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Antimicrobial Preservatives

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Antimicrobial Preservatives
Melgardt de Villiers, PhD
Definition
Uses of Preservatives
When Is It Not Necessary to Add a Preservative?
When Are Preservatives Contraindicated?
Alternative Strategies When Preservatives Are Needed But Are Contraindicated
Properties of the Ideal Preservative
Antimicrobial Preservatives Listed in the USP 30–NF 25
Preservatives for Oral Dosage Forms
Preservatives for Topical Preparations
Preservatives for Ophthalmic Preparations
Need for Preservative-free Formulations
I. DEFINITION
Antimicrobial preservatives are substances added to nonsterile dosage forms to protect them from micro-
biological growth or from microorganisms that are introduced inadvertently during or subsequent to the
manufacturing process. In the case of sterile articles packaged in multiple-dose containers, antimicrobial
preservatives are added to inhibit the growth of microorganisms that may be introduced from repeatedly
withdrawing individual doses.
Antimicrobial agents should not be used as a substitute for good manufacturing practices or solely to
reduce the viable microbial population of a nonsterile product or to control the presterilization bioburden
of multidose formulations during manufacturing (1).—USP
II. USES OF PRESERVATIVES
A. Preservatives should be added to extemporaneously compounded preparations when there is a
possibility of microbial contamination and growth, either at the time of preparations or during
use by the patient or caregiver (1,2).
B. In USP Chapter 1151Pharmaceutical Dosage Forms, antimicrobial agents are explicitly men-
tioned and required for most dosage forms containing water and for one nonaqueous system,oph-
thalmic ointments (3).
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CHAPTER
OUTLINE
16
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1. Emulsions (including semisolid or ointment-type emulsions):“All emulsions require an antimi-
crobial agent because the aqueous phase is favorable to the growth of microorganisms.
2. Suspensions: “Suspensions intended for any route of administration should contain suitable
antimicrobial agents to protect against bacteria, yeast, and mold contamination.
3. Oral solutions:“Antimicrobial agents to prevent the growth of bacteria, yeasts, and molds are
generally also present.
4. Ophthalmic solutions: “Each solution must contain a suitable substance or mixture of sub-
stances to prevent the growth of, or to destroy, microorganisms accidentally introduced when
the container is opened during use.Where intended for use in surgical procedures,ophthalmic
solutions, although they must be sterile,should not contain antibacterial agents, since they may
be irritating to the ocular tissues.
5. Ophthalmic ointments: “Ophthalmic ointments must contain a suitable substance or mixture
of substances to prevent growth of, or to destroy, microorganisms accidentally introduced when
the container is opened during use, unless otherwise directed in the individual monograph, or
unless the formula itself is bacteriostatic.
C. The requirements for antimicrobial agents in parenteral products are treated in USP Chapter 1,
Injections (4):
A suitable substance or mixture of substances to prevent the growth of microorganisms must be added to
preparations intended for injection that are packaged in multiple-dose containers,regardless of the method
of sterilization employed, unless one of the following conditions prevails:(1) there are different directions
in the individual monograph; (2) the substance contains a radionuclide with a physical half-life of less than
24 hours; (3) the active ingredients are themselves antimicrobial. Such substances are used in concentra-
tions that will prevent the growth of or kill microorganisms in the preparations for injection (4).
III. WHEN IS IT NOT NECESSARY TO ADD A PRESERVATIVE?
A. The preparation will be used immediately.This assumes that the preparation is made using appro-
priate techniques that avoid contamination while it is being made and administered.
B. No water is present.Generally, microorganisms require water for growth, so products that contain
no water,such as tablets, powders, and hydrocarbon ointments, are not media for growth. Excep-
tions to this rule include ophthalmic ointments and nonaqueous injections when the USP specif-
ically requires an antimicrobial agent.
C. The pH of the medium is either 3 or 9.
Note:Though this pH range for inhibition of growth holds true for most microorganisms,certain
resistant molds have been shown to grow in media with pH 3 (5).
D. Ingredient(s) that have antimicrobial properties are already present in the formulation.
IV. WHEN ARE PRESERVATIVES CONTRAINDICATED?
A. Neonates
B. Ophthalmic solutions intended for use in eyes during eye surgery, with non-intact corneas, or for
intraocular injection
C. Parenteral products with volumes 30 mL
V. ALTERNATIVE STRATEGIES WHEN PRESERVATIVES ARE NEEDED BUT ARE
CONTRAINDICATED
A. Prepare a single dose and use immediately.
B. Prepare a limited quantity that will be used within a short time period, store under refrigeration,
and label with a short expiration period.
