Peptides refer to biologically active molecules that contain at least two amino acids, interlinked by a peptide bond. Unlike large proteins, they are small in size and a typical chain will usually not go past 100 amino acids.
Since peptides are highly selective and also known to have relatively safe characteristics, their pharmacological profiles have always appealed to the research world. They are readily available in the human body where they play diverse biological roles.
For most applications, they mainly act as regulatory and signaling molecules in various physiological processes. Back in the day, the instability of peptides limited their use in the design and development of human drugs, but due to technological breakthroughs, the instability challenges have all been overcome, greatly increasing the broad application of peptides.
Current technologies have made it possible for peptides to be modified, new ones to be created, and the current ones to be improved in terms of stability, to overcome some of the inherent pharmacological weaknesses they are known to have.
Additionally, advancements in peptide manipulation have greatly increased the bioavailability of peptides, which has also led to an increase in their efficacy. In this text, we will explore the current developments, as well as the use of peptides as therapeutics.
Therapeutic peptides – mode of action
Anti-viral peptides can potentially be used as a treatment of prophylaxis for a variety of viral complications. These peptides work mainly by interacting with the virus particles or by targeting viral replication steps in the life cycles of the viruses.
Research scientists have already explored a variety of approaches to try and use peptides to inhibit the onset of viral diseases, such as dengue fever. Some of these approaches include targeting the host cell attachment factors or the cell receptors, the structural components of the virus, and the non-structural proteins.
The drugs that were originally designed for these three main targets use a variety of mechanisms to stop the actions of the virus. When the host attachment factors or cellular receptors are targeted, it will not be possible for the viral proteins to attach or bind themselves to the host cells.
As a result, this will stop their subsequent entry into the host. Drug candidates designed to target the viral structural proteins, may, on the other hand, interfere with the binding process of the viruses to the host cells, subsequently leading to the inhibition of the virus, hence, its entry into the host cells.
Also, it should be noted that non-structural proteins are vital components of the replication process and any attempts to come up with drugs that will work against non-structural proteins will without a doubt interfere with the viral replication cycle, which will effectively lead to the eradication of the virus from the host.
Entry inhibitors targeting the host cells
One of the best ways for inhibiting infections by viruses, is to simply block cellular receptors, their attachment factors, or just copy their cellular receptors. When this is done, the viruses will be stopped from getting attached to or entering the host cells. This is usually what is used as the very first barrier towards blocking viral infection.
Several studies agree that this is a viable approach to stopping viral infections. For example, in a study conducted by Pugach et al. in 2008, it was observed that small molecules were successfully used to inhibit HIV-1 when it was bound to CCR5 co-receptor of the host cell.
In yet another study, it was observed that lipomyristolated peptide containing a total of 47 homologous amino acid residues of the virus responsible for causing hepatitis B, could be bound to the NTCP of host cells.
The present DENV attachment factors or receptors present on the mammalian cells were critically observed during a study by Cruz-Olivier et al. in 2015. According to this study, it was revealed that some of the most important receptors or attachment factors are the cell-specific adhesion molecules known as 3-grabbing nonintegrin – DC-SIGN.
Presently, there are a plethora of small molecules that have been discovered as receptor antagonists, or simply mimics of DENV. However, in all the studies that have been conducted in this field, peptide inhibitors with the ability to block DENV infections by attaching to the receptors, are yet to be found. This has left a huge research gap, and one which researchers need to look into keenly for more specific answers.
Glycosaminoglycans (GAG) is another vital and well known DENV receptor. Within the GAG family, we find the heparin sulfate – HS, which is known as the most ubiquitous member of the family. Studies suggest that HS was used as the very first interactive attachment factor that made it possible for DENV to bind to the second receptor.
The study went to great lengths to demonstrate that DENV-HS can interact through positively charged E residues with the most common types, being Lys295 and Lys291 receptors that were able to bind to the negatively charged HS. The study also identified many heparin mimetics that could block DENV infection. One of these heparin mimetics was heparin sulfate mimetic, which greatly increased the survival rate of DENV-infected rats by up to 45%.
In yet another study, fucoidan – a sulfated polysaccharide, was obtained from Cladosiphon – marine algae showed the ability to inhibit DENV-2 infection by simply binding around the virus’ envelope. These and other similar studies have come to the general conclusion that inhibitors designed to target host cellular receptors can be less prone to develop resistance when they are compared to those that are specifically designed to target the viruses.
Entry inhibitors targeting Envelop proteins
The onset of the viral infection cycle is always characterized by the interactions of the viral structural proteins, usually conducted by the E protein of the attachment factors or the host cell receptors, which are usually responsible for facilitating the virus’ entry. The DENV E protein has a size of 53DA and it features three different domains, including domain I E, immunoglobulin-like domain - EII, and dimerization domain, EIII. These domains are then covered by a membrane proximal stem, as well as a transmembrane anchor.
