In Vitro Sensitivity Analysis of the Gastrointestinal Dissolution Profile of Weakly Basic Drugs in the Stomach-to-Intestine Fluid Changing System: Explanation for Variable Plasma Exposure after Oral Administration

An in vitro methodology for simulating the change in the pH and composition of gastrointestinal fluid associated with the transition of orally administered drugs from the stomach to the small intestine was developed (the stomach-to-intestine fluid changing system (the SIFC system)). This system was applied to in vitro sensitivity analysis on the dissolution of weakly basic drugs, and the obtained results were discussed in relation to the intrasubject variability in the plasma exposure in human bioequivalence (BE) study. Three types of protocols were employed (steep pH change: pH 1.6 FaSSGF → pH 6.5 FaSSIF, gradual pH change: pH 1.6 FaSSGF → pH 6.5 FaSSIF, and high gastric pH: pH 4.0 FaSSGF → pH 6.5 FaSSIF). Regardless of the protocols and the forms of drug applied in active pharmaceutical ingredient powder or formulation, dissolution profiles of pioglitazone after fluid shift were similar and the final concentrations in FaSSIF were approximately equal to the saturation solubility in FaSSIF, supporting its small intrasubject variance in human BE study. In contrast, dissolved concentration of terbinafine in the SIFC system became less than half in the high gastric pH protocol than that in other protocols, suggesting the fluctuation of gastric pH as one of the factors of high intrasubject variance of terbinafine in human. Plasma exposure of telmisartan was highly variable especially at the high dose. Although the dissolution of telmisartan in the SIFC system was greatly improved by formulation, it considerably fluctuated during fluid shift especially at the high dose, which corresponds well to in vivo results.