VI. PROPERTIES OF THE IDEAL PRESERVATIVE
A. Effective at a low, nontoxic concentration against a wide variety of organisms
B. Chemically stable under normal conditions of use over a wide pH and temperature range
C. Soluble at the required concentration
D. Compatible with a wide variety of drugs and excipients
E. Free from objectionable odor, taste, color, or stinging
F. Nontoxic and nonsensitizing both internally and externally at the required concentration
G. Reasonable cost
H. Unreactive (does not adsorb, penetrate, or interact) with containers or closures
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VII. ANTIMICROBIAL PRESERVATIVES LISTED IN THE USP 30–NF 25
A. Table 16.1 gives the articles listed as antimicrobial preservatives in the USP 30–NF 25 (6). Some
chemicals, though not listed by the USP as preservatives, have antimicrobial properties and may
be useful as preservatives in formulated preparations.
B. The preservatives used most commonly in extemporaneous compounding are described in sec-
tions VIII through X of this Chapter.
1. To facilitate selection of an antimicrobial preservative when formulating a preparation,the agents
are organized by suitability for route of administration (e.g., oral,topical, and ophthalmic).
2. The descriptions and solubilities presented here give a composite of information from the
Chemistry and Compendial Requirements section of the USP DI Vol. III (7), The Merck Index
(8), the Handbook of Pharmaceutical Excipients (2), and other references as cited. For additional
information on each agent, consult the Handbook of Pharmaceutical Excipients.
C. In reading and interpreting the current chapter, note that this text employs the usual convention of
using upper-case first letters for words designating official USP-NF articles (e.g., Alcohol, Purified
Water) and lower-case first letters for words designating the chemical substances (e.g.,ethanol, water).
VIII. PRESERVATIVES FOR ORAL DOSAGE FORMS
A. Alcohols and glycols
1. Ethyl Alcohol
CH
3
CH
2
OH MW 46.07
a. Official articles of ethyl alcohol that are suitable for oral use:
(1) Alcohol USP
(2) Dehydrated Alcohol USP
(3) Dehydrated Alcohol Injection USP
(4) Diluted Alcohol NF
Note: Rubbing Alcohol USP is a product containing ethyl alcohol, but it may not be
used orally. It contains denaturants that are toxic orally.
b. Description and solubility: See Chapter 15,Pharmaceutical Solvents and Solubilizing Agents
c. Effective concentration
(1) Effective concentration depends on the pH of the solution and the amount of “free
water.
Note: “Free water” is the water in a preparation that is not bound by interaction with
other molecules. Originally, the term was used to represent the amount of water in a
preparation that is not tied up by interaction with sucrose in the ratio of 85 g of sucrose
to 45 mL of water.This 85:45 ratio of sucrose to water is called the USP Syrup Equiva-
lent because it is the proportion of these ingredients in the formula for Syrup NF (Sim-
ple Syrup), which is a saturated,self-preserving solution. Obviously, other dissolved mol-
ecules can also reduce the activity of water. For example, a salt, such as potassium
chloride, or a highly hydrated polymer, such as methylcellulose, also binds water in an
Chapter 16 Antimicrobial Preservatives 205
Table 16.1 ANTIMICROBIAL PRESERVATIVES LISTED IN THE USP 30-NF 25
Benzalkonium Chloride Benzalkonium Chloride Solution Benzethonium Chloride
Benzoic Acid Benzyl Alcohol Butylparaben
Cetrimonium Bromide Cetylpyridinium Chloride Chlorobutanol
Chlorocresol Cresol Ethylparaben
Methylparaben Methylparaben Sodium Phenol
Phenoxyethanol Phenylethyl Alcohol Phenylmercuric Acetate
Phenylmercuric Nitrate Potassium Benzoate Potassium Sorbate
Propylparaben Propylparaben Sodium Sodium Benzoate
Sodium Dehydroacetate Sodium Propionate Sorbic Acid
Thimerosal Thymol
Source: 2007 USP 30/ NF 25. Rockville, MD: The United States Pharmacopeial Convention Inc. 2006: Front
Matter—NF:Excipients.
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aqueous solution, but the degree of these interactions has not been documented. It is
important to realize that these interactions do occur, and when these ingredients are in
a liquid preparation, they do effectively reduce the “free water” in that product.
(2) The alcohol concentration should be 15% (Acid)—17.5% (Neutral or mildly
alkaline) of the free water (9).
(3) Though the 15% to 17.5% range seems high, the percentage actually needed for most
preparations is far less because these percentages apply only to the “free water.” Consider
the following example:
Mineral Oil Emulsion USP (10)
Mineral Oil 500 mL
Acacia 125 g
Syrup 100 mL
Vanillin 40 mg
Alcohol 60 mL
Purified Water, a sufficient
quantity to make 1,000 mL
If you add the approximate volumes of all the ingredients (in the case of the solids—
acacia and vanillin—you have to estimate the powder volume) and subtract this quan-
tity from 1,000 mL, you get the approximate free water volume: 1,000 mL [500
100 60 ~40 (powder volume estimate)] 300 mL.This is the amount of water that
must be preserved with alcohol.The quantity of ethyl alcohol in this prescription is
95% 60 mL 57 mL.This quantity represents only 5.7% of the total product, but it
is 19% of the free water (19% 300 mL 57 mL).
Some syrups are protected from microbial growth by virtue of their high solute
concentrations that create an unfavorable osmotic environment for these organisms.
