Mechanism of action and effects
Levomycetin (Chloramphenicolum) is a classic broad-spectrum antibiotic. Its main site of action is the 50S ribosomal subunit of the bacterial cell. By binding to it, the drug disrupts protein synthesis at the translation stage. By its type of action, it is a bacteriostatic antibiotic: it does not kill microorganisms directly, but halts their growth and reproduction, allowing the immune system to cope with the infection on its own.
Pharmacokinetics and metabolism
The biotransformation of the drug takes place in the liver and involves two important pathways. The first is the reduction of the nitro group involving microsomal enzymes. The second is biosynthetic conjugation reactions, specifically glucuronidation (binding to endogenous glucuronic acid).
A critical pharmacokinetic feature of levomycetin is enterohepatic circulation (circulatio enterohepatica). The drug is excreted by the liver (hepar) into the bile as inactive glucuronides. Entering the intestinal lumen (lumen intestini), these conjugates undergo hydrolysis (cleavage of the bond). This releases the active lipophilic substance, which is easily reabsorbed through the intestinal wall into the portal venous system (vena portae) and returns to the liver. This cyclic process prolongs the antibiotic's action and causes the final elimination of a portion of the dose via the intestine to be significantly delayed.
Drug interactions
The drug requires extreme caution in combination therapy due to two types of interaction:
- Pharmacokinetic interaction: Levomycetin inhibits the activity of hepatic microsomal enzymes. This slows down the metabolism of drugs broken down in the liver (e.g., the indirect anticoagulant warfarin or the antiepileptic drug phenytoin). Their half-life ($t_{1/2}$) increases, sharply raising the risk of toxicity.
- Pharmacodynamic antagonism: As a bacteriostatic agent, levomycetin weakens the effect of bactericidal antibiotics (penicillins, aminoglycosides), whose targets are actively dividing cells.
Side effects and contraindications
- Hemolytic anemia: In patients with a genetic deficiency of erythrocyte glucose-6-phosphate dehydrogenase (a pathology common in tropical and subtropical regions), drug metabolism proceeds with the formation of quinone. This causes acute erythrocyte hemolysis.
- Pediatrics: The drug is strictly contraindicated in newborns during the first weeks of life. In the embryonic period and in infants, liver enzyme systems (especially conjugation processes involving glucuronyl transferase) are immature. This leads to fatal accumulation of toxic metabolites.
- Lactation: Levomycetin passes into breast milk, reaching concentrations of about 50% of plasma levels. Even microdoses can cause allergic or toxic reactions in the infant, making maternal drug intake an absolute contraindication to breastfeeding.
Formulations and prescription rules
According to the prescription reference guide, the drug is available in the following forms:
- Tablets 0.25 and 0.5 g;
- Sustained-release tablets 0.65 g;
- Capsules 0.1, 0.25, and 0.5 g;
- Eye drops (solution in vials 0.25% 10 ml).
Prescribed orally at a dose of 0.25–0.5 g.
Example of a tablet prescription: Rp.: Tab. Laevomycetini 0.5 D.t.d. N. 20 S. Take 1 tablet 3 times a day orally.
Example of an eye drops prescription: Rp.: Sol. Laevomycetini 0.25% - 10 ml D.S. 1-2 drops in both eyes 3 times a day.
Briefly: answers to topical questions
What adverse hematologic effects does levomycetin cause?
Levomycetin causes bone marrow suppression, including reversible and irreversible damage. The main adverse effects on the blood system include:
- Hemolytic anemia — develops in glucose-6-phosphate dehydrogenase deficiency due to quinone formation and subsequent hemolysis.
- Reversible suppression of erythropoiesis — a dose-dependent complication.
- Aplastic anemia — a rare, potentially fatal idiopathic complication characterized by pancytopenia (suppression of erythroid, myeloid, and thrombocytic lineages).
What specific toxic syndrome develops in newborns receiving levomycetin?
Newborns receiving levomycetin develop «grey baby syndrome». This specific toxic complication occurs with the use of high doses of the drug. The pathogenesis of the syndrome is associated with the immaturity of hepatic metabolic enzyme systems in the first weeks of life, specifically glucuronyl transferase deficiency. Because of this, detoxification is impaired, and toxic metabolites of the drug accumulate in the infant's body.
The metabolism of which drugs slows down when co-administered with levomycetin?
Co-administration with levomycetin slows down the metabolism of drugs that are broken down in the liver. Such medications include:
- Phenytoin.
- Warfarin.
The slowing of their metabolism occurs because chloramphenicol inhibits hepatic microsomal enzyme activity. This leads to an increased half-life of these drugs and raises the risk of toxicity.
Which microorganisms are sensitive to levomycetin (spectrum of antimicrobial activity)?
Levomycetin is a broad-spectrum antibiotic. It is active against the following groups of microorganisms:
- Gram-positive bacteria (G+).
- Gram-negative bacteria (G-).
- Intracellular parasites.
In clinical practice, the drug is specifically used to treat infections caused by yersiniosis and typhoid fever pathogens (including Salmonella typhi).
With which antibiotic groups does levomycetin exhibit pharmacodynamic antagonism?
Levomycetin exhibits pharmacodynamic antagonism with bactericidal antibiotics that act on dividing cells. These drug groups include:
- Penicillins
- Aminoglycosides
The mechanism of this interaction is that chloramphenicol, being a bacteriostatic agent, halts bacterial growth and division, thereby weakening the therapeutic effect of bactericidal drugs whose targets are actively multiplying microorganisms.
Levomycetin binds to the 50S subunit: the Roman numeral L stands for 50, which matches the first letter of the Latin name — Laevomycetinum.
Class and exam questions
Why should Levomycetin not be combined with penicillins?
Levomycetin is a bacteriostatic agent; it halts cell division. Penicillins are bactericidal drugs acting only on actively dividing bacteria. Pharmacodynamic antagonism ensues.
What is the reason for the high toxicity of the drug in newborns?
In infants during the first weeks of life, liver enzyme activity, particularly glucuronyl transferase, is reduced. Consequently, conjugation reactions are impaired, and toxic metabolites accumulate in the body.
How does enterohepatic circulation affect the pharmacokinetics of Levomycetin?
The drug is excreted in the bile as glucuronides, which are hydrolyzed in the intestine back into the active substance and reabsorbed into the bloodstream. This prolongs the antibiotic's action.
What hematological complication does Levomycetin cause in populations living in the tropics?
In individuals with hereditary erythrocyte glucose-6-phosphate dehydrogenase deficiency, the drug is metabolized to quinone, provoking hemolysis and the development of hemolytic anemia.
What else to study on this topic
- Protein synthesis on ribosomes: translation and antibiotic targets
- Hepatic microsomal enzymes: inducers and inhibitors
- Conjugation reactions (Phase II of drug biotransformation)
- Pharmacogenetics: Enzyme deficiency and atypical drug reactions
- Perinatal pharmacology: Excretion of drugs into breast milk
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