Have you ever wondered how that pill you are taking for migraines really end up where it is supposed to be? Absorption of drugs into the body is a complicated process, but it is interesting to know a little bit about the different methods by which our body absorbs the medication we take. It can help us appreciate why it can take ten years or more to develop a drug when we consider that each tissue type and each condition requires a drug that responds to very specific characteristics in order for it to work. One of those being the method of absorption. Some diseases can actually impair how or how much we absorb, resulting in some patients experiencing toxic effects at a regular dose.
There are essentially four mechanisms by which we absorb drugs into our body:
The first one is called passive diffusion of fat soluble drugs. This indicated that the drug is soluble in fat and therefore has no trouble passing the fatty layer (membrane) around our cells. It therefore moves from an area with high concentration (inside the stomach or bowels) into an area with low concentration (the cells). Examples of drugs that use this mechanisms are diazepam, phenytoin and propranolol. These are an anxiolytic, anticonvulsant and a B-adrenoceptor blocker respectively.
The second mechanism is through passive diffusion of drugs that are less fat soluble (i.e., they are more soluble in water than in fat). These have to be small so they can pass protein channels embedded in our cell membranes called aquaporins. Examples of drugs that use this route are nicotine, salts (like litium carbon trioxide), caffeine and ephedrine (the official name for “Sudafed”).
Another method of drug absorption is through so-called facilitated diffusion. This is used by molecules that are not fat soluble, but too large to pass through the small pores in our cell membranes. Basically the molecule has to attach to another protein on the surface of the cell, which then “flips over” and releases the drug molecule inside the cell. Examples of drugs that use this method are L-dopa, to treat Parkinson’s disease, and antimetabolite nucleotides used in cancer treatments. In order for this to happen the drug molecule has to resemble a molecule the body already makes to “fool” the receptor protein.
The last method of drug absorption discussed is active transport. This costs energy and requires a high degree of structural specificity for the molecule. I.e. it only works with certain types of drugs, and only when the molecules have a very specific structure the proteins in our body can “recognize”. This is actually often used to remove drugs, rather than absorb then. An example of this is penicillin: it is removed by active transport from the kidney and deposited into the urine for elimination because the body considers it a “poison”.