Raffinose Crystallization During Freeze-Drying and Its Impact on Recovery of Protein Activity

No HeadingPurpose. To study i) phase transitions in raffinose solution in the frozen state and during freeze-drying and ii) evaluate the impact of raffinose crystallization on the recovery of protein activity in reconstituted lyophiles.Methods. X-ray powder diffractometry (XRD) and differential scanning calorimetry (DSC) were used to study the frozen aqueous solutions of raffinose pentahydrate. Phase transitions during primary and secondary drying were monitored by simulating the entire freeze-drying process, in situ, in the sample chamber of the diffractometer. The activity of lactate dehydrogenase (LDH) in reconstituted lyophiles was determined spectrophotometrically.Results. Raffinose formed a kinetically stable amorphous freeze-concentrated phase when aqueous solutions were frozen at different cooling rates. When these solutions were subjected to primary drying without annealing, raffinose remained amorphous. Raffinose crystallized as the pentahydrate when the solutions were annealed at a shelf temperature of –10°C. Primary drying of these annealed systems resulted in the dehydration of raffinose pentahydrate to an amorphous phase. The phase separation of the protein from the amorphous raffinose in these two systems during freeze-drying resulted in a significant reduction in the recovery of LDH activity, even though the lyophile was amorphous.Conclusions. Annealing of frozen aqueous raffinose solutions can result in solute crystallization, possibly as the pentahydrate. The crystalline pentahydrate dehydrates during primary drying to yield an amorphous lyophile. Raffinose crystallization during freeze-drying is accompanied by a significant loss of protein activity.     

Effect of Product Temperature During Primary Drying on the Long-Term Stability of Lyophilized Proteins

Our objective was to investigate the effect of performing primary drying at product temperatures below and above Tg′ (glass transition temperature of the freeze-concentrated phase) on the long-term stability of lyophilized proteins. Two protective media differing in the nature of the bulking agent used (amorphous or crystalline) were selected. Several lyophilization cycles were performed by using various combinations of shelf temperature and chamber pressure to obtain different values of product temperature during primary drying. The antigenic activity of the proteins was measured after lyophilization and after 6 months of storage at 4°C and 25°C. After 6 months of storage and regardless of the protective medium, the losses of antigenic activity of both toxins increased from 0% when primary drying was performed at a product temperature lower than Tg′ and to 25% when the product temperature was higher than Tg′. The use of partially crystalline systems makes it possible to withstand high primary drying temperatures (above Tg′). However, the shelf life of lyophilized proteins may be decreased when the amorphous phase including the protein and the stabilizing molecule changes to the viscous state.     

Physical Stability and Protein Stability of Freeze-Dried Cakes During Storage at Elevated Temperatures

The relationship between physical stability of freeze-dried cakes and protein stability during storage was studied using -galactosidase as a model protein and inositol as an excipient. Amorphous samples freeze-dried from solutions containing the enzyme and various concentrations of inositol in sodium phosphate buffer (50 mM, pH 7.4) were stored for 7 days over P₂O₅ at 40 to 70°C. Structural collapse and inositol crystallization were observed in some of the samples, depending on the formulation and storage temperature. The physical stability of freeze-dried samples was also studied by differential scanning calorimeter (DSC). Inositol showed a protein-stabilizing effect when its amorphous form was retained during storage, regardless of structural collapse. However, crystallization of inositol during storage removed its stabilizing effect. Addition of water-soluble polymers such as dextran, Ficoll and carboxymethyl cellulose sodium salt (CMC-Na) preserved activity of the enzyme by preventing inositol crystallization.     

Effect of pH, Counter Ion, and Phosphate Concentration on the Glass Transition Temperature of Freeze-Dried Sugar-Phosphate Mixtures

PURPOSE: The aim of the present work is to study the interaction of phosphate salts with trehalose and sucrose in freeze-dried matrices, particularly the effect of the salts on the glass transition temperature (Tg) of the sugars. METHODS: Freeze-dried trehalose and sucrose systems containing different amounts of sodium or potassium phosphate were analyzed by differential scanning calorimetry to determine the Tg and by Fourier-transform infrared spectroscopy (FTIR) analysis to evaluate the strength of the interaction between sugars and phosphate ions. RESULTS: Sucrose-phosphate mixtures show an increase in Tg up to 40 degrees C in a broad pH range (4-9) compared to that of pure sucrose. Sucrose-phosphate mixtures exhibit a higher Tg than pure sucrose while retaining higher water contents. Trehalose-phosphate mixtures (having a Tg of 135 degrees C at a pH of 8.8) are a better option than pure trehalose for preservation of labile materials. The -OH stretching of the sugars in the presence of phosphates decreases with increase in pH, indicating an increase in the sugar-phosphate interaction. CONCLUSIONS: Sugar-phosphate mixtures exhibit several interesting features that make them useful for lyophilization of labile molecules; Tg values much higher than those observed for the pure sugars can be obtained upon the addition of phosphate.     

