Mechanisms of Water Permeation and Diffusive API Release from Stearyl Alcohol and Glyceryl Behenate Modified Release Matrices

This work aims to develop complimentary analytical tools for lipid formulation selection that offer insights into the mechanisms of in-vitro drug release for solid lipid MR excipients. Such tools are envisioned to aide and expedite the time consuming process of formulation selection and development.

Highlights

• Nuclear Magnetic Resonance Spectroscopy and diffusometry are used to define water permeation rates and the diffusion coefficients of permeated water and a model compound, caffeine, embedded in the solid lipid matrices stearyl alcohol and glyceryl behenate.

• Stearyl alcohol and glyceryl behenate were found to differ with respect to the porous environment occupied by permeated water and the solubilized model drug.

• Placebo and active containing matrices are shown to differ significantly in water permeation rates and mechanisms. Caution is suggested in using placebo preparations to rank order active containing formulations.

• A model is presented that accounts for the relative rates of water permeation and diffusive release. This model helps to identify the rate limiting step for drug release. A Peclet-like number is proposed as a parameter to distinguish between rate limiting steps.

Two pharmaceutically relevant solid lipid excipients are investigated, stearyl alcohol and glyceryl behenate, which are generally known to exhibit faster and slower relative release rates, respectively. Nuclear magnetic resonance spectroscopy and diffusometry are used, along with water uptake and dissolution experiments to help distinguish between two proposed in-vitro release mechanisms for crystalline caffeine from these matrices: 1) rate limiting movement of the wetting front through the particle, and 2) rate limiting diffusive release of the active from the wetted particle. Findings based on water permeation rates, API diffusion coefficients and kinetic modeling suggest that the rate limiting steps for caffeine release from these matrices are different, with stearyl alcohol being co-rate limited by movement of the wetting front and diffusive release of API, whereas glyceryl behenate is more strictly limited by diffusive release of API from the wetted matrix. A Peclet-like number is proposed to describe the different regimes of rate limitation for drug release. NMR spectroscopy and diffusometry are demonstrated to be useful tools for elucidating mechanisms of API release from crystalline drug/lipid mixtures and have significant potential value as screening tools in MR formulation development. Continue reading here