Transdermal Drug Delivery Expert Speaks At PSC Graduate Student Seminar
Presentation highlights approaches in assessing the bioavailability of topical drugs and examines what research is needed to enhance transdermal drug transport.
By Malissa Carroll
June 3, 2013
On May 20, Richard Guy, PhD, Professor of Pharmaceutical Sciences at the University of Bath in the United Kingdom delivered a special lecture to students in the Pharmaceutical Sciences (PSC) Graduate Program at the University of Maryland School of Pharmacy. This lecture, sponsored by the School’s student chapter of the American Association of Pharmaceutical Scientists, spotlighted advances in the field of transdermal drug delivery and addressed key problems that researchers encounter in their quest to develop new drugs that can be delivered across the skin.
“Dr. Guy is a pioneer in the field of transdermal drug delivery,” says Audra Stinchcomb, PhD, professor in PSC, whose research also focuses on transdermal drug delivery. “He was responsible for the creation of the Potts-Guy equation, which is used to predict the dermal absorption of most compounds, and is one of the most respected members of the dermal absorption research field.”
Guy received his master’s degree in chemistry from Oxford University and his doctorate in pharmaceutical chemistry from the University of London. In 2004, he joined the University of Bath as a professor in pharmaceutical sciences after holding appointments at the University of California, San Francisco and the University of Geneva. At the University of Bath, his research focuses on skin barrier function characterization, transdermal drug delivery, enhancement of percutaneous absorption, iontophoresis, non-invasive biosensing, and the prediction and assessment of skin penetration and topical bioavailability.
“I’ve spent my career studying drug delivery across the skin, because I think the skin is a fascinating membrane,” says Guy. “However, the problem with skin is that its primary purpose is to serve as a protective barrier. Whether you want to deliver drugs to treat skin conditions, such as psoriasis or eczema, or you want to use a patch to deliver drugs systemically, the skin poses a barrier that you must figure out how to overcome.”
To begin his presentation, Guy outlined the key questions that researchers encounter when developing new drugs for transdermal delivery, including how to identify the important rules that must be applied to select the best drugs for transdermal delivery, how to determine the bioavailability of topically applied drugs for local effect or to deliver them below the skin for systemic effect, and how to identify the benefits associated with the application of certain novel technologies.
“The best compound for topical or transdermal effect is not necessarily the most potent molecule in a typical cell-based screening process,” says Guy. “Both inherent activity and the potential for the molecule to transport across the barrier to reach its target must be considered. In other words, a highly potent compound that is unable to penetrate the skin will not be effective when applied topically.”
He addressed current approaches used to measure the bioavailability of topical drugs, including techniques such as microdialysis and tape stripping, which are currently the subject of most attention.
Tape stripping involves the use of adhesive tape to remove thin layers of an individual’s stratum corneum, the top-most layer of skin. “Tape stripping can provide a lot of information, which can teach us about the key parameters related to the transdermal delivery of drugs,” says Guy.
However, although Guy noted the promise of microdialysis, which uses a fiber placed under the skin to determine drug concentration, as a method to measure the bioavailability of topical drugs, he noted that it is typically a very difficult procedure to perform and can produce results with great variability.
“Topical formulations are not always as effective as they should be,” says Guy. “Once applied and massaged into the skin, constituents of the formulation may diffuse with the drug into the barrier or evaporate from the surface. This means that a residual film of the remaining vehicle containing the drug is left on the skin. The solubility and release of the drug from this film resulting from the ‘metamorphosis’ of the formulation may be quite different than that from the original preparation.”
To conclude his presentation, Guy addressed potential ways to enhance the transport of transdermal drugs and offered an objective look at “minimally invasive” technologies to porate the skin; thereby, increasing the rate and extent of drug delivery.