Drug release from lipid-based implants: elucidation of the underlying mass transport mechanisms

The aim of this study was to better understand the mass transport mechanisms involved in the control of drug release from lipid-based implants. Different types of triglyceride-based cylinders were prepared by compression. Glycerol-trilaurate, -trimyristate, -tripalmitate and -tristearate were used as model lipids, lysozyme and pyranine as model drugs. The effects of several formulation and processing parameters on the resulting drug release kinetics in phosphate buffer pH 7.4 were studied and the obtained results analyzed using Fick's second law of diffusion. Interestingly, lysozyme release from implants prepared by compression of a lyophilized emulsion (containing dissolved drug and lipid) was found to be purely diffusion-controlled, irrespective of the type of triglyceride. In contrast, the dominating release mechanism depended on the type of lipid in the case of pyranine-loaded implants prepared by compression of a lyophilized lipid-drug solution: with glycerol-trilaurate and -tristearate the systems were found to be purely diffusion-controlled, whereas also other mass transport phenomena are of importance in glycerol-trimyristate and -tripalmitate-based devices. Similarly, changes in the size of the compressed lipid-drug particles, drug loading and compression force significantly affected the underlying release mechanisms. The addition of a drug-free, poly(lactic-co-glycolic acid) (PLGA)-based coating around the implants delayed the onset of pyranine release for about 20 days. Interestingly, the subsequent drug release was purely diffusion-controlled, irrespective of the type of triglyceride. Also the addition of different amounts (and particle size fractions) of saccharose to pyranine-loaded implants led to purely diffusion-controlled drug release.