Formulation components--their freezing and drying. The profitability of a lyophilized product begins with the development of the formulation, not at the start of manufacturing.
NOTE: This is the first part of a two-part article. Part 2 is also available for on-line viewing.
A formulation can be defined as a liquid medium in which one or more active components (chemical or biological) remain in a stable environment and maintain specified potency limits for some period of time. For example, a vaccine formulation may still be effective and safe to use after several days of storage at 4°C. After about a week of storage, it must be discarded. However, if the kinetic clock for degradation of the vaccine could be slowed so that 1 second was extended to 1 hour, then the useful life of the formulation would be increased to almost 20 years.
Lyophilization stabilizes the formulation by slowing the kinetic clock of the degradation process. It alters the clock by removing the solvent component or components to levels that no longer support chemical reactions or biological growth. This removal is accomplished, first, by freezing the formulation, that is, separating the solutes from the solvent or solvents and immobilizing any solvent in the interstitial region between the solvent crystals. Then the solvent is removed by sublimation (primary drying) and next by desorption (secondary drying).
The nature of the formulation determines:
Because time is equated to productivity, the formulation affects not only the process and drying equipment, but also the cost of manufacturing. As process time increases, profitability decreases.
The formulation consists of three basic components--active ingredient, excipient, and solvent system. In general, the active ingredient in the pharmaceutical industry is defined by its potency and, in the diagnostic industry, by its reactivity. Depending on means of production, there may be variations in the composition of the active component from batch to batch.
The formulation of a product affects the time required to process it and, ultimately, the cost of the product. Here, products are readied for lyophilization at Medicus Technologies (West Chester, PA).
Excipients serve several functions.1,2 They primarily provide a stable liquid environment for the active ingredient for some finite time. The excipient may also cryoprotect the active ingredient during the freezing process.35 In the freezing of formulations containing biological organisms, the formation of ice within can lead to the organism's destruction by cell membrane rupture. Sucrose, glucose, and dextran are excipients used to cryoprotect organisms.2,4
The excipient may also serve only as a bulking agent.2 When solid concentrations of a formulation reach <2%, the resulting cake may have poor structural qualities and leave the container during the drying process. The addition of bulking agents such as mannitol and dextran strengthen cake structure. The role of the solvent system is often overlooked. Most formulations are totally aqueous solutions, although others contain solvents such as tertiary butyl alcohol to increase the solubility of some compounds. The solvent system is removed during drying, but its thermal properties have a major impact on the cosmetic properties of the final product (Figure 1).
Figure 1. Typical lyophilization cakes. (A) uniform distribution of constituents; (B) nonuniform distribution of constituents in the cake with a crust or glaze on the upper surface; (C) a cake with poor self-supporting structural properties; (D) a cake showing signs of collapse; (E) example of meltback; (F) disappearing cake, i.e., dissolution of the cake by excess water; and (G) puffing resulting from incomplete freezing of the matrix before evacuation of the dryer.
Freezing. Formation of ice during freezing results in dramatic changes in concentrations of the active ingredient and the excipient or excipients of the formulation. As an example, consider the freezing of an isotonic (0.9% w/v) NaCl aqueous solution. At a temperature of 20°C, the frozen matrix consists of ice crystals interlaced with a 23% NaCl solution. At a temperature of <23°C, the interstitial region in the matrix consists of a eutectic mixture of NaCl * 2H2O and ice crystals. (Eutectic temperature is a point on a phase diagram where the temperature of the system or the concentration of the solution at the point cannot be altered without changing the number of phases present.)
In most formulations (>99%; based on thermal analysis of >1000 formulations at Phase Technologies, Inc., Conshohocken, PA) containing an active ingredient and an excipient, freezing greatly increases the concentration of the active ingredient and the excipient or excipients, but does not produce a well defined eutectic mixture. Instead, freezing produces a complex, glassy system that may be homogeneous or heterogeneous. This complex system, at this time, can only be produced in the interstitial region of ice crystals as a result of the freezing process.
