The Upside of Transdermal Delivery
Aside from invasive injections, transdermal delivery offers the greatest potential efficiency for drug delivery.
Advantages that transdermal delivery offers over oral, sublingual, topical or inhalation1:
- Improved bioavailability
- Skip 1st Pass metabolism
- Uniform blood plasma levels
- Fewer drug interactions
- Smaller doses needed
The biggest barrier to these transdermal benefits is the largest organ in the body – our own skin. Very simply, the skin is made of three main layers: epidermis, dermis, and subcutaneous tissue. The outer layers of the epidermis – the stratum corneum - provides the most resistance to molecular penetration.
Once a molecule is able to penetrate the stratum corneum, its target destination is:
- The lower layers of the viable epidermis - where the molecule can act on nerve endings or other receptors for localized treatment.
- The dermis - where the molecule will begin to accumulate and then become steadily absorbed by the peripheral blood vessels and carried into systemic circulation.
Most compounds do not posses the physiochemical criteria to permeate the stratum corneum, making transdermal delivery a challenge. An ideal molecule for transdermal delivery has the following properties: Low molecular weight (<500 Da). Adequate lipophilicity (logPo:w ~1-3). A low melting point.2 Transdermal products are specifically formulated and developed to optimize these conditions.
In part two of this series we will discuss advancements in molecular technology and the impact that it may have in transforming unfavorable molecules to ideal candidates for transdermal delivery.
In part three – we will discuss how patch technology could take these modified molecules two steps further.
- MD Edge, April 9, 2010 Transdermal Patches: Benefits May Outweigh Risks, R. Guilford-Blake
- Curr Drug Deliv. 2019 Jun; 16(5): 444–460