Mechanism of Action
Metronidazole (Metronidazolum) functions as a classic prodrug. Its bioactivation occurs strictly intracellularly via the reduction of its nitro group.
This process requires a specific condition: a high negative oxidation-reduction (redox) potential within the cell. In the human body, such a potential is characteristic exclusively of anaerobes and microaerophiles (which include protozoa).
Once activated, the reduced forms of the drug exhibit potent cytotoxic effects: they irreversibly bind to parasite proteins, cell membranes, and DNA. This causes severe damage to intracellular structures and inevitable microbial cell death.
Pharmacokinetics and Drug Interactions
The primary route of administration is oral, but alternative routes are widely used in clinical practice: intravenous and rectal (including intravaginal application).
From a clinical pharmacology standpoint, it is critically important to remember that metronidazole is an inhibitor of CYP3A4 / CYP3A5 isoenzymes. These are part of the major enzyme system metabolizing the majority of drugs. Administration of metronidazole slows down metabolism and increases the blood concentration of numerous substrates, among which are:
- Cardiovascular drugs: amiodarone, verapamil, diltiazem, nifedipine, losartan, lidocaine, quinidine.
- Statins: lovastatin.
- Psychotropics: diazepam, midazolam.
- Hormones: glucocorticoids, testosterone, ethinyl estradiol.
- Others: warfarin, cyclosporine, sildenafil.
Indications and Pharmacotherapy Strategy
The drug possesses combined activity and is used for several specific infections:
1. Amebiasis Metronidazole is a universal tissue amebicide. It acts on vegetative forms of amebae regardless of localization (intestines, liver, other organs), but does not affect cysts and has low efficacy against luminal forms.
- For acute invasive interstitial amebiasis (amebic dysentery) and hepatic amebiasis, combination therapy is used: systemic metronidazole is prescribed first to eliminate tissue forms, followed mandatory retreatment with a luminal amebicide (diloxanide) to clear residual organisms from the intestinal lumen.
2. Trichomoniasis Used for the eradication of Trichomonas vaginalis. Both systemic (tablets) and local (vaginal suppositories) forms are utilized.
3. Giardiasis The pathogen localizes in the upper small intestine and bile ducts, frequently causing cholecystitis. Metronidazole is one of the primary agents for giardiasis eradication.
4. Helicobacter pylori Eradication Because H. pylori plays a key role in the pathogenesis of peptic ulcer disease, proton pump inhibitor (PPI) monotherapy is insufficient. Metronidazole is included in combination regimens alongside PPIs (lansoprazole, rabeprazole) and other antibiotics (amoxicillin, clarithromycin).
Side Effects
Administration of the drug is frequently accompanied by dyspeptic symptoms (gastrointestinal disturbances) and the development of a characteristic metallic, bitter taste in the mouth.
A severe and dangerous complication when regimens are violated is the disulfiram-like effect. Metronidazole inhibits the enzyme acetaldehyde dehydrogenase. If a patient consumes alcohol, the oxidation of ethanol is halted at the toxic acetaldehyde stage, triggering severe acute intoxication.
Formulations and Administration
The following formulations and dosages are used in clinical practice:
- Tablets: 0.25 g and 0.5 g. Taken orally with meals, without chewing. Standard regimen: 3 tablets (750 mg) 3 times daily.
- Vaginal suppositories: 0.1 g and 0.5 g. Applied intravaginally at 0.5 g.
- Solution for infusion: 0.5% in 20 ml and 100 ml vials. Administered via intravenous drip at 0.5–1.0 g over 30–40 minutes.