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| home > pmps > spring 2003 > modified-release oral solid dosage forms - part ii: microstructure and release mechanisms |
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Pharmaceutical Manufacturing and Packing Sourcer
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This second part of the article reviews the effects of matrix and coating formulation and formation on the design of microstructure and release mechanisms of modified release oral solid dosage forms (tablets and particles). The microstructure of each system as defined by porosity, channel tortuosity, flexibility and durability, depends on the physical and chemical parameters of the polymers and drugs used. Mechanicals appear to be similar to those of ceramic/polymer composites but their effects on drug release are still under investigation. Drug release from the bulk of matrix and coating systems involves two major mechanisms. The erosion rate of the matrix determines the drug release rate in matrices governed by erosion or dissolution: (dM/dt)=S(dx/dt)f(C), where (dM/dt) is the drug release rate, S is the surface area, (dx/dt) is the matrix erosion rate and f(C) is the drug concentration gradient. The diffusion through a barrier membrane describes drug release in insoluble coatings via Fick's second law of diffusion: (dM/dt)=DSK(Cd-Cr)/h, where (dM/dt) is the drug release rate, D is the diffusion coefficient, S is the exposed surface area, K is the partition coefficient, Cd-Cr is the drug concentration gradient and h is the coating thickness.
Drug delivery at zero rate provides uniform drug adsorption and distribution into systemic circulation so that the drug level is maintained at the therapeutic window to minimise side effects and/or reduce the frequency of administration. Elucidation of the mechanisms of drug release from various matrix and coating systems is therefore critical in order for improvements to be realised. Many researchers have sought to design zero release mechanisms. Hydrophilic matrices have been reported to display a time release dependent profile due to an increase in the diffusion path length as the matrix dissolves, swells and releases drug.
This inherent limitation leads to first-order kinetics (1). One can therefore understand the significance of examining the release mechanisms of the various oral solid dosage forms. Form design has a tremendous impact on the drug release characteristics. The fine form design at the molecular level as defined by porosity, channel tortuosity, flexibility and durability is the so called 'microstructure'.
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