Pilot-Scale Lyophilization Characterization
This section addresses the development of a design space at the pilot scale for lyophilization using a DOE approach.11,12 All pilot-scale lyophilization runs were performed using a mixed load of active and placebo material in a BOC Edwards (IMA,
Tonawanda, NY) lyophilizer with 2 m2 shelf area. Thermocouples were used to monitor the temperature profile of the material during the cycle. For selected runs,
the sublimation rate was determined by sealing selected vials with stoppers during the course of primary drying and measuring
the weight loss as a function of time.
Table 2. Design of experiments strategy for lyophilization process characterization
Based on the outcome of the risk assessment discussed in the previous section, it was determined that the following parameters
must be characterized: primary drying temperature, secondary drying temperature and time, chamber pressure, and freeze ramp
rate. The approach to the DOE (Table 2) was as follows: 1) Evaluate the main effect of the freeze ramp rate. The first two
experiments examined the effect of varying the freeze ramp rate on product temperature, sublimation rates, and moisture content.
As shown in Table 3, no significant difference in product attributes was found among the different freezing rates. 2) Because
freeze ramp rate did not impact product attributes, this parameter was excluded from subsequent characterization studies and
multivariate characterization was performed on the primary drying temperature, the secondary drying temperature and time,
and the chamber pressure. It should be noted that if a main effect of the freeze ramp rate was seen, this would have been
included as a factor in the multivariate studies. In all runs, primary and secondary drying times were kept the same as the
initial target cycle (Table 1).
Table 3. Freeze ramp rate study and results
Experiments 3–7 provided a half factorial experimental design around primary drying temperature, secondary drying temperature,
and chamber pressure. The results of this multivariate DOE are shown in Table 4. The low and high extremes tested for primary
drying temperature, secondary drying temperature, and chamber pressure were –11 °C to 1 °C, 15 °C to 25 °C, and 70 to 140
Table 4. Results of DOE with primary and secondary drying parameters
Run #5, with elevated temperature and pressure, saw an increase in sublimation rate (+34%) and average product temperature
(+15%) relative to the target cycle. In contrast, Run #4, with both primary drying parameters lowered, saw a decrease in sublimation
rate (–25%) and average product temperature (–16%), compared with the target cycle. Both Run #3 (–, +, +) and Run #6 (+, –,
+) exhibited similar average product temperatures and comparable drying rates. All cycles produced a solid off-white-colored
cake with no collapse or meltback. Based on these experiments, an initial design space can be constructed for primary drying
(blue region in Figure 6). The process outputs and product attributes from the DOE were quantitatively assessed using the
statistical analysis program JMP (Version 8, SAS, Cary, NC) and yielded the statistical model shown in Equation 1 (eq 1) below.
The dependence of product temperature during primary drying on shelf temperature and chamber pressure (a largely linear additive
effect with some interaction) is described by eq 1, in which Tp is the product temperature, PDT is the primary drying temperature, and CP is the chamber pressure. For the parameter space outside the initial primary drying design space region shown in Figure 6,
eq 1 can be used to obtain extrapolated values for product temperature and sublimation rates. The red curve in Figure 6 was
obtained by identifying the primary drying temperature and chamber pressure values from eq 1 that yield a product temperature
of –15 to –16 °C (slightly below the collapse temperature for this formulation). The relevant output for secondary drying
is residual moisture, and thus based on results from Table 4. The experimentally demonstrated design space for secondary drying
was a shelf-temperature range of 15–25 °C and a chamber pressure of 70–140 mTorr.
Figure 6. Experimental initial design space for primary drying and schematic of hypothetical process limits
Because the primary drying step is where the product is at greatest risk of impact relative to the process control capability
of the equipment, the subsequent sections will focus on developing the primary drying design space further.
Figure 7. Steps involved in the use of the mechanistic process model to expand the primary design space for lyophilization