Another property of frozen matrix is the degree of crystallization (D), the ratio of the quantity of ice formed during the freezing process to the total freezable water in the formulation (Jennings TA, "Lyophilization Seminar" [Notes], Conshohocken, PA, Phase Technologies, Inc., p 26, 1994). As D approaches 1 (Figure 2), most water is in the form of ice crystals, and only a small quantity forms part of the interstitial region. The ice crystals interconnect to form vapor paths. With decreasing values of D (e.g., D = 0.5), the volume of glassy interstitial region approaches that of the ice crystals.
Figure 2. Illustration of a frozen matrix of a formulation in which the degree of crystallization approaches 1.
In the frozen state, the mobility of the water in the glassy interstitial region approaches 0, and the formulation is considered completely frozen. As the temperature of the matrix increases, the glassy interstitial region softens, the electrical resistivity of the interstitial region decreases, or the conductivity of the system increases. Such a change in the electrical nature of the matrix is associated with the onset of mobile water within its interstitial region. As temperature further increases, the interstitial region slowly takes on liquidlike characteristics, while surrounding ice crystals remain frozen.
In most glassy systems, onset temperature for the mobility of the water in the interstitial region is not as sharp and well defined as that for a eutectic mixture. The onset temperature for the mobility of water in the matrix interstitial region is referred to as collapse temperature. This definition differs from that of MacKenzie, who called collapse temperature "disappearance of the freezing pattern, more or less extensive flow of the residual material, and the generation of new patterns."6
Drying. For lyophilization to occur, the solvent is first removed by sublimation while the temperature of the frozen matrix is maintained below the eutectic or collapse temperature of the formulation. This is the primary drying process. The chamber pressure and product and shelf temperatures, during primary drying, are based on the formulation's eutectic or collapse temperature. A lyophilized cake is typified by Figures 1A and 1B. The resulting cake volume approaches the original fill-volume.
Primary drying at temperatures greater than that of the collapse or eutectic temperature of the formulation (sometimes referred to as vacuum drying or cryodrying) can lead to a product typified by Figures 1D and 1E, respectively. Figure 1D illustrates some collapse of the cake resulting from the presence of mobile water in the matrix interstitial region during primary drying. Figure 1E, though, illustrates meltback, a result of liquid states in the interstitial region.
After primary drying, the residual moisture on the resulting cake surface is reduced to levels that no longer support biological growth and chemical reaction. This process is secondary drying. The reduction of moisture in the cake during secondary drying is accomplished by increasing the shelf temperature and reducing the partial pressure of water vapor in the container. The required partial pressure of water vapor and shelf temperature are ascertained from stability studies of lyophilized or vacuum-dried products having varied amounts of residual moisture.
1. Bashir J, and Avis KE, "Evaluation of Excipients in Freeze-Dried Products for Injection," Bull Parent Drug Assoc, 27:6883, 1973.
2. Wang YJ, and Kowal RR, "Review of Excipients and PH's for Parenteral Products Used in the United States," Bull Parent Drug Assoc, 34:452462, 1980.
3. Greaves RIN, "Fundamental Aspects of Freeze-drying Bacterial and Living Cells," in Aspects Théoriques et Industriels de la Lyophilisation, Rey L (ed), Paris, Herman, pp 407410, 1964.
4. Smith AU, "Some Problems in Supercooling and Freezing Living Cells, Tissues and Organisms," in Aspects Théoriques et Industriels de la Lyophilisation, Rey L (ed), Paris, Herman, pp 257278, 1964.
5. MacKenzie AP, "Comparative Studies on Freeze-drying Survival of Various Bacteria, Gram Type, Suspending Medium and Freezing Rates," in International Symposium on Freeze-drying of Biological Products, Washington, DC, Develop. Biol. Standard, 36 S, Basel, Switzerland, Karger, pp 263277, 1977.
6. MacKenzie AP, "Collapse during Freeze-drying--Qualitative and Quantitative Aspects" in Freeze Drying and Advanced Food Technology, Goldblith SA, Rey L, and Rothmayr WW (eds), New York, Academic Press, pp 277306, 1975.
Continue on to Part 2 of this article.