Oral Delivery of Proteins and Peptides: Concepts and Applications - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 488

In the duodenum, apart from pH, enzymatic degradations make conditions peril-
ous for ingested peptides and proteins. Two classes of pancreatic proteases, consisting
of endopeptidases and exopeptidases, are present in the duodenum. Endopeptidases
include trypsin, chymotrypsin, and elastase; and exopeptidases include carboxy-
peptidase A [57] . Proteins and peptides are degraded into smaller peptides, and this
sequence is continuous in the brush border, the cytosol of the enterocyte, and even in
the lysosomes and other cell organelles. Amiopeptidases are from the exopeptidase
family and act on the N-terminal of peptides [58] . Leucine aminopeptidase and ami-
nopeptidases N, A, and B are generally present in the intestine. Apart from the areas
these peptidases reach, there are areas in the jejunum and ileum where amiopepti-
dases activity is about 20-30% of the aminopeptidases activity in other neighboring
areas. Such areas are known as Peyer's patches and are a potential targeting site for
the delivery of proteins and peptides [59-64] .
A wide variety of peptidases is present in brush border membranes of different sites.
Such peptidases are present in different concentrations, and their target sites differ as
well. An understanding of the distribution of peptidases in the GI tract and of their tar-
get site provides an opportunity for targeted oral delivery of peptides and proteins [65] .
10.4.3 Cellular Barriers
Transcellular and paracellular are the major pathways involved in the transfer of pep-
tides and proteins across the epithelial barriers. In the transcellular pathway, peptides
and proteins move across the epithelia through intracellular transfer from the lumen
to the blood stream. In this case, proteins and peptides either transfer through a spe-
cific uptake mechanism or follow simple partitioning from the aqueous lumen con-
tent to the lipid membrane, and from there to the aqueous blood stream. Lipophilic
molecules are generally transported through partition, whereas hydrophilic molecules
require a specific transport mechanism.
The paracellular pathway involves the transfer of peptides and proteins through the
space present between the adjacent cells. This space has radius 8 Å, so only smaller
peptides can pass through the space. The only hindrance in the paracellular pathway is
the tight intracellular junction of the villus cells [66] . The paracellular pathway avoids
degradation of peptides and proteins by proteases present in the cells. Peptides and pro-
teins have a log P value of less than zero, suggesting that they follow the paracellular
pathway. Use of a penetration enhancer significantly improves the transport of peptides
through paracellular pathways. Penetration enhancers such as zonula occludens toxin
(a protein from vibrio cholera), Pz peptide, and chitosan reversibly open tight junctions
between intestinal cells [67,68] . Once tight junctions have been opened, transport is
enhanced not only for drugs but also for potentially toxic or unwanted molecules pres-
ent in the GI tract [69,70] .
Nonpolar compounds with a molecular weight (MW) of 3-4 kDa are absorbed
orally to a significant extent because of their desired partition coefficient. Although
the tight junctions of villus cells have been reported to be generally impermeable to
molecules with radii of more than 11-15 Å, peptides have some conformational flex-
ibility, and even larger molecules can perhaps permeate the tight junctions [71] .