Preparations in solution form is easily measured, easily delivered and provide the most uniform dose. Water is always the preferred solvent. This is why, vast majority of products are prepared as aqueous solutions or as powder to be reconstituted with aqueous diluent. However, it is not always possible to formulate drugs in a completely aqueous system because of the limited aqueous solubility or the deleterious effect of water on the stability of certain drugs. For manufacturing of the sterile preparations, two solvent systems are available and include the aqueous and non-aqueous vehicles.
Aqueous vehicles
A) Water for injection
Water for injection (WFI) is prepared by distillation of deionized water. It is the most widely used solvent for sterile preparation. The WFI should meet certain specifications. It should be apyrogenic and should not contain more than 1mg/100 ml of total solids and may not contain added substances. Microorganism, dissolved organic and inorganic substances are the most common contaminations found in water. Inorganic substances are removed by distillation, deionization. Osmosis is another procedure used to remove the inorganic material. Membrane and depth filters are used to remove organic materials.
Filtration, chilling or heading and or re-circulation of water are used to reduce microbial growth and prevent pyrogen formation that occurs in a static deionization system. To inhibit microbial growth, WFI must be stored at either 5oC or 60oC-90oC if to be held for
24 hr. The USP also lists sterile water for injection and bacteriostatic water for injection.
B) Sterile water for injection
Sterile water for injection must be sterile and is intended to be used as a solvent, vehicle or diluent for already sterilized and packaged injectable medications. Thus sterile water for injection should not be placed in container greater than one litre. Like bacteriostatic water for injection, for the sterile water for injection, higher levels of the solid content are allowed.
C) Bacteriostatic water for injection
Bacteriostatic water for injection, unlike water for injection must be sterile. A bit higher levels of solids are allowed in this vehicle because of the possible leaching of glass constitutes during manufacturing or storage. Bacteriostatic water for injection contains one or more suitable antimicrobial agents. It must not be placed in container larger than
30 ml. This is to prevent the administration of large quantities of bacteriostatic agents (such as phenol) which could become toxic if large volume of solution was administered.
D) Dextrose and sodium chloride
Five percent dextrose or 0.9% sodium chloride and a variety of other electrolytes and nutrient solutions can be used in place of sterile water for injection or bacteriostatic water for injection for reconstitution or administration of drugs. However, the stability and compatibility of the active pharmaceutical ingredient and the added substances must be considered before their usage. Sodium chloride injection UP is a sterile isotonic solution of sodium chloride in water for injection. It contains no antimicrobial agents. The sodium and chloride ion contents of the injection are approximately, 154 mEq per litre. The solution may be used as a sterile vehicle in preparing solutions or suspension of drugs for parenteral administration.
E) Ringer’s injection
The Ringer’s injection USP is a sterile solution of sodium chloride, potassium chloride and calcium chloride in water for injection. The three agents in the Ringer’s solutions are present in the concentrations similar to that found in physiological fluids. The solution is employed as a vehicle for other drugs or used alone as an electrolyte replenisher and fluid extender.
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