However, more dilute syrups are good media for microbial growth and require the addi-
tion of preservatives.This is another example in which alcohol or another preservative
should be added to preserve the free water.
(4) One commonly seen guideline of alcohol content for preservation of liquid formula-
tions is 5% to 10% alcohol. In using this figure, one should be aware of its origins: It
comes from a rough estimate of the free water available in the “average” product.There-
fore, if you have a prescription order that contains a large proportion of free water, you
should add extra alcohol for adequate preservation.
2. Propylene Glycol
C
3
H
8
O
2
MW 76.09
a. Official article Propylene Glycol USP
b. Description and solubility: See Chapter 15, Pharmaceutical Solvents and Solubilizing
Agents.
c. Effective concentration
(1) In most situations,propylene glycol is effective at a concentration of 10% w/v, although
inhibition of growth of certain molds requires up to 30% w/v (11,12).
(2) Propylene glycol potentiates several other preservatives. A 2% to 5% concentration of
propylene glycol, though ineffective as a sole preservative, potentiates the effect of a
methyl and propyl paraben combination.Various combinations tested were found to be
effective against bacteria, molds, and yeast but were ineffective against bacterial spores
until they entered the vegetative stage (13).This effect on paraben efficacy is especially
useful because the concentration at which the parabens exert their antimicrobial effect
is so close to their solubility.Though parabens are particularly useful for preserving vul-
nerable syrups that have a neutral pH, because of their poor water solubility, it is often
difficult to dissolve a sufficient concentration of parabens to achieve adequate preserva-
tion. In this study, both a 2% and a 5% concentration of propylene glycol allowed the
OH
HO
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minimum inhibitory concentration of methylparaben to be reduced from 0.18% to 0.1%
when combined with 0.02% propylparaben (13).
3. Glycerin
C
3
H
8
O
3
MW 92.01
a. Official article Glycerin USP
b. Description and solubility: See Chapter 15, Pharmaceutical Solvents and Solubilizing
Agents.
c. Effective concentration
(1) Though glycerin preserves at concentrations 50%, at lower concentrations it may
actually act as a nutrient for some microorganisms.This is because glycerin’s activity as
an antimicrobial agent depends solely on an osmotic effect rather than any innate toxi-
city to microorganisms (13).
(2) Though glycerin is not often used as a sole preservative,it is frequently used with alco-
hol to reduce the volume of alcohol necessary to preserve a preparation.
4. Benzyl Alcohol
C
7
H
8
O MW 108.14
a. Official article Benzyl Alcohol NF
b. Description: colorless liquid;faint, aromatic odor; sharp, burning taste; neutral pH;flammable
c. Solubility: 1 g/30 mL water; freely soluble in 50% alcohol; miscible with alcohol and fixed
and volatile oils
d. Effective concentration: bacteriocidal at 1% to 2% (14)
e. Benzyl alcohol is listed here because it is approved for oral products;however,it is not typ-
ically used in these preparations because of its sharp, burning taste. It is used frequently in
manufactured parenteral products, in which it is especially useful because of its local anes-
thetic properties. It may be used in topical preparations and, in fact, has been used thera-
peutically as a local anesthetic to relieve itching when mixed with equal parts of alcohol and
water (15).Though listed as an approved preservative for ophthalmic products (2), it is not
commonly found in ophthalmic preparations (16).
f. Benzyl alcohol is most effective at pH less than 5 and has minimal activity at pH 8 and
above. It is incompatible with methylcellulose and its activity is reduced by nonionic sur-
factants, such as polysorbate 80 (2).
B. Organic acids
1. Benzoic Acid/Sodium Benzoate/Potassium Benzoate
a. Official articles include the following:
(1) Benzoic Acid USP
C
7
H
6
O
2
MW 122.12
(2) Sodium Benzoate NF
(3) Potassium Benzoate NF
b. Descriptions
(1) Benzoic acid: white crystals, scales, or needles; slight aromatic odor; volatile at warm
temperatures; pK
a
4.19; pH of a saturated solution 2.8
(2) Sodium benzoate: white granular or crystalline powder; practically odorless; slightly
hygroscopic but stable in air. Has an unpleasant sweet and salty taste. Sodium benzoate
is the most commonly used salt of benzoic acid.
OH
O
OH
OH
HO OH
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(3) Potassium benzoate: white granular or crystalline powder; practically odorless; stable in
air
c. Solubilities
(1) Benzoic acid: 1 g/300 mL of water (0.33%), or 3 mL of alcohol; soluble in fixed oils
(2) Sodium benzoate: 1 g/2 mL of water, or 75 mL of alcohol.
(3) Potassium benzoate: soluble in water and alcohol
d. Effective concentration
(1) Benzoic acid is the active preservative form, and its effective concentration is 0.1% to
0.3% (17).
(2) Because its effective concentration is so close to its solubility, benzoic acid is often dis-
solved as the sodium salt (sodium benzoate), which is very water-soluble.The amount
present as the active form is then dependent on the pH of the solution.This amount can
be estimated using the Henderson–Hasselbalch equation.
