(W1130-08-53) Impact of Lyoprotectant Thermal Properties on Batch Heterogeneity through Various Processing Technologies: Annealing vs. Controlled Ice Nucleation

Purpose: Lyoprotectants are commonly used in lyophilization to control the freezing process. However, spontaneous ice nucleation can result in batch heterogeneity, causing variations in product properties within a batch. To mitigate batch heterogeneity, two adjusted thermal treatments based on the thermal properties of the lyoprotectant - annealing and controlled ice nucleation - are used. However, a comparison of these methods is still lacking. Batch heterogeneity can be identified by observing the variation of primary drying time and pore size distribution of lyophilized cakes. The objectives of this study are to determine the differences between annealing and controlled ice nucleation on pore size and size distribution of cakes made of lyoprotectant, as well as to investigate their efficacy in mitigating batch heterogeneity.

Methods: To prepare the modeling solution, a 5% w/v sucrose solution was mixed in 2 mL cut serum vials with a fill depth of 7 mL. Ice nucleation was induced at a shelf temperature of −10°C, and two post-nucleation cooling rates (5°C/min and 0.2°C/min) were studied using a MicroFD® from Millrock Technology. Annealing at −5°C for 1 hour was used as a comparison. The frozen solutions were subjected to primary drying at shelf temperatures ranging from −35°C to −25°C and 50 mTorr to achieve comparable product temperatures of 39.0±0.5°C. For secondary drying, the shelf was heated at 0.1°C/min at 30°C for 4 hours, and LyoSim® blocks were utilized to minimize edge effects during the process. The pore size and size distribution were measured using low-pressure mercury porosimetry (LPMIP), and the variance of pore size distribution was assessed in the lyophilized cakes. Additionally, X-ray micro computed tomography (XRµCT) was used to measure pore size distribution and pore wall thickness from various locations within each lyophilized cake.

Results: From LPMIP results, lyophilized cakes produced with controlled ice nucleation had a smaller standard error in median pore diameter and a smaller quartile ratio compared to the annealed cakes (Table 1). Post-nucleation/annealing shelf cooling rates had little influence on the standard error of median pore diameter or quartile ratio. XRµCT images showed no significant difference of pore size distribution in the center of lyophilized cakes produced by different freezing conditions. However, the pore size distribution and pore wall thickness at the radial edge of the cake did vary with freezing protocol. The group subjected to ice nucleation control had a smaller median pore size at the edge than in the center of the cake while annealing produced cakes with a median pore diameter at the edge equal to or larger than at the center.

Conclusion: Compared to annealing at -5°C, controlled ice nucleation at -10°C resulted in a more homogenous pore structure of lyophilized cakes. The group subjected to ice nucleation control exhibited a narrower pore size distribution than those subjected to annealing. The width of pore size distribution and the homogeneity of median pore size were unlikely dependent of the post-nucleation/annealing shelf cooling rates. Annealing led to large pores near the cake edge while controlled ice nucleation produced small pores. Overall, controlled ice nucleation was found to be more effective than annealing in reducing batch heterogeneity.

Acknowledgements: Funding from the Dane Kildsig Center for Pharmaceutical Processing Research (CPPR) is greatly appreciated.

Summary of pore distribution (by LPMIP)