and studies have shown that they can differentiate between MRSA and MSSA strains. Studies also suggested that they can be protective in mice models infected with MRSA, as well as selectively eliminating the cariogenic bacterium in multispecies biofilms, without having any effects on the surrounding non-cariogenic oral streptococci.
A lot of investigations have also been done to the generation of targeted antimicrobial peptides, with the use of conjugation as a classical antibiotic to a host defense peptide sequence. This was to increase its activity and selectivity against the bacteria that express themselves at the targets of conventional antibiotics.
The vancomycin-peptide conjugates of magainin 2, makes for a relevant example. These peptides were constructed using copper-catalyzed azide-alkalyne cyclo-addition. This approach led to a hybrid antibiotic that had two different pharmacophores that could use the dual activity of the classical agent against resistant bacteria. Another example where hybrid antibiotics had been used in such a manner, is in the generation of antimicrobial peptide prodrug candidate. With these candidates, the classical antibiotic was targeted to act as a promoiety, instead of being an active agent.
A good example of this, is the conjugation of cephalosporin to a host defense peptides sequence. When this was done, cephalosporin reversibly modulated one of the activity determinants of the parent peptide. The net charge could be selectively released from the conjugate through the hydrolysis of cephalosporin’s lactam ring. Such a reaction displays the main process of the operations of the antibiotic resistance, usually witnessed in Gram-negative pathogens.
There has been a proposal for a pro-drug modification with a promising strategy for potentiating the systematic applications of host defense peptides, as one of the most effective ways of overcoming low molecular weight drug candidates, as well as that of the lipopeptide polymyxin. It is also worth pointing out that discoveries have been made about certain natural approaches for regulating and controlling the activity of certain innate immunity peptides. It is believed that the actions of these natural mechanisms may be artificially copied to generate peptide sequences that possess three domains – the parent sequence of a host defense peptide, a linker cleavable by a target disease-associated protease, and an oligo-glutamyl fragment. It may also be possible to obtain both the activity and toxicity differentials between the parent peptide, and a neutrophil elastase-dependent propeptide, however, enzymes with a bacterial origin that portray narrow substrate specificities with the absence of mammalian homologs, may be targeted in order to activate a propeptide for systematic administration.
It is also possible to target antimicrobial peptides using their loading in nanoparticulate systems that have the selective delivery capacity. These may include liposomes, such as stealth liposomes, nanospheres, polymers, carbon nanotubes, and nanocapsules, among others. With these, the nanoscale sizes are what is used to determine their drug loading capacities, as well as for prolonging their circulation times.
With their structure, their cargo will be adequately protected from metabolic degradation, and also reduce their toxicity levels, since they will not be interacting with the plasma proteins, as well as the surfaces of the host cells. Also, with this approach, it is possible to environmentally control the release of the cargo, as well as modifying the surfaces of the nano-carriers with the targeting moieties to make it possible for selective delivery to very specific tissues and cells – even through the blood-brain barrier.
In conclusion, regarding the high costs of production of the candidates, which have now been moderated through the use of advanced technologies, as well as advanced production methods, including recombinant expression in heterologous microbial systems. These may also be used in the production of peptides obtained from natural amino acids, as well as the application of solid-phase peptide synthesis, which has greatly lowered the cost of peptide production, allowing for the production of multi-tone peptides annually.
The therapeutic potentials of host defense peptides, is a subject that should be explored beyond the anti-inflammatory and anti-infective applications of the peptides in cancer therapies. In reality, innate immunity peptides can be used as active prokaryotic cells, due to their high ionic lipid content, which works so well against mitochondrial membranes, and on the various structures of the malignant cells.
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