Comprehensive Powder Rheological Characterization of Fifteen Lactose-Based Co-Processed and Single-Component Excipients Using FT4 Powder Rheometry and European Pharmacopoeia Methods: A Multi-Parameter Comparative and Correlative Study

Background/Objectives: Co-processed excipients (CPEs) are designed for direct compression through particle engineering, yet comprehensive powder rheological profiles systematically comparing advanced and traditional characterization methods remain limited. This study characterized fifteen lactose-based excipients using European Pharmacopoeia (Ph. Eur.) methods and the complete Freeman FT4 Powder Rheometer measurement suite, establishing a correlation framework linking particle-level attributes to macroscopic flow behavior. Methods: Fifteen excipients were characterized for bulk and tapped density, compressibility index, flow time (Ph. Eur. 2.9.16), and angle of repose (Ph. Eur. 2.9.36). Particle size and shape were measured by dynamic image analysis. FT4 measurements comprised stability and variable flow rate testing, consolidation, aeration, compressibility, permeability, shear cell, and wall friction at three surface roughness. Pearson correlation matrices were computed across all 53 parameters. Results: Classical flow indices classified most CPE as good-to-satisfactory, failing to discriminate materials with fundamentally different dynamic flow profiles. FT4 testing revealed a fourfold range in Basic Flowability Energy (624–2107 mJ), a ninefold range in flow function coefficient (4.3–35.8), and wide aeration sensitivity differences (Aeration Ratio: 1.9–283.7). Strong correlations were identified between Specific Energy and compressibility index (r = 0.85), cohesion and Flow Rate Index (r = 0.79), and Normalized Aeration Sensitivity and pressure drop (r = 0.86). Within-family comparisons (Tablettose 70/80/100, FlowLac 90/100) revealed that particle size distribution breadth is a more critical flow determinant than median size alone. Conclusions: Combining FT4 rheometry with pharmacopoeial testing provides substantially greater discriminating power than either approach alone, enabling rational excipient selection for direct compression formulation.