Effect of experimental pH on the in vitro permeability in intact rabbit intestines and Caco-2 monolayer

The effect of experimental (apical) pH on absorptive permeability (Pe) was investigated in animal intestinal tissues and Caco-2 cell monolayers to examine whether the introduction of physiological pH such as 6.5 relates to the better prediction of animal intestinal Pe. Transport studies were conducted in a 24-well transwell for Caco-2 and diffusion chambers for rabbit intestinal permeability. Twenty-four test compounds were chosen (seven acidic, seven basic, eight neutral, and two zwitterionic) and Pe was measured at a 100 μM donor concentration with two apical pHs, Krebs Bicarbonate Ringer's buffer (pH 7.4) and 4-morpholineethanesulfonic acid (MES) buffer (pH 6.5). Samples were collected over a 90-min interval and analyzed by LC/UV, LC/MS, or LSC. Upon the apical pH change from 7.4 to 6.5, Caco-2 Pe of acidic and basic compounds changed significantly, whereas rabbit intestinal Pe did not change possibly by the presence of mucous layer. When the intestinal Pe was correlated with pH 6.5 or 7.4 Caco-2 Pe, the correlation of pH 6.5 duodenum and jejunum Pe with pH 6.5 Caco-2 Pe was very poor. However, pH 7.4 Caco-2 Pe correlated relatively well with pH 6.5 duodenum and jejunum Pe and pH 7.4 ileum and colon Pe. The results suggested that pH 7.4 Caco-2 Pe is a good qualitative predictor for physiological intestinal permeability from duodenum to colon.