STUDY ON THE GLYCOSYLATION AS A SYNTHETIC METHOD TO BOOST MEDICINAL PEPTIDE BIOAVAILABILITY
Dr. Ajay Kumar
Department of Chemistry, N. A. S College, Meerut, India
Abstract - It is believed that glycosylation of peptides is a viable technique for altering the physicochemical characteristics of peptide medicines as well as for increasing their absorption across biological membranes. It is discussed in this article how several techniques for the synthesis of glycoconjugates may be used, as well as current developments in the development of glycosylated peptide therapies. Furthermore, the effects of glycosylation on the ability of peptides to cross existing barriers that prevent oral and cerebral delivery of peptides are discussed in this paper.
Keywords: Biological, Therapies, Peptides.
1 INTRODUCTION
As a result of their high activity, target specificity, low toxicity, and low non- specific and drug interactions with other drugs, peptides have the potential to be effective therapeutic agents in the treatment of a variety of disorders. In recent years, several attempts have been undertaken to enhance the pharmacological characteristics of peptide medications as well as transport them efficiently to their target areas, notably by non-parental delivery methods. The weak physiochemical characteristics of peptides, on the other hand, prevent them from being delivered efficiently. More significantly, oral peptide distribution can be difficult due to biological obstacles such as varying pH levels throughout the gastrointestinal tract (GIT), the presence of proteases, and physical obstacles.
For instance, the phospholipid bilayer found in biological membranes prevents peptide medicines from penetrating deeply enough into intestinal cells. Moreover, insufficient absorption and quick breakdown by proteolytic enzymes are two additional barriers that contribute to the limited oral bioavailability of peptides (less than 1–2 percent). Different solutions have been investigated in order to overcome these limitations, and they may be divided into two broad groups: (1) chemical modification of peptides, and (2) formulation of peptides (including use of absorption enhancers). Peptide modifications such as glycosylation, Pegylation, lipidation, and cyclisation are examples of chemical techniques that can be used to alter the pharmacological profile of therapeutic peptides. When peptides are subjected to chemical
modifications, such as the attachment of glycosyl units, they can exhibit a variety of changes in their characteristics, including changes in their conformational structures and changes in their chemical, physical, and biochemical properties as well as their functions.
2 STRATEGY OF GLYCOSYLATION FOR PEPTIDE DELIVERY
The insertion of carbohydrate moieties into peptides alters their physiological characteristics, which can result in an increase in their bioavailability. A number of beneficial properties associated with peptide glycosylation include: (1) targeting specific organs and increasing bio- distribution in tissues, (2) enhancing penetration through biological membranes, (3) increasing metabolic stability and lowering the clearance rate, (4) receptor-binding, (5) protecting amino acid side chains from oxidation, and (6) maintaining and stabilizing the physical properties of peptides, such as precipitation, aggregation, thermal and kinetic denaturation. Because glucose transporters are found on the surface of biological membranes, sugar-peptide conjugation can also promote the active transport of modified molecules across cell membranes by targeting glucose transporters on the surface of biological membranes. The beneficial effect of glycosylation on the pharmacokinetic characteristics of native peptides results in an improvement in the oral absorption and bioavailability of the peptides in question. For example, glycosylated somatostatin exhibits high oral action and is a pioneering example. When compared to the original peptide, the oral
bioavailability of the modified peptide increased significantly, resulting in a significantly greater inhibitory impact on the release of growth hormone after oral administration of the modified peptide.
2.1 Strategies for Site-Specific Glycosylation of Peptides
2.1.1 N- and O-linked Glycosylation Natural Nand O-linked glycosylation occurs when carbohydrates are bonded to polypeptide chains. Co-translational or post-translational modifications can attach. An amide bond is formed by N- linked glycosylation of asparagine residue's amine group. The oxygen atom in Ser or Thr side chains forms an ether connection with the carbohydrate moiety in O-linked glycopeptide (Fig. 1A).
Glycopeptide and glycoprotein synthesis can be done chemically or chemo- enzymatically. To make N- or O-linked glycopeptides, two typical chemistries are direct and convergent (Fig. 1B). In the direct technique, the pre-synthesised glycosylated amino acid is sequentially linked to the elongating peptide utilising SPPS. SPPS uses two chemical methods:
Fmoc and Boc. The intense acidic state of Boc-chemistry impacts the glycosidic
bonds in common oligosaccharide.
Stepwise synthesis of lengthy peptides above 50 residues is problematic owing to incomplete couplings and epimerisation.
This causes side products and low end product yield. To solve this issue, convergent approaches such as on-resin linked glycopeptide and Lansbury aspartylation are used. The convergent technique is utilised for N-linked glycopeptide synthesis since O- glycosylation is not possible. In these convergent approaches, the glycosylamine unit is condensed to a free Asp residue on a peptide. The main drawbacks of convergent techniques are peptide racemisation and aspartimide formation.
