Peptide applications may soon be as varied as peptides themselves. Indeed, cell-penetrating peptides (CPP) have served to deliver various molecules and particles into cells. Biomedical research is vastly improving and gaining ground due to the use of CPPs and synthetic peptides. Disease diagnostics as well as future drug components show a very bright future via delivery of these therapeutic molecules such as nucleic acids, drugs, and imaging agents, to cells and tissues. From small chemical drugs to large plasmid DNA, these peptides are relatively easy to synthesize. They are also very functional and can be readily characterized. CPPs can be manipulated in order to obtain large levels of gene expression, gene silencing, and even tumor targeting. Once they are functionalized or chemically altered, they can create effective delivery methods in order to target certain corrupt cells or tissues.
In addition to the medical and pharmaceutical arena, synthetic peptides (throughout we will interchangeably refer to synthetic peptides and cell-penetrating peptides [CPP]) have found their places in biochemistry, molecular biology, and immunology. Synthetic peptides are extremely useful in studies regarding polypeptides, as well as:
• Peptide hormones
• Hormone analogues
• Cross-reacting antibody preparation
• Design of novel enzymes
The design of synthetic peptides was begun mainly due to the availability of secondary structure prediction methods, and by the discovery of finding protein fragments that are >100 residues can assume or maintain their native structures as well as activities. Here it is important to note that, due to the biological activity of peptides, they must take on conformational aspects which mirror their conformational properties. These are all determined by:
• Amino acid sequences
• Polarity of the medium
• Ligand interactions, such as: nucleic acids, receptors, metal ions
Taking all of this into account, peptide chemists and the like have been able to perform experimental and computational techniques in order to focus on peptide conformational stability and the accompanying and resulting dynamics of such. Molecular biologists, on the other hand, perform protein engineering processes at the level of DNA, but this work was initiated by peptide chemists. Peptide chemists were also the first to prepare synthetic peptides (a/k/a peptide libraries) in vast amounts.
Model proteins and synthetic mini-proteins have been used to observe structure-activity relationships. All of this work has catapulted scientists into present-day studies of peptide applications, for which they are varied, including biotech companies who have discovered new peptides that hold valuable pharmacologic properties. The routine synthesis of large polypeptides or small proteins of 30-100 amino acids has enhanced peptide applications.
Synthetized Bioactive Peptides
Synthetic peptides and proteins also have their place in the market for which they have yielded billions of USDs. New chemical entities (NCE) have remained steady over the last decade, other than peptide and protein NCEs, which have been increasing. Synthetic peptides can positively influence functions and conditions of humans. In fact, several useful properties for human health using bioactive peptides have been realized, such as for these activities:
All peptides exist in limited quantities within natural settings or nature. Therefore, researchers have re-produced them in their labs, and these are known as synthesized bioactive peptides. As we can see, their applications and properties have been applied to various areas. Synthesized peptides are categorized by class and the mechanisms that they serve.
Classes of Biosynthetic Peptides & Peptidomimetics
Delving further into the particular classes of peptides is important. In order to better understand these classes, first we will explore peptidomimetrics, which are known as compounds that contain crucial elements (pharmacophore) which mimic a protein in 3D space, or a natural peptide. They maintain a striking ability to interact with designated biological targets. They then create the same biological effect as their natural counterparts. Peptidomimetrics serve to avoid some of the issues that are associated with natural peptides, such as their duration of activity and unavailability. In addition, properties such as potency or receptor selectivity can be improved, giving mimicking proteins the ability to aid in drug discovery. Now, for the classes of biosynthetic peptides and peptidomimetics:
Gonadorelin Super-agonists: These peptides have been used to treat endocrine cancers, most notably breast and prostate cancers. Many of the pharmaceutical products either being tested in clinical trials or currently being administered to cancer patients have not yet been replaced by gonadorelin antagonists, but this market is expected to grow steadily in the next few years.
Somatostatin Analogues: Product sales in this class are also growing in the market. There are two main products in the market that contain somatostatin agonists. They are expected to be released in generic forms fairly soon, but the formulations that are currently registered will be on the market for several years to come.
Angiotensin Converting Enzyme (ACE) Inhibitors: Many of these have not been categorized as peptides since they are primarily replacement dipeptides that are developed in mass quantities by conventional, organic synthesis. Though, if we look closely at this process, peptide synthesis is, in fact, a specialized branch of organic chemistry. Nevertheless, the two main drugs in this class are Enalapril and Lysinopril, and new non-peptide angiotensin II receptor antagonists have been introduced as well. This is a larger class than gonadorelin super-agonists, and certainly much bigger than the somatostatin analogues classification. Currently there are over 15 launched products in this class, and counting.
HIV Protease Inhibitors: This sequence was deduced via cDNA sequences, but it was the synthesis by Dan Veber and Steve Kent that permitted the precise determination of its enzymatic activity as well as the design of synthetic vasopressin analogues. The most widely used agent of this family is Desmopressin and its therapeutic usage is for enuresis.
Calcitonins: This class of agents treats osteoporosis. Calcitonins come from three main products, and they are salmon, human and eel calcitonin. Though, some research has indicated that salmon calcitonin was 10 times stronger than human calcitonin, but this turned out to be erroneous as each of these are considered equally potent. This large peptide contains 32 amino acids.