Though the functions of EI are yet to be fully understood, it has been demonstrated that it can be involved in processes that deal with structural rearrangements of E protein during the process of internalization. The EII has a special segment known as fusion peptide. This is the peptide believed to be responsible for the activities of viral fusion.
The EII is also believed to have serotype-conserved epitopes that are heavily involved with the E protein dimerization. Studies conducted in the past have suggested that EIII made a great contribution to receptor recognition. This process is very important for viral attachment, which is also another important process for viral entry into the host cells.
Also, EIII is believed to contain serotype-specific neutralizing epitopes. This is because they are greatly involved with receptor recognition and attachment. Additionally, they play a vital role in the fusion of viral and cellular membranes that usually lead to the release of viral genomic RNA used in the replication of the viruses. Due to all these, the E protein is currently seen as a potent antiviral target to help stop the entry of the viruses into the host cells.
With the use of x-ray, cryo-election microscopy, crystallography, and nuclear magnetic resonance spectroscopy, researchers have been able to identify DENV E structural proteins. With the advancement of knowledge in this field, as well as technological advancements, it has become possible for researchers to determine the E structure of the DENV in high resolution, which has in turn provided them with vital information for the development and design of antiviral drugs. Several studies have come up with a variety of methodologies including the use of silico drug design for screening novel antiviral peptides against various E proteins.
Entry inhibitors targeting C proteins and prM/M
Most of the studies in this field have always had their focus on DENV E protein because of how the virus is structured, and because E proteins cover a huge surface area of the viral particle, and also because there is extensive knowledge available regarding the E protein. However, C proteins and prM proteins are also potential candidates worth looking into when researching anti-viral peptides.
Think of the prM protein as the predecessor of the M protein. Its hydrophilic N-terminal is known to be responsible for coding the pr portion of the prM protein. It is believed that this protein is responsible for protecting the E protein from any changes that it may encounter in the maturation pathways when it is in an acidic environment. During a study, it was demonstrated that viruses that contained the prM protein were more resistant in environments that had low pH. When the matured virons were released, it was observed that the pr was separated. This action, hence, left the M and the E proteins sitting on the surface of the mature DENV.
The C protein, on the other hand, contains approximately four helical regions, with the structure having a high net charge, featuring an asymmetric distribution of all the residues present on the surface of the C protein.
The opposite surface, however, has a hydrophobic region believed to be able to interact with lipids. With such a configuration, the C protein is a vital ingredient in the virus’ assembly, since it makes the encapsulation of the RNA genome possible, which then leads to the formation of the nucleocapsid. Apart from the general understanding of the role played by the C protein in viral RNA assembly, several studies have shown that the hydrophobic region of the C-terminal protein has a signal sequence that is responsible for anchoring the protein inside the endoplasmic reticulum.
Replication and translation inhibitors targeting NS proteins
Studies involving viral proteases have determined they are very good candidates for inhibitory targets. For example, in the treatment of an HIV infection, it was shown that protease inhibition was a very successful approach in dealing with such an infection.
During a study, it was observed that the HIV-1 protease cleaved the translated polypeptide chain into smaller functional protein groups. This action then made it possible for the virus particles to mature. When the protease is inhibited, it will not be possible for the immature virus particles to grow into the mature virion.
As a result, viral replication will be effectively blocked. Over the years, there are several HIV-1 protease inhibitors that have been discovered and deployed in a variety of clinical studies. Also, NS5, NS2B, and NS3 have been deployed in a variety of enzymatic activities targeted at treating DENV infection, hence, making them highly potent targets for antiviral therapies.
After applications in treading DENV infections, it is possible to translate the viral genome to give a polyprotein with seven non-structural and three structural proteins. With the polyprotein, it is possible to cleave it into various proteins during the process of maturing the virus. This is usually done on the luminal side of the endoplasmic reticulum.
Possible limitations of antiviral peptides
There are various roadblocks that make it a challenge to successfully use peptides as therapeutic drugs. The major ones, however, are associated with their poor stability, as well as the bioavailability of the peptides. When peptides are not modified, it has been observed that they degrade very fast when placed in human serum, and this leads to low in vivo activities.
But that notwithstanding, there are various types of chemical manipulation that can be used to alter the physicochemical properties of the peptides to increase their bioavailability and stability so that they don’t degrade fast when placed in human serum. Such chemical manipulations have been reviewed by a variety of experts, including Gentilucci et al.
In some of the studies, it was observed that conjugation to polymers can increase the molecular weight of the peptides, hence, improving their peripheral stability. Through such manipulations, it is believed that most of the current shortcomings of using peptides as therapeutic drugs will be a thing of the past, giving way for peptides to be used in the treatment of a variety of conditions that have not been possible in the past.
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