The first tier decision point for the classification of a product is based on tissues to which the active is delivered. The
broad categories are gastrointestinal tract, tissues or body fluids by injection, mucosal membranes, skin surface, or lungs
(8). Although parallels can be drawn to the classification of nasal, ophthalmic, otic, urethral, and vaginal dosage forms
(i.e., the mucosal membrane delivery route), this article's discussion is limited to dosage forms applied to the skin surface.
The skin surface delivery route is further categorized as topical or transdermal. Topical is for the localized treatment of a dermatological condition, and transdermal refers to products that use the skin surface as a portal for the systemic delivery of active pharmaceutical ingredients.
Although we think of semisolids for topical delivery and patches for transdermal delivery, it is possible to have transdermal
gels and topical patches. An example of a transdermal gel is AndroGel (testosterone gel, Solvay Pharmaceuticals), 1% of which
"provides continuous transdermal delivery of testosterone" (9). This article does not include transdermal products, rather
it will be limited to topical dosage forms applied to the skin surface. In addition, products applied to normal skin for localized
effect will not be discussed. Examples of this type of product includes Emla cream (lidocaine 2.5% and prilocaine 2.5%, AstraZeneca)
and Lidoderm (lidocaine patch 5%, Endo Pharmaceuticals) (10, 11). The method for classifying topical dosage forms is clear
for products applied for skin effects but can be ambiguous for products applied for localized effects (i.e., APIs targeted
to underlying muscle, tendons, nerves, etc). Because bioequivalence to some of these localized effects products can be established
using plasma assays, it is suggested that products applied to the surface of normal skin for localized effect be categorized
as transdermal dosage forms.
CDER Data Standards Manual list of dosage forms for the delivery of pharmaceutical actives to the skin
Is the dosage form a liquid or semisolid?
Once the skin surface is determined as the delivery route, a topical dosage form for dermatological application is further
categorized using a series of questions (4). The first question in the classification decision tree is, Is the dosage as supplied,
and during use, either a liquid or a semisolid? If the answer is no, then the dosage form is an aerosol, cloth, lipstick,
patch, plaster, poultice, powder, soap, sponge, spray, stick, or swab.
A list of liquid or semisolid dosage forms defined by CDER Data Standards Manual are provided in the sidebar "CDER Data Standards Manual list of dosage forms for the delivery of pharmaceutical actives to the skin." These topical dosage forms include jelly, liniment,
oil, rinse, salve, shampoo, and tincture. Some of these terms could be considered obsolete for drug products. For example
jelly, liniment, salve, and tincture could be better classified as solution, paste, or cream. The sidebar "Dosage forms to
deliver pharmaceutical actives to the skin" contains a similar list of terms taken from the glossary published by the USP
Pharmaceutical Dosage Forms Expert Committee (PDF EC) (5). The list of terms and definitions was divided into two parts, with
the second part including "definitions of nonpreferred and/or outdated terms" (5). It should be noted that the dosage forms
titled cleanser, pads, pledgets, scalp application, and wash have been used for approved FDA prescription products, despite
these terms not being defined in either the PDF EC article or in CDER Data Standards Manual (5, 6).