Solid lipid extrudates as sustained-release matrices: the effect of surface structure on drug release properties

The study focused on the structural characterization of sustained-release lipid matrices prepared by solid lipid extrusion. Drug-containing lipid extrudates were locally analyzed in order to identify differences between the chemical and structural composition of surface and core elements. Independent of the lipid the dissolution from the outer extrudate surfaces was slower compared with dissolution from surfaces prepared by cutting the extrudate. The burst effect was higher for the cross-sections indicating more drug was exposed on these surfaces. The release from glycerol trimyristate (Dynasan 114) extrudates was slower compared with glycerol palmitostearate (Precirol ATO 5) extrudates. By solid-state analysis using DSC, ATR-FTIR and SEM measurements the differences between surface material and core material could be attributed mainly to morphological differences. Chemical differences between the core and the outer surface were not relevant. The differences between the surfaces might be explained by the friction induced temperature increase during extrusion in the die plate. The obtained results and a proposed scheme were used to explain the influence of different formulation/processing parameters, such as drug particle size and milling on the drug dissolution behaviour. Small drug particles and intact extrudates are a means of minimizing the burst release.