Do Transdermal Antiepileptic Drugs Work the Same as Oral Medications?

Heidi Barnes Heller, DVM, DACVIM (Neurology), clinical associate professor of neurology/neurosurgery at the University of Wisconsin-Madison School of Veterinary Medicine, explains how transdermal antiepileptic drugs work in cats—and why the results weren't the same in dogs.

Heidi Barnes Heller, DVM, DACVIM (Neurology): My research is focused on transdermal and alternative routes of administration for antiepileptic drugs or anticonvulsants. And so we formulated a novel formulation of a transdermally applied or topically applied drug for use in cats that are epileptic that is phenobarbital. So we've put it through all of the various testing that I just described to show that it is absorbed, it is absorbed in the therapeutic range, it is trackable by blood samples—but it acts very differently than when you give it as an oral drug and you have to monitor differently, you have to keep track of their seizure control, and you need to keep track of their environment pretty carefully when you're using a transdermal compared to when you're using an oral therapy.

And so, there's not something that I have commercially made available, in the sense that I am not a producer of drugs I am a researcher, but there is transdermally available product that we have published how we have made it, what our compounding is of that drug, so that others can have it compounded. But there isn't a product where I could write down and say 'transdermal phenobarbital' and send it to a pharmacy and they know what that means. So you have to have the formulation or the recipe if you will. It's actually a lotion that you rub. So we dispense at an individual milligram dose for that animal. And then they rub it into the pinna—the ear—with a gloved hand, of course, because it's supposed to be absorbed through the skin. And so you rub it in with a gloved into the ear, either splitting the dose and putting it in each ear or just per ear.

I've only vetted it in cats. I tried single dose application in dogs and it failed immensely, but that wasn't surprising because we know that transdermals, if you apply them once, don't terribly work well. If you apply them multiple times, they have a chance to break down the barriers in the skin to allow absorption more readily and they can create drug depots, or little pockets of drug within the skin itself that allows it to slowly release over time, but it takes time to have that happen. So you really have to let that drug hit its steady state before it is even really probably useful for that animal. That's the same thing for oral, you still have to let it reach a steady state before you would consider it to be effective, if you will. But the transdermal in dogs, the utility that we saw was a dog that say is vomiting and can't take its oral medication, and yet I don't need to hospitalize it for its vomiting to give it IV phenobarbital for example. Why don't I just patch it through with some transdermal phenobarbital for a day or 2 until the vomiting has resolved for whatever the reason for the vomiting was. And that is not a practical use as we discovered, with transdermal phenobarbital at least, in dogs. They tolerated it fine, they are much easier to apply it to, but it wasn't absorbed.