Effect of surfactants, gastric emptying, and dosage form on supersaturation of dipyridamole in an in vitro model simulating the stomach and duodenum.

Abstract

The purpose of this study was to investigate the influence of gastric emptying patterns, surfactants, and dosage form on the supersaturation of a poorly soluble weakly basic drug, dipyridamole, using an in vitro model mimicking the dynamic environment of the upper gastrointestinal tract, and, furthermore, to evaluate the usefulness of this model in establishing correlations to in vivo bioavailability for drugs with solubility/dissolution limited absorption. A simulated stomach duodenum model comprising four compartments was used to assess supersaturation and precipitation kinetics as a function of time. It integrates physiologically relevant fluid volumes, fluid transfer rates, and pH changes of the upper GI tract. Monoexponential gastric emptying patterns simulating the fasted state were compared to linear gastric emptying patterns simulating the fed state. The effect of different surfactants commonly used in oral preparations, specifically, sodium lauryl sulfate (SLS), poloxamer-188, and polysorbate-80, on dipyridamole supersaturation was investigated while maintaining surface tension of the simulated gastric fluids at physiological levels and without obtaining artificial micellar solubilization of the drug. The supersaturation behavior of different dose strengths of dipyridamole was explored. Significant levels of dipyridamole supersaturation were observed in the duodenal compartment under all the different in vivo relevant conditions explored. Dipyridamole supersaturation ratios of up to 11-fold have been observed, and supersaturation has been maintained for up to 120 min. Lower duodenal concentrations of dipyridamole were observed under linear gastric emptying patterns compared to mononexponential gastric emptying. The mean duodenal area under concentration-time curves (AUC60min) for the dipyridamole concentration profile in the duodenal compartment is significantly different for all the surfactants explored (P < 0.05). Our investigations with the different surfactants and comparison of dosage form (solution versus suspension) on the precipitation of dipyridamole revealed that crystal growth, rather than nucleation, is the rate-limiting step for the precipitation of dipyridamole. A linear dose-response relationship was found for the mean in vitro duodenal area under concentration-time curves (AUC∞) in the dose range of 25 mg to 100 mg (R(2) = 0.886). This is in agreement with the pharmacokinetic data of dipyridamole reported in the literature. The simulated stomach duodenum model can provide a reliable and discriminative screening tool for exploring the effect of different physiological variables or formulations on the supersaturation/precipitation kinetics of weakly basic drugs with solubility limited absorption. The amount of drug in solution in the duodenal compartment of the SSD correlates to bioavailability for the weakly basic drug, dipyridamole, which has solubility limited absorption and undergoes supersaturation/precipitation.