Lyophilization: A Primer - Pharmaceutical Technology

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Lyophilization: A Primer
Optimized freeze-drying cycles can offer scientific and business advantages.

Pharmaceutical Technology
Volume 37, Issue 5, pp. 42-45, 93

Freezing. The material is frozen. The rate of freezing, and the final temperature to which the material is lowered, both have a significant impact on the quality of the final product. The rate at which the temperature is lowered affects the structure of the ice matrix, which has an impact on the ease of flow of the sublimated vapor out of the sample. Annealing, a technique of raising and then lowering the temperature of a frozen material, can be used to encourage crystallization or to provoke a more favorable ice structure.

In delicate materials such as proteins, there is a risk of damage from ice crystal growth. In general, the faster the rate of freezing, the larger the ice crystals formed and the greater the risk of damage. A slower freezing cycle will result in smaller crystals that cause less damage, but the resulting structure will cause a greater impediment to the flow of vapor and therefore slow the drying process.

During the freezing stage, it is vital that the material is cooled below its critical temperature (Tcrit) to ensure it is fully frozen. Every formulation has a different Tcrit that is affected by the combination and proportions of the elements within it, such as the solvent, excipients, and the active ingredient. It is vital that the critical temperature is determined for every different formulation. Knowing the Tcrit not only makes it easy to ensure that the Tcrit is achieved during freezing, but also means that energy is not wasted by taking the temperature lower than required. Methods for determining Tcrit are discussed below.

Primary drying. The frozen material is initially dried by sublimation. During primary drying the pressure of the drying chamber is reduced to a very low level, while the temperature is raised slightly to allow the solvents to sublime. Throughout this stage the temperature must be kept below the critical temperature (Tcrit) so that the material does not melt or its structure collapse.

One of the effects of sublimation is cooling of the product, which slows the process of drying. The rate of sublimation can decrease by as much as 13% for each unnecessary 1C decrease in temperature (1). To counter this cooling and provide energy to drive the sublimation process, heat is added through the freeze-dryer shelf. The energy transfer during primary drying must be balanced so that sufficient heat is used to encourage sublimation without risking collapse.

Figure 2: A selection of vials containing the same freeze-dried material. The fill depth of all four vials was identical before processing. The three vials to the right have all undergone serious process defects.
Collapse is the most serious processing defect in freeze drying, resulting in reduced shelf life, reduced stability, decreased product activity, and poor reconstitution (see Figure 2).

Secondary drying. Secondary drying is a desorption process that removes any solvent that is left chemically bound in the material after primary drying. The moisture level at the beginning of this stage may be around 510%, with a final moisture content of typically less than 5%.

Figure 3: A simplified freeze-drying chart, showing the variations in temperature and pressure throughout the lyophilization cycle.
To facilitate the desorption process, the temperature is raised and the pressure reduced to a minimum (see Figure 3). This is the slowest phase of the lyophilization process. Depending on the final moisture level required, it could last several days. Therefore, any increases in efficiency can have a significant impact on manufacturing throughput.


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