Transdermal delivery to veterinary patients, especially cats, is an answer to an age-old prayer from both veterinarians and pet owners. To deliver medication non-invasively is truly a landmark in veterinary pharmacotherapy. Transdermal delivery of medications has been well documented in veterinary medicine for ivermectin, selamcctin, fipronil, nitroglycerin, imidacloprid, and fentanyl. The desire to extrapolate other drugs to transdermal delivery is great, however, not every drug utilized in veterinary pharmacotherapy is suitable for transdermal delivery. While many drugs can be delivered transdermally to veterinary patients, many drugs cannot.
A recent FDA concept paper on transdermal drug delivery suggests that the maximum amount of drug delivered by a transdermal patch could not exceed 1 mg per square centimeter. Considering the surface area of the pinnae on any cat, it is unlikely that more than 50 mg of any drug could likely be absorbed transdermally. Prior to compounding any drug in a transdermal delivery dosage form, the veterinarian and compounding pharmacist must carefully consider the pharmacology of the drug in the target species as well as identify specific objective assessment parameters to determine safety and efficacy. The veterinary care team must also determine if the risk of drug exposure to the client during administration precludes use of a transdermal dosage form. Transdermally-delivered drugs obviously bypass hepatic portal first pass extraction, and extrapolating oral doses to transdermal ones can result in toxicity. To date there have been no published pharmacokinetic, safety or efficacy studies on compounded transdermal medications used in veterinary patients. Until controlled studies provide evidence for safety and efficacy, the compounding pharmacist and veterinarian must use their best scientific judgment to predict whether a particular drug is suitable for transdermal delivery in a given patient. When transdermal agents are used, comprehensive data should be catalogued to determine clinical success or toxicity. For example, therapeutic blood concentrations to quantify serum drug levels should be conducted to determine drug disposition for those drugs with established therapeutic concentrations (e.g. phenobarbital, theophylline, aminoglycosides). Any diagnostic results should also be catalogued for a response to transdermal therapy (e.g. blood glucose, blood pressure, serum T4 levels, and urine culture results). Ideally, transdermal dosage forms should only be used when traditional oral and injectable routes have been exhausted without achieving therapeutic success. This article presents considerations for transdermal use of several veterinary medications including pharmacokinetic considerations and possible assessment parameters for determining toxicity and efficacy. Presentation of this information is meant only as a catalyst for thought and in no way endorses the safety or efficacy of any of the drugs listed when administered transdermally.