M.  Alyami   1  

School of Life and Health sciences

(Peptide structure) hhttp://www.rosaceasupportgroup.com

DR. Zheng By Mohammad Alyami 109068062

Therapeutic applications of peptides

 

Drug  Discovery  Module  PHM  008    

M.  Alyami   2  

Background Peptides have increasingly become important compounds that are used for treating many

diseases. Peptides consist of amino acids linked by peptide bonds. They can be found naturally or synthesised for example thousands of peptides have been isolated from natural sources such as animals, plants microorganisms and they also can be synthesised in laboratories. They are divided into linear and cyclic peptides according to their chemical structure. In nature the majority of isolated peptides are cyclic peptides that are characterised by stable configurations. As a result of their stability they are considerably more potent biological activities than linear peptides. Peptides can be found in different number of amino acids and different sequences ranging from 2 to 50 amino acids. Dipeptide is the shortest peptides chain that is only formed of two amino acids followed by tripeptides (3 amino acids) and tetrapeptides (4 amino acids). Peptides can be distinguished from proteins on the basis of size in which proteins are more complex containing more than fifty monomers units. Peptides are made in every living organism possessing variety of biochemical functions. For instance, they act like enzymes, hormones, antibiotics, receptors, etc. They also regulate most of the physiological function by playing endocrine or paracrine roles at some sites and neurotransmitters or growth factors at others. Consequently, peptides have been isolated, made and used as drugs in treatment and diagnosis variety of disease. Examples of area that already have used peptides as therapy are haematology, neurology and endocrinology (Edwards et al, 1999).

A number of peptide has been used therapeutically for the treatment of many medical conditions. For instance, oxytocin was the first used synthetic peptides, introduced in 1953, that plays vital roles in sexual reproduction. Moreover, insulin was given approval to be manufactured by recombinant fermentation method in 1974. Recently, peptides have been gaining the passion of scientists and the drug companies as a result of their low toxicity and high selectivity. Numbers of peptides that are approved clinically for using as therapies in various diseases are growing providing great contributions to healthcare. Peptides drugs estimated on the top of 100 best selling approved medications (Vlieghe et al, 2010).

Although the importance of peptides in medical filed has been emerged, parenteral administrations are still the common delivery method for peptides, as they cannot be given orally. The reason refers to the break down process of peptides in the gastrointestinal tract after orally administration. As a result, focus on alternative route of administration increased complies with the increased peptides usage as therapies including buccal, inhaled, transdermal and intranasal routes as well as protective liposomal system.

Peptides manufacture

Therapeutic peptides are driven from three sources: (i) natural from plants, animals or humans; (ii) isolated from genetic or recombinant library (iii) made chemically. However, the increase of using unnatural amino acids and pseudo peptides bonds has helped chemical synthesis to enlarge with an expanded potential for intellectual property. Furthermore, large-scale production of small and medium size peptides has been available by chemical synthesis, which is considered to be better than biotechnological methods of recombinant DNA and biocactaysis. Manufacture process of therapeutic peptides has also become easier and quicker by using Merrifield solid-phase peptide synthesis (SPPS) method (Vlieghe et al, 2010).

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Solid phase synthesis Solid-phase peptide synthesis (SPPS) technique is the most commonly used technique for

peptides synthesis. In SPPS a small solid bead is treated with a linker. A sequence of reactions is performed to which the desired peptide is made. After building needed peptide, a special reagent is used such as trifluroacetic acid or anhydrous hydrogen fluoride to cleave formed peptide from bead (Castanho and Santos 2011).

Figure 1: Solid phase technique; this figure shows the repetitive stepwise process: activation,

coupling, deprotection, washout and cleavage of chemical peptide synthesis. (Adapted from http://www.sigmaaldrich.com.)

Gene cloning

Since natural sources are limited and chemical synthesis method is only suitable for short peptides, gene-cloning technique is used to manufacture long peptides and proteins as well. Four essential steps are used in this technique; obtaining DNA encoding the therapeutic intended peptides; inserting the DNA into a vector; introducing vector into a host cell and grow the cells in a specific culture and in the last step peptides are isolated and purified (Castanho and Santos 2011).

