STUDIES ON DRUG SOLUBILIZATION MECHANISM IN SIMPLE MICELLE SYSTEMS

Poor aqueous solubilities of drug candidates limit the biopharmaceutical usefulness in either oral or parenteral dosage forms. Lipid assemblies, such as micelles, may provide a means of enhancing solubility. Despite their usefulness, little is known about the means by which micelles accomplish this result. The goal of the current dissertation is to provide the molecular level understanding of the mechanism by which simple micelle systems solubilize drugs. Specifically, the location, orientation and amount of the drug molecules in micelle systems are the focuses of the work. Three series of model drugs, steroids, benzodiazepines and parabens, in three surfactant systems with anionic, cationic and neutral hydrophilic headgroups were studied. Solubilization power of each micelle system for each model drug was determined by equilibrium solubility. The observed strong surface activities of model drug at hydrocarbon/water interface and the ability of the drugs to compete with surfactants for the model oil/water interface lend support to the hypothesis that drug molecules are mainly solubilized in the interfacial region of the micelles. A surface-localized thermodynamic model that considered the surfactant-drug competition at micelle surface was successfully applied to predict the micelle/water partitioning coefficients. The predictions were made without the use of adjustable parameters in the case of both dilute and concentrated solutions. The orientation of drug at micelle surface was determined by matching calculated occupied areas by solutes at oil/water interface using molecular modeling method to the experimental values. To look into the micro-structure of micelles, twodimensional and diffusion (or PGSE) NMR techniques were employed to detect the specific drug-surfactant interactions and the micelle sizes influenced by model drugs and electrolytes