(3) Benzoic acid/sodium benzoate is ineffective in solutions with a pH above 5.This means
that in deciding the effectiveness of this preservative, you must know the pH of the liq-
uid you wish to preserve. Most fruit-flavored and cola fountain syrups have pHs around
3 and are effectively preserved with benzoic acid/sodium benzoate. Depending on
method of preparation, Syrup NF has a pH of 5 to 6.5, so it may or may not be ade-
quately protected. Methylcellulose 1% gel has a pH of approximately 6,and sodium car-
boxymethylcellulose 1% gel has a pH of about 7, so these would probably not be pre-
served.The monographs of some official liquid preparations give pH ranges for these
liquids.The pH of manufactured liquid drug products is often given in the product pack-
age insert. An alternative easy way to determine the approximate pH of a liquid prod-
uct or preparation is to use pH paper or a pH meter.This is illustrated in the sample pre-
scription quality control section for numerous water-containing preparations in Part 5,
Nonsterile Dosage Forms and their Preparation, of this book.
(4) Its effectiveness may be reduced by such nonionic surfactants as polysorbate 80.
(5) Benzoic acid/sodium benzoate is widely used as a preservative, especially in foods. It has
most of the properties of an ideal preservative. Its biggest drawback is the pH depend-
ence of its effectiveness.
2. Sorbic Acid/Potassium Sorbate
a. Official articles include the following:
(1) Sorbic Acid NF
C
6
H
8
O
2
MW 112.13
(2) Potassium Sorbate NF
b. Descriptions
(1) Sorbic acid: white crystalline powder; pK
a
4.76
(2) Potassium sorbate: white crystals or powder
c. Solubilities
(1) Sorbic acid: listed in Remington:The Science and Practice of Pharmacy as 1 g/1,000 mL of
water (0.1%);The Merck Index gives a water solubility of 0.25% (1 g/400 mL) at 30° and
3.8% (1 g/26 mL) at 100°.The solubility in alcohol is 1 g/10 mL and in propylene gly-
col, 1 g/19 mL.
(2) Potassium sorbate:very soluble in water,1 g/4.5 mL, and moderately soluble in alcohol,
1 g/35 mL
d. Effective concentration
(1) Sorbic acid is the active form, with an effective concentration of 0.05% to 0.2% (17).
(2) Sorbic acid has properties and problems that are similar to those of benzoic acid. Like
benzoic acid, sorbic acid has an effective concentration that is very close to its solubil-
ity. Because of this, the salt form, potassium sorbate, which is very soluble in water, is
often used. As with benzoic acid, the amount that is present in the active acid form is
dependent on the pH of the solution. Because sorbic acid has a slightly higher pK
a
than
benzoic acid, it has a higher ratio of active to inactive form at the pH levels common to
oral products. It reportedly has little antimicrobial activity above pH 6 (18).
O
OH
H3C
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(3) Sorbic acid is widely used as a preservative in foods. It is one of the least toxic preserv-
atives, with a reported oral LD
50
in rats of 7.36 g/kg (7). Though it has low toxicity
orally, irritation of the skin in topical products has been reported.
C. Esters of p-Hydroxybenzoic Acid (Parabens)
1. Official articles of parabens include the following:
a. Methylparaben NF
C
8
H
8
O
3
MW 152.15
b. Methylparaben Sodium NF
c. Propylparaben NF
C
10
H
12
O
3
MW 180.20
d. Propylparaben Sodium NF
2. Descriptions
a. Methylparaben: small, colorless crystals, or white, crystalline powder; practically odorless;
slight burning taste
b. Methylparaben sodium: white, hygroscopic powder
c. Propylparaben: small, colorless crystals, or white powder
d. Propylparaben sodium: white, odorless, hygroscopic powder
3. Solubilities
a. Methylparaben: 1 g/400 mL of water (0.25%), or 3 mL of alcohol. It is soluble in glycerin
(1 g/60 mL) and propylene glycol (1 g/5 mL) but insoluble in mineral oil; solubility in fixed
oils varies.
b. Propylparaben: 1 g/2,500 mL of water (0.04%), or 1.5 mL of alcohol
c. The sodium salts are more soluble, but they are formed only at a relatively high pH (~9),
and at this pH, the molecule is fairly unstable because of hydrolysis of the ester group.
4. Effective concentration
a. Methylparaben: 0.05% to 0.25% (The lower part of the range is effective only when used
in combination with another preservative, such as propylparaben, benzyl alcohol, or propy-
lene glycol.)
Methylparaben has a water solubility that is essentially identical to its effective concen-
tration. Because it is a rather hydrophobic powder, it is somewhat difficult to dissolve in
aqueous solutions. If the product will tolerate a small amount of alcohol or propylene gly-
col, the powder can first be dissolved in a minimal amount of alcohol or propylene glycol.
