Preservation of Medicines Using Antimicrobial Agents: Basic Principles

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Chapter: Pharmaceutical Microbiology : Microbial Spoilage, Infection Risk And Contamination Control

An antimicrobial ‘preservative’ may be included in a formulation to minimize the risk of spoilage and preferably to kill low levels of contaminants introduced during storage or repeated use of a multidose container.


PRESERVATION  OF  MEDICINES  USING ANTIMICROBIAL  AGENTS:  BASIC  PRINCIPLES

 
INTRODUCTION

 

An antimicrobial ‘preservative’ may be included in a formulation to minimize the risk of spoilage and preferably to kill low levels of contaminants introduced during storage or repeated use of a multidose container. However, where there is a low risk of contamination, as with tablets, capsules and dry powders, the inclusion of a preservative may be unnecessary. Preservatives should never be added to mask poor manufacturing processes.

 

The properties of an ideal preservative are well recognized: a broad spectrum of activity and a rapid rate of kill; selectivity in reacting with the contaminants and not the formulation ingredients; non-irritant and non-toxic to the patient; and stable and effective throughout the life of the product.

 

Unfortunately, the most active antimicrobial agents are often non-selective in action, interacting significantly with formulation ingredients as well as with patients and microorganisms. Having excluded the more toxic, irritant and reactive agents, those remaining generally have only modest antimicrobial efficacy, and no preservatives are now considered sufficiently non-toxic for use in highly sensitive areas, e.g. for injection into central nervous system tissues or for use within the eye. A number of microbiologically effective preservatives used in cosmetics have caused a significant number of cases of contact dermatitis, and are thus precluded from use in pharmaceutical creams. Although a rapid rate of kill may be preferable, this may only be possible for relatively simple aqueous solutions such as eye drops or injections. For physico-chemically complex systems such as emulsions and creams, inhibition of growth and a slow rate of killing may be all that can be realistically achieved.

 

In order to maximize preservative efficacy, it is essential to have an appreciation of those parameters that influence antimicrobial activity.

 

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