Insights on the role of excipients and tablet matrix porosity on aspirin stability.

Excipient-moisture interaction can be a critical attribute in determination of product stability. This study aimed to investigate influence of integrating excipients having different moisture interaction into moisture sensitive drug formulations. Aspirin was formulated with maize starch (MS), microcrystalline cellulose (MCC) and calcium hydrogen phosphate dihydrate (DCP). The excipients were evaluated for their inherent moisture content and water activity. Tablets fabricated at different compression pressures were exposed to 40 °C, 75% relative humidity for a stipulated period before analyzing for aspirin degradation. The results revealed that while MS had higher moisture content, the water activity was relatively low. Consequently, MS tablets had lower aspirin degradation than MCC and DCP tablets. In contrast, high water activity of DCP resulted in greater aspirin degradation. This was despite the low moisture content of DCP. Influence of tablet porosity on aspirin degradation was minimal. This illustrated the fugacity of moisture, possessing high thermodynamic activity and physical spatial delimitation would not suppress its distribution. The findings suggested that excipients possessing high water retentive capacity could potentially be useful as internal tablet desiccants by acting as a moisture scavenger. This study also highlights the importance of water activity in preformulation studies related to the choice of excipients.