Prediction of oral absorption of low-solubility drugs by using rat simulated gastrointestinal fluids: the importance of regional differences in membrane permeability and solubility.

PURPOSE: This study aimed to develop a novel approach for predicting the oral absorption of low-solubility drugs by considering regional differences in solubility and permeability within the gastrointestinal (GI) tract. METHODS: Simulated GI fluids were prepared to reflect rat in vivo bile acid and phospholipid concentrations in the upper and lower small intestine. The saturated solubility and permeability of griseofulvin (GF) and albendazole (AZ), a drug with low aqueous solubility, were measured using these simulated fluids, and fraction absorbed (Fa) at time t after oral administration was calculated. RESULTS: The saturated solubility of GF and AZ, a drug with low aqueous solubility, differed considerably between the simulated GI fluids. Large regional differences in drugs concentration were also observed following oral administration in vivo. The predicted Fa values using solubility and permeability data of the simulated GI fluid were found to correspond closely to the in vivo data. CONCLUSION: These results indicated the importance of evaluating regional differences in drug solubility and permeability in order to predict oral absorption of low-solubility drugs accurately. The new methodology developed in the present study could be useful for new oral drug development.

Species differences in the dissolution and absorption of griseofulvin and albendazole, biopharmaceutics classification system class II drugs, in the gastrointestinal tract.

It is well known that large differences exist in the bioavailability of orally administered drugs between species. Dissolution is the first step in the oral absorption of solid drugs. In this study, we measured the in vivo luminal concentrations of griseofulvin (GF) and albendazole (AZ), Biopharmaceutics Classification System (BCS) class II drugs, and the GF fraction absorbed (Fa) in rats. Then, we compared the GF Fa in rat with that in other species reported previously to evaluate differences in drug dissolution and oral absorption. The Fa of GF has been reported to decrease from 80% to 40% with an increase in the oral dose in dogs and humans, because the rate-limiting step for absorption shifts from dissolution to solubility. However, such non-linearity was not observed in rats that were administered doses in the same ranges as those in humans, and the Fa values in rats were higher than those in dogs or humans. The in vivo luminal concentration of GF after oral administration in rats was much higher than the saturated solubility of GF in fasted-state simulated dog (FaSSIF(dog)) or human intestinal fluid (FaSSIF(human)). Furthermore, oral administration of AZ showed similar tendencies of interspecies differences in dissolution and oral absorption.

Regional differences in the components of luminal water from rat gastrointestinal tract and comparison with other species.

PURPOSE: The bile acids, phospholipids, inorganic ions, and pH in luminal fluid are very important factors for the dissolution and oral absorption of solid drugs. In this study, we evaluated the regional differences in these factors in the rat GI tract. The solubility of griseofulvin, a poorly water-soluble drug, in the luminal fluid in each segment was also measured. In addition, the data from rats were compared with those from other species published previously to evaluate the species differences in the composition of luminal fluid. METHODS: Rat abdomen was opened and residual water was sampled from each region of GI tract to measure the various components concentrations. RESULTS: The total bile acid and phospholipid concentrations were much higher in the lower jejunum and upper jejunum, respectively, than in the other regions. The solubilities of griseofulvin in the lower jejunal fluid (153-260 ug/mL) were about 1.5-2 times higher than those in the upper jejunal fluid (99-146 ug/mL). The regional differences in inorganic ions and pH were also observed. As for species differences, the total bile acid and phospholipid concentration in rats GI tract were much higher than those of dogs and humans. CONCLUSION: These informations about the regional differences and species differences of the components in the GI fluid should be very useful to consider dissolution and oral absorption of solid drugs.