Drug Administration part VI

Dermatologic Agents

Pharmaceutical preparations applied to
the outer skin are intended either to
provide skin care and protection from
noxious influences, or to serve as a
vehicle for drugs that are to be absorbed
into the skin or, if appropriate, into the
general circulation .

Skin Protection
Protective agents are of several kinds to
meet different requirements according
to skin condition (dry, low in oil,
chapped vs moist, oily, elastic), and the
type of noxious stimuli (prolonged exposure
to water, regular use of alcoholcontaining
disinfectants , intense solar irradiation).
Distinctions among protective
agents are based upon consistency, physicochemical
properties (lipophilic, hydrophilic),
and the presence of additives.

Dusting Powders are sprinkled onto
the intact skin and consist of talc,
magnesium stearate, silicon dioxide
(silica), or starch. They adhere to the
skin, forming a low-friction film that attenuates
mechanical irritation. Powders
exert a drying (evaporative) effect.

Lipophilic ointment (oil ointment)
consists of a lipophilic base (paraffin oil,
petroleum jelly, wool fat [lanolin]) and
may contain up to 10 % powder materials,
such as zinc oxide, titanium oxide,
starch, or a mixture of these. Emulsifying
ointments are made of paraffins and
an emulsifying wax, and are miscible
with water.

Paste (oil paste) is an ointment
containing more than 10 % pulverized
constituents.

Lipophilic (oily) cream is an emulsion
of water in oil, easier to spread than
oil paste or oil ointments.

Hydrogel and water-soluble ointment
achieve their consistency by
means of different gel-forming agents
(gelatin, methylcellulose, polyethylene
glycol). Lotions are aqueous suspensions
of water-insoluble and solid constituents.

Hydrophilic (aqueous) cream is an
emulsion of an oil in water formed with
the aid of an emulsifier; it may also be
considered an oil-in-water emulsion of
an emulsifying ointment.
All dermatologic agents having a
lipophilic base adhere to the skin as a
water-repellent coating. They do not
wash off and they also prevent (occlude)
outward passage of water from
the skin. The skin is protected from drying,
and its hydration and elasticity increase.
Diminished evaporation of water
results in warming of the occluded skin
area. Hydrophilic agents wash off easily
and do not impede transcutaneous output
of water. Evaporation of water is felt
as a cooling effect.
Dermatologic Agents as Vehicles
In order to reach its site of action, a drug
must leave its pharmaceutical preparation
and enter the skin, if a local effect
is desired (e.g., glucocorticoid ointment),
or be able to penetrate it, if a
systemic action is intended (transdermal
delivery system, e.g., nitroglycerin
patch, ). The tendency for the drug
to leave the drug vehicle is higher
the more the drug and vehicle differ in
lipophilicity (high tendency: hydrophilic
D and lipophilic V, and vice versa). Because
the skin represents a closed lipophilic
barrier, only lipophilic
drugs are absorbed. Hydrophilic drugs
fail even to penetrate the outer skin
when applied in a lipophilic vehicle.
This formulation can be meaningful
when high drug concentrations are required
at the skin surface (e.g., neomycin
ointment for bacterial skin infections).