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Table 1: Some important peptides hormone that are released by endocrine glands are:

Hormones Amino acids Numbers

Function

Insulin

51 (21+30 AA)

Lowers blood glucose level, promotes glucose storage as glycogen and fat. Fasting decreases insulin production.

Adrenocorticotropic hormone

39

Stimulates production of adrenal cortex steroids (cortisol and costicosterone)

Vasopressin 9

Increases the reabsorption rate of water in kidney tubule cells (antidiuretic hormone)

Oxytocin 9

Causes contraction of mammary gland cells to produce milk and stimulation of uterine muscles during childbirth.

Angiotensin II 8

Regulates blood pressure through vasoconstriction

Therapeutic applications of peptides

Therapeutic uses of peptides have been significantly emerged since 1970s, when Novartis lypressin peptide agent. Since then, peptides have been increasingly considered as attractive therapeutic agents. The specificity and biological activity of peptides can help to decrease drug-drug interaction and drug accumulation inside tissues. Peptides have been evolved for treatment variety of different diseases such as cardiovascular diseases, diabetes and cancer as well as their applications is growing rapidly for treating other therapeutic areas notably.

Anticancer peptides

Over the years, peptides have been promising therapeutic agents in cancer treatment. Currently, more than 60 approved peptide drugs (leuprolide, goserelin, and octreotide) are available gaining over than $16 million of annual sales. Drugs of peptides are available in the market acting by different strategies. For instance, they can act directly as drugs (e.g., as angiogenesis inhibitors)

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or tumor targeting agents that carry cytotoxic drugs and radionuclides, hormones, and vaccines (Aina et al 2002).

Direct therapeutic agents of anticancer peptides are becoming more popular and preferable. Since they can be functioned to target many different receptors for example inhibition of enzymes, proteins, angiogenesis, protein-protein interactions, and gene expression or signal transduction pathways. For instance, cilengitide is an antiangiogenic agent derived from RGD peptide. It displays selectivity for αv integrins. It is currently under investigations in clinical trial phase II for glioblastoma and refractory brain tumors in children. Also, some peptides that are naturally made in thymus gland have been found to have antiangiogenic activity like dipeptide, L-glutamine L-tryptophan (Thundimadathil 2012. It was found that BN/GRP (bombesin/gastrin-releasing peptide) peptides have the ability to selectively bind to the G-protein-coupled receptors on the cell surface G-protein-coupled receptors on the cell surface causing growth of tumor cells in human models and murine. Therefore, it has been suggested that the secretion of BN/GRP resulted from the progression of prostate cancer to androgen independence (Cescato et al 2008).

Figure 2: different options of cancer treatments using peptides agents (adopted from

Thundimadathil 2012).

Peptides hormone: LHRH Agonists and Antagonists The use of LHRH agonist ((luteinizing hormone-releasing hormone)) is considered the best

example of the application of peptides in cancer treatment. For instance, buserelin, leuprolide, goserelin, and triptorelin are LHRH agonist agents, which are mainly used for treating patients with prostate cancer (wirth etal 1999). These peptides agents cause downregulation of LHRH receptors in the pituitary gland inhibiting the release of follicle-stimulating hormone (FSH) and LH and a decrease in testosterone production concomitantly. However, LHRH antagonists are more efficient than LHRH agonists as they can competitively block LHRH receptors producing an immediate inhibition of FSH and LH. Cetrorelix, abarelix and Degarelix are examples of approved LHRH antagonists agents (Debruyne etal 2006).

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Peptide as cytotoxic drug carriers Peptides are also used to deliver a cytotoxic drug to a cancer cell expressing the

corresponding peptide receptor. These peptides are known as cell targeting peptides as they can target cells expressing their receptors, specifically. Cytotoxic peptides conjugated with hormonal peptides like bombesin, LHRH, and somatostatin can be targeted to specific tumors and therefore are more efficient and selective therapies. For instance, AEZS-108 , links a peptide LHRH with the doxorubicin (chemotherapeutic agent) to directly target cells that express LH-RH receptors. An example of carrier peptide is AN-201 and doxorubicin (Szepeshazi et al 2001). Antidiabetic peptides.