Methylparaben has many properties of an ideal preservative, including activity over a wide
pH range, 4 to 8 (2,16,17).Its major problem is its poor water solubility. Its effectiveness is
enhanced by 2% to 5% propylene glycol (13,19).
b. Propylparaben: 0.02% to 0.04% (Most effective when used in combination with another
preservative such as methylparaben.A concentration of 0.035% is the maximum acceptable
concentration for parenteral products.)
Propylparaben would be a great preservative if it were more water-soluble.It is rarely used
by itself because it is impossible to get enough dissolved for sufficient preservative action. It is
most often used in combination with methylparaben. Combinations of paraben esters have a
synergistic effect.A combination of 0.18% of methylparaben to 0.02% propylparaben is com-
mon and has been approved for use as a preservative for certain parenteral products. Like
methylparaben, propylparaben is hydrophobic and is difficult to dissolve in water. Further-
more, propylparaben is used in such a small amount that for the small quantities used in
HO
O
CH3
O
HO
OCH3
O
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extemporaneous formulation, an aliquot is required. If the product will tolerate a small
amount of alcohol or propylene glycol, the powder can first be dissolved in a minimal
amount of alcohol or propylene glycol. One possible stock solution has the following for-
mula:
Methylparaben 9 g
Propylparaben 1 g
Propylene Glycol qs ad 100 mL
Two milliliters of this stock solution in each 100 mL or 100 g of preparation will provide
0.18% methylparaben, 0.02% propylparaben, and approximately 2% propylene glycol.
5. Parabens are FDA-approved for use in a wide range of dosage forms, including oral, topical,
rectal, vaginal, urethral, ophthalmic, nasal, inhalation, irrigation, and parenteral products (16).
Though not common, parabens can cause hypersensitivity reactions, most commonly delayed-
contact dermatitis. Irritation has also been reported when parabens are used ophthalmically and
parenterally (2).
IX. PRESERVATIVES FOR TOPICAL PREPARATIONS
A. Alcohols and glycols
1. Ethyl alcohol: See earlier description and in Chapter 15, Pharmaceutical Solvents and Solubi-
lizing Agents.
One official article of ethyl alcohol that is not suitable for oral use but may be used topi-
cally is Rubbing Alcohol USP. It is described in Chapter 15.
2. Isopropyl Alcohol CH
3
CHOHCH
3
C
3
H
8
O MW 60.10
a. Though isopropyl alcohol may not be used internally because of its toxicity, it is safe for
external use and is an effective preservative for topical products.
b. Official articles, Descriptions, and Solubility: See Chapter 15, Pharmaceutical Solvents and
Solubilizing Agents.
c. Effective concentration: Same as ethyl alcohol
3. Propylene Glycol: See section VIII.
4. Glycerin: See section VIII.
5. Benzyl Alcohol: See section VIII.
B. Organic acids
1. Benzoic Acid: See section VIII.
2. Sorbic Acid: See section VIII.
C. Esters of p-Hydroxybenzoic Acid (Parabens): See foregoing description.
D. Organic mercurial derivatives
1. Official articles
a. Phenylmercuric Acetate NF
C
8
H
8
HgO
2
MW 336.74
b. Phenylmercuric Nitrate NF
Phenylmercuric Nitrate is a mixture of phenylmercuric nitrate and phenylmercuric
hydroxide (20).
N
HgO O
O
O
CH3
HgO
O
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c. Thimerosal USP
C
9
H
9
HgNaO
2
S MW 404.81
d. Thimerosal Topical Solution USP
e. Thimerosal Tincture USP
2. Descriptions
a. Phenylmercuric acetate: white crystalline powder; odorless; pH of a saturated solution 4
b. Phenylmercuric nitrate: white crystalline powder with mild aromatic odor; affected by light.
Saturated solutions are acid to litmus.
c. Thimerosal:light cream-colored, crystalline powder with a slight odor;affected by light; pH
of a 1% solution 6.7
d. Thimerosal Topical Solution USP: clear liquid; sensitive to some metals; affected by light;
pH, 9.6–10.2.
e. Thimerosal Tincture USP: transparent,mobile liquid with the odor of acetone and alcohol;
sensitive to some metals; affected by light; alcohol content, 45.0% to 55.0%.
3. Solubilities
a. Phenylmercuric Acetate (PMA):listed in Remington’s Pharmaceutical Sciences, 18th ed., as 1 g/
180 mL water,225 mL alcohol; The Merck Index gives 1 g/600 mL water,soluble in alcohol
and acetone.
b. Phenylmercuric Nitrate (PMN): listed in Remington’s Pharmaceutical Sciences, 18th ed., as 1 g/
600 mL water, slightly soluble in alcohol or glycerin; The Merck Index gives 1 g/1,250 mL
water,slightly soluble in alcohol, moderately soluble in glycerin; soluble in propylene glycol.
Note: The Handbook of Pharmaceutical Excipients states for both compounds that the com-
pendial values and laboratory values on solubility vary considerably.This can be seen in the
foregoing values.
c. Because PMN and PMA are used in very dilute concentrations, stock solutions are useful.
