Oral delivery of microencapsulated proteins.

Abstract

While the neonatal mammalian small intestine is able to absorb macromolecules (Walker and Isselbacher, 1974; Walker, 1979), in the mature gut the oral bioavailability of polypeptide drugs is generally poor (Lee et al., 1991; Humphrey and Ringrose, 1986; Pusztai, 1989). This low bioavailability is not surprising given the large number of barriers in the gastrointestinal tract to the uptake of intact peptides and proteins. Proteolytic enzymes in the stomach, the intestinal lumen, and the brush border of the enterocytes digest proteins by hydrolysis. Most digestion in the small intestine is due to the proteolytic enzymes trypsin, chymotrypsin, and carboxypeptidase, all from the pancreas (Castro, 1981; Matthews et al., 1968). Any remaining peptides larger than three amino acids are further hydrolyzed in the brush border. Tripeptides are hydrolyzed extracellularly, by the brush border enzymes, or, if absorbed, by cytoplasmic amino peptidases (Nicholson and Peters, 1977). Besides enzymatic barriers, there are significant physical barriers to peptide drug absorption in the intestinal wall. Within the intestinal mucosa, the membrane of the microvilli consists of the typical trilaminar arrangement observed in biological membranes: two molecular layers of lipid with the hydrocarbon tails oriented inward and the hydrophilic heads on the outer part of the protein-coated membrane. The