On the chemical and processing stability of pharmaceutical solids Solid form dependent water presenting capacity and process induced solid form transformation

Abstract: There is a need for improving our knowledge and understanding about formation mechanisms and nature of amorphous state in order to prevent the unintended presence of disorder in solid pharmaceutical products and reduce the related stability issues. The suggested theory that water binding capacity of amorphous cellulose affects the chemical stability of hydrolysis sensitive drugs in formulations with cellulose based excipients needs a clarification and water-cellulose interaction profiles need to be examined.  This thesis has addressed these questions.Chemical, mechanical and thermal methods have been used to create partially or predominantly amorphous solids. Mechanisms and the pathways for transformation to amorphous phase and the characteristic qualities of this phase is studied in order to give some tools to predict, to control or prevent the creation of disorder in a crystalline structure. The water interaction with amorphous pharmaceutical materials has been studied to improve stability of hydrolysis sensitive drugs.  The transition to amorphous state during handling of pharmaceutical material, referred to as mechanical activation in processes like blending, mixing and compression is substantially a consequence of vitrification. The process is described as creation of hot spots where friction caused by particle sliding raise the temperature above the melting point of the material. The fast cooling process promotes creation of a local disordered molecular arrangement. It is possible to decrease the degree of amorphisation and undesired stability problems by reducing the friction and inhibit the creation of crystal defects during processing. The glass-forming propensity is an inherent material characteristic related to molecular size and structure and is not process dependent. Molecules with a couple of aromatic rings are often poor glass-formers. Less symmetrical, branched molecular structures with presence of electronegative atoms are more readily transformed to and exist in amorphous state when handled and stored at temperatures below their glass transition temperature. The interaction profile of cellulose with water is strongly dependent on solid state structure of cellulose. Crystallinity is the key parameter in water presenting capacity of cellulose. Amorphous regions have a capacity to bind the water and decrease water mobility and in that way reduce cellulose water presenting capacity despite higher moisture content in partially amorphous cellulose compared to crystalline cellulose. The fact that higher amorphous content decreases cellulose water presenting capacity is a promising lead to improve stability of hydrolysis sensitive drugs in compositions with cellulose. This knowledge could be applicable to other pharmaceutical materials as the differences between crystalline and amorphous states of material are generally the same for different kind of materials.