What are the advantages or disadvantages of transdermal (patch) absorption of medication versus oral/gastrointestinal absorption?
There is an increasing useage of transdermal patches for varying medications, especially analgesics and antiinflammatories, not to mention thermal heat and nicotine. How does a physician determine which is the better choice: patch or pill?
In virtually most cases, making the pill is easier R&D wise. The basis of making patches and topical creams comes from the observations and often requests of physicians who find that IV or pill based drug delivery aren’t suiting the needs of a patient. Very often, this starts with doctors making their own creams before a company commercializes it. This only explains where the draw comes from but doesn’t explain the rationale.
Topical creams and patches offer a different way of controlling the pharmacokinetics of a drug. An ideal pill is taken once-a-day because it is a simple concept to remember and understand. However, if there are side effects at high plasma concentrations or if the metabolism is very quick and it is difficult to make a continuous release drug (as in Ambien CR), then patients will have to take the pill multiple times a day which is either annoying, easy to skip, or difficult for overnight situations.
Patches and creams allow for control of both the dose and the time. Twice the size of a patch means twice the dosage. When you want to stop dosing, you remove the patch. While the drug is still in your system your plasma concentration is already fairly low. This is a common way for smokers to control their nicotine dose.
Patches and creams allow for continuous release. The mechanism of drug delivery is based on diffusion of the drug through the patch to your skin and into your blood stream. This is different from an IV injection which is an instantaneous dose or a pill which is a time-delayed but concentrated dose depending on where the drug is absorbed. Patches and creams allow you to keep a low plasma concentration which avoids side effects and the continuous release gives you a larger AUC ie. a longer time of effective dosing. Opioid drugs like morphine are ideal candidates for patches.
Patches and creams allow for site-specific dosing. Assume that you have eczema, you don’t need to take whole body drugs to attack your small skin infection. A cream would be simpler. Lets say that your knee is inflamed. You would want your drugs to go there and not necessarily messing around with the rest of your fragile body. You now affect the absorption and distribution of the drug.
Patches and cream avoid first-pass metabolism. Possibly the drug doesn’t absorb in the GI tract well and effectively has a low bioavailability. In addition, the drug may be rapidly metabolized in the liver. While the bioavailability of a patch is also fairly low, depending on where the patch is placed, it could avoid those two routes of metabolism and elimination.
There are also some other silly things that often define the success of a drug. Things like taste or color come to mind. Maybe it is very difficult to make a liquid-based formulation for kids.
The reasons why you won’t want this is because the pill is much easier to make. The skin is evolved to be a chemically resistant barrier and it is a very difficult surface to force drugs through. Furthermore, there is a large optimization problem of designing the matrix that contains the drugs and the chemicals that interact with the skin, all things that are case-by-case issues for each drug.
The R&D behind patches and creams are also a pain in the neck. Franz Diffusion Cell systems that replicate the skin are very tricky assays to use. Furthermore imagine testing this on the poor lab mice in a consistent manner. Frequently, you will only see new patches for drugs that are already FDA approved.
For those skeptical about the good will of pharma companies, I will add that making a pill into a patch is a fairly clever way of creating new patents on expiring and expired drugs and many companies have gone this route as a method of re-purposing. It is also a viable business strategy to make patents that will force a larger drug company to in-license it to you. It would not surprise me that drug companies will continue to make patches.