Lipid nanoparticles: Drug localization is substance-specific and achievable load depends on the size and physical state of the particles

Lipid nanoemulsions and -suspensions are being intensively investigated as carriers for poorly water soluble drugs. The question on where model compounds or probes are localized within the dispersions has been the subject of several studies. However, only little data exists for pharmaceutically relevant molecules in dispersions composed of pharmaceutically relevant excipients. In this work, the localization of drugs and drug-like substances was studied in lipid nanoemulsions and -suspensions. Conclusions about the drug localization were drawn from the relations between lipid mass, specific particle surface area and drug load in the dispersions. Additionally, the achievable drug loads of the liquid and the solid lipid particles were compared. Nanoemulsions and -suspensions comprised trimyristin as lipid matrix and poloxamer 188 as emulsifier and were prepared with different well-defined particle sizes. These pre-formed dispersions were passively loaded with either amphotericin B, curcumin, dibucaine, fenofibrate, mefenamic acid, propofol, or a porphyrin derivative. The physico-chemical properties of the particles were characterized; drug load and lipid content were quantified by UV spectroscopy and high performance liquid chromatography, respectively. For all drugs the passive loading procedure was successful in both emulsions and suspensions. Solid particles accommodate drug molecules preferably at the particle surface. Liquid particles can accommodate drugs at the particle surface as well as in the core; the distribution between the two sites is drug specific. It is also drug specific whether solid or liquid particles yield higher drug loads. As a general rule, smaller particles led to higher drug loads than larger ones. Propofol and the porphyrin derivative displayed eutectic interaction with the lipid and crystal growth after loading, respectively.