Structural attributes of model protein formulations prepared by rapid freeze-drying cycles in a microscale heating stage

Abstract

Downscaled freeze-drying was demonstrated to be a valuable alternative for formulation development and optimization. Although the pore structure is known to exert a major influence on the freeze-drying cycle, little is known about the ones of microscale preparations. This study describes morphology evaluation methods for lysozyme formulations prepared in one microscale processing option and the assessment of fundamental product quality criteria. Scanning electron microscopy (SEM) revealed cooling rate dependent pore size variations at the nucleation site which diminished as the rate increased. Micro-X-ray computed tomography (μ-CT) showed that porosity generally increased in the sample from bottom to top, the pore size fractions shifted toward larger pores in elevated sample levels, and horizontal homogeneity was found throughout each sample with minor deviations in the bottom region. Furthermore, the event of microcollapse could be identified and quantified. Low residual moisture was achieved repeatedly and the procedure did not influence the post freeze-drying bioactivity. This microscale heating stage is a valuable option to reduce overall cycle times and cost, and to prepare freeze-drying formulations with high reproducibility. The mapping tools permit a quick but detailed insight into the structural features resulting from the process environment and processing conditions.

Keywords: Freeze-drying; Lysozyme; Micro-X-ray computed tomography; Microcollapse; Microscale; Morphology; Scanning electron microscopy.