Factors Affecting Preservative Efficacy: A wide range of antimicrobial preservatives is available for the preservation of pharmaceutical formulations. The preservative must prevent accidental contamination and serious or even dangerous decomposition of the product throughout its accepted strange life. The major reasons for a preservative not attaining an effective concentration in the aqueous phase are its interaction with emulgents, solubility in oil, suspended solids, and interaction with the container or pH of the formulation.
Factors Affecting Preservative Efficacy
1. Interaction with formulation components:
Hydrocolloids such as methylcellulose, polyvinylpyrrolidone, alignates, and tragacanth can interact with preservatives and diminish their activity. Many emulgents are used in pharmaceutical preparations to produce elegant applications. Interactions may occur between preservatives and the emulsified oil phase and with emulgent molecules or micelles. The nature of oil, oil-water ratio, and type of concentration of emulgent, influence the concentration of preservative needed to protect the system. Many tablet additives cause problems in tablet preservations due to their interaction with added preservatives. Therapeutically active ingredients (sulphadimidine, kaolin, magnesium trisilicate) in the form of suspended solids also reduce preservative concentration by absorption.
2. Properties of the preservatives:
The distribution of the preservative must be homogeneous and more solubility in the bulk phase is preferable in a multi-phase system. Some chemicals such as chlorobutol may hydrolyze on storage if the pH is unfavorable. Preservatives may react with substances leached from the container and lose their antimicrobial activity.
3. Effect of containers:
Formulations packed in glass containers can be expected to retain their preservative content if the closure is airtight. Preservatives may penetrate through the plastic container and interact with it. Rubber also reacts with many preservatives but is still used for teats and closures. Containers or closures may cause the contamination of pathogens. Screw-capped containers and corks are a common source of mold spores.
4. Type of microorganisms:
Plant products may contain pathogenic microorganisms from the soil e.g. Clostridium species, and Bacillus anthracis. These soil microorganisms can cause spoilage of pharmaceutical products. Soil organisms are common in dust which may gain access to preparation during processing or packaging. Many products prepared from animal sources may contain pathogens like Salmonella typhi. Spores of tetanus and gas gangrene have been isolated from gelatin.
5. Influence of pH:
Adjustment of the pH of a solution may affect the chemical stability and the activity of the preservative. Benzoic acid (weak acid preservative) mainly requires to be predominantly in an undissociated form to exert antimicrobial activity. This act has a pka value of 4.2, and an ambient pH, below this, is needed for efficient preservative activity. The majority of preservatives are less dependent upon pH, although cationic active quarternary ammonium compounds are more active at high pH values.
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