Non-Isothermal and Isothermal Crystallization of Sucrose from the Amorphous State

The crystallization of a model compound, sucrose, from the amorphous solid state has been studied non-isothermally using differential scanning calorimetry to determine crystallization temperature, Tc, and isothermally at 30°C by subjecting samples to 32.4% relative humidity and gravimetrically monitoring water vapor uptake and subsequent loss with time due to crystallization. From the measurement of glass transition temperature, Tg, and melting temperature, Tm, for sucrose alone and in the presence of absorbed water it was possible to predict Tc and thus to directly relate the plasticizing effects of water to its tendency to promote crystallization. Colyo-philization of sucrose with lactose, trehalose, and raffinose, all having Tg values greater than that of sucrose, increased Tc significantly, even at levels as low as 1 – 10% w/w. In the isothermal studies the time required for crystallization to commence, due to the plasticizing effects of water, i.e., the induction time, assumed to be mostly affected by rates of nucleation, was greatly increased by the presence of the additives at these low levels, with raffinose producing a greater effect than lactose and trehalose. Similarly, these additives reduced the rate of water loss, i.e., the rate of crystal growth, but now no significant differences were noted between the three additives. The possible relationships of nucleation and crystal growth and the effects of additives on molecular mobility are discussed.     

Effect of Freezing Rate on the Stability of Liposomes During Freeze-Drying and Rehydration

Purpose. In the present study we examined the effect of the freezing protocol on carboxyfluorescein (CF) retention in liposomes after freeze-drying and rehydration. Methods. Liposomes were frozen slowly at 0.5°C/min, or quickly by submerging the samples in boiling nitrogen before freeze-drying. The thermal behaviour of the frozen dispersions was analysed by Modulated Temperature Differential Scanning Calorimetry (MTDSC). The dried cakes were analysed by SEM, MTDSC and FTIR. The % encapsulated CF of the (re)hydrated liposomes was determined by fluorimetry after GPC, their vesicle size was measured by the Dynamic Light scattering Technique and their bilayer transition was studied by DSC. Results. Slow freezing resulted in a markedly higher CF retention after freeze-drying and rehydration as compared to quick freezing. The effect of the freezing rate depended on the lipid composition and was most pronounced for rigid liposomes. The damage caused by quick freezing did not occur after a freezing/thawing cycle. The freezing protocol did not influence the interaction between the phospholipids and the lyoprotectants (sucrose, trehalose or glucose) in the freeze-dried state. However, analysis by DSC of dipalmitoylphosphatidylcholine (DPPC): dipalmitoylphosphatidylglycerol (DPPG) =10:1 and DPPC liposome dispersions showed that the freezing protocol affected the bilayer melting characteristics of these liposomes after freeze-drying and rehydration. Conclusions. A proper design of the freezing protocol is essential to achieve optimal stability of rigid liposomes during a freeze-drying and rehydration cycle.     

Protein Stability During Freezing: Separation of Stresses and Mechanisms of Protein Stabilization

Although proteins are often frozen during processing or freeze-dried after formulation to improve their stability, they can undergo degradation leading to losses in biological activity during the process. During freezing, the physical environment of a protein changes dramatically leading to the development of stresses that impact protein stability. Low temperature, freeze-concentration, and ice formation are the three chief stresses resulting during cooling and freezing. Because of the increase in solute concentrations, freeze-concentration could also facilitate second order reactions, crystallization of buffer or non-buffer components, phase separation, and redistribution of solutes. An understanding of these stresses is critical to the determination of when during freezing a protein suffers degradation and therefore important in the design of stabilizer systems. With the exception of a few studies, the relative contribution of various stresses to the instability of frozen proteins has not been addressed in the freeze-drying literature. The purpose of this review is to describe the various stages of freezing and examine the consequences of the various stresses developing during freezing on protein stability and to assess their relative contribution to the destabilization process. The ongoing debate on thermodynamic versus kinetic mechanisms of stabilization in frozen environments and the current state of knowledge concerning those mechanisms are also reviewed in this publication. An understanding of the relative contributions of freezing stresses coupled with the knowledge of cryoprotection mechanisms is central to the development of more rational formulation and process design of stable lyophilized proteins.     

