Frog intestinal sac: a new in vitro method for the assessment of intestinal permeability

The aim of this study was to evaluate a new experimental protocol utilizing isolated frog intestinal sacs for the assessment of intestinal drug permeability in humans. Segments of approximately 5.0 cm in length were used for these experiments. The intestinal sacs were filled with a solution of the appropriate drug in frog Ringer (FR) and immersed in a vial containing fresh FR. The transport was monitored for a period ranging from 2 to 5 h by moving the intestinal sac at each time point to a new vial containing fresh medium (Method A). Alternatively, according to Method B, at predetermined times aliquots of receiving mixture were taken up without removing the intestinal sac, and replaced with fresh drug-free FR. In all cases, the samples were analyzed by HPLC. A series of 20 noncongeneric drugs, predominantly absorbed by passive diffusion mechanism, was examined. The results indicate that drugs completely absorbed in humans had Papp values greater than 1 x 10(-6) cm/s, while drugs absorbed <90% had Papp values lower than 1 x 10(-6) cm/s. By plotting Log Papp values against percent human absorption, an approximately sigmoidal relationship was obtained. The frog intestinal sac method was evaluated as a permeability model to classify the 20 studied drugs into the Biopharmaceutics Classification System (BCS). By comparing the predictions made by this new approach with those reported in literature, it can be concluded that a satisfactory biopharmaceutical classification may be based on the Papp values determined by the frog intestinal permeability. Molecular properties relevant to passive intestinal permeability were considered to evaluate the correlation with the frog intestinal permeability rates recorded. Good relationships were observed when hydrogen-bonding parameters were expressed as a function of the Log Papp values. It can be concluded that the in vitro permeability coefficient deduced from isolated frog intestinal experiments can be used for predicting peroral absorption in humans for passively absorbed compounds. Computational methods for prediction of frog intestinal permeability may be applied in a highly simplified manner.