Immunostimulating Peptides: These peptides have been produced since the 1980s. One of the most known of this class was thymopentin, a sequence of one of the Immunostimulating thymic hormones, but it was taken off the market due to reimbursement policies by insurance companies. Another thymic hormone, thymosin &alpha-1 continues to be used in South America, Southern Asia, and the Middle East.
Other peptides that have been around even longer, such as the natural human angiotensins and oxytocins, as well as ACTH-(124) are still being produced and used. Let it be said once again that the market for peptides and proteins is steadily growing. The triggering of certain receptors or the modulation of enzymatic activities can be accomplished in specific and targeted ways by using these potent tools to guard against, and defeat human diseases.
Specific tumor antigens have advanced the field of medicine, specifically oncology, to target cancer cells. Cancer vaccines have been developed as a result of heightened knowledge regarding the molecular basis of antigen recognition. This has resulted in human class I or class II major histocompatibility (MHC) motifs which bind to these human classes. Here, synthetic peptides have been used for these cancer vaccines while presenting the following advantages of their uses:
• Ease of construction and production
• Chemical stability
• Lack of infectious or oncogenic potential
• Improved manipulation of immune responses via epitopes that stimulate T-cell subsets
• Improved effectiveness for generating immune responses to self-proteins
With the primary aim being that of eliciting immune responses to tumor antigens, this is an exciting area for cancer treatment where peptide immunization is playing a huge role. Evidence largely points to T-cells being tolerant to dominant epitopes of self-proteins, but they might also respond to subdominant epitopes (an epitope is a site on an antigen surface molecule where a single antibody molecule binds).
Class I peptide-based vaccines: Cancer patients can be administered (be vaccinated with) class I peptide-based vaccines. Several clinical trials have involved vaccinating patients who've been diagnosed with certain cancers. This originated from various tumor antigens which resulted in patients being immunized so as to boost their immunities to self-tumor antigens. Peptide-based vaccines have offered a thorough and superior model for assessing the ability of these immunizations by measuring immunity that is generated with assays. These assays measure class I, T-cell responses.
Class II peptide-based vaccines: Patients with cancer can also be immunized with class II peptide-based vaccines. Cancers that are treated with interferon to upregulate class I vaccines were found to not always respond to the vaccines. In these instances, GM-CSF was used as an adjuvant for immunizing patients. GM-CSF is a maturation factor for skin dendritic cells as well as Langerhans cells, and may allow for more effective peptide epitope presentation than standard adjuvants used in Class I peptide-based vaccinations. Hence, they are characterized as Class II peptide-based vaccinations.
Continued clinical trials using multiple tumor antigen specific peptides for both Class I and Class II-derived epitopes are showing much promise for the treatment of cancers. Clinical trials of single peptides have shown that cancer patients can be vaccinated against self-tumor antigens, with some studies showing positive early results. On the horizon are continuing efforts for multiple peptide vaccinations for the prevention and treatment of malignant human cancers. Other efforts include the focus on improving the immunogenicity of individual MHC-binding peptides.
Nanotechnology is known as molecular manufacturing. Nanotubes are grown in laboratories and they hold thermal and electrical properties that are used in chips, for instance. Nanotubes also have the ability to be used as semiconductors with the possibility of replacing silicon. Peptide nanotubes (PNTs) are surfacing as one of the most compelling nanostructures within the field of nanotechnology. These smart self-assemblies have various applications such as:
• Stimulus-response materials
• Nanoreactor designs
PNT synthesis and production are extremely up and coming as they are under extensive study. Novel biomedical applications also include smart nanodevices and innovative drug delivery systems. By controlling peptide-nanoparticle interactions, it will be possible to produce more sophisticated bio-functionalized materials with even more enhanced properties and nanostructures. More research, however, needs to be done in determining peptide-nanoparticle interactions, and designing more precise computer modeling to help predict and guide peptide-nanomaterial binding, among other crucial nano-processes.
Antimicrobial Peptides for Food Safety
Antimicrobial peptides have also found their place as a beneficial application in the area of biomedical devices, food processing equipment, and food preservation. In the latter, peptides can be incorporated into materials that create an antimicrobial packaging (Appendini and Hotchkiss, 2002). This type of packaging serves to sustain food safety and quality by reducing bacterial growth on product surfaces (Soares, et al., 2009). It works to inhibit, reduce, or retard the growth of unhealthy microorganisms within the food. Specific food preservation is enacted via peptide synthesis and peptide bioproperties, making food safer for public consumption.
Advantages and Disadvantages of Peptides as Drugs
For all their usefulness and varied applications, synthesized peptides and CPPs, when used as drugs, do have disadvantages. For instance, they have to be injected, or they must consist of specific formulations due to their low bioavailability. As well, synthesis is very costly, but this could change with increased production of necessary products such as protected amino acids, coupling reagents, and resins. However, their advantages far outweigh any disadvantages, which include the fact that, to be effective, only small doses of peptide are needed, and the total amount to be produced is small. They also have low systemic toxicity since they do not accumulate within body tissues, organs and blood due to short half-lives.
New non-injectable peptide (and protein) formulations are being developed such as non-degradable implants, liposomes, inhalation, oral administration and more. While dozens of companies are supporting new formulations, not all are focusing on peptides and proteins. It is considered this will change as more and more research reveals the steady benefits of synthesized peptides for human therapeutics, diagnostics, and treatments, as well as nanotechnology, food safety and food quality.
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