*3.1.3 Tight junctions between adjacent epithelial cells*

*Chitin and Chitosan - Physicochemical Properties and Industrial Applications*

adequate therapeutic concentration in the body [58].

*3.1.1 The presence of proteolytic enzyme and pH of GIT*

**3.1 Challenges in oral peptide delivery**

peptide bonds through hydrolysis [4, 58].

*3.1.2 The intestinal barrier to drug absorption*

restricted in the presence of mucus [54].

der Waals interaction.

the layer of mucus is shed [60].

peptide will be reduced.

peptide [59].

Due to the physical and chemical instability of protein in the GIT, they would not achieve an acceptable therapeutic bioavailability. The nature of the GIT as great physiology to digest food will be a barrier for protein drugs to penetrate through the membrane. The challenges and strategies to improve the protein drug delivery through oral administration need to be considered to ensure the drug achieves

GIT is the hollow organs include the oral cavity, stomach, small intestine, large intestine, and colon. Each part of the GIT is varying in pH. However, most proteins are stable at neutral pH and tend to undergo protein denaturation at the extreme changes in pH [48]. A building block of protein is sensitive to pH. In the presence of hydrochloric acid in the stomach, hydrolysis will occur and the disulphide bonds of

Acidic pH environment activates the conversion of pepsinogen into pepsin [59], by transferring hydrogen ion (H+). Pepsin is responsible for the breakdown of peptide bonds which will interfere with the structure and stability of the

In the small intestine, the pancreatic juice is secreted in the duodenum. This

atic enzymes consist of amylase, lipase, and protease, which are responsible for the digestion of lipid and peptide. Protease catalyses the proteolysis rate, which cleaves

The presence of chymotrypsin and peptidase in the jejunum interferes with peptide absorption in the epithelial membrane [48]. Peptide drugs are digested before it reaches the membrane and the fraction of the undigested peptide will reduce. This physiological function will lower the possibility for the therapeutic concentration of

The layers of the epithelial cell of the intestine are covered by mucus or mucin glycoproteins [55]. These glycoproteins will form a gel layer that covers the surface of the intestinal cell. The diffusion rate for the peptide to the epithelial membrane is

One of the intestinal mucosal epithelial cells is the goblet cell. Goblet cells are responsible for secreting mucin 2 in the intestine and Mucin-5b in the colon. Lubricate the passage for chime is the main function of the mucus layer, protecting

The overexpression of mucin will interfere with the pharmacokinetics of drugs. The higher the concentration of the mucin, the lower the ability of a drug to diffuse through the membrane. Drugs and mucin interact through hydrophobic and Van

Large amounts of enzymes present within the mucus layer increase the tendency

to digest peptide drugs [58]. The ionic strength, pH, and chyme content in the intestine will affect the charge density in the mucin [54]. The presence of a charged group on mucin interact ionically with charged particles and immobilised them in the mucus. The immobilisation of peptides leads to the clearing from the tract when

−

) [48]. The pancre-

juice contains pancreatic enzymes and bicarbonate ions (HCO3

peptides to be achieved in the systemic circulation [58].

the epithelium from mechanical damage of GIT [54].

**60**

Tight junctions are protein complexes that existed within the adjacent epithelial cells [48]. It prevents leakage and restricts the flux of substances through the paracellular pathway [61]. They consist of transmembrane protein with extracellular domains called Claudin 4 (CLDN4). CLDN4 protects the paracellular physiological function of GIT [62].

The linkage of CLDN4 domains with zonula occludens 1 will result in the connection of cytoskeleton components through linker protein [62]. The components include actin, myosin and microtubules which involve in the contraction of muscle, upon phosphorylation. The contraction leads to cellular tightness, hence, reduce the permeability of substance into the cell.

#### **3.2 Strategies to improve oral peptide delivery system using chitosan**

The effective delivery of oral peptide drugs can be achieved by altering the formulation for maximum solubility, avoid enzymatic degradation and enhance the absorption of drugs through the intestinal epithelial cell [63]. For the sake of preventing enzymatic degradation or inactivation, the addition of enzyme or protease inhibitor is a great approach (**Figure 5**). Proteolytic enzymes are responsible for cleaving protein molecules into an inactive amino acid chain. Protease inhibitors such as aprotinin and chromostatin can be used to prevent the inactivation of protein drugs [7].

As discussed earlier, protein molecules show poor permeability through various mucosal surfaces and biological membranes. The improvement of membrane permeability can be achieved by the inclusion of a permeation enhancer into the formulation. Permeation enhancers are either tight junction selective or membrane perturbing [61].

Chitosan and its derivatives have been used as an enzyme inhibitor, permeation enhancer and mucoadhesive agent [30]. With the different mechanism, modification, and preparation technique, this polymer also involves in the encapsulation of peptide drugs into the nanoparticulate system [60], which protect them from harsh GIT environment.

**Figure 5.** *Strategies to improve oral peptide delivery system (adapted from [53]).*