Insulin is the first well-known peptide, which was isolated from a pig, to be used therapeutically in treatment of diabetes. Following, some development process for the natural form, lispro insulin was developed insulin in which rapid absorption and dissociation can be achieved. Also, scientific research has not been terminated where a number of studies have investigated the rule of other peptides in diabetes treatment. Amylin and pramlintide peptides were found to work as gastric emptying inhibitors (Young et al, 1996). These peptides have been discovered to be valuable for treating diabetes type 1 and 2.

Moreover, glucagon-like peptide–1 (GLP–1) which is an endogenous hormone, can suppress glucagon level and delay gastric emptying as well as stimulating insulin releases. GLP-1 has been reported to increase peripheral glucose disposal and decrease appetite (Flint et al 1996), an example of glucagon-like peptide-1 agonist is exenatide that was approved in 2005. Antiobesity peptides

In 2008, the global obesity number climbed to reach more than 1.4 billion adults (WHO 2013). Accordingly, studies have found that some peptides have antiobesity activity. For examples, in a study that obese patients were treated with exogenous leptin is an adipose tissue peptide hormone, which decreases appetite and increase metabolic rate (Montague et al, 1997) Hormonal peptides

Many hormonal peptides are used clinically for the treatment of medical conditions. For instance, octreotide is an octapeptide that inhibits growth hormone, insulin and glucagon mimicking the effect of somatostatin hormone. Octreotide is used as a therapy for variety of disorders such as acromegaly, acute pancreatitis, upper-gastrointestinal bleeding, and gastrointestinal fistulae dumbing syndrome, secretory diarrhea and Zollinger- Ellison syndrome. Lanreotide and sandostatin LAR are novel analogues of octreotide that developed to have better stability and longer acting (Vlieghe et al 2010). Moreover, Peptides have been also used to treat cancer for example Abarelix is the first luteinizing hormone releasing hormone antagonist. Degarelix is an example of hormonal peptides that is currently used to treat prostatic cancer by inhibiting testosterone production. Furthermore, A Growth hormone releasing peptides, which is called ghrelin, is used to simulate the release of growth hormone from pituitary. Premature labour peptides

Premature labour is a medical condition associated with serious complications on the neonates such as respiratory distress syndrome, jaundice, hypoglycemia, etc. Atosiban is synthetic peptides

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that act as a competitive antagonist of oxytocin. It can be uses as a therapy for treatment premature labour in which birth is delivered before 37 weeks gestation (Fullerton et al, 2011). Anti HIV peptides

Peptides have been found to have anti HIV activity. Currently, anti HIV agents that are made of peptides are available in the markets. For instance, enfuvirtide is a peptidic antireoviral agent, indicated for treatment of HIV-1 infection. It can be administered with other antireovirals inhibiting viral infusion.

Table 2: some approved peptides drugs from 2000 to 2012. Approval year Generic name Indication

2000

Atosiban

Premature labour

Bivalirudin Unstable angina

VIP Erectile dysfunctions

2001 Nesiritide Congestive heart failure

Triptorelin Hormone-responsive cancer

2002 Teriparatide Osteoporosis

2003 Abarelix Prostate cancer

Enfuvirtide HIV-1

2004 Zicontide Severe and chronic pain

2005 Exenatide Diabetes, Type 2

Pramlintide Diabetes, Type 1 and Type 2

2007 Lanreotide Agromegaly

2008 Degarelix Prostate cancer

Icatibant Hereditary angioedema

2009 Liraglutide Diabetes, Type 2

2010 Tesamorelin Lipodystrophy in HIV

Sinapultide RDS in premature infants

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2012 Pasireotide Cushing’s disease

Peginesatide Anemia in CKD with dialysis

Carfilzomib Refractory multiple myeloma

Afamelanotide Erythropoietic protoporphyria

Linaclotide Constipation in IBS

Lixisenatide and teduglutide

Diabetes, Type 2 and Adult short bowel syndrome.

