Using density changes to monitor blending with magnesium stearate by terahertz time-domain spectroscopy

Magnesium stearate (MgSt) is among the most common excipients and the most common lubricant in solid oral products. It is primarily added to tablet formulations to ease ejection during tablet compression. While commonly present in low concentrations, the addition of MgSt substantially affects the final tablet properties. Its impact is further not only concentration dependent but also varies with exposure of the formulation to shear, which worst-case results in over-lubrication. The presented study investigated the applicability of terahertz time-domain spectroscopy (THz-TDS) to monitor the shear-induced blend densification of microcrystalline cellulose blended with MgSt over a range of concentrations (0.3, 0.7, and 1.0 %). The effect of shear was investigated by variation of blending times (5 - 20 min) in a diffusion blender. THz-TDS measurements of the powder blends were acquired in transmission by measuring directly through the mixing container. The refractive index at terahertz frequencies was found to be sufficiently sensitive to resolve the densification of the blend with increased blending times. Thus, THz-TDS blend density measurements can be used as a surrogate parameter to evaluate the total shear exposure of a blend. Considerations regarding implementation are discussed. In the context the approach was integrated with the well-described THz-TDS-based tablet porosity analysis into a unified model to monitor and predict the tensile strength. Including the THz-TDS measurement on the blend allowed for a more accurate description of the tensile strength, reducing the root mean squared error by over 40 % (0.33 MPa). The possibility of monitoring the density changes of a blend non-invasively makes THz-TDS a promising process analytical technology approach for controlling the total shear impact on lubricated blends and tablet quality.