Comparison of wax and glyceride solid lipid nanoparticles (SLN)

The present study compares solid lipid nanoparticles (SLN) formulated with either wax or glyceride bulk material. While most published data deal with glyceride SLN, little knowledge is reported on wax carriers. The two types were compared with respect to drug encapsulation efficacy, particle size distribution after production and storage, and crystal packing. The inclusion of retinol as a model drug was investigated. Retinol is chemically unstable in water and rather stable in lipid phases. Thus, rapid degradation of retinol indicates rapid drug expulsion from the carrier. Good stability indicates an effective drug encapsulation in the lipid phase of the nanoparticles. Particle size distribution was measured by laser diffractometry. Subcell packing and assignment of polymorphic forms was investigated by WAXS measurements. Glyceride SLN showed good drug encapsulation, while physical stability was poor. In contrast, wax SLN possessed good physical stability but lacked sufficient drug encapsulation in the solidified state. These differences were attributed in part to different crystal packing. Less ordered crystal lattices favour successful drug inclusion, as in the case of glyceryl monosterate and glyceryl behenate SLN. The highly ordered crystal packing of wax SLN comprised of beeswax or cetyl palmitate, for instance, leads to drug expulsion, but also to superior physical stability.