CYP2C9 isoenzyme
In clinical pharmacology, the CYP2C9 isoenzyme holds a special place because it ensures the metabolism of a number of vital drugs. The molecules interacting with it are divided into three main groups depending on the nature of their effect on the enzyme or their dependence on it.
First and foremost, it is necessary to highlight substrates — those medicinal substances whose structural transformation occurs directly under the action of this isoenzyme. This extensive category includes nonsteroidal anti-inflammatory drugs, the classic representative of which is ibuprofen (Ibuprophenum). The enzyme also actively metabolizes antiepileptic drugs such as phenytoin. It plays an important role in the biotransformation of hypoglycemic agents (e.g., tolbutamide) and oral anticoagulators, among which warfarin stands out.
The rate of enzymatic action is not a constant value. There are inductors capable of stimulating and significantly accelerating enzymatic activity. According to pharmacological characteristics, rifampicin and phenobarbital act as potent inductors for this system.
On the other hand, there are inhibitors — chemical compounds that slow down or completely block the function of the isoenzyme. In the case of this enzymatic system, inhibitors include medications such as diclofenac, various sulfonamide preparations, and cimetidine.
CYP2C19 isoenzyme
The next key component of the metabolic system is the CYP2C19 isoenzyme. Like the previous enzyme, it exhibits strict specificity toward certain chemical structures and drug groups.
The list of substrates for this isoenzyme is quite diverse. It includes benzodiazepine drugs, in particular the tranquilizer diazepam. Additionally, the enzyme is responsible for the metabolism of certain nonsteroidal anti-inflammatory drugs (naproxen) and cardiological beta-blocker drugs (propranolol). Gastroenterological drugs such as proton pump inhibitors (omeprazole) and antiplatelet agents (clopidogrel) also undergo biotransformation via this exact pathway.
Regarding the inductors of this isoenzyme, they are completely identical to those affecting CYP2C9: rifampicin and phenobarbital significantly increase the metabolic activity of the system, making the enzyme work faster.
The group of inhibitors includes the antidepressant fluoxetine and the proton pump inhibitor omeprazole. An interesting feature of omeprazole is the autoinhibition effect: this drug acts simultaneously as a substrate and an inhibitor, meaning it independently suppresses the activity of the enzyme that performs its own metabolism.
Clinical significance of polymorphism (using clopidogrel as an example)
To understand the clinical significance of the cytochrome P-450 system, it is extremely important to examine the phenomenon of genetic polymorphism. This phenomenon is most vividly and clearly demonstrated by clopidogrel, a widely used antithrombotic agent.
The main feature of this drug is that it is a prodrug. The initial molecule is pharmacologically inactive. For the drug to begin preventing thrombus formation, it must undergo a mandatory metabolic activation step in the patient's body. The CYP2C19 isoenzyme is specifically responsible for this critically important process.
Polymorphism of the genes encoding this enzyme leads to drastically different metabolic rates among various patients. In practice, physicians encounter two main types of deviations:
- Reduction or complete loss of CYP2C19 activity. In this scenario, the normal activation of the prodrug is impaired. As a result, an insufficient amount of the active substance enters the systemic circulation, leading to a sharp decrease in the expected therapeutic (antithrombotic) effect.
- Increased CYP2C19 activity. In this case, excessive, pathologically rapid activation of the administered drug occurs. Due to an excessively high concentration of the active substance in the blood, the patient's risk of developing severe bleeding increases manifold.
Briefly: answers to topical questions
Which nonsteroidal anti-inflammatory drugs are metabolized by the cytochrome P-450 system?
The cytochrome P-450 system metabolizes the following nonsteroidal anti-inflammatory drugs, which act as substrates for various isoenzymes:
- Ibuprofen (Ibuprophenum) — metabolized with the participation of the CYP2C9 isoenzyme.
- Naproxen — serves as a substrate for the CYP2C19 isoenzyme.
Additionally, in individuals abusing alcohol, the induction of the CYP2E1 isoenzyme leads to increased sensitivity to nonsteroidal anti-inflammatory drugs.
Which drugs are substrates of the CYP3A4 isoenzyme?
The substrates of the CYP3A4 isoenzyme are:
- Cardiological drugs — amiodarone, verapamil, diltiazem, nifedipine (Nifedipinum), losartan, lidocaine, quinidine.
- Statins — lovastatin.
- Hormonal agents — glucocorticoids, testosterone, progesterone, ethinylestradiol.
- Psychotropic agents — diazepam, midazolam.
- Antibiotics and antifungals — macrolides, ketoconazole.
- Immunosuppressants — cyclosporine (Cyclosporinum).
- Other drugs — warfarin, cocaine, sildenafil, ritonavir.
- Antineoplastic drugs — gefitinib and erlotinib.
- 2nd generation antihistamines.
Which drugs are metabolized by the CYP2D6 isoenzyme?
A wide range of psychotropic and other drugs are metabolized by the CYP2D6 isoenzyme. The main substrates of this isoenzyme include:
- Opioids — Codeine (Codeinum).
- Psychotropic drugs — Haloperidol, Clozapine, Tricyclic antidepressants.
- Cardiological drugs — Metoprolol, Timolol.
- Proton pump inhibitors — Omeprazole.
A feature of this isoenzyme is the absence of known inductors.
What are the inhibitors of the CYP3A4 isoenzyme?
There are numerous drug and food inhibitors of the CYP3A4 isoenzyme. The main inhibitors include:
- Antifungal agents (azoles) — Ketoconazole, Itraconazole, Metronidazole.
- Antibiotics — Macrolides (Erythromycin, Clarithromycin), Ciprofloxacin.
- Cardiological drugs — Verapamil, Diltiazem, Quinidine.
- Antiviral agents — Protease inhibitors (Ritonavir causes the strongest inhibition).
- Other drugs — Cimetidine.
- Dietary inhibitors — Furanocoumarins (dihydroxybergamottin, etc.) found in grapefruit juice, lime, and pomelo.
To memorize the common inductors of the CYP2C9 and CYP2C19 isoenzymes (Rifampicin and Phenobarbital), use a simple association: "Reactive Enzymes" — Rifampicin and Phenobarbital accelerate (induce) the work of cytochromes.
Class and exam questions
What is auto-inhibition using isoenzymes as an example?
Autoinhibition is a process in which a drug suppresses the function of the enzyme that metabolizes it. A classic example is omeprazole, which is simultaneously a substrate and an inhibitor for CYP2C19.
Why is genetic polymorphism so dangerous when taking clopidogrel?
Clopidogrel is a prodrug requiring activation by the CYP2C19 isoenzyme. With reduced enzyme activity, the drug does not work and fails to protect against blood clots, whereas with increased activity, it is excessively activated, posing a threat of bleeding.
Which drugs act as universal inducers for CYP2C9 and CYP2C19?
Potent general inductors that accelerate the work of both discussed isoenzymes are the antituberculosis antibiotic rifampicin and the antiepileptic drug phenobarbital.
What else to study on this topic
- Mechanisms of drug interactions between warfarin and CYP2C9 isoenzyme inductors.
- Pharmacokinetic features of diazepam against the background of CYP2C19 inhibitors use.
- Clinical consequences of the concomitant use of omeprazole and clopidogrel.
- Effect of sulfonamides on the metabolism of the hypoglycemic agent tolbutamide via the CYP2C9 system.
Master the entire topic in the «Pharmacology» course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on the «Cytochrome P450 System» and 5,304 other pages on the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 116 video lessons on the subject
Other topics in "General Pharmacology"
Free video lessons: pharmacologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.