Many pharmaceuticals, either by accident or design, may exist in a total or partially amorphous state. Consequently, it is essential to have an understanding of the physico-chemical principles underpinning the behaviour of such systems. In this discussion, the nature of the glassy state will be described, with particular emphasis on the molecular processes associated with glass transitional behaviour and the use of thermal methods for characterising the glass transition temperature, Tg. The practicalities of such measurements, the significance of the accompanying relaxation endotherm and plasticization effects are considered. The advantages and difficulties associated with the use of amorphous drugs will be outlined, with discussion given regarding the problems associated with physical and chemical stability. Finally, the principles of freeze drying will be described, including discussion of the relevance of glass transitional behaviour to product stability.
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