by Beau Harger, PharmD, PCCA Clinical Compounding Pharmacist/Training Instructor

Popular compounds such as transdermal methimazole for hyperthyroidism and transdermal buprenorphine for pain in felines are common in the veterinary world. Not only has this delivery been a staple for pets like cats and dogs, in some instances, this method may be used for some exotic species as well. For example, delivery through the skin has been used under the wing in birds.

Key Considerations

The site of transdermal application is important. The ideal site has a thin stratum corneum with minimal hair follicles, adequate blood perfusion and is difficult for the patient to access (i.e., the pinna in felines and canines).¹ Another key consideration is the selection of the base vehicle. PCCA Lipoderm^® had been the industry-leading base used in permeation-enhancing compounded preparations for many years. While Lipoderm had met the need for an alternative dosing form, the industry was seeking anhydrous options for stability and extended-beyond use dates (BUDs) without compromising the skin-permeation performance.

Innovation Meets Demand

PCCA introduced PermE8^® Anhydrous Gel, an anhydrous, permeation-enhancing base, in 2018. PermE8 is classified as an anhydrous base due to water activity below 0.6 (Aw < 0.6).² Comparison testing between Lipoderm and PermE8 Anhydrous Gel was conducted in vitro using a human skin model that measured the skin percutaneous absorption of ketoprofen. Test results indicated that PermE8 performed comparably to Lipoderm and significantly outperformed Anhydrous Lipoderm^®.^2,3 Traditionally, anhydrous bases didn’t have a smooth feel; however, PermE8 offers a soothing, silky texture that makes application more enjoyable for patients. It was also formulated without preservatives and may be an option for those pets with sensitive skin.

Important Considerations

It’s important for pharmacists and veterinarians alike to understand that not all drugs are good candidates for transdermal delivery.

Important considerations include:

• Molecular weight of the drug
• Intended use of the preparation
• Dosing delivery
• If the drug has a narrow therapeutic index (i.e., warfarin, digoxin)
• Irritation potential of the drug on the patient’s skin
• Risk of drug transference to the caregiver/owner⁴

Drugs with large molecular weights may not pass through the skin and antibiotics could produce low serum levels, resulting in antibiotic resistance. If the veterinarian recommends proceeding with transdermal administration, it is important that monitoring parameters are established to assess the effectiveness of the preparation.

Compounding Tips

When compounding with PermE8, we recommend not exceeding a 20% powder load. This could include the pure API alone or with commercially available drug tablets or capsules containing excipients. Appropriate wetting agents are propylene glycol, diethylene glycol monoethyl ether or ethyl alcohol at a 5-10% concentration. We also recommend using an electronic mortar and pestle (EMP) on a high setting before and after milling to ensure full incorporation of wetting agent and API.

Permeation-enhancing formulations offer options for veterinary patients who otherwise may not receive treatment. Compounding pharmacies that provide these therapies should consider and have access to the newest technology — for people as well as pets — to optimize their influence in their community and in the compounding industry.

PCCA members with clinical services access may contact our Clinical Services team for answers to questions about permeation-enhancing topical preparations, as well as other compounding concerns.

References

1. Forsythe L. E. (2017). Feline Transdermal Formulation Considerations. International journal of pharmaceutical compounding, 21(6), 446–452.
2. PCCA PermE8® Anhydrous Gel. PCCA Document #99608.
3. PCCA (2018) Evaluation of the in vitro Human Skin Percutaneous Absorption of Ketoprofen in PCCA PermE8™ Anhydrous Gel vs. PCCA Lipoderm® PCCA Document #99732.
4. Eichstadt, L. R., & Davidson, G. S. (2014). To compound or not to compound: a veterinary transdermal discussion. International journal of pharmaceutical compounding, 18(5), 366–369.

These statements are provided for educational purposes only. They have not been evaluated by the Food and Drug Administration, and are not to be interpreted as a promise, guarantee or claim of therapeutic efficacy or safety. The information contained herein is not intended to replace or substitute for conventional medical care or encourage its abandonment.