Excipient-induced salt disproportionation (conversion from salt form to free form) in the solid state during storage or manufacturing is a severe formulation issue that can negatively influence product performance. However, the role of excipient properties on salt disproportionation and mechanisms of proton transfer between salt and excipients are still unclear. Moreover, knowledge about the formation of disproportionation products and the consequent impact of these reactions products on the disproportionation process is still inadequate. In the present study, three commonly used lubricants (sodium stearate, calcium stearate, and magneisum stearate) were mixed with a hydrochloride salt as binary mixtures to examine their different capabilities for inducing salt disproportionation at a stressed storage condition (40°C 65% RH). The overall objective of this research is to explore factors influencing the kinetics and extent of disproportionation including surface area, alkalinity, hygroscopicity, formation of new species etc. In addition, we also aim to clarify the reaction mechanism and proton transfer between the model salt and stearates to provide insight into the in-situ formed reaction products. We found that the properties of stearates significantly affect the disproportionation process in the initial stage of storage, while properties of the reaction products negatively affect the hygroscopicity of the powder mixture promoting disproportionation during longer-term storage. In addition, lubrication difference among three stearates was evaluated by performing compaction studies. The findings of this study provide an improved understanding of the proton transfer mechanism between the ionized form of an API and excipients in solid dosage forms. It also provides pragmatic information for formulation scientists to select appropriate lubricants and other excipients, and to design robust formulations.

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