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Anti-epileptic drugs (part 1)
For most people with epilepsy, the treatment for their seizures includes anti-epileptic drugs (AEDs). But what do these drugs do? Anti-epileptic drugs (AEDs) aim to stop seizures from happening. In part one of AEDs, we look at what AEDs aim to do, and how they get to the ‘site of action’.
What do they do (and what don’t they do)?
AEDs do not cure epilepsy or treat the reason why epilepsy has started. They are taken to try and stop the symptoms of epilepsy – the seizures. They aim to stop seizures from happening, and do not stop a seizure once it has started. They are prophylactic (preventative), and are usually taken either once or twice a day. Before we look at how AEDs work on the brain to stop seizures (in the next Back2Basics), we need to consider how they get to the brain in the first place.
From start to finish
To work, drugs need to get from where you take them to where they start to work. They need a route to get there and a waste-removal system to get rid of them afterwards. So, let’s look at this journey, the transport, and the stops along the way. And let’s use the analogy of a trip from home to work.
Departure – the route of administration
There are various ways medications are taken, by mouth, by injection (into the vein, muscle, or just under the skin) or by suppository (up the bottom). AEDs are usually taken orally (by mouth), and are tablets, capsules, liquids and syrups.
Taking the AEDs is the first part of the journey – like getting into a car at home.
From mouth to stomach
The next leg of the journey is driving from home (the mouth) to the train station (the stomach) down the oesophagus.
Absorption in the stomach
For drugs to be able to get around the body to the places where they need to work (the site of action) they need transportation and they need a route.
Once swallowed, AEDs go into the stomach. Digestive juices in the stomach help to break down food and, in this case, the tablets containing the medication.
The tablets break down, releasing the medication which can then pass through the wall of the gut into the bloodstream (absorption), and be transported around the body. The quicker the medicine gets into the bloodstream the quicker it can get to work. The AEDs getting from the gut into the bloodstream is like boarding the train.
Once a medicine is absorbed it is bioavailable, which means that it is in a state where it can act and do the job it is supposed to.
Getting to the site of action
Once in the bloodstream, AEDs are carried to the site of action: in this case, the brain.
How well and quickly the drugs get to their site of action depends on how well the part of the body is supplied with blood (how good the transport links are), and how easily the drug gets from the blood stream into the part of the body (how busy the train is).
Although the brain has a good supply of blood, there is a barrier between the blood and the brain that helps protect the brain from infections and toxic chemicals. This means that drugs do not pass easily into the brain. To use our train analogy, this is like only being able to exit the train from some, not all, doors.
We’ll find out more about how AEDs work at the site of action (at work) in the next Back2Basics.
And afterwards – the journey home
Once drugs have played their active role, they start to break down (metabolise) so that they can be excreted from the body (passed out through the digestive tract, like with food and drink) or in the urine. How long before AEDs start to be metabolised varies from one to another and is referred to as half-life.
To be excreted in the urine, AEDs have to be broken down so that they can dissolve in water, and then the kidneys can get rid of them. Some AEDs become inactive when they are metabolised.
Most AEDs are metabolised in the liver (hepatic metabolism) where they are changed into water-soluble metabolites with the help of different enzymes. Some AEDs – gabapentin, vigabatrin, levetiracetam and pregabalin – are not metabolised, not affected by hepatic enzymes and they are excreted in the same form in the urine.
Glossary
Absorption: where a substance moves from one part of the body to another, for example moving from the stomach into the blood stream.
Bioavailable: a form of substance (in this case an AED) that is in its active state and it can start working. Excreted/excretion the process where substances are removed from, or exit, the body, such as through urine.
Enzymes: substances that help a reaction to take place without themselves changing.
Half life: the length of time it takes for half of the drug to be inactivated and excreted.
Hepatic: relating to the liver.
Metabolise/metabolism: the process where substances are broken down so that they can be excreted from the body. Metabolism is usually hepatic (in the liver) but can happen in other parts of the body. It usually involves different types of enzymes. Once metabolism happens the resulting substances are metabolites.
Site of action: the place in the body where the drug works. In the case of AEDs this is the brain.
© Epilepsy Society
December 2007