Convenient concentrations of aqueous stock solutions are 1:2,000 and 1:10,000.
d. Thimerosal: 1 g/1 mL water, 8 mL (Merck) or 12 mL (Remington) alcohol
4. Effective concentrations
a. Phenylmercuric Acetate and Nitrate: may be used for topical products in a range of
0.002% to 0.01%. The usual concentration for ophthalmic solutions is 0.002% to
0.004%, with 0.004% the maximum allowed for eye products (21).
Used primarily to preserve parenterals and eye and nasal products.All mercurial com-
pounds can be sensitizing.
b. Thimerosal: 0.001% to 0.02%. The maximum acceptable concentration for ophthalmic
products is 0.01% (21); the maximum for parenteral products is 0.04%.
5. Incompatibilities
a. Phenylmercuric acetate and nitrate precipitate with halides and anionic emulsifying
agents, suspending agents, and drugs such as penicillin and fluorescein.They also are
incompatible with tragacanth, starch, talc, silicates, sodium metabisulfite, aluminum and
other metals, ammonia and its salts, amino acids,sulfur compounds, rubber,and some plas-
tics. Disodium edetate and sodium thiosulfate may cause inactivation of PMN or PMA.
Activity may be lost because of sorption onto polyethylene surfaces of containers, clo-
sures, or droppers (2,7).
b. Thimerosal is a sodium salt that precipitates in acidic solutions. It also is incompatible with
aluminum and other metals, silver nitrate, sodium chloride solutions, lecithin, phenylmer-
curic compounds, large cations such as quaternary ammonium compounds, thioglycolate,
and proteins. Sodium metabisulfite and the EDTA compounds can reduce its antimicrobial
effectiveness. In solution, it may sorb to some plastics and rubber closures (2,7).
E. Salts of quaternary ammonium bases
Note:Though the quaternary ammonium preservatives may be used in oral dosage forms, they
are listed here because they are used primarily in topical and ophthalmic drug products and prepa-
rations.
HgS
ONa
H3CO
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1. Official articles
a. Benzalkonium Chloride NF
Benzalkonium Chloride is a mixture of alkyl-benzyldimethylammonium chlorides with the
general formula [C
6
H
5
CH
2
N(CH
3
)
2
R]Cl, in which R represents a mixture of alkyls,
including groups beginning with n-C
8
H
17
and extending through higher homologs, with
n-C
12
H
25
,n-C
14
H
29
, and n-C
16
H
33
providing the majority of R groups (20).
b. Benzalkonium Chloride Solution NF
Benzalkonium Chloride Solution contains benzalkonium chloride in solution.The solution
may also contain a suitable coloring agent and may contain not more than 10% alcohol.This
solution should not be mixed with ordinary soaps or with anionic detergents as they may
decrease or destroy its bacteriostatic activity (20).
c. Benzethonium Chloride USP
C
27
H
42
ClNO
2
MW 448.08
d. Benzethonium Chloride Concentrate USP
Benzethonium Chloride Concentrate contains not less than 94.0% and not more than
106.0% of the labeled amount of benzethonium chloride (C
27
H
42
ClNO
2
).
e. Benzethonium Chloride Topical Solution USP
Benzethonium Chloride Topical Solution is an aqueous solution of the chemical (10).
f. Benzethonium Chloride Tincture USP has the formula shown here (10):
Benzethonium Chloride 2 g
Alcohol 685 mL
Acetone 100 mL
Purified Water,a sufficient quantity to make 1,000 mL
g. Cetylpyridium Chloride USP
C
21
H
38
ClNH
2
O MW 358.00
h. Cetylpyridinium Chloride Topical Solution USP
Cetylpyridinium Chloride Topical Solution is a solution of the chemical (10).
2. Descriptions
a. Benzalkonium Chloride (BAC): white or yellowish-white amorphous powder,thick gel or
gel-like pieces; hygroscopic; mild odor with a very bitter taste; slightly alkaline (pH 5–8
for a 10% w/v solution). Solutions foam when shaken.
b. Benzethonium Chloride: white crystals; mild odor with a bitter taste.The pH of a 1% solu-
tion 4.8–5.5.
CH2
N+
CH2CH3
14
CI-
O
O
H3CH3C
H3CH3C
H3CN
+
CI-
CH3
CH3
NR
CI-
+
R=C8H17 to C18H37
212 Part 4 Pharmaceutical Excipients
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c. Cetylpyridinium Chloride: white powder; mild odor with a bitter taste
d. Cetylpyridinium Chloride Topical Solution USP: clear, colorless solution with an aromatic
odor and bitter taste; may be colored if dye is added
3. Solubilities
a. Benzalkonium Chloride: very soluble in water, alcohol, and acetone
b. Benzethonium Chloride: 1 g/mL of water, alcohol, or acetone
c. Cetylpyridinium Chloride: very soluble in water and alcohol
4. Effective concentrations
a. Benzalkonium Chloride: 0.004% to 0.02% (22).The usual concentration for preser-
vation is 0.01%, with 0.013% the maximum allowed for ophthalmics (21).The pre-
servative action of Benzalkonium Chloride is somewhat unpredictable at the 0.01% con-
centration used for ophthalmic solutions. Its bacteriocidal properties are improved by the
addition of 0.1% Edetate (EDTA). It can be irritating to the eye, and its solutions may be
sensitizing. Benzalkonium chloride is available as 1:750, 10%, 17%, and 50% w/v aqueous
solutions and as a 1:750 tincture.
