Beyond glass transitions: studying the highly viscous and elastic behavior of frozen protein formulations using low temperature rheology and its potential implications on protein stability.

PURPOSE: A novel application of oscillatory shear rheology was used to directly monitor global phase behavior of protein formulations in the frozen state and study its correlation with physical instability of frozen protein formulations. METHODS: Oscillatory rheology was used to measure changes in rheological parameters and to identify mechanical softening temperature (Ts*) and related properties of an IgG2 mAb formulation. Rheological measurements were compared to DSC/MDSC. Physical stability of IgG2 formulations was monitored by SE-HPLC. RESULTS: Rheological parameters and Ts* of an IgG2 formulation were sensitive to physical/morphological phase changes during freezing and thawing. Ts* of the frozen formulation was a function of concentration of protein and excipient. Complex modulus, G*, and phase angle, δ, for IgG2 at 70 mg/mL in a sucrose-containing formulation showed the system was not completely frozen at -10°C, which correlated to stability data consistent with ice-induced protein aggregation. CONCLUSIONS: We report the first application of oscillatory shear rheology to study phase behavior of IgG2 in a sucrose-containing formulation and its correspondence with physical stability not explained by glass transition (Tg'). We provide a mechanism and data suggesting that protein instability occurs at the ice/water interface.

Micrometer-scale particle sizing by laser diffraction: critical impact of the imaginary component of refractive index.

PURPOSE: This study evaluated the effect of the imaginary component of the refractive index on laser diffraction particle size data for pharmaceutical samples. METHODS: Excipient particles 1-5 microm in diameter (irregular morphology) were measured by laser diffraction. Optical parameters were obtained and verified based on comparison of calculated vs. actual particle volume fraction. RESULTS: Inappropriate imaginary components of the refractive index can lead to inaccurate results, including false peaks in the size distribution. For laser diffraction measurements, obtaining appropriate or "effective" imaginary components of the refractive index was not always straightforward. When the recommended criteria such as the concentration match and the fit of the scattering data gave similar results for very different calculated size distributions, a supplemental technique, microscopy with image analysis, was used to decide between the alternatives. Use of effective optical parameters produced a good match between laser diffraction data and microscopy/image analysis data. CONCLUSIONS: The imaginary component of the refractive index can have a major impact on particle size results calculated from laser diffraction data. When performed properly, laser diffraction and microscopy with image analysis can yield comparable results.