Freeze residual water content

Figure 2 The rate of water loss and residual water content during freeze drying. Moxa-lactam disodium formulated with 12% mannitol in aqueous solution at 30% solids 10 mL tubing vials with a solution fill depth of 1 cm. The chamber pressure was 0.05 torr (6.6 Pa). (From Ref. 2.)...

Studies with the freeze dried DPPC liposomes in trehalose solution showed, that not Tg )f the amorphous sugar is the critical temperature during storage, but the bilayer transition emperature Tm. for the lyposomes determines the short term stability of the formulation. With trehalose as lyoprotectant and a low residual water content, Tm proved to be 10 to 30 °C below the onset of T . 30 min heating above Tm but well below T% decreased the retention of CF after rehydration. Tm< after the heating was reduced from 40 to 80 °C to below 25 °C. 

Keeping the residual water content of the lyophilized liposomes at minimum may also increase the shelf life of freeze-dried formulation and prevent increases of vesicle size on rehydration [45]. A dehydration-rehydration protocol has been introduced by Kirby and Gregoriadis [46] to produce a mixture of OLVs and MLVs, with entrapment efficiencies between 40% and 50% being reported. Ambisome is an antifungal freeze-dried liposome formulation that is currently available in the maiket for treatment of systemic fungal infections [47]. 

Table 4 Residual Water Content in Three Freeze-Dried Products Measured by Different Methods ...

Genin, N., Rene, F Corrieu, G 1996. A method for on line determination of residual water content and sublimation end point diuing freeze drying. Chem. Eng. Process. 35 255-263. 

After freezing the product, most of the solvent is removed by sublimation during the primary drying. In this stage it is necessary to monitor product temperature and the residual water content in such a manner that the control system can both optimize... 

As highlighted in the previous sections about monitoring and control of a vial freeze-drying process, one of the most important issues that has to be taken into account for product quaUly monitoring is batch heterogeneity during the process both product temperature and residual water content in the various vials are not uniform, there exists a wide (or narrow) distribution around a mean value for both variables. 

In addition to the additives used in a formulation to help stabilize the protein to freezing, the residual moisture content of the lyophilized powder needs to be considered. Not only is moisture capable of affecting the physicochemical stability of the protein itself, equally important is the ability of moisture to affect the Tg of the formulation. Water acts as a plasticizer and depresses the Tg of amorphous solids [124,137,138]. During primary drying, as water is gradually removed from the product, the Tg increases accordingly. The duration and temperature of the secondary drying step of the lyophilization process determines how much moisture remains bound to the powder. Usually lower residual moisture in the finished biopharmaceutical product leads to enhanced stability. Typically, moisture content in lyophilized formulations should not exceed 2% [139]. The optimal moisture level for maximum stability of a particular product must be demonstrated on a case-by-case basis. 

The water is not crystallized to its maximum and can be removed during freeze drying only with difficulty, or not at all. The residual moisture content remains undesirably high. 

Merika [3.51] emphasized from his 17 years of experience with the quality control of freeze dried transplants the importance of sterility and residual moisture control as the decisive characteristics. Furthermore, the leak tightness of the storage containers was constantly controlled. Merika did not measure the product temperature during drying, but controled the process by measuring water vapor pressure and temperatures of the shelves and the condenser. The residual moisture content after 2 years of storage must be below 5 %. All products were sterilized by gamma radiation. 

For lyophilized preparation kits containing reducing agents, such as "mTc kits, a test for moisture content can be necessary. Residual water in the freeze-dried pellet may lead to oxidation of the reducing agent. 

Figure 1 Schematic evolution of the freeze-drying process. Temperatures (upper curve) and water content (lower curve) are indicated versus time. In the temperature diagram cs = maximum temperature of complete solidification 7 = minimum temperature of incipient melting = absolute limit for fast process = maximum allowed temperature for the dry product RMF, final requested residual moisture.

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