Solid State Characterization of Pharmaceuticals: Advanced Analytical Techniques and Processing Approaches

Solid-state characterization (SSC) plays a fundamental role in the development of pharmaceutical products by influencing their stability, manufacturability, and therapeutic performance. This review provides a comprehensive overview of the major analytical techniques used to characterize pharmaceutical solids, with emphasis on polymorphs, amorphous solid dispersions (ASDs), and protein-based solid formulations. The principles, applications, advantages, and limitations of differential scanning calorimetry (DSC/mDSC), X-ray powder diffraction (XRPD), solid-state Fourier transform infrared spectroscopy (ssFTIR), Raman spectroscopy, solid-state nuclear magnetic resonance (ssNMR), and solid-state hydrogen–deuterium exchange mass spectrometry (ssHDX-MS) are critically discussed. In addition, the review examines the role of drying technologies, including lyophilization, spray drying, and spray freeze drying, in preserving the structural integrity and stability of pharmaceutical solids. The comparative evaluation demonstrates that no single analytical technique can comprehensively characterize complex solid-state systems; instead, complementary techniques are required to evaluate crystallinity, molecular interactions, thermal behaviour, protein conformation, and long-term stability. This review further discusses the importance of selecting analytical methods according to formulation type and characterization objectives, particularly for ASDs and protein formulations where subtle structural changes significantly influence product quality. Overall, integrating complementary solid-state analytical techniques with Quality by Design (QbD) principles provides a robust framework for formulation development, process optimization, regulatory compliance, and the development of stable, safe, and high-quality pharmaceutical products.