b. Benzethonium Chloride: 0.01% to 0.02% (22).The usual preservative concentration and
the maximum allowed for ophthalmics is 0.01% (21). Benzethonium Chloride is
available as the powder.
c. Cetylpyridinium Chloride: 0.01% to 0.02% (22). Cetylpyridinium Chloride is available as
the powder.
5. Incompatibilities: The salts of quaternary ammonium bases have many incompatibilities. For
example, Martindale:The Extra Pharmacopeia lists the following incompatibilities for benzalko-
nium chloride: soaps,anionic drugs and detergents, nonionic surfactants in high concentrations,
citrates, iodides, nitrates, permanganates, salicylates, silver salts, tartrates, fluorescein sodium,
hydrogen peroxide, kaolin, lanolin, some sulfonamides, some components of commercial rub-
ber mixes, and boric acid 5% (but not less than or equal to 2%) (23).The Handbook of Pharma-
ceutical Excipients also lists zinc oxide and zinc sulfate (2). Cetylpyridinium Chloride, but not
Benzalkonium Chloride, is reported to be inactivated by methylcellulose (24).
X. PRESERVATIVES FOR OPHTHALMIC PREPARATIONS
A. All ophthalmic products must be sterile.Preparations in multi-dose containers must contain a suit-
able preservative to prevent the growth of microorganisms that may be introduced inadvertently
into the product during use.
B. Preservatives commonly used in commercial ophthalmic products are given in Table 16.2 (25).
Chlorobutanol, which is not used for oral or topical preparations, is described here. Preservatives
that are frequently used in extemporaneous compounding of ophthalmic preparations but that are
also used in oral and topical preparations have been described previously.
C. Chlorobutanol
C
4
H
7
Cl
3
O (anhydrous) MW 177.46
and/or (hemihydrate) 186.46
1. Official article Chlorobutanol NF (20)
2. Description: colorless to white crystals with a camphor-like odor and taste
CI3COH
H3CCH
3
Chapter 16 Antimicrobial Preservatives 213
Table 16.2 PRESERVATIVES COMMONLY USED IN OPHTHALMIC PRODUCTS
Benzalkonium Chloride Benzethonium Chloride Cetylpyridium Chloride
Chlorhexidine Chlorobutanol Disodium ETDA
Methylparaben Phenylethyl Alcohol Phenylmercuric Acetate
Phenylmercuric Nitrate Propylparaben Sodium Benzoate
Sodium Propionate Sorbic acid Thimerosal
Source: Handbook of Nonprescription Drugs, 13th ed. American Pharmaceutical Association, Washington DC,
2002.
LWBK127-C16[203-215].qxd 10/19/2008 12:34 PM Page 213 Aptara Inc.
3. Solubility: 1 g/125 mL water, although it is somewhat difficult to dissolve.Also, 1 g/0.6 mL
alcohol and 10 mL glycerin. It is freely soluble in volatile oils.
4. Effective concentration: 0.5% (21)
5. Incompatibilities
a. Is incompatible with silver nitrate and the sodium salts of sulfonamides
b. Hydrolyzes to hydrochloric acid in solutions with pH’s at or above neutrality; should be
used in solutions buffered at 5.0–5.5
c. Activity may be lost because of sorption onto polyethylene or rubber surfaces of ophthalmic
containers or droppers.
d. Is inactivated by the macromolecules polysorbate 80 and polyvinylpyrrolidone, but not by
methylcellulose (24). Also interacts with carboxymethylcellulose, with resulting reduced
antimicrobial activity (2)
6. Chlorobutanol is used for ophthalmic and parenteral products. It is not used for oral prepara-
tions because of its camphor-like odor and taste. Its use as an ophthalmic preservative is lim-
ited because of its instability except at acid pH and because it acts slowly in killing organisms.
XI. NEED FOR PRESERVATIVE-FREE FORMULATIONS
A. It is well known that traditional preservatives are effective in preventing microbial growth. How-
ever, there have been reports of irritation from some preserved products (26,27).
B. Disodium edetate (EDTA) and benzalkonium chloride (BAC) are often present as preservatives
or stabilizing agents in nebulizer solutions used to treat asthma and chronic obstructive pulmonary
disease (28). However, benzalkonium chloride is a potent bronchoconstrictor when inhaled in
concentrations similar to those in which it is present in these solutions. Inclusion of BAC
(together with EDTA) in the ipratropium bromide (Atrovent) nebulizer solution resulted in para-
doxic bronchoconstriction in some asthmatic patients and an overall reduction in bronchodilator
efficacy.The use of preservative-free bronchodilator nebulizer solutions does not result in clini-
cally significant bacterial contamination if they are dispensed in sterile unit-dose vials in volumes
and concentrations that do not require modification by the user (28).
