Antimicrobial agents must be studied with respect to compatibility with all other components of the formula. In addition,
their activity must be evaluated in the total formula. It is not
uncommon to find a particular agent effective in one formulation but ineffective in another, possibly due to the effect of
various components of the formula on the biological activity
or availability of the compound; for example, the binding and
inactivation of esters of p-hydroxybenzoic acid by macromolecules, such as Polysorbate 80, or the reduction of phenylmercuric nitrate by sulfide residues in rubber closures. A physical
reaction encountered is that bacteriostatic agents are sometimes removed from solution by rubber closures.
Protein pharmaceuticals, because of their cost and/or frequency of use, are preferred to be available as multiple dose
formulations (e.g., Human Insulin, Human Growth Hormone,
Interferons, Vaccines, etc.). However, several proteins are reactive with antimicrobial preservative agents (e.g., Tissue
Plasminogen Activator, Sargramostim, and Interleukins) and,
therefore, are only available as single dosage units. Phenol and
benzyl alcohol are the two most common antimicrobial preservatives used in peptide and protein products. Phenoxyethanol
is the most frequently used preservative in vaccine products.
Single-dose containers and pharmacy bulk packs that do not
contain antimicrobial agents are expected to be used promptly
after opening or discarded. The ICH/CPMP guidelines (http://
www.ema.europa.eu/docs/en_GB/document_library/Scientific_
guideline/2009/09/WC500003476.pdf) require that products
without preservatives be used immediately, although some
package inserts define immediate use as within 3 hours after
entering the primary package, or a longer usage period must be justified. Large-volume, single-dose containers may not contain
an added antimicrobial preservative. Therefore, special care
must be exercised in storing such products after the containers
have been opened to prepare an admixture, particularly those
that support the growth of microorganisms, such as total parenteral nutrition (TPN) solutions and emulsions. It should be
noted that, although refrigeration slows the growth of most microorganisms, it does not prevent their growth.
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