News on cytochrome P450 in the liver

Cytochrome P450 proteins influence drugs degradation in the body, but are also triggers of drug interactions. What influence do polymorphisms have on drug effects?

Was sind Cytochrome P450?

Cytochrome P450 proteins influence the degradation of drugs in the body, but are therefore also triggers of various drug interactions. What influence do polymorphisms have on the effect of drugs?

Article translated from the original German version

In 1958, researchers discovered a central pigment in the liver. The enzyme plays a key role in the degradation of foreign substances. In experiments, it binds toxic carbon monoxide (CO) to its reduced iron ion.

The name P450 is derived from the light absorption of the CO-cytochrome iron ion complex in reduced form. In the photometer, it shows a very typical maximum (peak) in the UV range at 450 nm. Hence the name cytochrome (short CYP) P450. Cytochromes P450 (CYP) are found in practically all living organisms, animals, plants, fungi and bacteria. In humans, CYP are mainly found in the liver, but also in the intestines, kidneys and lungs.

Cytochrome P450 ensures the detoxification of xenobiotics

Many poorly water-soluble substances, i.e. also numerous drugs, are "detoxified" in the liver via the process of biotransformation, i.e. they are broken down and excreted. These foreign substances (xenobiotics) are provided with a polar (positively or negatively charged) group through oxidation, reduction, hydrolysis or hydration and therefore made more hydrophilic ("water-loving") and more water-soluble. The products are then conjugated with endogenous substances such as glucuronic acid and then easily excreted via the kidneys or bile.

Cytochrome P450 is also responsible for the "toxification" of foreign substances

In most cases, foreign substances are detoxified in the body. Unfortunately, however, substances that were originally less reactive can also be "poisoned" into highly reactive ones. The benzo[a]pyrene produced by smoking and barbecuing is hydroxylated by P450 to the highly active benzo[a]pyrene diol epoxide. This attaches itself to DNA. This can ultimately lead to cancer. Benzo[a]pyrene is therefore "poisoned".

In humans, 57 different cytochromes P450 have been found so far. CYP3A4 metabolizes more than half of all drug substances; as the name suggests, it belongs to family 3, subfamily A, and is the 4th enzyme of this subfamily. Other cytochromes P450 important for the degradation of drugs are CYP2D6, CYP2C9, CYP2C19, and CYP1A2.

In humans, about 15 different cytochromes P450 are involved in drug metabolism. Some figures: For example, CYP3A4 participates in the metabolism of over 50% of drugs (such as erythromycin, cyclosporine, nifedipine). 15% of all drugs are broken down by CYP2D6, but 75-80% are broken down by P450. 5-10% of Europeans are barely able to break down and excrete certain ß-blockers, antidepressants and other drugs such as bufuralol, imipramine, and debrisoquine.

Cytochrome P450 form polymorphisms

Research in recent years has shown that several of these drug-metabolizing cytochromes P450 have so-called polymorphisms. For example, it has been shown that CYP2D6 is defective in 5-10% of the European (so-called Caucasian) population. This strongly reduces the degradation of drugs such as debrisoquine and bufuralol and can therefore lead to considerable side effects. On the other hand, there are patients who have multiple copies of this gene, resulting in very rapid degradation of the corresponding drugs.

There are numerous active substances known that can inhibit cytochromes P450. The most effective inhibitors of CYP3A4 include azole antifungals such as ketoconazole, HIV protease inhibitors such as indinavir or nelfinavir, and macrolide antibiotics such as clarithromycin.

But foods such as grapefruit juice can also impair enzyme activity (see figure).  CYP2D6 is inhibited, for example, by the antidepressants paroxetine, fluoxetine and bupropion.

Induction takes time: cytochrome P450 and nutrition

Enzyme induction is the increased formation of biotransformation enzymes through increased transcription of the corresponding genes. Enzyme induction is the basis for pharmacokinetic tolerance, i.e. the decrease in effect due to increased degradation of the pharmaceutical. It is well known that the effectiveness of sleeping pills decreases with constant use.

The induction of CYP enzymes leads to increased formation of the respective enzyme. However, the effect is not immediate: The maximum enzyme induction is only observed after two to three weeks and it can last for more than four weeks after discontinuation of the inducer. Inducers of CYP3A4 are, for example, rifampicin, barbiturates and carbamazepine, but also natural remedies such as St. John's wort extracts. Other cytochromes P450 are also increased in their activity by food and stimulants: broccoli and Brussels sprouts, but also smoking, can lead to increased formation of CYP1A2. The inhibition of cytochrome P450 by grapefruit juice is a classic in food-drug interactions.

Especially in the case of substances with a narrow therapeutic range, CYP induction can lead to loss of effect with quite serious consequences, e.g. unwanted pregnancy due to pill failure. Some cytochromes P450, for example CYP2D6 and CYP2C19, have a large genetic variability. Depending on the genetic CYP variants of a patient, his or her so-called "metabolizer status" can be derived, the speed with which he or she can metabolise the corresponding pharmaceutical agents:

Having the genotype determined in order to establish the influence of cytochrome P450

Is the patient a fast or slow metaboliser? A genetic test can provide information. There are often significant differences in the frequency of the individual metaboliser types between different ethnic groups. In Sweden, for example, only 1 to 2% and in Germany 3% of the population are ultra-fast metabolisers, whereas in North Africa their share is 20 to 29%.

image.5.pngThe variable functioning of the CYP450 enzymes is a possible reason why, at the same dosage of a drug, the intensity and duration of effects and side effects can vary greatly depending on the patient. If a corresponding drug treatment does not respond or is poorly tolerated, a precise genotyping of the individual patient can be useful. For example, with the help of the AmpliChip® CYP450 biochip from Roche, it is possible to diagnose the individual metabolisation type for CYP2D6 and CYP2C19 via a polymerase chain reaction (PCR). The test only requires a blood sample from the patient.

Molecular biology can therefore be used to clearly analyse these gene defects. Patients can then be given the appropriate individual dose of medication. However, such molecular genetic tests for individual drug therapy are currently not widely used in Germany due to cost reasons.

References:
1. Bernhard R (2021) in: Bioanalytik für Einsteiger (Bioanalytics for Beginners) (2nd edition) Springer Spektrum Heidelberg
2. Bernhardt R (2004) Sitzungsberichte der Leibniz-Sozietät (minutes of proceedings of the Leibniz Society) 67, 69–84