Stabilizing Effect of Four Types of Disaccharide on the Enzymatic Activity of Freeze-dried Lactate Dehydrogenase: Step by Step Evaluation from Freezing to Storage

Purpose In order to understand the stabilizing effects of disaccharides on freeze-dried proteins, the enzymatic activity of lactate dehydrogenase (LDH) formulations containing four types of disaccharide (trehalose, sucrose, maltose, and lactose) at two relative humidity (RH) levels (about 0 and 32.8%) was investigated after three processes: freeze-thawing, freeze-drying, and storage at three temperatures (20, 40, and 60°C) above and/or below the glass transition temperature (T _g). Materials and Methods The enzymatic activity was determined from the absorbance at 340 nm, and T _g of the samples was investigated by differential scanning calorimetry. Results At each RH condition, T _g values of sucrose formulations were lower than those of other formulations. Although effects of the disaccharides on the process stability of LDH were comparable, storage stability was dependent on the type of disaccharide. All the formulations were destabilized significantly during storage at temperature above T _g. During storage at temperature below T _g, the LDH activity decreased with increases in the storage temperature and moisture. Maltose and lactose formulations showed significant destabilization with the change of color to browning. Conclusions Taking the storage stability of freeze-dried proteins under the various conditions (temperature and RH) into consideration, trehalose is better suited as the stabilizer than other disaccharides.     

药品冷冻干燥过程中的玻璃化作用

叙述了玻璃化药品的特点、玻璃化转变温度以及实现药品玻璃化的方法;分析了玻璃化对冷冻干燥过程中药品质量及稳定性的影响。     

Effect of Controlled Ice Nucleation on Stability of Lactate Dehydrogenase During Freeze-Drying

Several controlled ice nucleation techniques have been developed to increase the efficiency of the freeze-drying process as well as to improve the quality of pharmaceutical products. Owing to the reduction in ice surface area, these techniques have the potential to reduce the degradation of proteins labile during freezing. The objective of this study was to evaluate the effect of ice nucleation temperature on the in-process stability of lactate dehydrogenase (LDH). LDH in potassium phosphate buffer was nucleated at ?4°C, ?8°C, and ?12°C using ControLyo? or allowed to nucleate spontaneously. Both the enzymatic activity and tetramer recovery after freeze-thawing linearly correlated with product ice nucleation temperature (n = 24). Controlled nucleation also significantly improved batch homogeneity as reflected by reduced inter-vial variation in activity and tetramer recovery. With the correlation established in the laboratory, the degradation of protein in manufacturing arising from ice nucleation temperature differences can be quantitatively predicted. The results show that controlled nucleation reduced the degradation of LDH during the freezing process, but this does not necessarily translate to vastly superior stability during the entire freeze-drying process. The capability of improving batch homogeneity provides potential advantages in scaling-up from lab to manufacturing scale.     

Evaluation of the Physical Stability of Freeze-Dried Sucrose-Containing Formulations by Differential Scanning Calorimetry

Freeze-dried samples of sucrose with buffer salts, amino acids, or dextran have been analyzed with differential scanning calorimetry (DSC) to evaluate the use of DSC thermograms in predicting the physical storage stability. The glass transition temperature, T _g, of the amorphous cake, crystallization, and melting of sucrose are observed with DSC. T _g appeared to be an important characteristic of the physical stability of the amorphous freeze-dried cake. A storage temperature above T _g results in collapse or shrinkage of the cake, which for a sucrose-based formulation, may be accompanied by crystallization of the sucrose. The T _g of the amorphous sucrose is influenced by other components present in the cake. Dextran-40 raised T _g, while the addition of glycine to the formulation lowered T _g. The residual moisture content strongly influences T _g, since water acts as a plasticizer of the system; the higher the moisture content, the lower the T _g and the less physically stable the freeze-dried cake. Crystallization of amorphous sucrose is shown to be inhibited by high molecular weight components or ionic compounds. DSC analysis of freeze-dried cakes proved to be a powerful tool in formulation studies.     

