Transport of a Series of D-Phenylalanine—Glycine Hexapeptides Across Rat Alveolar EpitheliaIn Vitro

Abstract

The effect of lipophilicity on the absorption of peptides from the lungs was investigated. D-phenylalanine (F)-glycine (G) hexapeptides were synthesised to differ, predominantly, only in their lipophilicity. Rat alveolar type II cells were isolated and cultured on plastic, or polycarbonate filters; by day 6 they had de-differentiated to an alveolar type I-like epithelium. The permeability of the monolayers to the hexapeptides was determined. The hexapeptides were metabolically and chemically stable for greater than 24h in the presence of the cells. They did not adhere to the cell culture plastic and were associated only to a low extent with the cell monolayer. The apical to basolateral permeability coefficients for D-F1G5, D-F2G4, and D-F3G3 were 2.19+/-0.53, 1.75+/-0.42 and 2.20+/-0.56 x 10(-7) cm s(-1) respectively. The permeability of the monolayers to D-F1G5 and D-F2G4 was concentration and direction independent, however for D-F3G3 the monolayer was more permeable in the basolateral to apical direction. There was no correlation between the lipophilicity of the hexapeptides and permeability coefficients: other physicochemical parameters did not predict hexapeptide transport. Lipophilicity does not appear to control the transport of hexapeptides across the alveolar epithelium probably as a consequence of the peptides being transported via the paracellular route.

Routes of Delivery for Biological Drug Products

“…The proteolytic activity in the epithelial lining fluid presents a major barrier to pulmonary delivery of therapeutic peptides because these compounds are considered to be absorbed primarily by the paracellular route [134,135]. Enzymatic hydrolysis of small natural peptides can bevery high, resulting in low bioavailabilities unless they are chemically engineered (blocked) to inhibit peptidases [136].…”

Further Evidence of Extensive Pulmonary First-pass Ester Hydrolysis after Airways Administration in Rats

“…The lungs are known to express a wide range of enzymes capable of metabolizing biotechnological drugs (Wall & Lanutti 1993;Forbes et al 1997). The absorption of biotechnological drugs from the lungs is undoubtedly diffusion-limited; that is, the alveolar epithelium presents a substantial barrier to pulmonary absorption even for small peptides (Evans et al 1998). This suggests that even though peptidase expression in the lungs is relatively low, extensive ®rst-pass metabolism of biotechnological drugs is likely to occur.…”