This has led to several solutions. Chen and Tolbert disclosed an on-resin convergent synthesis using a 2- phenylisopropyl protecting group to produce glycosylation sites for linking big high mannose oligosaccharides to peptides to prevent aspartamide production. Deprotecting aspartic acid residues with allyl esters and 4-[N-[1-(4, 4-dimethyl-2, 6-dioxocyclohexylidene)-3- methylbutyl]-amino] benzyl (Dmab) is also an effective approach for increasing N- linked glycopeptide yield.
Fig. 1 (A) O-linked and N-linked glycosylated amino acids, (B) direct and convergent strategies for glycopeptides synthesis.
2.1.2 Chemical Glycosylation
Several chemical techniques for attaching carbohydrate units to distinct amino acid residues at the N-terminus of the peptide sequence have been established. The anomeric carbon of the carbohydrate was changed by ethanoic acid and linked to the N-terminus of -melanocyte-
stimulating hormone octapeptide analogue NAPamide through SPPS (Fig.
2). Succinic acid replaces the azide derivative of the sugar moiety at the anomeric carbon and is connected to the N-terminus of the peptide by a peptide bond (Fig. 3).
Fig. 2 Trans-acetylation of galactose through allyl and aldehyde intermediates.
Anomeric carbon modified by ethanoic acid conjugates galactose to a peptide.
Fig. 3 Reduction of azide followed by succinic anhydride treatment produced succinic acid derivative building block.
2.2 Physicochemical Effects of Glycosylation on Peptides
Pharmacokinetic and metabolic destiny of peptide medicines are affected by their physical characteristics. Peptide backbone conformation can be altered by glycosylation. Based on their findings, Lin et al. found that sugar modifications including mannose, galactose, and N- acetylgalactosamine (GalNAc) had distinct effects on the polypeptide chain's structural characteristics. So the prion's mannosylation has an anti-aggregation action. The location of the glycosyl unit in the peptide structure affects the
conformation of the peptide backbone and hence the biological characteristics of the modified peptides. As an example, glucocorticoid receptor-binding affinity was reduced and bioactivity was lost when GalNac was added to calcitonin peptide.
3 NEUROPEPTIDE THERAPEUTICS The blood brain barrier (BBB), enzymatic digestion, and liver clearance have all impeded successful transport of neuropeptides to the CNS for neurological diseases. Glycosylation has been found to promote therapeutic peptide brain
delivery. Enkephalins, endorphins, and dysorphins are all pharmacologically active when administered this way. The analgesic activity of glycosylated opioid peptides such as endomorphin-1 (orally) and enkephalin (intraperitoneally) has been found to be superior than intact peptides and conventional analgesics.
Enkephalin is an antinociceptive pentapeptide with a short half-life in blood and no capacity to cross the BBB.
When added to Leu-enkephalin amide, Ser(Glc) increased the peptide's permeability across the BBB in mice. This glycosylated analogue was antinociceptive like morphine.
3.1 Radiopharmaceuticals
Glycosylation may help improve the biodistribution and pharmacokinetics of radiolabeled peptides used in diagnostic and therapeutic applications. Radio labeled bombes in peptide derivatives may
be used in cancer cell imaging and peptide receptor treatment. They have undesirable pharmacokinetics, such as hepatic buildup and hepatobiliary excretion. The addition of a glucose moiety (through a triazole group) to radiolabeled bombesin analogues decreased abdominal accumulation and boosted tumour absorption without compromising peptide internalization. (Fig 4) In order to improve the hydrophilic properties of radiolabeled Tyr(3)-octreotide peptide for use in diagnostic imaging and cancer treatment, glycosylation was used.
Using glucose, maltose, and maltotriose to modify the peptide increased renal clearance and decreased liver and belly accumulation. This makes Tyr(3)- octreotide analogues (especially maltose and glucose conjugated peptides) ideal for somatostatin receptor-expressing tumour imaging and treatment.
Fig. 4 Glycated [99mTc(CO)3] labeled bombes in analogue Melanoma researchers studied the effects
of galactose, glucose, and maltotriose on- pharmacokinetic MSH's characteristics.
The glycosylated analogues have good nanomolar and subnanomolar binding affinities to melanocortin receptor 1, which is over expressed in melanoma cells in vitro. Overall, the -melanocyte- stimulating hormone peptide analogue containing a galactose unit at the N- terminus showed the best pharmacokinetic profile for melanoma targeting.
4 CONCLUSION
The tuning of peptide-based therapies' pharmacological characteristics is essential for the successful development of these treatments. It is possible to employ glycosylation to improve the
therapeutic action of peptide medicines by adjusting the pharmacokinetic features of the medications. Increased membrane permeability across biological membranes and higher proteolytic stability against digestive enzymes can result from the insertion of carbohydrate moieties into the sequence of peptides, according to recent research. Because of the enormous therapeutic potential of glycoconjugates, numerous approaches were developed, each of which had a substantial influence on the creation of carbohydrate-modified peptide medicines. Further research into the effects of glycosylation on the pharmacological characteristics of peptides is still needed in order to rationally design glycopeptides with increased biological activity.
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