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
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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