Identifying Optimal Drug Loading in Stable Amorphous Solid Dispersion Formulations: A Rheological Approach

Synthetic polymers play a crucial role in enhancing the performance of drug formulations by enabling controlled release, improving bioavailability, and ensuring stability. Copovidone is frequently employed as an excipient in hot-melt extrusion (HME) to augment the solubility of active pharmaceutical ingredients (APIs). However, this method faces two significant challenges: identifying the optimal copovidone/API composition (the solubility of the crystalline API in the copovidone matrix) and the inadequate time-dependent stability of some amorphous APIs in the copovidone matrix. To overcome these issues, it's crucial to accurately assess the solubility and miscibility of the crystalline drug within the polymer, which is essential for developing a stable polymer/API amorphous system. This study utilized Plasdone^™ S630 Ultra copovidone, a widely used solubility enhancer, to solubilize the poorly water-soluble active pharmaceutical ingredient (API) nifedipine. Several amorphous copovidone/nifedipine extrudates with drug loadings ranging from 10 to 60% w/w were produced via HME at temperatures above nifedipine's melting point to optimize the copovidone/API ratio for stability and homogeneity. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) measurements confirmed that all extrudates were amorphous regardless of drug loading (even at 60% w/w), as indicated by the presence of a single glass transition temperature and broad diffraction patterns. The extrudate stability and homogeneity were evaluated through a heat-cool temperature cycle using a rheometer. At lower drug loads, the extrudates exhibited homogeneity with consistent rheological behavior during the rheological temperature cycles. Medium drug loads led to slight rheological deviations, indicating a heterogeneous structure with API-rich areas. In contrast, high drug loads exhibited larger changes in rheological properties, likely indicating the dynamic formation of API nuclei. Although all extrudates exhibited enhanced solubility compared to their crystalline forms, those with higher drug loading showed poor physical stability, as evidenced by API recrystallization during accelerated-condition storage. In conclusion, this designed rheological study helps formulation scientists determine the optimal polymer/API composition (semi-phase diagram). This diagram and subsequent stability tests reveal that thermodynamic stability depends on the formulation composition, underscoring the importance of determining the API's solubility in the polymer.