(M0930-01-01) Graphical Design Space for the Primary Drying Phase of Freeze Drying: Factors Affecting the Dried Product Layer Resistance

Purpose: The objectives of this study were to challenge the assumption in previous work that the resistance of dried product layer, R_p, is independent of process conditions within the design space of a formulation during lyophilization and to gain better understanding of the relationship between R_p and formulation components.

Methods: The graphical design space for primary drying of a formulation is developed based on the first principles of heat and mass transfer and the conservation of energy. A key factor in calculating the design space is R_p. Previous data (limited) suggested that R_p is independent of process conditions. In the present investigation, three formulations containing bovine serum albumin as the model protein (10 mg/mL), were chosen to represent: (a) an amorphous system containing sucrose (3% w/v), (b) a crystalline system containing 3% mannitol, and (c) a mixed system containing sucrose and mannitol at 3% w/v, respectively. The formulations also contained L-histidine (30 mM) at pH 6.0. Formulations were filled into a 10 mL Schott Fiolax vial with a fill volume of 5 mL and partially stoppered. A conservative lyophilization cycle was conducted for each formulation to collect the mass flow rate (MFR) data and individual design spaces were then established using an excel-based primary drying model. The design space for Formulation 1 is provided in Figure 1. A series of lyophilization cycles was then conducted using process conditions that resided within the individual design space (indicated by colored ovals in Figure 1) and the resultant product temperature and resistance of dried product layer (R_p) values were compared between the individual cycles of each formulation.

Results: The data indicated that the R_p was component dependent with the mannitol formulation exhibiting higher R_p values than the sucrose formulation (data not shown). The product resistance values for Formulation 1 ranged from 4 Torr.h.cm².g^-1 to 13 Torr.h.cm².g^-1 for the process conditions examined (Table I) and 9 Torr.h.cm².g^-1 and 12 Torr.h.cm².g^-1 for Formulation 2 (data not shown). Interestingly, when mannitol was retained amorphous, the formulation exhibited a lower R_p, similar to that of the sucrose formulation. For the mixed system (Formulation 3), the R_p range was 7 Torr.h.cm².g^-1 and 9 Torr.h.cm².g^-1 (data not shown). An inverse relationship between sublimation rate and R_p was observed (Table I). The relationship between formulation components and MFR and R_p for Formulations 1 and 2 is shown in Figure 2 when lyophilized under identical process conditions. Shrinkage and microcollapse observed in Formulation 1 presumably contributed to lowering of R_p. Conversely, crystallization of mannitol is hypothesized to facilitate a decrease in the size of the ice porous structure by making the water vapor flow path tortuous, thereby increasing the R_p of mannitol formulations. Within the “acceptable” zone of the individual design space, R_p was dependent on the process condition with more aggressive shelf temperature cycles resulting in lower R_p. Specific Surface Area examination of freeze-dried solids demonstrated an inverse relationship with solids processed under more aggressive conditions providing higher specific surface area. Freeze-dried solids of crystalline formulations consistently exhibited higher surface areas than the amorphous formulations.

Conclusion: The data demonstrated that sucrose exhibited the lowest R_p and the mannitol formulation exhibited the highest resistance. The mixed formulation containing both mannitol and sucrose exhibited intermediate process attributes. Within the same formulation, the R_p showed an inverse correlation with sublimation rates. The data demonstrated that the assumption of constant R_p within the acceptable range of a design space is incorrect. A consistent pattern of decreased resistance under more aggressive process conditions was observed. The lowering of R_p due to shrinkage and microcollapse appears to be unique to sucrose formulations since when lyophilized in TopLyo^TM vials, the mannitol formulations exhibited shrinkage, but the R_p remained the same as when the product was dried in a standard Schott vial. Specific surface area data were in agreement with the R_p and sublimation rate data with the sucrose formulation freeze-dried solids exhibiting lower surface area than that of the mannitol formulation. The SSA data indicated an inverse correlation with the shelf temperature with solids from aggressive cycles exhibiting lower specific surface areas. The data indicate that R_p is dependent of the process conditions, and it is prudent to choose process conditions that provide maximum sublimation rates.

References: 1. Srinivasan, J.M.; Sacha, Gregory, A.; Nail, S.L., “The Graphical Design Space for the Primary Drying Phase of Freeze Drying: Factors Affecting the Dried Product Layer Resistance”, International Journal of Pharmaceutics, 630 (2023) 122417.
2. Bogner, R.; Gong, E.; Kessler, W.; Hinds, M.; Manchanda, A.; Yoon, S.; Liu, H.; Marx, R.; Zhao, J.; Sharma, P.; Bhambhani, A.; Stanbro, J.; Alexeenko, A.; Kazarin, P. “A Software Tool for Lyophilization Primary Drying Process Development and Scale-Up Including Process Heterogeneity, I: Laboratory _Scale Model Testing. AAPS Pharm. Sci. Tech. 22, 274 (2021).
3. Patel, S. M.; Pikal, M. J. “Lyophilization Process Design Space”. J. Pharm. Sci. 102(11), 3883-3887 (2013).
4. Nail, S. L.; Searles, J. A. “Elements of Quality by Design in Development and Scale-Up of Freeze-Dried Parenterals”. Biopharm International. 21(1), 40-45 (2008).
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Figure 1. Design Space and Processing Conditions Evaluated within the Design Space for Formulation 1.


Table I. Process Parameters for Lyophilization Cycles Conducted Within the Design Space of the Formulation 1


Figure 2. A comparison of MFR and Rp for Formulations 1 and 2 when lyophilized using identical process parameters is provided. The sucrose formulation (Formulation 1) exhibited a constant Rp until the end of primary drying when it increased steeply (green line), whereas the Rp for Formulation 2 (mannitol) increased steadily as primary drying progressed (blue line). This trend is reflected in the MFR data which remained relatively constant during sublimation for Formulation 1 (dark blue) but decreased steadily for Formulation 2 (orange line).