Probing Nanoscale Features of Electrospun Amorphous Solid Dispersions Using Advanced Biophysical Analysis

Amorphous solid dispersions (ASDs) can significantly enhance the solubility of poorly soluble active pharmaceutical ingredients (APIs). However, the typical thermodynamic instability of ASDs is a significant barrier to commercial success, and understanding API recrystallization behavior is crucial for enhancing stability. While most techniques do not allow the effective exploration of early-stage crystallization, small-angle neutron scattering (SANS) has shown promise in the study of nanoscale drug domains in polymer-drug composite materials. In this study, aspirin was incorporated in electrospun polycaprolactone (PCL) fibers at loadings from 10-30% w/w. Electron microscopy revealed the fibers to have smooth surfaces and diameters of 2-3 μm. X-ray diffraction and differential scanning calorimetry revealed the presence of crystalline drug at 20 and 30% loadings. Infrared spectroscopy indicated the presence of intermolecular interactions between the drug and the polymer; however, these interactions were insufficient to completely inhibit drug crystallization when the drug was supersaturated. SANS revealed heterogeneous nanoscale structures within the fibers, characterized by concentration-dependent drug-rich domains in the polymeric matrix. Their precise solid-state nature (amorphous versus crystalline) could not be determined by SANS, though X-ray diffraction indicated the presence of crystalline material at higher drug loadings. The SANS data also indicated the growth of drug aggregates after exposure to water. In vitro drug release tests suggested that the size of the drug domains could influence drug release behavior. Overall, this study provides a number of new insights into the crystallization of APIs in electrospun ASDs, which can be used to help guide the future development of more effective medicines.