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Freeze-drying (lyophilization) is a comparatively expensive and time-consuming process and a great deal of emphasis is placed upon improving efficiency and minimizing costs by optimization of the conditions used for freeze-drying.
Optimizing Primary Drying
Increasing the product temperature during primary drying, in which the bulk or “free” solvent is removed, increases the rate of sublimation significantly. However, depending on the properties of the constituent components, most formulations will undergo frozen state transitions that lead to increased solute mobility which typically prevents the solute phase from maintaining a cohesive structure upon removal of the solvent. Degradation of the dried structure usually compromises the designated criteria for final product quality and therefore the temperature at which these transitional events take place may represent a limitation of the permissible product temperature during primary drying. Before attempting to optimize the conditions for freeze-drying it is important to establish good understanding of the frozen state behavior of the formulation. The presence of multiple excipients and buffering agents in addition to the active ingredient can make this complex, especially when both crystalline and amorphous components are present.
Crystalline Products and Eutectic Point
Solutes that adopt a crystalline conformation will typically form a separate crystalline phase, exhibiting behavior independent of other structures. When crystalline structures within the frozen matrix become sufficiently energetic they melt to form a eutectic liquid. This transition, known as the eutectic melting point (Teu), will impact the structural integrity of the solute phase and therefore it is important to establish the Teu of crystal structures contained within the frozen matrix.
Solutes that do not adopt a crystalline conformation will typically form an amalgamated “amorphous mass” (although phase separation has been observed) which will exhibit a shift from a rigid glass-state to a more mobile rubber-state in a phenomenon known as the glass transition (Tg’). Knowledge of the temperature at which this transition occurs is important as the increased mobility of the amorphous phase directly impacts the structural integrity of the solute phase leading to the possibility of viscous flow (known as collapse) upon removal of the solvent.
Frozen State Analysis
Biopharma’s Lyotherm is an integrated differential thermal analysis (DTA) and electrical impedance analysis (Zsinψ) instrument which can be used for the characterization of liquid formulations intended for freeze-drying. Differential Thermal analysis reveals thermal transitions (endo- or exo-thermic events) taking place within the formulation including the glass transition (Tg’), crystallization, and melting events, which can occur in crystalline solutes as well as in the frozen solvent phase. Electrical impedance is derived by a function of resistance, inductance and capacitance and represents a measure of the ‘molecular mobility’ of the sample. The technique is extremely effective at detecting significant changes to material structure such as crystallization, eutectic melting point and glass transitions, but is also able to detect very subtle softening and/or stabilization events which may not be detected using other methods yet may still have a significant impact on the freeze-drying properties of the material. Studies conducted at Biopharma Technology have revealed that in many cases impedance analysis is able to detect subtle transitions within the frozen materials which were not observed in either DTA or DSC analysis of the same material.
Collapse Temperature and Freeze Drying Microscopy
In some cases transitions within the frozen material do not necessarily coincide with the physical degradation of dried structure. It has been observed that the temperature at which the solute phase loses structural integrity, designated the “collapse temperature” (Tc), can exceed the measured Tg’ by as much as 10°C. In such cases, the Tg’ does not represent the limit of the permissible product temperature and therefore using it as a basis for optimization would likely result in less efficient, conservative freeze-drying. For this reason, other techniques such as freeze-drying microscopy (FDM) are used to build a more complete picture of dried structure formation.
The Lyostat freeze-drying microscope can be used to identify the temperature at which collapse takes place. A specialized cryostage allows the freeze-drying of a small sample of the formulation to be observed and recorded. By controlling the temperature of the frozen material the user can accurately determine the Tc or Teu of the formulation.
It has been shown that in some cases where multiple solute phases are present and/ or phase separation is observed, it is possible for one phase to provide a stable “scaffold” onto which the other phases could collapse. This phenomenon, known as micro-collapse, may not always be detectable using FDM but could still adversely affect the quality of the product. In some cases cycles optimized using the measured collapse temperature of the material may still exhibit poor long-term stability and loss of activity, even when the critical temperature of the formulation has not been exceeded and the criteria for residual moisture content and aesthetic qualities of the dried cake are met. This may be an indication that increases in solute mobility associated with the glass transition or other softening events, undetectable by freeze-drying microscopy (FDM), are affecting quality of the product.
The critical temperature obtained from visual assessment is arguably more representative of the behavior of the formulation during primary drying and can provide a more relevant benchmark for the limitation of product temperature. Nevertheless it is important to use a range of analytical techniques to provide a full picture of the frozen state behavior of the formulation, to ensure that the freeze drying process is cost-efficient and effective.
Over the past 15 years Biopharma has worked with over 1000 different products. Biopharma assists customers with all aspects of freeze drying:
- Product characterization, including Lyostat and Lyotherm analysis
- Formulation and cycle development
- Scale-up and production
- Training courses
- Analytical instruments, including installation, qualification and operator training
For more information about Lyostat and Lyotherm contact Mervyn Middleton, +44 1962 841092, firstname.lastname@example.org
Tags: collapse temperature, critical temperatures, DSC, eutectic point, glass transition, Lyostat, lyotherm