Chitosan is a biocompatible polysaccharide of natural origin that can act as a permeation enhancer. In this study, we used an integral in vitro/in vivo correlation approach to: a) investigate polysaccharide-mediated absorption kinetics of the peptide drug octreotide across mammalian airway epithelium, b) assess formulation toxicity, c) correlate the mechanism of permeation enhancement. The 20% and 60% N-trimethylated chitosan derivatives (TMC20 and TMC60) were synthesized by alkaline methylation using chitosan as starting material. Octreotide was administered in control buffers or in 1.5% (w/v) gel-phase formulations of pH 5.5 for chitosan and pH 7.4 for TMCs. In vitro, reconstituted Calu-3 cell monolayers were used for trans-epithelial electrical resistance (TEER), transport and cytotoxicity assays. Intratracheal instillation in rats was used to determine octreotide kinetics and formulation toxicity in vivo. Chitosan, TMC20 and TMC60 decreased TEER significantly and enhanced octreotide permeation in vitro by 21-, 16- and 30-fold. In vivo, sustained release properties of the formulations were observed and the bio-availability was enhanced by 2.4-, 2.5- and 3.9-fold, respectively. Interestingly. we found a linear in vitro/in vivo correlation between calculated absorption rates (R-2 = 0.93), suggesting that the permeation enhancement by polysaccharides, both in vitro and in vivo, proceeds via an analogous mechanism. Cell viability and histology studies showed that the TMCs are safer than chitosan and that Calu-3 cell monolayers are a valuable model for predicting paracellular transport kinetics in airway epithelia. Additionally, cationic polysaccharides are promising enhancers for peptide drug absorption with prospect for sustained-release formulations. (c) 2005 Elsevier B.V. All rights reserved.