Caco-2 cell permeability and stability of two D-glucopyranuronamide conjugates of thyrotropin-releasing hormone

Caco-2 cell permeability and stability assays were used as an in vitro model to study the intestinal epithelial transport and stability of two analogues of thyrotropin-releasing hormone (TRH; Pyr-His-Pro-NH2). Peptide 1 (Pyr-His-Pro-D-glucopyranuronamide) was more permeable across the Caco-2 cell monolayer compared with the permeability of the parent TRH peptide (P-app = 5.10 +/- 1.89 x 10(-6) cm/s c.f, P-app = 0.147 +/- 0.0474 x 10(-6) cm/s respectively). The permeability of peptide 1 was improved threefold by attaching a 2-aminooctanoic acid moiety to the N-terminus to form peptide 2 (2-aminooctanoic acid-Gln-His-ProD-glucopyranuronamide) (P-app = 16.3 +/- 2.47 x 10(-6) cm/s). The half-life for both peptide 1 and peptide 2 was similar to 20 min in a homogenate of Caco-2 cells compared with the half-life of TRH which is similar to 3 min. It was concluded that the permeability of peptides 1 and 2 was enhanced because of their increased stability, while the higher permeability of peptide 2 compared with peptide 1 may be attributed to its increased lipophilicity which results in enhanced passive diffusion. (c) 2007 Elsevier Ltd. All rights reserved.