Have you ever thought about applying a transdermal preparation to the inside of an animal's ear or another hairless area as an alternate route of systemic administration? It's quick and easy, and many medications are compatible with transdermal bases. Transdermal delivery is particularly useful for animals who should not be stressed due to cardiovascular or hypertensive illness. Also, it is appreciated by owners who no longer have to deal with an animal who resists being medicated, and the resulting scratches! We can also prepare topical medications for application at the site of inflammation or infection
Various alternative dosage forms permit medication to be absorbed via non-oral routes to meet an animal's specific needs. Although the parenteral and rectal routes are traditional alternatives to oral administration, transdermal absorption offers many advantages.

For example:
  • When medication is absorbed directly into the bloodstream without first entering the gastrointestinal system, a smaller amount of active ingredient may be required for therapeutic effect.
  • Direct application and absorption at the target site can mean higher tissue levels and lower blood levels of various medications. Side effects such as GI irritation can be eliminated.
  • Various types of drug interactions may be avoided when one or more interacting medications are administered transdermally.

A substantial number of references exist in human medical literature with regard to the efficacy of transdermal administration of non-steroidal anti-inflammatory drugs and other types of analgesics, antiemetics, and other medications. We can compound transdermal and topical medications using a suitable base, and add penetrant enhancers if desired.
Oral administration of atenolol at a median dose of 1.1 mg/kg every 12 hours (range, 0.8 to 1.5 mg/kg) in cats induced effective plasma concentrations at 2 hours after treatment in most cats. Transdermal administration provided lower and inconsistent plasma atenolol concentrations. Further studies are needed to find an effective formulation and dosing scheme for transdermal administration of atenolol.

“In theory, the transdermal route of administering medications has many potential advantages. It is noninvasive and not demanding technically, avoids first-pass hepatic metabolism and gastrointestinal breakdown, has potential for sustained release formulations, and can be administered over a large surface area. Transdermal administration of medication has been shown to achieve blood concentrations of drug that are considered to be therapeutic (eg, fentanyl) or efficaciously affect physiologic surrogates (eg, methimazole, nitroglycerine, and lidocaine). Feasibility of transdermal medication varies on a drug-by-drug basis.”

Discussion: In spite of these results, investigators did not conclude that transdermally administered atenolol is not feasible.Because two cats did achieve therapeutic blood concentrations of atenolol after transdermal administration, the authors called for further research to find a transdermal formulation and dosing regimen for atenolol that will consistently result in plasma atenolol concentrations of >260ng/ml.Investigators offered several considerations for future studies. This study utilized a hydrophilic carbomer/propylene glycol/glycerin gel vehicle which has been used in human delivery of transdermal medications. As pluronic lecithin organogel (PLO) is the transdermal vehicle used almost exclusively in veterinary medicine, investigators encouraged future transdermal atenolol research utilizing PLO as the vehicle.Investigators also noted that higher doses of atenolol (3mg/kg) have been reported to consistently result in blood levels providing adequate adrenergic blockade at 12 hours in all cats studied.Since the median atenolol dose administered in this study was 1.1mg/kg, researchers suggest studying transdermal atenolol at the 3.3mg/kg dose.

Because daily oral administration of atenolol to cats is challenging and often results in a lack of compliance, a non-invasive dosage form such as transdermal atenolol will most likely result in better compliance, less stress to the cat, and reveal a positive therapeutic effect.

Am J Vet Res. 2008 Jan;69(1): 39-44. Comparision of pharmacodynamic variables following oral versus transdermal administration of atenolol to healthy cats. Click here to read the PubMed abstract of this article.
The aim of a study conducted by Buijtels et al. of Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, Utrecht University, The Netherlands, and presented at the 16th ECVIM-CA Congress, 2006, was to develop a carbimazole gel for application at the inner pinna of the ear and to study its effectiveness in cats with hyperthyroidism. The results of this study indicate that twice daily administration of carbimazole gel at the inner pinna of the ear is an effective treatment of cats with hyperthyroidism.

Tijdschrift voor Diergeneeskunde. 2006; 131(13):478-82 [Transdermal carbimazole for the treatment of feline hyperthyroidism] Click here to read the PubMed abstract of this article