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What Conditions must Drugs satisfy for them to be Deliverable via the Transdermal route? I assume a high $\log{P}$ (lipophilicity) would be one, are there any others?

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Whether a drug can be delivered transdermally? I think that carriers can be formulated to carry a small molecule through the skin into circulation, so I don't even know about lipophilicity. I believe if you spend enough money, you can overcome many of these problems. I think that pharmacology might be just as important.

I think the first condition of a transdermal administration is that the drug cannot be effectively administered orally. This is the most cost effective route go get a pharma compound into the body. It requires no devices, no fancy coatings or vaporizors.

Reasons that a pill (or its less popular cousins the suppository and sublingual tablet) cannot be used as the delivery system for a drug include effect of pH and other degradation of the compound in the GI tract, absorption rate from the GI tract and other factors such as effect of food in the system, other drugs which might interact in the system.

Another advantage of absorption through the GI tract is that this is the physiological system which is meant to be ingested and introduced into the body - its relatively fault tolerant, even if less of the drug actually reaches the blood stream. There's more, but that's not a bad outline.

After pills and the like, the second choice is intravenous administration, which puts 100% of the drug into the circulatory system. This has its own disadvantages, requires a clinical setting and the needle and breaking of the skin are not ideal in many cases.

If the drug needs continuous and non clinical administration there are many other means of drug delivery, including transdermal administration. Differentiating these methods often includes making sure the drug is available specifically to the part of the body that needs it. E.g. topical administration; putting on a medicated cream for a skin condition or use of an inhaler which delivers the drug directly to the lung for an asthmatic condition.

Transdermal administration has its own specific advantages; continuous distribution of the compound over long periods of time. Disadvantages include: poor ADME properties for biologicals like antibodies and other proteins, low delivery concentrations where you need a large amount added (epinephrine, and maybe antibiotics come to mind).

Examples of successful transdermal delivery are testosterone gels and the nicotine patch. Both of which apply continuous controlled low dosage that is important there.

There are three typical routes of drug absorption in transdermal delivery. The drugs can diffuse through the membranes of dead cells of the stratum corneum, which works if the drugs are lipophilic enough. They can diffuse around these cells, through the intercellular spaces, which can work for more polar drugs although this path is not usually as efficient. The most extreme means are patches which contain tiny needles that can deliver straight through the protective layer of the skin. With enough economic investment probably any sort of compound could be introduced through a patch.

Most of this is just cribbing off the notes from this lecture on coursera: Fundamentals of Pharmacology. Its probably outlined on any pharmacology text.

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What about fentanyl? Fentanyl can be delivered orally yet it is also available in transdermal patches. Nicotine is water soluble but it is also lipophilic which is how it manages to cross the BBB so easily. –  Brenton Horne Feb 9 '13 at 19:50
    
I tried to address this in edits. I don't claim to cover all transdermal delivery, just trying to answer the question at the fairly general level it was being asked. I would have to do some digging re: fentenyl. I might have time tonight. –  shigeta Feb 9 '13 at 21:15
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