History and classification
Unlike true antibiotics, synthetic agents are created entirely in chemical laboratories. The historical timeline dates back to the early 20th century, when Paul Ehrlich developed arsphenamine. Later, in 1935, Gerhard Domagk discovered the therapeutic properties of Prontosil ("red streptocide"). Inside the human body, this substance broke down, releasing the active component—sulfonamide.
Modern synthetic drugs are divided by indication into antibacterial, antifungal, and antiprotozoal agents (antiviral drugs are traditionally classified as a separate independent group). Basic chemical classes include:
- Sulfonamides
- Quinolones and fluoroquinolones
- Nitroimidazoles and imidazoles
- Nitrofuran derivatives
- Oxazolidinones
Sulfonamides: double blockade mechanism
The basis of the structure of sulfonamides is the para-amino group. Their mechanism of action is based on structural similarity with para-aminobenzoic acid (PABA). They act as competitive antagonists: the bacterium mistakenly captures the drug instead of PABA, which completely blocks the synthesis of folic (tetrahydrofolic) acid. Since this acid is a precursor of purines and pyrimidines, microorganisms lose the ability to reproduce (a bacteriostatic effect is observed).
Today, single-ingredient drugs of this group are rarely used due to high microflora resistance, toxicity, and lower efficacy compared to modern antibiotics. However, the combination drug co-trimoxazole (sulfamethoxazole combined with trimethoprim) has found widespread use. Trimethoprim blocks another enzyme in the same metabolic chain. Such a double blockade produces a powerful synergistic effect, changing the type of action to bactericidal. The drug is actively prescribed for urinary tract infections caused by gram-negative flora.
Evolution of quinolones and fluoroquinolones
The first generation of this group (classic quinolones, a prominent representative being nalidixic acid, used since 1962) had a narrow spectrum of activity and was intended exclusively for the treatment of urinary tract infections. A serious drawback was the rapid development of bacterial resistance.
Modern fluoroquinolones (ciprofloxacin, norfloxacin) contain fluorine atoms in their molecule, which dramatically improved their pharmacological properties:
- Type of action: bactericidal.
- Spectrum: ultraspread, including Pseudomonas aeruginosa, mycobacteria, and intracellular parasites.
- Pharmacokinetics: penetrate blood-tissue barriers excellently, creating therapeutic concentrations in organs and tissues.
- Safety: characterized by good tolerability and a low risk of resistance development.
Special-purpose and reserve drugs
Nitroimidazoles (e.g., metronidazole) possess unique selectivity: they are reduced and converted into a bactericidal form only inside anaerobic cells. Their targets are obligate anaerobes and protozoa (Entamoeba histolytica, Giardia, Trichomonas).
Nitrofurans (furazolidone) are agents with a broad bactericidal spectrum. Their most important feature is that they accumulate in high concentrations in urine. Due to this, they are used as effective urinary antiseptics.
Oxazolidinones (linezolid) were created as "heavy artillery" to combat severe infections. They are active against gram-positive flora (acting bacteriostatically on staphylococci and bactericidally on a number of other microbes). The main clinical niche of this group is the treatment of infections caused by multiresistant pathogens: methicillin-resistant Staphylococcus aureus (MRSA), as well as pneumococcal and enterococcal strains that have acquired resistance to penicillin and vancomycin.
Briefly: answers to topical questions
What is the molecular mechanism of action of fluoroquinolones?
The molecular mechanism of action of fluoroquinolones consists in inhibiting bacterial topoisomerases at the DNA strand break stage. The drugs stabilize the DNA-enzyme complex, which leads to the destruction of the DNA polymer.
- In gram-negative bacteria, the main target is DNA gyrase (type II topoisomerase).
- In gram-positive bacteria, the main target is topoisomerase IV.
Fluoroquinolones suppress enzymes until the second segment of DNA passes through the break. As a result, double-strand DNA breaks are formed, making replication and transcription impossible. The subsequent cellular response to the breaks (SOS response) leads to cell death, providing a bactericidal effect.
What side effects are typical for sulfonamides?
Sulfonamides are characterized by allergic and skin reactions, organ toxicity, and systemic disorders. The main side effects include:
- Allergic and skin reactions — fever, pruritus, rash, photosensitization.
- Organ toxicity — dyspeptic disorders (gastrointestinal tract) and hepatotoxicity (liver).
- Hematopoiesis disorders — anemia, leukopenia, thrombocytopenia.
- Kidney damage — crystalluria occurring in an acidic urine reaction.
- Drug-induced pathologies — periarteritis nodosa.
- Specific complication in newborns — kernicterus, occurring due to the displacement of bilirubin from its bond with albumin, leading to severe toxic damage to the brain.
Which drugs belong to first-generation quinolones?
Generation I quinolones include classic non-fluorinated drugs:
- Nalidixic acid (Negram, Nevigramon).
- Oxolinic acid (Gramurin).
- Pipemidic acid (Palin).
Classic generation I quinolones are characterized by a narrow spectrum of action, rapid development of resistance, and use in urinary tract infections predominantly caused by gram-negative bacteria. Oxolinic and pipemidic acids are analogues of nalidixic acid, possessing similar pharmacological action and higher activity.
What is the exact mechanism of the bactericidal action of nitroimidazoles after their reduction in the cell?
The exact mechanism of the bactericidal action of nitroimidazoles after the reduction of their nitro group is implemented through two main pathways of cell damage.
- Respiratory impairment — the reduced drug incorporates into the respiratory chain and blocks flavoproteins (electron carriers), which causes a failure of respiratory processes and cell death.
- DNA damage — upon reduction of the nitro group, free radicals are formed, exerting a direct toxic effect on DNA.
In addition, the reduced forms of the drug bind to the pathogen's proteins, membranes, and DNA, causing severe damage to them, which provides a pronounced cytotoxic and bactericidal effect.
To remember the targets of nitroimidazoles (metronidazole), use the association "A-P": they are strictly active against Anaerobes and Protozoa.
Class and exam questions
How do synthetic antimicrobial agents differ from antibiotics?
Synthetic agents have no natural analogs and are created exclusively by chemical means in the laboratory, whereas true antibiotics are of natural or semisynthetic origin. However, their mechanisms of bacterial destruction are similar.
Why is co-trimoxazole more effective than conventional sulfonamides?
It consists of two components (sulfamethoxazole and trimethoprim), which sequentially block the synthesis of bacterial folic acid at different stages. This provides mutual reinforcement (synergism) and a bactericidal effect.
What is the main advantage of fluoroquinolones over early quinolones?
The introduction of a fluorine atom into the molecular structure expanded their spectrum of action, made the drugs bactericidal, provided an excellent ability to penetrate tissue barriers, and sharply reduced the frequency of microbial resistance development.
What are oxazolidinones used for in clinical practice?
Linezolid is used in extreme cases for the treatment of severe infections caused by multiresistant gram-positive flora against which other drugs are powerless (e.g., infections caused by MRSA strains or vancomycin-resistant enterococci).
What else to study on this topic
- Mechanisms of bacterial resistance formation to classical sulfonamides
- Pharmacokinetics of fluoroquinolones: features of overcoming blood-tissue barriers
- Biochemical bases of synergism in combination drugs (using co-trimoxazole as an example)
- Molecular mechanisms of activation of nitroimidazoles in an anaerobic environment
- The problem of multiresistant strains of gram-positive cocci in modern surgery and intensive care
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