Studies on the transport of thyrotropin-releasing hormone (TRH) analogues in Caco-2 cell monolayers

The transport mechanisms of thyrotropin-releasing hormone (TRH) and its pharmacologically active analogues ((3-methyl-His(2))TRH (MeTRH), taltirelin, montirelin, azetirelin) across Caco-2 cell monolayers were characterized. The results of kinetic analysis showed a linear relationship between the concentration (over the range 0.5-10 mM) and apical-to-basolateral transport rate of these agents. The permeability coefficients (P(app)) of these agents were not substantially different from each other, and their P(app) ratios of the basolateral-to-apical over the apical-to-basolateral transport were close to one (0.73-1.23). The cellular transport of [(3)H]MeTRH at low concentrations (3-15 nM) showed a linear relationship between the concentration and transport rate. The transport of [(3)H]MeTRH in Caco-2 cell monolayers was neither affected by TRH nor TRH analogues, and there was little difference in P(app) values between [(3)H]MeTRH and [(14)C]mannitol. The cell-per-medium ratio of [(3)H]MeTRH in the cellular uptake experiment was similar to the value of [14C]mannitol. A large excess of TRH and MeTRH did not significantly influence cell-per-medium ratios of [(3)H]MeTRH in Caco-2 cell monolayers. The k'(IAM) value, which represents lipophilicity, was decreased in the following order: montirelin > taltirelin > TRH > azetirelin, and the values varied from 0.234 to 1.028. These results indicate that a paracellular passive diffusion may be the major route for the transport of TRH and its analogues in Caco-2 cell monolayers.