Mechanism of Action and Selectivity
The antimicrobial effect of nitrofurans is dose-dependent: depending on their concentration, they act bacteriostatically or bactericidally. Notably, their activity increases in an acidic environment (e.g., at low urine pH).
The process of microbial cell destruction occurs in several stages:
- Activation. Reduction of the nitro group occurs inside the bacterium.
- Metabolite formation. Highly active amino derivatives are produced.
- Target damage. Metabolites disrupt cellular respiration, inhibit RNA replication, destroy DNA structure, and alter protein conformation (including ribosomal proteins).
The basis of human safety lies in the reaction kinetics: bacterial enzyme systems reduce nitrofurans significantly faster than mammalian host cells.
Spectrum of Activity and Main Drugs
The group features a broad spectrum of activity. Nitrofurans are effective against Gram-positive bacteria (staphylococci, streptococci, enterococci) and Gram-negative bacteria (Escherichia coli, Shigella, Salmonella, Vibrio cholerae). They are also active against protozoa (Giardia, Trichomonas), Chlamydia, and some viruses. An important advantage is their efficacy against strains resistant to several antibiotics.
Drug classification is based on pharmacokinetics and primary indications:
- Nitrofurazone (Furacilin). The most common antiseptic in the group. Used topically (aqueous solutions) for rinsing the mouth and throat in tonsillitis and stomatitis, washing wounds, mucous membranes, and serous or joint cavities. Used as drops for conjunctivitis and purulent otitis media. It does not irritate tissues and stimulates tissue regeneration.
- Chinifuryl (Chinifuryl). Available as an ointment. Prescribed for purulent-inflammatory skin lesions: furuncles, carbuncles, infected burns, and postoperative wound suppuration.
- Systemic agents. These include nitrofurantoin (Furadonin), furazolidone, furazidin (Furagin), and nifuroxazide (Enterofuryl).
Safety Profile and Toxicity
Adverse effects during systemic administration occur with a high frequency.
- Gastrointestinal tract: nausea, vomiting, diarrhea. Risk of pancreatitis and pseudomembranous colitis.
- Neurotoxicity: headache, dizziness, asthenia, peripheral polyneuropathies.
- Hepatotoxicity: chronic hepatitis, cholestatic jaundice.
- Hematological disorders: granulocytopenia, hemolytic anemia.
- Allergic reactions: ranging from skin manifestations, chills, and myalgia to severe conditions (anaphylactic shock, lupus-like syndrome).
- Pulmonary toxicity: a specific entity known as "nitrofuran lung". Acute reactions manifest as cough, bronchospasm, chest pain, pulmonary infiltrates, and pulmonary edema. The chronic form leads to interstitial pulmonary fibrosis. The mechanism involves the generation of free oxygen radicals during the reduction of the nitro group, triggering lipid peroxidation and damaging alveolar cell membranes.
Resistance and Contraindications
Acquired microbial resistance to nitrofurans develops rarely and very slowly. However, natural resistance is characteristic of anaerobic bacteria, Pseudomonas aeruginosa, Klebsiella, and Enterobacter.
The use of nitrofurans is strictly contraindicated in:
- Genetically determined glucose-6-phosphate dehydrogenase deficiency (high risk of hemolysis).
- Severe impairment of excretory and metabolic organ function (renal and hepatic failure).
- Pregnancy (third trimester).
Note: Long-term administration carries a risk of developing renal malignancies.