Effect of Moisture on the Stability of a Lyophilized Humanized Monoclonal Antibody Formulation

Purpose. To determine the effect of moisture and the role of the glass transition temperature (T_g) on the stability of a high concentration, lyophilized, monoclonal antibody. Methods. A humanized monoclonal antibody was lyophilized in a sucrose/histidine/polysorbate 20 formulation. Residual moistures were from 1 to 8%. T_g values were measured by modulated DSC. Vials were stored at temperatures from 5 to 50°C for 6 or 12 months. Aggregation was monitored by size exclusion chromatography and Asp isomerization by hydrophobic interaction chromatography. Changes in secondary structure were monitored by Fourier transform infrared (FTIR). Results. T_g values varied from 80°C at 1% moisture to 25°C at 8% moisture. There was no cake collapse and were no differences in the secondary structure by FTIR. All formulations were stable at 5°C. High moisture cakes had higher aggregation rates than drier samples if stored above their T_g values. Intermediate moisture vials were more stable to aggregation than dry vials. High moisture samples had increased rates of Asp isomerization at elevated temperatures both above and below their T_g values. Chemical and physical degradation pathways followed Arrhenius kinetics during storage in the glassy state. Only Asp isomerization followed the Arrhenius model above the T_g value. Both chemical and physical stability at T T_g were fitted to Williams-Landel-Ferry (WLF) kinetics. The WLF constants were dependent on the nature of the degradation system and were not characteristic of the solid system. Conclusion. High moisture levels decreased chemical stability of the formulation regardless of whether the protein was in a glassy or rubbery state. In contrast, physical stability was not compromised, and may even be enhanced, by increasing residual moisture if storage is below the T_g value.     

The Relationship Between Protein Aggregation and Molecular Mobility Below the Glass Transition Temperature of Lyophilized Formulations Containing a Monoclonal Antibody

Purpose. To find out if the physical instability of a lyophilized dosage form is related to molecular mobility below the glass transition temperature. Further, to explore if the stability data generated at temperatures below the glass transition temperature can be used to predict the stability of a lyophilized solid under recommended storage conditions. Methods. The temperature dependence of relaxation time constant, , was obtained for sucrose and trehalose formulations of the monoclonal antibody (5 mg protein/vial) from enthalpy relaxation studies using differential scanning calorimetry. The non-exponentiality parameter, , in the relaxation behavior was also obtained using dielectric relaxation spectroscopy. Results. For both sucrose and trehalose formulations, the variation in with temperature could be fitted Vogel-Tammann-Fulcher (VTF) equation. The two formulations exhibited difference sensitivities to temperature. Sucrose formulation was more fragile and exhibited a stronger non-Arrhenius behavior compared to trehalose formulation below glass transition. Both formulations exhibited <2% aggregation at t values <10, where t is the time of storage. Conclusions. Since the relaxation times for sucrose and trehalose formulations at 5°C are on the order of 10⁸ and 10⁶ hrs, it is likely that both formulations would undergo very little (<2%) aggregation in a practical time scale under refrigerated conditions.     

Effects of Sucrose and Trehalose on the Preservation of the Native Structure of Spray-Dried Lysozyme

Purpose. To investigate the effects of sucrose, trehalose, sucrose/dextran mixtures, and sucrose/trehalose mixtures on the preservation of the native structure of spray-dried lysozyme in the solid state. Methods. The intensity of the -helical band and the melting enthalpies (H_m ) of spray-dried lysozyme in the dried form and in aqueous solution were obtained using second derivative FTIR and differential scanning calorimetry (DSC) respectively. Results. The intensity of the -helical band and the H m of spray-dried lysozyme obtained were linearly correlated and both suggest that the stabilization of lysozyme in the dried form was excipient concentration-dependent with a close to maximum stabilization being conferred by sucrose or trehalose at a mass ratio 1–2 (sugar:enzyme). Sucrose appeared to be more effective than trehalose on a weight by weight basis whilst stabilizing effects of dextran/sucrose or trehalose/sucrose mixtures were found to be additive. Conclusion. Dehydration during spray drying was considered the main stress to the denaturation of lysozyme. A major effect of the sugars in protecting lysozyme against dehydration was attributable to hydrogen bonding between the sugar and protein molecules, which lead to an increase in the change in the negative value of the free energy between native and denatured states.     

