Drug solubility in lipid nanocarriers: Influence of lipid matrix and available interfacial area.

Amongst other strategies for the formulation of poorly water-soluble drugs, solubilization of these drugs in lipid-based formulations is a promising option. Most screening methods for the identification of a suitable lipid-based formulation fail to elucidate the role interfacial effects play for drug solubility in disperse systems. In a novel screening approach called passive drug loading, different preformed lipid nanocarrier dispersions are incubated with drug powder. Afterwards, undissolved drug is filtered off and the amount of solubilized drug is determined. The aim of this study was to identify parameters for drug solubility in pure lipids as well as for drug loading to the lipid-water interface of lipid nanoparticles. Using passive loading, the solubility of eight poorly water-soluble drugs in seven lipid nanocarriers varying in particle size or lipid matrix was investigated. Drug solubility in the nanocarriers did not follow any apparent trend and different drugs dissolved best in different carriers. Drugs with a melting point below approximately 150°C displayed distinctly better solubility than higher melting drugs. Additionally, relating the specific lipid nanocarrier surface area to the drug solubility allowed drawing conclusions on the drug localization. Fenofibrate, dibucaine and, less distinctly also clotrimazole, which all melt below 150°C, were predominantly located in the lipid droplet core of the nanoparticles. In contrast, the five remaining drugs (betamethasone valerate, flufenamic acid, itraconazole, ketoconazole, mefenamic acid) were also located at the lipid-water interface to different, but substantial degrees. The ability to account for drug loading to the lipid-water interface is thus a major advantage of passive loading.