Drug Metabolism

In my previous post, I mentioned the ADME acronym outlining the four major processes that happen to drugs: Absorption, Distribution, Metabolism and Excretion. In the previous post, I covered absorption and distribution. Now I'm going to cover metabolism- hence the title of this post.

1) Show an understanding of the term “biotransformation” and the importance of this phenomenon to mammalian organisms.

"Biotransformation," or metabolism, is the modification of drugs and other substances by chemicals in the body. This is important for several reasons. Firstly, just like enalapril, which I mentioned in my previous post, less active or inactive forms of a drug can be converted to more active forms. Secondly, biotransformation often serves to make drugs easier for the body to excrete, often by making them more water-soluble (for excretion in the kidneys). This prevents bioaccumulation and toxicity.

There are many types of reactions involved in biotransformation. Many of the enzymes required are located in the endoplasmic reticulum of liver cells, but they may be found in other locations as well. The most common types of reactions are oxidation, sulfation and glucuronidation (the addition of glucuronic acid, which is a carboxylic acid derived from glucose). Other less common reactions include acetylation, glutathione conjugation (glutathione is a tripeptide), methylation, amino acid conjugation, reduction and hydrolysis.

2) Identify the main classes of oxidative drug metabolism, showing an understanding of the CYP450 system in terms of basic enzymology, genetics and role in human drug metabolism.

There are two main types of oxidative drug metabolism: one in which oxygen is added and one in which hydrogen is removed. An example of an enzyme that catalyses the first type is Cytochrome P450, which I'm going to talk about very shortly. An example of an enzyme that catalyses the first type is alcohol dehydrogenase, which apparently we get to play with in the lab this week. Yay?

Okay, so back to the topic of Cytochrome P450, or CYP450 for short. There are four main families of CYP450, imaginatively named CYP1, CYP2, CYP3 and CYP4. The first three are the most important in the metabolism of xenobiotic (i.e. foreign substances like drugs) substances, while CYP4 is mostly used in the metabolism of fatty acids. All four types contain a Fe atom attached to a haem group, kinda like haemoglobin. There are also many different subtypes (a.k.a. isoforms) of CYP450. One of the most important CYP450 isoforms in human drug metabolism is called CYP3A4.

In order for CYP450 to do its job, it requires the electron carrier NADPH (which has made a cameo appearance in my posts before) and oxygen gas. CYP450 is located with NADPH oxidoreductase in the endoplasmic reticulum of liver cells. I have a very limited understanding of how this works but the NADPH oxidoreductase appears to move an electron from NADPH to CYP450, allowing it to carry out its job. In the end one of the oxygen atoms from O2 inserts into the drug, while the other joins with the H+ from NADPH to form water.

The activity of CYP450 can be induced by the introduction of drugs and certain other substances. Essentially, drugs bind to certain receptors in the cytosol known as xenosensors. This drug-xenosensor complex can then enter the nucleus, where it acts as a transcription factor and induces the synthesis and thus increases the activity of CYP450. It usually takes around a week for the effects to be seen. These effects include faster metabolism of the drug, which means that the drug leaves the body more rapidly. Hence more doses of the drug may need to be taken to have the desired effect.

CYP450, like other enzymes in the body, is also prone to competitive inhibition: when multiple substances are "competing" for the same enzyme. This may slow down or completely inhibit metabolism of drugs. In fact, CYP450 inhibition is a major cause of drug-drug interactions (DDIs). When metabolism is slowed down, the body may not be able to eliminate the drug as rapidly, leading to accumulation and toxicity. Alternatively, the body may not be able to convert an inactive drug into its active form, so that the patient may not experience the benefits.

An example of this CYP450 inhibition is the coadministration of fluoxetine (Prozac) and codeine. Fluoxetine inhibits CYP2D6, one of the isoforms of CYP450. Codeine, however, also relies on CYP2D6 to be converted into morphine and eventually morphine-6-glucuronide, both of which are actually responsible for the analgesic effect of codeine. Hence, coadministration of the two drugs means that codeine doesn't get converted into morphine and the patient doesn't experience relief from their pain.

CYP inhibition sounds annoying, but it can actually be used to have a beneficial effect. For example, Stribild, which is a cocktail of anti-HIV drugs, includes a CYP450 inhibitor. This CYP450 inhibitor stops the other drugs from being metabolised too quickly, so the patient only needs to take Stribild once a day.

There are several other factors that can affect the action of CYP450. One of these is genetic polymorphisms, such as deletions, insertions and repeats in the CYP450 gene. This leads to the rise of normal metabolisers (NM), poor metabolisers (PM), intermediate metabolisers (IM) and ultra metabolisers (UM) within a population.

3) Identify the main classes of conjugative drug metabolism, including sulfation & glucuronidation.

Conjugation is simply the addition of another molecule or substituent onto a drug. I've already mentioned several types, but I'm going to go into slightly more detail on sulfation and glucuronidation as they are the most common types of conjugative reactions. (By more detail I mean I'm just going to list some enzymes and cofactors. Enjoy.)

As previously mentioned, glucuronidation is the addition of glucuronic acid onto a drug. It is catalysed by UDP-glucuronosyltransferase, which uses UDP-glucuronic acid as a cofactor.

Sulfation is literally just the addition of a sulfate group (SO₃). It usually happens on -OH and -NH₂ groups. It is catalysed by sulfotransferase, which is located in the cytosol of certain cells. The cofactor here is PAPS (3'-phosphoadenosine 5'-phosphosulfate), which donates the sulfate group. (I've mentioned PAPS before in an earlier post about protein modifications.) Products formed from sulfation are usually very water-soluble.