Caco-2 cell permeability and stability of two d-glucopyranuronamide conjugates of thyrotropin-releasing hormone Original Research Article

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 (Papp = 5.10 ± 1.89 × 10−6 cm/s c.f. Papp = 0.147 ± 0.0474 × 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-Pro-d-glucopyranuronamide) (Papp = 16.3 ± 2.47 × 10−6 cm/s). The half-life for both peptide 1 and peptide 2 was ∼20 min in a homogenate of Caco-2 cells compared with the half-life of TRH which is ∼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.