Oral Delivery of Proteins and Peptides: Concepts and Applications - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 485

Figure
10.3 Mechanisms for
the transport of drugs
paracellular transport is limited, as tight junctions comprise only about 0.01% of the
total absorptive surface area of the intestine (villi) [38] .
10.3.2 Transcellular Absorption
Drugs pass through the intestinal epithelial cells (enterocytes) and require partitioning
across both the apical and the basolateral membranes. The transcellular permeability of
a peptide is a complex function of various physicochemical properties, including size,
lipophilicity, hydrogen bond potential, charge, and conformation [39] . This route is lim-
ited to the transport of relatively low-molecular-weight lipophilic drugs. Furthermore,
studies in humans have demonstrated that absorption by the transcellular route
decreases significantly in the colon (small intestine  ascending colon  transverse
colon), whereas no such gradient exists for the paracellular route [40] .
10.3.3 Carrier-Mediated Transport
Drugs interact with a specific transporter or carrier, in which the drug is transferred
across the cell membrane or entire cell and then released from the basal surface of