Determination of Glass Transition Temperature and in Situ Study of the Plasticizing Effect of Water by Inverse Gas Chromatography

Purpose. To use an inverse gas chromatographic (IGC) method to determine the glass transition temperature (Tg) of some amorphous pharmaceuticals and to extend this technique for the in situ study of the plasticizing effect of water on these materials. Methods. Amorphous sucrose and colyophilized sucrose-PVP mixtures were the model compounds. Both IGC and differential scanning calorimetry (DSC) were used to determine their Tg. By controlling the water vapor pressure in the IGC sample column, it was possible to determine the Tg of plasticized amorphous phases. Under identical temperatures and vapor pressures, the water uptake was independently quantified in an automated water sorption apparatus. Results. The Tg of the dry phases, determined by IGC and by DSC, were in very good agreement. With an increase in the environmental relative humidity (RH), there was a progressive decrease in Tg as a result of the plasticizing effect of water. Because the water uptake was independently quantified, it was possible to use the Gordon-Taylor equation to predict the Tg values of the plasticized materials. The predicted values were in very good agreement with those determined experimentally using IGC. A unique advantage of this technique is that it provides complete control over the sample environment and is thus ideally suited for the characterization of highly reactive amorphous phases. Conclusions. An IGC method was used (a) to determine the glass transition temperature of amorphous pharmaceuticals and (b) to quantify the plasticizing effect of water on multicomponent systems.     

Investigations into the Stabilization of Drugs by Sugar Glasses: III. The Influence of Various High-pH Buffers

Purpose. To study the effect of the high-pH buffers ammediol, borax, CHES, TRIS, and Tricine on the glass transition temperature of the freeze concentrated fraction (Tg) of trehalose/buffer and inulin/buffer solutions at pH 6.0 and pH 9.8. Also, the glass transition temperature (Tg) of sugar glasses obtained after freeze drying of these solutions was elucidated. Additionally, the effect occurring during the freezing process on the pH of the various buffers was investigated. Furthermore, the stability of alkaline phosphatase (AP) incorporated in these sugar glasses prepared from solutions at pH 9.8 was evaluated. Methods. The Tg and Tg were measured using differential scanning calorimetry (DSC), and the change of pH during freezing was estimated by using an indicator solution added to the respective solutions. The enzymatic activity of AP after freeze drying and storage at 60°C was evaluated by an enzymatic activity assay. Results. It was found that the Tg and Tg of the samples investigated are strongly influenced by the presence of the buffer. On freezing, only minor changes of the pH were observed. The samples with the lowest Tg and the samples containing buffers that formed complexes with the sugars showed the poorest stability of the AP. Conclusions. The stabilizing capacities of sugars that are currently recognized as excellent stabilizers for proteins during drying and storage can be completely lost if certain high-pH buffers such as ammediol, borax, and TRIS are used at high concentrations. Loss of stabilizing capacities can be ascribed to strong depression of the Tg and Tg or to complex formation.     

Stability of the Thrombolytic Protein Fibrolase: Effect of Temperature and pH on Activity and Conformation

The effect of temperature and pH on the activity and conformation of the thrombolytic protein fibrolase was examined. Fibrolase maintained proteolytic activity over 10 days at room temperature (22°C). At 37°C, greater than 50% of the proteolytic activity was lost within 2 days and no activity remained after 10 days. Circular dichroism (CD) spectra at elevated temperatures showed that alphahelical structure was lost in a cooperative transition (T _m of 50°C at pH 8). Structural changes were detected by NMR prior to unfolding which were not observable by CD, and the T _m determined by NMR was 46°C at pD 8. The effect of pH on the proteolytic activity and structure of fibrolase was examined over the pH range from 1 to 10. Activity was maintained at neutral to alkaline pH values from pH 6.5 to pH 10.0 but decreased substantially in acidic media. While CD spectra indicated little variation in secondary structure over the pH range 5 to 9, significant differences were noted at pH 2 to 3. The melting temperature of fibrolase decreased to 43°C at pH 5. Protein concentrations determined over the pH range 1 to 10 showed an apparent solubility minimum at pH 5.0, which did not correspond to the isoelectric point of 6.5. Explanations for these observations are proposed.     

Measurement of the Kinetics of Protein Unfolding in Viscous Systems and Implications for Protein Stability in Freeze-Drying

