Practical aspects of lyophilization using non-aqueous co-solvent systems

Non-aqueous co-solvent systems have been evaluated for their potential use in the freeze-drying of pharmaceutical products. The advantages of using these non-aqueous solvent systems include: increased drug wetting or solubility, increased sublimation rates, increased pre-dried bulk solution or dried product stability, decreased reconstitution time, and enhancement of sterility assurance of the pre-dried bulk solution. Conversely, the potential disadvantages and issues which must be evaluated include: the proper safe handling and storage of flammable and/or explosive solvents, the special facilities or equipment which may be required, the control of residual solvent levels, the toxicity of the remaining solvent, qualification of an appropriate GMP purity, the overall cost benefit to use of the solvent, and the potential increased regulatory scrutiny. The co-solvent system that has been most extensively evaluated was the tert-butanol/water combination. The tert-butanol possesses a high vapor pressure, freezes completely in most commercial freeze-dryers, readily sublimes during primary drying, can increase sublimation rates, and has low toxicity. This co-solvent system has been used in the manufacture of a marketed injectable pharmaceutical product. When using this solvent system, both formulation and process control required optimization to maximize drying rates and to minimize residual solvent levels at the end of drying. Other co-solvent systems which do not freeze completely in commercial freeze-dryers were more difficult to use and often resulted in unacceptable freeze-dried cakes. Their use appears limited to levels of not more than 10%.