Formulations: Ointments and Pastes
Dermatological formulations are among the most frequently compounded products
because of their wide range of potential uses. These include solutions (i.e.,
collodions, liniments, aqueous and oleaginous solutions), suspensions and gels,
emulsions, lotions, and creams. Lotions can be either suspensions or emulsions
but are fluid liquids that are typically used for their lubricating effect.
Creams are emulsions and are typically opaque, thick liquids or soft solids
used for their emollient properties. Creams also have the added feature that
they tend to "vanish" or disappear with rubbing. Distinctions between
lotions and creams are open to individual interpretation.
Other dermatological formulations that are not commonly compounded include
aerosols, dusting powders, and devices such as transdermal patches, tapes, and
gauzes. These formulations are typically manufactured.
This chapter will deal with two other dermatological formulations, ointments
and pastes. Pastes have more solid material in them than ointments. These two
formulations are also termed "semisolids" because they appear to be
solid but still have fluid properties.
Local and Systemic Effects of Dermatological Formulations
Regardless of the formulation, all dermatological formulations are applied
to the skin.
The skin is the largest and heaviest organ in the body and accounts for about
17% of a person's weight. Its major function is to protect the underlying organ
systems from trauma, temperature, humidity, harmful penetrations, moisture,
radiation, and microorganisms. It is composed of three layers of stratified
tissue: epidermis, dermis, and subcutaneous tissue. The thickness of the skin
is 3 - 5 millimeters. The thickness of the skin varies with the different parts
of the body. The thickest parts of the skin are the palms and soles and the
thinnest parts are the eyelids and genitals. Within the structure of the skin
are several skin appendages: hair follicles, sebaceous glands, sweat glands,
In normal skin, the epidermal cells are continually replenished by the formation
of initially viable cells from the basal germinative layer. As the new cells
develop, they displace the outer epidermal cells. The outer layer is called
the stratum corneum and these cells are sloughed off to the environment. As
the cells migrate to become the stratum corneum, they become flattened, lose
their nuclei, and the organized cell contents becomes replaced with keratin
fibrils. The turnover time from germination to sloughing is about 21 days.
It is the stratum corneum that is the barrier to drug penetration through the
skin. The stratum corneum is approximately 10 microns thick. It can swell to
approximately three times its original thickness and absorb about five times
its weight in water. When the stratum corneum hydrates, it becomes more permeable.
Therefore, occlusive dressings are often used to hydrate the stratum corneum
and increase the penetration of certain drugs. Dermatoses such as eczema and
psoriasis can also hydrate the stratum corneum and increase the absorption of
Dermatological formulations produce a local drug effect either on or in the
skin. Besides the specific therapeutic action of incorporated active drugs,
the formulations also as serve as protectants, lubricants, emollients, or drying
agents. Examples of treatments using dermatological formulations include minor
skin infections, itching, burns, diaper rash, insect stings and bites, athlete's
foot, corns, calluses, warts, dandruff, acne, psoriasis, and eczema.
Some dermatological formulations are intended to systemically deliver a drug.
Percutaneous (through the skin) absorption is the result of three competing
- the potential of the drug to cross the stratum corneum
- the potential of the drug to leave the formulation
- the influence of the formulation on the stratum corneum.
Although the percutaneous absorption of drugs is a complex process, several
generalizations are possible:
- More drug is absorbed when the formulation is applied
to a larger surface area.
- Formulations or dressings that increase the hydration
of the skin generally enhance percutaneous absorption.
- The greater the amount of rubbing in or inunction of
the formulation, the greater the absorption.
- The longer the formulation remains in contract with
the skin, the greater will be the absorption.
Percutaneous administration affords an ease of administration not found in
other routes, and patient compliance is generally good with dermatological formulations.
There is also the possibility of continuous drug administration exemplified
by the transdermal patches. In addition, dermatological formulations can be
easily removed if necessary.
The major disadvantage of this route of administration is that the amount of
drug that can be absorbed will be limited to about 2 mg/day. This may become
a significant limitation if the route is being considered for systemic therapy.
However, several chemicals have been found that promote the percutaneous absorption
of drugs. These "penetration enhancers" improve the solubility of
the active drug in the stratum corneum and facilite the diffusion of the drug
through this barrier into the systemic circulation. Below is a list of penetration
enhancers used in dermatological formulations. (reference 1) Other commonly
used enhancers include DMSO (dimethyl sulfoxide), urea, and triethanolamide.
||methanol, ethanol, propanol, octanol
||myristyl alcohol, cetyl alcohol, stearyl alcohol
||myristic acid, stearic acid, oleic acid
|Fatty Acid Ester
||isopropyl myristate, isopropyl palmitate
||propylene glycol, polyethylene glycol, glycerol
||sodium lauryl sulfate
||benzalkonium chloride, cetylpyridinium chloride
||Spans®, Tweens®, poloxamers, Miglyol®