C. In the case of nasal spray delivery, preservatives can be irritating to the patient mucosa, causing
some unpleasant itching, but more seriously can also slow down or even stop the mucociliary
clearance that is an essential natural mechanism for the protection of the upper airways (26,27).
D. Parabens are used as preservatives in many thousands of cosmetic, food, and pharmaceutical products
to which the human population is exposed.Though recent reports of the estrogenic properties of
parabens have challenged current concepts of their toxicity in these consumer products, the ques-
tion remains as to whether any of the parabens can accumulate intact in the body from the long-
term, low-dose levels to which humans are exposed. Initial reported studies showed that parabens
can be extracted from human breast tissue and detected by thin-layer chromatography (29).
E. Thimerosal is a preservative that has been used in some vaccines since the 1930s,when it was first
introduced by the Eli Lilly Company.At concentrations found in vaccines, it met the requirements
for a preservative as set forth by the United States Pharmacopeia; that is,it kills the specified chal-
lenge organisms and is able to prevent the growth of the challenge fungi.However,owing to pub-
lic perceptions of mercury and its toxicity, the use of mercury-containing preservatives in vaccines
has declined markedly since 1999, and the FDA is continuing its efforts toward reducing or
removing thimerosal from all existing vaccines.In this regard,all vaccines routinely recommended
for children 6 years of age or younger and marketed in the United States contain no thimerosal
or only trace amounts (1 µg or less mercury per dose), with the exception of inactivated influenza
vaccine,which was first recommended by the Advisory Committee on Immunization Practices in
2004 for routine use in children 6 to 23 months of age.
214 Part 4 Pharmaceutical Excipients
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1. The United States Pharmacopeial Convention Inc.Chapter 51
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2. Rowe R, Sheskey P, Weller P, eds. Handbook of pharmaceutical
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3. The United States Pharmacopeial Convention Inc. Chapter
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4. The United States Pharmacopeial Convention Inc. Chapter 1
2007 USP 30/NF 25. Rockville, MD:Author, 2006; 34.
5. Barr M,Tice LF.The preservation of aqueous sorbitol solutions.
JAPhA Sci Ed 1957; 46: 221–223.
6. The United States Pharmacopeial Convention Inc. 2007 USP
30/NF 25. Front Matter—NF: Excipients. Rockville, MD:
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7. USP DI vol. III, 27th ed. Englewood, CO: Thompson,
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8. O’Neil MJ, ed.The Merck Index, 13th ed. Rahway, NJ: Merck
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9. Gabel LF. The relative action of preservatives in pharmaceutical
preparations. JAPhA 1921; 10: 767–768.
10. The United States Pharmacopeial Convention Inc. 2007 USP
30/ NF 25. USP Monographs. Rockville, MD:Author, 2006.
11. Rae J.The preservative properties of ethylene and propylene gly-
col. Pharm J 1938; 140:517.
12. Barr M,Tice LF.A study of the inhibitory concentrations of glyc-
erin-sorbitol and propylene glycol-sorbitol combinations on the
growth of microorganisms.JAPhA Sci Ed 1957; 46: 217–218.
13. Prickett PS, Murray HL, Mercer NH. Potentiation of preserva-
tives (parabens) in pharmaceutical formulations by low concen-
trations of propylene glycol. J Pharm Sci 1961; 50: 316–320.
14. Leszczynska-Bakal H, Smmazynski T. Preservation of pharma-
ceutical preparations by chemical compounds with antibacterial
activity. Paper presented at Symposium on Preservatives used in
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15. Swinyard EA, Harvey SC. In: Hoover JE, ed. Remington’s phar-
maceutical sciences, 14th ed. Easton, PA: Mack Publishing Co.,
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16. Inactive Ingredient Guide (redacted) January 1996.
http://www.fda.gov/cder/drug/iig/default.htm .Accessed Janu-
ary 2008.
17. Entrekin DN. Relation of pH to preservative effectiveness: I.
Acid media. J Pharm Sci 1961; 50:743–746.
18. Eklund T. The antimicrobial effect of dissociated and undissoci-
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383–389.
19. Aalto TR, Firman MS, Rigler NE. p-Hydroxybenzoic acid esters
as preservatives.JAPhA Sci Ed 1953; 42: 449–457.
20. The United States Pharmacopeial Convention Inc. NF Mono-
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21. FDA Advisory Review Panel on OTC Ophthalmic Drug Prod-
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22. Allen LV. Preservatives and compounding. US Pharmacist 1994;
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23. Reynolds JEF,ed. Martindale:The extra pharmacopoeia, 30th ed.
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26. Furrer P, Mayer JM, Gurny R. Ocular tolerance of preservatives
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Chapter 16 Antimicrobial Preservatives 215
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Article
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