Viewed from different angles, the administration of drugs through the skin offers incomparable advantages

• Avoid first-pass hepatic metabolism which allows reducing the intake dose to achieve a therapeutic effect
• Easy administration compared with injectables, intranasal, and even oral dosage forms when the swallowing process is altered
• Accurate dosage metering, especially when referring to transdermal patches
• Easy medication management for patients and caregivers
• Wide administrations periods, ranked from hours to one a week
• Non-invasive
• Controlled drug release particularly useful in painkillers chronic administration and hormone replacement therapies
• Faster dosage interruption compared to other extended-release medications

In this article, it is solely considered “Passive” transdermal dosage forms, where the drug, freely diffuses through the skin due to a chemical gradient acting as a “driven force”. Other “Active” dosage forms require additional energy to deliver the drug. Make sense to mention within this group, the iontophoretic devices where, an electric current, forces the ionized drug to pass through the skin. Microneedles and electroporation cause a mechanical or thermal alteration of stratum corneum porous to improve the passage of drugs.

In this article, we particularly are focused on patches rather than on gels and cremes. Nevertheless, should be recognized that semisolid dosage forms have been demonstrated to be safe and efficient to deliver hormones, corticoids and anti-inflammatories, becoming key medicines for the treatment of specific diseases.

Although the benefits of transdermal delivery appear to be outstanding for a large number of drugs and therapies, only a small number of transdermal products are available worldwide.

The main reason for the limited existence of drugs administered transdermally is a consequence of the characteristics of the APIs involved, specifically, the size of the molecule and the therapeutic dose. In fact, not all APIs are likely to be administered through the skin to produce a scientifically proven therapeutic effect.

The landscape for “Passive” transdermal administration suddenly drops into small-size molecules, dismissing from the scenario a lot of current, highly efficient therapies, involving medium and big size molecules.

In general terms, it is accepted that an API is feasible to be administered through the transdermal route if meets the following attributes:

• Molecular weight < 400 D
• Daily dose < 20 mg/day
• Melting point < 200 ºC
• log P_O/W: –1.0 and 4.0
• Slightly water soluble
• Half-life < 10 h
• No irritant and no sensitizing

Permeability of a drug is defined as the amount diffusing through one square centimeter of intact skin during a finite period of time. Thus, the dose delivered can be metered by adjusting the area of the skin affected and the involved period of time. Should be considered that there is a rational threshold, limiting, from the wear and comfortability of the patient, the acceptable size of a transdermal patch.

In line with the size, the patches offer and clear advantages over gels and cremes since the area of skin involved is very well defined by the patch size while in semi-solids, the area affected is more difficult to be limited either in single or in repeated dose administration.

The following table summarizes the most relevant characteristics of APIs marketed worldwide from transdermal patches