Purpose The aim of the study is to determine the degree of coupling between protein unfolding rate and system viscosity at low temperatures in systems relevant to freeze-drying.Methods The cold denaturation of both phosphoglycerate kinase (PGK) and β-lactoglobulin were chosen as models for the protein unfolding kinetics study. The system viscosity was enhanced by adding stabilizers (such as sucrose), and denaturant (guanidine hydrochloride or urea) was added to balance the stabilizing effect of sucrose to maintain the cold denaturation temperature roughly constant. The protein unfolding kinetics were studied by both temperature-controlled tryptophan emission fluorescence spectroscopy and isothermal high-sensitivity modulated differential scanning calorimetry (MDSC) (T_zero). Viscometers were used to determine the system viscosity. To verify the predictions of structure based on protein unfolding dynamics, protein formulations were freeze-dried above the glass transition temperatures, and the protein structures in dry products were determined by fluorescence spectroscopy of reconstituted solids by extrapolation of the solution data to the time of reconstitution.Results Empirical equations describing the effect of sucrose and denaturant (urea and guanidine hydrochloride) on protein cold denaturation were developed based on DSC observations [X. C. Tang and M. J. Pikal. The Effects of Stabilizers and Denaturants on the Cold Denaturation Temperature of Proteins and Implications for Freeze-Drying. Pharm. Res. Submitted (2004)]. It was found that protein cold denaturation temperature can be maintained constant in system of increasing sucrose concentration by simultaneous addition of denaturants (urea and guanidine hydrochloride) using the empirical equations as a guide. System viscosities were found to increase dramatically with increasing sucrose concentration and decreasing temperature. The rate constants of protein unfolding (or the half-life of unfolding) below the cold denaturation temperature were determined by fitting the time dependence of either fluorescence spectroscopy peak position shift or DSC heat capacity increase to a first-order reversible kinetic model. The half-life of unfolding did slow considerably as system viscosity increased. The half-life of PGK unfolding, which was only 3.5 min in dilute buffer solution at −10°C, was found to be about 200 min in 37% sucrose at the same temperature. Kinetics of protein unfolding are identical as measured by tryptophan fluorescence emission spectroscopy and by high-sensitivity modulated DSC. The coupling between protein unfolding kinetics and system viscosity for both proteins was significant with a stronger coupling with PGK than with β-lactoglobulin. The half-lives of PGK and β-lactoglobulin unfolding are estimated to be 5.5 × 10¹¹ and 2.2 years, respectively, even when they are freeze-dried in sucrose formulations 20°C above T_g′. Thus, freeze-drying below T_g′ should not be necessary to preserve the native conformation. In support of this conclusion, native PGK was obtained after the freeze-drying of PGK at a temperature more than 60°C above the system T_g′ in a thermodynamically unstable system during freeze-drying.Conclusions Protein unfolding kinetics is highly coupled with system viscosity in high viscosity systems, and the coupling coefficients are protein dependent. Protein unfolding is very slow on the time scale of freeze-drying, even when the system is freeze-dried well above T_g′. Thus, it is not always necessary to freeze-dry protein formulations at temperature below T_g′ to avoid protein unfolding. That is, protein formulations could be freeze-dried at product temperature far above the T_g′, thereby allowing much shorter freeze-drying cycle times, with dry cake structure being maintained by the simultaneous use of a bulking agent and a disaccharide stabilizer.     

蔗糖铁注射液浊点测定方法的改进

目的：建立一种客观评价蔗糖铁注射液浊点的测定方法。方法：在一定浓度的样品溶液中，持续滴加0.1 mol·L^-1的盐酸溶液，边滴加边搅拌，用浊度仪测定溶液的浊度，当溶液的浊度在0.2～0.8 NTU之间时，停止搅拌，记录溶液pH值，即为样品的浊点。结果：用仪器法测定蔗糖铁注射液的浊点，RSD<1.0%。结论：该方法能够有效地避免主观因素对浊点测定的干扰，且准确可靠。     

The Importance of Understanding the Freezing Step and Its Impact on Freeze-Drying Process Performance

The freeze-drying process is a combination of 2 equally important processes, freezing, and drying. In the past, the effort was mainly focused on optimizing the drying process without considering the possible effects of the freezing step. During freezing, a solution undergoes several physical changes, including a supercooling state. The degree of supercooling of a solution dictates the ice habit (size, number, and morphology) during freezing, which impacts the subsequent drying process, such as the resistance to water vapor flow. Therefore, heterogeneous degree of supercooling leads to heterogeneous ice habits and, in turn, to heterogeneous drying behavior. This poses significant challenges during freeze-drying process development, optimization, and scale up. Hence, controlling the degree of supercooling significantly improves freeze-drying process design. The aim of the current review is to gather existing information on the physicochemical phenomena involved in the freezing process and how these phenomena impact the subsequent drying step of the freeze-drying process. In addition, modification of the freezing process and different techniques used to actively control the degree of supercooling during freezing will be reviewed and discussed. Their impact on freeze-drying process performance will be also addressed.