Influence of Magnesium Stearate on the Homogeneity of a Prednisone-Granule Ordered Mix

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Abstract
The use of ordered systems has been advocated in the formulation of microdose delivery systems to improve and maintain drug homogeneity during mixing. This study considered the effect of a lubricant such as magnesium stearate on the degree of homogeneity and stability of a preformed prednisone-granule ordered mix. Micronized prednisone was mixed with starch-lactose granules to produce an ordered mix of satisfactory homogeneity. Magnesium stearate in concentrations above and below the theoretical surface saturation of the granule caused negligible change in the degree of homogeneity. Sieve analysis of the mix and subsequent analysis of size fractions for prednisone allowed the prednisone distribution within the mix to be determined. Prednisone was found to be associated with the granules in all the mixes; the magnesium stearate did not compete for the surface adsorption sites and did not dislodge the drug from the granule surface, during mixing and mild demixing conditions. However, a decrease in surface adsorbed prednisone occurred in all mixes (with and without magnesium stearate) under more severe segregating conditions. Recent research in drug homogeneity studies in microdose tablets has highlighted serious problems in dosage variation1. Drugs are frequently micronized to improve their release from the solid dosage form. Micronization produces drug particles which are extremely cohesive and interactive. In practice, the adequate mixing of micronized powders with other excipients may be difficult to achieve since this cohesiveness produces aggregation of drug particles and interaction of the drug with the mixer surfaces. In recent years some research effort has been applied to using the interactive nature of drug particles to improve the homogeneity of mixes2,3. Controlled adsorption of a micronized drug particle onto a carrier particle to produce an “ordered unit” has been shown to minimize segregation within the mix4,5. Some of the factors affecting the degree of homogeneity of “ordered mixes” have been studied6,7. However, little research has been conducted on the influence of other excipients on the homogeneity and stability of preformed ordered mixes. A cautionary note on the use of magnesium stearate in ordered mixtures indicated that the lubricant may displace salicylic acid from a sucrose carrier under conditions of segregation8. The purpose of this study, therefore, was to evaluate the influence of a third component such as magnesium stearate on the degree of homogeneity and stability of a preformed prednisone-starch lactose granule ordered system during a mixing process.