won’t it be more efficient if delevered transdermally ? because it will directly get absorbed into blood stream whereas in oral route, it has to go through gastric channel and liver thus decreasing its efficiency.
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Because to be absorbed transdermally, any molecule (drug) needs to be somewhat lipophilic (Lipid or fat ‘loving’) and non-polar. This is to be able to pass through the cell membranes of skin and other tissues which are made of phospholipids which are non-polar (and like dissolves like). Molecules also need to be small enough to enter other paracellular pathways (between cells) for absorption in the case that they cannot pass through biological tissues. This is the short version, there is a lot of nuance to drug absorption, which is why a lot of research is done to formulate products for optimal transdermal absorption. However, the drug in question needs to be an ideal candidate at first, or else no amount of formulation will help.
The skin is pretty good at keeping things _out_ of the blood stream. That is actually the primary purpose of skin – to keep the outside separate from the insides.
Some stuff can be absorbed through the skin, sure, but not everything can and even the things that can be are often absorbed slower than an injection or a pill.
We use transdermal patches where they make sense, but we can’t use them for everything because the skin is good at its job.
Medication delivery is a pretty big challenge. The goal is to get the medicine to the right tissue and minimize its effects everywhere else.
The types of drugs that can be delivered transdermally are typically very small molecules and water resistant. It allows them to diffuse across cell membranes all the way to the blood where it can then be pumped throughout the rest of the body. This represents quite a small amount of medications.
There are also some limitations to oral medications. First, they have to be absorbed without being broken down in the gut. For example, that eliminates oral delivery for anything that ends with “-ab” which are digested in the gut and have to be given via IV. After the drugs are absorbed, they go straight to the liver before the heart can pump them anywhere else. Any drug that is metabolized by the liver will lose some effectiveness that way. This is the reason nitroglycerin is often administered by putting a tablet under your tongue. When it diffuses into the blood it goes straight to your heart and gets distributed to the entire body quickly, bypassing the initial liver breakdown.
TL;DR every method of drug delivery has its benefits and limitations. Some drugs can be adapted for multiple delivery methods and others cannot.
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