Diagnostic peptides

Myocytes release B-type natriuretic peptide (BNP) as a result of acute dypnea in congestive heart failure patients, as a vasodilator hormone. Natriuretic peptide NP is apolypeptide hormone that is released in response to myocardial stretch and overload. NP includes arterial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP). The release of NP is considered as a compensatory mechanism. European Society of Cardiology has established the use of NP assay during CHF diagnosis in the first step. Accordingly, an early diagnosis for this any elevation is helpful to manage patients and reduce treatment cost in dypneic patients. Exogenous NP can be used rationally in CHF management (Ray et al 2008).

Moreover, chromogenic peptide substrate is used as a diagnostic tool to detect any fluctuating for the blood enzymes either presence or absences that play a vital role in blood clotting and blood pressure.

Table 3: some reasons that leads to elevate BNP and NT-proBNP levels Coronary artery disease Acute respiratory distress syndrome Congestive heart failure (CHF) Septic shock Right or Left ventricular dysfunction Acute pulmonary embolism Chronic obstructive pulmonary disease Subarachnoid hemorrhage Ischemic and hemorrhagic stroke Renal failure Liver cirrhosis Hyperthyroidism

Antimicrobial peptides AMBs Antimicrobial peptides are gaining much interest as their broad spectrum, rapid action, and

low susceptibility for bacterial resistance. They also can act by different mechanisms. They have broad-spectrum antimicrobial activity against variety of organisms such as gram positive, gram negative and, fungi and virus. They are produced as a result of invasion microbe in order to protect a host. Generally, antibacterial peptides classified into non-ribosomally synthesized peptides or ribosomally synthesized peptides (RAMPs). Examples of non-ribosomally synthesized antibacterial

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peptides are Gramicidin, bacitracin, polymyxin B, and vancomycin. AMBs can be also divided into four different groups based on their structures α-helical peptides, β-sheetpeptides, extended peptides, and loop peptides.

Indolicidin is an example of extended peptides composed of 13 amino acid acting by disrupting bacterial membrane. Pexiganan is used for treating mild to moderate diabetic patients foot ulcer infections, which is a novel peptide antibacterial as synthetic magainin 2. However, some limitations in developing AMBs such as high cost, the AMBs labile features as well as their oral potential toxicity. Nisin is a lantibiotic antibacterial peptide used as food preservative. Alamethicin is made of 20 amino acids. (Epand et al, 1999).

Current challenges and limitations of peptides

Therapeutic peptides have many limitations, which contribute to restrict their actions or uses. The main drawback is low oral bioavailability; parenteral administration is required. Moreover, the liver and kidney remove peptides very rapidly. Also, they have a short half-life since they undergo degradation by proteolytic enzymes of the digestive system and blood plasma. They additionally suffer from crossing physiological barriers as result of their hydrophilicity. Furthermore, high conformational flexibility of peptides could contribute to activate different targets leading to serious adverse effects. Additionally, peptides production is relatively costly (Vlieghe et al, 2010).

Prospective for the future Its expected that uses of peptides will be increased as well as their application areas will

enhance as well. Correspondingly, they will be not used as a therapy only, they will be used as carriers to transport drugs that have low bioavailability to their targets and will be used to increase biological action of some medications by some innovative techniques. Furthermore, the commercial usage of antimicrobial peptides is expected to grow as well as using peptides to treat undruggable targets. It is highly predictable that a massive increase in peptides therapeutics once a suitable platform has been developed that can deliver short acting peptides efficient. Conclusion

Peptides are largely growing and expected to have a very bright future as diagnostic and therapeutic agents. Therapeutically, they are currently available for the treatment of a variety of diseases like cancer, diabetes, anaemia and constipation etc. As a result of the tremendous advancement in peptide synthesis, it is become possible to produce huge mount of therapeutic peptides and seek for opportunities for treating other medical issues. The reason that peptides have gained popularity is referred to that they can and target specific receptors and also used to decrease adverse effects of traditional therapies by delivering them selectively.

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