Preservatives are added to various dosage forms and cosmetic preparations to prevent microbial contamination. In parenteral and ophthalmic preparations, preservatives are used to maintain sterility in the event of accidental contamination during use. An ideal preservative would be effective at low concentrations against all possible microorganisms, be nontoxic and compatible with other constituents of the preparation, and be stable for the shelf life of the preparation. The ideal preservative does not exist, but there is quite a bit of experience with some of them. In some cases, combinations of preservative agents are used to approximate a mixture of ideal features.
p-Hydroxybenzoic Acid Derivatives
Esters of ^-hydroxybenzoic acid (parabens) have distinct antifungal properties. Their toxicity to the human host is typically low because they undergo rapid hydrolysis in vivo to ^-hydroxybenzoic acid, which is quickly conjugated and excreted. This property makes the parabens useful as preservatives for liquid dosage forms. The preservative activity generally increases with molecular weight, but the methyl ester is most effective against molds, whereas the propyl ester is most effective against yeasts. The more lipid-soluble propyl ester is the preferred preservative for drugs in oil or lipophilic bases.
Methyl ^-hydroxybenzoate, or methylparaben, is a white crystalline powder. It is soluble in water and alcohol but only slightly soluble in nonpolar organic solvents. Methylparaben is used as a safeguard against mold growth.
Propyl ^-hydroxybenzoate, or propylparaben, occurs as a white crystalline powder that is slightly soluble in water but soluble in most organic solvents. It is used as a preservative, primarily to retard yeast growth. Propylparaben sodium is a water-soluble sodium salt of the 4-phenol group. The pH of solutions of propylparaben sodium is basic (pH —10).
Butylparaben n-Butyl ^-hydroxybenzoate (butylparaben) occurs as a white crystalline powder that is sparingly soluble in water but very soluble in alcohols and in nonpolar organic solvents.
Ethyl ^-hydroxybenzoate (ethylparaben) is a white crystalline powder that is slightly soluble in water but soluble in alcohol and most organic solvents.
1,1,1-Trichloro-2-methyl-2-propanol is a white crystalline solid with a camphorlike aroma. It occurs in an anhydrous form and a hemihydrate form, both of which sublime at room temperature and pressure. Chlorobutanol is slightly soluble in water and soluble in alcohol and in organic solvents.
Chlorobutanol is used as a bacteriostatic agent in pharmaceuticals for injection, ophthalmic use, and intranasal administration. It is unstable when heated in aqueous solution, especially at pH greater than 7. Under these conditions, chlorobutanol undergoes elimination. Solutions with a pH of approximately 5 are reasonably stable at 25°C. Chlorobutanol is stable in oils and organic solvents.
Benzyl alcohol (phenylcarbinol, phenylmethanol) occurs naturally as the unesterified form in oil of jasmine and in esters of acetic, cinnamic, and benzoic acids in gum benzoin, storax resin, Peru balsam, tolu balsam, and some volatile oils. It is soluble in water and alcohol and is a clear liquid with an aromatic odor.
Benzyl alcohol is commonly used as a preservative in vials of injectable drugs in concentrations of 1% to 4% in water or saline solution. Benzyl alcohol has the added advantage of having a local anesthetic action. It is commonly used in ointments and lotions as an antiseptic in the treatment of various pruritic skin conditions.
Phenylethyl alcohol (2-phenylethanol, orange oil, rose oil, C6H5CH2CH2OH) is a clear liquid that is sparingly soluble in water (—2%). It occurs naturally in rose oil and pine-needle oil. It is used primarily in perfumery.
Benzoic acid and its esters occur naturally in gum benzoin and in Peru and tolu balsams. It is found as a white crystalline solid that slowly sublimes at room temperature and is steam distillable. It is slightly soluble in water (0.3%) but more soluble in alcohol and in other polar organic solvents. It has a pKa of 4.2. Benzoic acid is used externally as an antiseptic in lotions, ointments, and mouthwashes. It is more effective as a preservative in foods and pharmaceutical products at low pH (less than the pKa). When used as a preservative in emulsions, its effectiveness depends on both pH and distribution into the two phases.16
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