Investigations into the Diffusion Behaviour of Polypeptides in Native Intestinal Mucus with Regard to their Peroral Administration

Abstract

Beside enzymatic and absorption barriers, the diffusion barrier presented by the mucus layer covering the gastrointestinal epithelia has an important influence on the bioavailability of perorally administrated peptide and protein drugs. To obtain detailed information on this barrier, the diffusion behaviour of polypeptides in the range of 3.4 to 66 kDa and the influence of a mucolytic agent (N-acetylcysteine) was studied in native mucus gel from porcine small intestine during an incubation period of 5 h. Since the percentage amount of protein that passed through the mucus was almost constant (1 ± 0.5%) between molecular masses of 12.4 and 66 kDa but increases significantly below these values, the margin for polypeptides passing through a layer of mucus 1 mm thick has to be no more than 12.4 kDa. Smaller polypeptides, 6.5 and 3.4 kDa, exhibited approximately two- and threefold diffusion rates, respectively. In the presence of 2% N-acetylcysteine (w/v) the diffusion rate of all tested polypeptides could be increased significantly. Depending on their respective molecular sizes, the permeability of the native mucus could be increased by 50% for large sized molecules (66 kDa) up to approximately three times for comparable small molecules (6.5 kDa). The results demonstrate that the mucus covering gastrointestinal epithelia represents an almost insurmountable barrier to proteins with a molecular mass higher than 12.4 kDa but this can be reduced by the addition of appropriate mucolytic agents such as N-acetylcysteine.